Both galaxies and stars (see page 20-5) come in diminutive sizes to which the term "dwarf" is applied. A Dwarf galaxy can be irregular in shape but spheroidal, elliptical and compact varieties occur.
Cena-Centaurus is classed as a Dwarf Elliptical that is nearly spherical and possesses a huge ring of dust and gas around its "equator": Two further categories of dwarfs: gas-rich, with active star formation, and gas-poor, with dominantly old stars. The paucity of gas may mean that supernovas in the galaxy were able to drive off (expel) that gas because the low numbers of stars fail to provide enough gravitational stability to retain the extra-stellar gases. A Dwarf galaxy is typically around 6000 light years in maximum dimension, contains from 1 to 10 million solar masses of gas made up of atomic hydrogen (larger galaxies contain dominantly molecular hydrogen). The dwarfs may be very abundant throughout the Universe, and some current estimates hold them to be numerically more common than the billions of spiral and elliptical types (but because of their smaller sizes, most still remain undetected because of their much lower luminosities. Some Dwarf galaxies, if locally in sufficient numbers, likely merge to form larger galaxies that then evolve into Spirals or Ellipticals (again, see paragraphs near bottom of this page). Because of its small size, such a galaxy is more often detected as a source of infrared and radio radiation. However, a few have been imaged optically, such as NGC 4214 (top) and NGC 5253 (bottom): Embedded in these views are bunched clots of very hot, bright stars (each, typically, around 20 solar masses). These are created intermittently over short periods of time (for most of their history, dwarf galaxies tend to be quiescent in terms of new star production) at rates up to 100 times greater than spiral galaxies (in the latter, a new star is generated on average once a year over the entire system). They result in what are termed "starbursts", a situation that is a hallmark of dwarfs. The Arches cluster is a good example:
During the bursts, the entire gaseous cloud is heated (thus increasing its radiation and improving detectability). These sporadic bursts are postulated to be caused by interactions with larger galactic clouds or by collisions between two dwarfs. Because most of the dwarf's hydrogen gas is not converted into elements such as carbon, nitrogen, and oxygen (see page 20-7), this galaxy type is primitive, more like the galactic clouds in the early Universe. The importance of Dwarf galaxies is that they could be a major contributor to the as yet unaccounted for (missing) mass (see below) needed to control the Universe's expansion (see page 20-9). Clusters of very massive, bright (blue-white) stars are much rarer, since stars of this size tend to burn out far more rapidly. Here is an unnamed cluster of large stars which is probably not a starburst. Another category of small galaxies has been named UltraCompact Dwarfs (UCDs). These are as small as 120 l.y. across, yet contain tens of millions of smaller stars. They ae believed to be remnants of once larger galaxies that have been stripped of most earlier stars and nebular matter. A recent study of the Fornax cluster of galaxies (about 60 million l.y. away, visible in the southern hemisphere) has found 46 such UCDs. Consider this image: The POX 186 UCD is larger, about 900 l.y. across, and irregular in shape: Still other star groupings as seen now contain as few as ten thousand individuals to a million or more. They have been called halo clusters. Uncertainty persists as to whether these simply did not ever grow larger or represent once bigger clusters that have lost much of their population. This next image, made by a Palomar telescope involved in the on-going Sloan Digital Sky Survey (SDSS), shows one such cluster 5000 l.y. away in the Milky Way's halo in which mass is being removed as a pair of opposing tails owing to gravitational disruption as the group passes in a high angle orbit through the halo to distances where the central mass of stars exerts enough force to be pulling the cluster apart. Astronomers think now that shredding of halo clusters over time is the norm. Another category of galaxies is referred to as irregular or sometimes peculiar. Though usually having much fewer stars that the spiral or elliptical galaxies, these still contain millions of stars. This group of four, found at distances of several billions of light years, illustrate the seemingly poorly organized morphologies of irregulars: One type of irregular galaxy is thought to be caused by two galaxies in process of colliding (see below), as seen in this HST image: Another class of irregular galaxies is known as faint blue galaxies. The HST has located large numbers of these at distances from 3 out to 8 billion light years. Several faint blues appear in this mid-field image: The population of faint blue galaxies seems to increase the further out (back in time) a telescope can conduct an inventory. This accounts for the "faintness" of these hot star groups. That suggests these, and at least some of the irregular types, formed early in Universe time. Some never organized into the regular types or have been dissipated by burnout of most of their stars. This view is supported by HST's discovery of a galaxy approximately 13 billion light years from Earth, in the process of forming during the initial organization of galaxies. This galaxy, one of the most distant yet found, appears as an arcuate red smear, which is a distortion of its probably irregular shape by the process of gravitational lensing described in the Preface of this Section. Individual galaxies of the main types are separated by distances of up to millions
of light years. Despite these large separations, collisions between galaxies
(one tends to "pass" through the other) have been observed (see page 20-4) and may
well be common. Galaxies tend to be grouped in great clusters or even superclusters
(collections of proximate clusters) arranged commonly in elongated bands or
strings that began aligning about 1-2 billion years after time zero. The elliptical
galaxies are more abundant than spiral in these superclusters. Intergalactic
space between superclusters contains lower numbers of galaxies, in regions the
decrease may be so sharp as to warrant the term "voids", meaning a dearth of gtouped galaxies. The distribution of the different types of galaxies (each of which varies over a range of absolute magnitudes [expression of luminosity; see page 20-5] depends in part on the density of galaxies in a given volume of space. Consider this diagram:
Beyond clustered galaxies (see below), Spiral galaxies (S) are four to five times more common than Elliptical (E); both have similar magnitude ranges, although the compact Ellipticals usually are more luminous. The Irregular (Irr) galaxies and the more abundant Dwarf Ellipticals (dE) are fainter (smaller negative numbers), so that they are hard to detect; the numbers of such dwarfs have almost certainly been underestimated because of the difficulty in detecting them. The diagram
below displays the distribution of approximately 2500 bright galaxies within
a 10 Mpc thick wedge of outer space extending out to 300 Megaparsecs (Mpc) over
a declination ranging between 26.5° and 44.5° (a parsec is a distance
measure based on parallax methods; it is ~3.26 light years). This survey is
part of an on-going sky count being made by Margaret Geller and associates of
Harvard's Center for Astrophysics. The plot clearly indicates that many galaxies
organize along distinct linear clusters described as "filaments; strings; walls". In most clusters, the elliptical type of galaxy is much more common than are spirals - a reversal of the proportion when inventories outside of clusters are established (thus, in most of space spirals exceed ellipticals by a ratio of about 4.5 to 1).
Some volumes of space have low populations of clusters, indicating voids. The
reason for clustering is still uncertain but probably is set up by variations
in matter distribution during the early stages of expansion (perhaps even extending
back to fluctuations during the inflationary period in the early fractions of
the first cosmic second).
The 2MASS Sky Survey program, conducted jointly by the University of Massachusetts and CalTech, is plotting the location of galaxies observed in infrared light over the entire sky (both hemispheres). It has published maps of galaxies out to ever increasing distance from Earth, including this plot of galaxies out to 500 million light years from Earth:
In March of 2003 the 2Mass team released a "completed" survey of the entire sky out to the farthest reaches detected. Here is their map in which closer galaxies are in blue and the farthest in red (there is a dark area in the central part that represents the location of the Milky Way, after its star contribution had been subtracted). In making this color version, the 2.2 µm band = red; 1.6 µm = green; 1.2 µm = blue. Thus, in this rendition a sense of 3-D distance out towards the cosmic horizon is given by the color scheme: closer in is blue, intermediate is green, and the more distant galaxies are red.
In mid-2000 preliminary results of a more extensive count (~106,000) of galaxy distribution with distance in two slices of the celestial sphere (each about 75° across, 8 - 15° thick, and out to ~ 4 billion light years from Earth), known as the 2 Degree Field Redshift Galaxy Survey, was announced at the annual American Astronomical Union (AAU) meeting. This is the map presented there:
At first glance, the distribution is a bit illusory. At ever farther distances, this map seems to indicate a decrease in the numbers of galaxies, which would seem to defy the Cosmological Principle which says that the Universe appears to be isotropic and homogeneous at large scales. But, remember that current detectability decreases with increasing distances. And, perhaps also, the number of galaxies has been increasing with time so that closer to Earth, where one sees younger and younger events (light has not traveled as far and thus represents later departure from the source), there are newer galaxies added to those formed earlier than 4 billion years. The clusters of voids and filaments is masked somewhat at this scale owing to the large numbers involved, but this structure still persists when the map is examined in a large sheet. The structure is less evident when only galaxies close to the Milky Way are depicted, as in this image which includes galaxies at distances less than 1 billion light years.
One of the largest Sky surveys to date uses the APM (Automatic Plate Measurement) technique to image galaxies betwee Magnitude 17 to 22. Here is a composite made by the University of Nottingham that contains at least 2 million galaxies over a 100° portion of the sky outward from the Earth's South Pole. Individual galaxies can be resolved in enlargements but in this rendition the sky seems filled with galaxies (most rendered in red, indicating smaller size.
A website that lists many of the ongoing surveys of the distribution of galaxies and other large objects beyond the Milky Way is accessed by clicking on Sky surveys.
By far the most ambitious celestial survey (begun in 1998) is the Sloan Digital Sky Survey (SDSS), whose Home Page is accessed through this link. When completed, approximately a quarter of the sky will have been mapped in detail (including spectra, redshift data, etc.) out to considerable distances. The goal is to map and locate about 100 million celestial objects (stars, quasars, galaxies) and redshifts for one million of these. A project involving 8 institutions (University of Chicago is lead), the SDSS will rely on several telescopes, the principal being this 2.5 m instrument at Apache Point in the Sacramento Mountains of New Mexico (about 100 miles northeast of El Paso, TX). Results are being released piecemeal as more data are acquired. Here is a recent sky map from the SDSS effort: In each of both top and bottom views is an elongated cluster of thousands of galaxies. These have been termed "Great Walls" and can extend up to a billion light years in length. One of the Great Walls is shown in this plot: The University of Chicago Cosmological Physics website has a page devoted to the successive growth of galactic clusters, in which computer-generated schematic pictures indicate likely steps in the process from right after Recombination (at high z values, which denote the appearance of the farthest galaxies at early times in the young Universe (see discussion of redshift on page 20-9) through times near the present (low z values). Four of the panels taken from their web site are reproduced here:
This strong tendency for galaxies to cluster into linear or planar arrangements may owe its origin to the early moments of the Big Bang. Gravitational waves and/or other processes may have concentrated energy and mass in thin tubelike patterns called cosmic strings that extended for millions of light years in various directions within the growing Universe. Because of their higher densities, the strings served to attract and draw in matter that eventually organized into centers of star formation that developed into galaxies located around these narrow lines of stronger gravity. Proof of the existence of cosmic strings is still speculative. Cluster sheets like the "Great Wall" that intersect other sheets produce
Superclusters in a honeycomb-like network with dimensions exceeding 100 million light years (See the Virgo Cluster illustration on this page for another typical example).
The conditions favoring formation of networks of galaxies are still being worked out. It depends on the type of dark matter that was involved (see page 20-9) Two scenarios are being considered. In the Cold Dark Matter case, galaxies began as individuals and small groups and then organized into the filamentous networks now observed. In the Hot Dark Matter case, The filamenous strings of matter developed first and then broke up into individual anc clusters of later-forming galaxies. This next Hubble image shows the region called c10024+1654, noted for the various galaxies lying at that distance (about 5 million light years). The number of galaxies here is small but galaxy clusters can contain a thousand or more individual members. Another type of galactic clustering produces huge globular concentrations. The Coma Cluster is a grouping of several thousands of individual galaxies in a loosely spherical spatial distribution that is 1.5 million light years across. An x-ray image made by the orbiting Chandra telescope (see next page) shows this clustering. Between galaxies is low density intergalactic gas (mainly hydrogen) whose kinetic temperatures reach one million degrees Kelvin. Lower temperature clouds also can be discerned; these may eventually form new galaxies. The history of clustering shows a variable but fairly consistent pattern. The Chandra X-ray telescope has image Abell 2125, a cluster of clustered galaxies lying about 3 billion light years from Earth. In the image below, the bright large blobs of light in the upper left actually are a large cloud of hydrogen gas enveloping hundreds of (unresolved) galaxies. In the lower right is a diffuse cloud of hydrogen gas (again with individual galaxies unresolved) that is poorer in iron content; iron enriches in galaxies as stars produce the heavier elements, expel these during supernova events, and recollect the iron in newer stars. This galactic cluster is thus less evolved then the brighter, more concentrated iron-rich cluster in the upper left. As these galaxy surveys add more information about the structure of the Universe, it appears that clustering and filamentation - visually different from isolated galaxies - still seems to be more or less uniformly distributed over all observable space. Thus Einstein's Cosmological Principle, that the Universe is to a first approximation homogenous and uniform over a range of (volume) scales is obeyed as the largest scales are better observed and mapped. Also, continuing investigations are revealing that the cluster-filament structure was influenced from the outset of galaxy formation by dark matter (most astronomers favor the Cold type). The early Univeres was probably more clustered than today, since expansion has stretched out and even disrupted the initial filaments and/or has combined some of these. The Hubble Space Telescope has greatly increased the number of galaxies that can now actually be seen; cataloging of a significant fraction of the new HST individuals in the northern celestial hemisphere is underway and more detailed observations in the southern hemisphere have been initiated. But, because of the huge numbers involved, only estimates of the total can be made by sampling segments
of the observable Universe and then extrapolating throughout the celestial sphere
encompassing the Earth. Thus, an unsophisticated (lower limit) guess at the total number of stars in our Universe can be set forth as the product of the approximately 10 billion (1010) galaxies times the number of stars contained in typical
galaxies (another 1011) or at least 1021 stars. However, be advised that the outer reaches of the observable
Universe have yet to be seen, if indeed one can imagine such limits (there is
no proof or reasonable assumption that demands the Universe to be bounded or
finite).
Returning close to "home", the Solar System is embedded in a spiral galaxy (our star, the Sun, born about
5 b.y. ago, is positioned just over 1/2 out [about 27000 light years] from the
galactic center which lies visually within the constellation Sagittarius as
a backdrop). As seen from Earth the rest of the galaxy, with its high density
of nearby stars (closest to the Sun is Alpha Centauri, 4.2 l.y. away; see image on page 20-5), is named
colloquially the "Milky Way" (M.W.) (because the disc in which we are embedded
resembles a diffuse "milk"-like band across the night sky). Age estimates
for the Milky Way's inception fall to at least 11 billion years. Because nearly all galaxies are so much farther away, so that telescopes still cannot resolve stars within them nor can they see details of the internal structure and of surrounding objects within their halos, astronomers of necessity have intensively studied the M.W. and its environs with the supposition that it is fairly typical of most spiral galaxies. Hence its features are likely to recur in variations within the spiral galaxy class in general.
Like most spiral
galaxies, the bulk of the stars occurs within the spiral disc which extends to a diameter of about 100000 light years and has a mid-disk thickness of 3000 l.y. The maximum
number of stars (at least 100 billion) lie in the central region. Much lower numbers (besides globular clusters, about 50 million isolated individuals) are found in the "halo"
that extends above and below the disc. These are tied gravitationally to the
galaxy (which itself maintains its integrity of motion - and stays together
- owing to a great deal of mass, most of it invisible or non-luminous). The arms of the Milky Way are in rotational motion around its center; the Solar System makes a complete revolution about that center in 225 million years.
As a reminder of what we think is the organization of the Milky Way (and of spiral galaxies in general), we show a somewhat different version of the figure presented on the previous page, in which the structure of the central galactic disk is subdivided into distinct groupings of stars:
Thus, most of what we think we know about other galaxies and star behavior in the galaxies beyond comes from studies of our host galaxy. Because of its proximity - actually the Sun is within it - astronomers have obtained exceptional photos and images of the entire Milky Way. This one is spectacular. It was made by Sylvestre Lacblanc using a camera that took a continuous color-filtered photo for 40 minutes, keeping the camera moving in accord with the Earth's rotation. Taken in the Swiss Alps, he combined the celestial photo with a nighttime photo of the nearby mountains. To photograph other parts of the Milky Way, the observation must be made from the Southern Hemisphere. Here is a long segment of the M.W. seen from Africa in a photo taken by John Gleason. The several spiral arms of the Milky Way have been named. This is an artist's painting of the Milky Way, with the Sun and several other named stars located within their respective arms: The location of the Sun is indicated in this generalized side view: As seen through a 16 inch telescope, a part of the Milky Way cutting across the constellation Sagittarius is shown to consist of many stars mixed with gaseous clouds: A large
part of the Milky Way is imaged in this infrared view obtained by the COBE (Cosmic
Background Explorer) satellite (described on page 20-9).
Much the same picture of the Milky Way, from the perspective of our position within it, is shown in the visible in this composite infrared image made by the 2Mass Sky Survey inventory described earlier. The number of stars (of all sizes and states) in the Milky Way approaches at least 100 billion and may be as much as 300 billion. One M.W. star has now been accurately dated to be about 13 billion years old, using a measurement of beryllium (synthesized in the first stars from low atomic number element fusion) of star composition determined spectrally. This means that the M.W. began to form (probably as a globular cluster that continued to grow in star numbers into the billions as it evolved into a spiral form) early during the half billion year interval after the Big Bang during which most of the galaxies organized. The age of the M.W. has been determined by several methods - one involves element concentrations (such as beryllium) in stars in the nearby NGC6397 globular cluster (there are many such clusters still in the galaxy).
A recent image of just a tiny segment (13 light years wide) near the center
of the Milky Way (in the vicinity of the Sagittarius constellation) was obtained
by the Hubble Space Telescope (see below), as seen here, with some of the individual stars being Red and Blue Giants (see page 20-5).
A somewhat different perspective on the core region is afforded by this optical telescope (the 8.2 meter VLT YEPUN) image, in which the two close-spaced yellow arrows point to the Sagittarius-A region which appears to be the M.W. center, at which evidence grows that a Black Hole exists there (see also page 20-5 for a radio telescope image of the immediate central region). Hot stars are blue, cooler red; clouds of dust show up as diffuse red areas. Infrared images show myriads of stars not always observable in visible light. The 6.5 m Magellan telescope in Chile produced this image, which is 6 l.y. across, with many of the stars shown not having any resolvable counterparts when observed at X-ray wavelengths: A great deal of dust obscures these stars in the central budge when viewed in visible light. In the Near IR, much of the dust becomes transparent to that radiation wavelength but a narrower central band of dust remains inpenetrable. On a grander scale, HST has imaged both the Milky Way's spiral disk and the myriad of stars that surround it as a halo. Thus: An announcement made on January 6, 2003 at the annual meeting of the American Astronomical Society tells of a new discovery about our galaxy. Just beyond the spiral arms of the M.W. evidence has been found for a ring (distinct from the halo) of about 500 million stars, in a denser concentration than its surroundings. Actually, observers have so far found only segments of this still not fully observed ring and are now engaged in searching for the remaining segments if this is indeed a ring. Still so new that speculation abounds, theorists have considered this evidence of remnants from an earlier collision with another, smaller galaxy. Here is the artist's conception of the ring, part of a press release from the AAS meeting (note the strong similarity to Hoag's Object, shown on the previous page as an actual HST image): It is practical to display stars close to the Sun and within our galaxy, and named or numbered galaxies themselves in a series of drawings and plots that better illustrate astronomers' conception of the large scale structure of galaxies throughout the Universe. This type of plot can be seen at Jerry Pool's An Observation of the Night Sky. Fourteen such maps, beginning with stars within 12 light years of the Sun and ending in deepest space, are accessible at this Internet site. Some of the plots are reproduced here, after downloading and improving their display characteristics (in resizing, the print tends to become poorly legible). Several others that show only galaxies appear below. Others that download as too large to display after reduction in size can be checked out at Pool's site, which retains legibility for all maps.
The first drawing shows only stars, out to a distance of 12 light years:
The next plot encompasses more stars out to 20 light years: The third map, extending to 250 light years, still contains only stars in our neighborhood in the Milky Way Galaxy, some of which are named here: The Milky Way is the central player in a cast of additional smaller, irregular galaxies and other stellar clusters that remain within the M.W. halo. Here is a map of some of these objects: Most prominent are two irregular satellite galaxies - the Large and Small
Magellanic Glouds - that are respectively about 163000 and 190000 light years away. These galaxies (among the dwarf group) are part of the Local Cluster described below and are themselves orbiting the Milky Way. When seen in a broad field (non-closeup) mode through a ground telescope, they appear as cloudy "smudges" that do not reveal their makeup as harboring millions of stars:
A closeup telescopic view of the Large
Magellanic Cloud, made by the Anglo-Australian Observatory, is shown here; note that it is more a small nebular mass than a distinct galaxy. Because of the irregularity of the arms (not in a spiral configuration) the LMC is also known as the Tarantula Galaxy.
The Small Magellanic Cloud is shown here in another telescope view made at a different United Kingdom Observatory: The Hubble Telescope reveals much more about the stars within these "Clouds". Here is a star cluster in the Large Magellanic Cloud: Most M.W. satellite galaxies are of the Dwarf category. Closest (about 50000 l.y.) to the Milky Way disk is the Sagittarius Dwarf galaxy, shown here in an optical telescope image: Our galaxy is part of the Local Group which includes galactic bodies in a volume of about 10 million light years in diameter. Including the M.W. there are three spiral galaxies, globular clusters, irregular and dwarf galaxies in the assemblage of galactic types. Here is a map of most of its (up to about 40) major members:
However, recent studies by Cal Tech astronomers using the Keck Telescope to track orbital paths of stars beyond the visible manifestation of Andromeda disclose that nearby stars follow the rotational motion of Andromeda, i.e., are part of the gravitational array of Andromeda stars. With this information the diameter of Andromeda has been adjusted to about 200000 l.y. This is a Keck image of the expanded galaxy, colored to indicate the extent of Andromeda when these stars are added:
Another prominent nearby spiral galaxy is the Triangulum Galaxy (M33), seen here: Beyond the Local Group is the nearby (60 million light years away) Virgo Supercluster (containing about 250 large galaxies
and up to 2000 smaller ones); the individual galaxies in this broad grouping are moving (separating) relative to each other as space itself expands. However, these clusters and a large number of other galaxies (e.g., Centaurus) in this region of the Universe, when their general directions of movements are plotted as vectors, appear to be converging on a center of mass (colloquially referred to as the "Great Attractor") which may itself define superclustering on an even larger scale. The three-dimensional appearance of the Virgo Supercluster can be visualized with the aid of this artist's rendition, which attempts to show how galaxies and galaxy clusters tend to be arranged in long filamentous "strings" containing the galaxies and hydrogen gas: This next group (map taken from the aforementioned website produced by Jerry Pool [see above]) shows only galaxies and contains the Virgo Cluster (found in the constellation that includes the star Virgo) which is part of the so-called local Supercluster: This (somewhat blurred) map identifies galaxies and galaxy clusters across a field of view 400 million years across.
We jump now to a map that carries out to 1,000,000,000 light years which includes the supercluster the Milky Way lies within: The last map is a flattened view to 2 billion light years from Earth. Both filament clusters and individual superclusters are shown; many have been named: What is the fate ("death") of a galaxy? While individual stars may "disappear" through loss of luminosity (see page 20-6), their "corpses" - neutron stars and black holes" remain. There is no evidence that galaxies are destroyed, although some lose their identity if they merge or collide with another galaxy. Since the beginning of galaxy formation, these stellar aggregates have been moving apart at rates that have varied with time. The ultimate fate of all galaxies as they continue to separate is that their smaller stars will finally die as they burn up their nuclear fuels, and there is insufficient hydrogen available within or near them to form significant number of new stars. Estimates of when this will occur generally place this burn-out as at least 50 billion years in the future, and some cosmologists argue for even longer periods of continued galactic survival. Several comments about galaxy ages: The majority we can detect through telescopes or other instruments are old: Evidence is growing that the first galaxies may have organized before the half billion year mark in the evolving Universe. The bulk now extant had their inceptions as protogalaxies in the first billion years of the Universe. Well-formed galaxies had evolved by 2 billion years and these were fully matured by 8 to 9 billion years ago. Yet, when we look out into space (and back into time), the oldest-appearing are near Earth and the youngest farthest away, even though the majority may have developed early. This paradox is illusive - caused by travel distances of light to Earth from galaxies at different parts of an expanding Universe - the rule is that the farther away a galaxy is from our observation point, the longer (greater time) light has taken to reach us and, as a corollary, the earlier is the stage of development of the more distant galaxies. This is treated in detail on pages 20-8 through 20-10. However, within any given galaxy, its component stars have a wide range of ages - some were destroyed long ago, some have survived from the youthful stages of a galaxy's history, and some have formed or are forming in recent cosmic history. The Hubble Telescope is capable of looking through observable space to the outer fringes of the observable Universe. Galaxies far out would be too faint to be seen during the usual exposure times. Thus, the HST scientists have initiated the Deep Field (also referred to as the Hubble Deep Field) program to try to obtain images of the more distant galaxies. This involves multiple observing sessions that provide repeated exposures, up to 10 or more days, through the Wide Field Camera of the same narrow segments of the celestial sphere, so that the superimposed images are additive in photon energy received. Many Deep Field views have been made, of which the one below is typical. However, most of the galaxies in this image are not in the farther regions of space. Galaxies ranging from close in to far out are included in the conical segment of the observed sky. The farthest generally appear as small shapeless objects that are faint to moderately bright (in a few views some can be separately with limited certainty into spirals or ellipticals). What one is also seeing in such a Deep Field image is also a time distribution of the galaxies (see previous paragraph): better defined galaxies are larger, more structured, and appear as they were at the cosmic time when light left them where/when located then in the expanding sphere of galaxies; these are also in more advanced stages of development; the fainter, more distant galaxies appear in the image at younger stages of development . Detecting very distant galaxies (seen as they were in the first few billion years after the B.B.), requires long exposures (hours to days) in which the telescope is programmed to dwell on the same small area of the sky even as the stars move as the Earth rotates. This next pair of images (provided by Prof. T. Shanks group at the University of Durham) used two earth-based telescopes that image a very small area of the sky. Most of the bright points are galaxies at various distances from Earth. The left image is made in visible light; the right in infrared light. Note the area within an open cross in the left view: nothing is evident within. In the right image a yellowish galaxy shows up (in the IR). Its distance, in terms of a redshift greater than 4, is at least 10 billion light years. The view below is oft cited as the "type" Hubble Deep Field image. It was made by repeated exposures totaling 10 days of acquisition time by the Advanced Camera for Surveys (ACS) pointed at the same small part of the sky. Of course, only a few of the star-like (actually galaxies) bodies are very distant. A logical question to ask here is what are the limits of "Deep". At first, the farthest galaxies were said to be about 10 to 12 billion light years away. Later, "Deep" came to have a 'deeper' meaning, namely those galaxies that were formed in the first billion years or so after the Big Bang. Some place anything "Deep" as between 12 and 15 billion light years away. As we stated two paragraphs above, only a fraction of the galaxies visible in a Deep Field view are really 12 or more billion years distant. One way to illustrate this is shown in this Hubble Deep Field image, in which the numbers shown next to galactic bodies are their computed red shifts (see page 20-9 for a review of this topic). Small numbers are only slight shifts and hence associate with closer galaxies; large numbers are more distant (that at z = 4 is at least 12 billion light years away).
Another of the best Deep Field images obtained to date was made by the Advanced Camera for Surveys (ACS) on the HST. This camera, peering through the Abell galaxy, has picked up many distant galaxies, several of which may be approaching 13 billion l.y. away, and hence less than 2 billion years old. Note the gravitational lensing (arcs) associated with several of the galactic objects; this lensing effect can cause an increased concentration of light, aiding in making the old galaxies more visible.. In this next Deep Field view, two red dots in the enlarged inset (see main image for outline) are early Universe galaxies as they appeared at approximately 12.6 billion (light) years before the present. These are among the most distant from Earth found to date. The bulk of these older galaxies have redshifts around 5-6. As of March 1, 2004 one of the oldest galaxy found so far, in terms of a redshift of close to 10, yields an age of 13.23 billion years, which is just 470 million light years after the Big Bang, assuming the 13.7 billion year age (WMAP estimate; see page 20-9) for that event is close to actuality. It was discovered using a ground-based telescope using the near IR segment of the spectrum. Consider this image:
If 14.7 billion years is the best estimate of the Universe's life span, then the deepest space telescopes have penetrated back in time (and space) is displayed in this composite of HST and a Spitzer Space Telescope images. The circumscribed feature is a massive galaxy (at least 10x larger than the Milky Way) as it appeared about 800,000 years after the Big Bang: In all these Deep Field images the galaxies identified as very far way (using mostly redshift measurements to gauge distances; see page 20-9) appear very small compared with images of the myriads of closer galaxies we have already seen. Some of the properties of the Deep Field types can be determined but much important data are not obtained owing to the low resolution which prevents measurements of important defining information. Thus, there is uncertainty about the general galaxy types, distribution of stars, galactic structure, etc. during the first billion years of cosmic time that will have to await the Next Generation Telescope (see page 20-2). The Deep Field Program has been supplanted by the GOODS [Great Observatories Origins Deep Survey]) program. This program looks at two much larger segments of the sky (60 times the area of the celestial sphere that was looked at by the DFP), one in the northern celestial hemisphere, the other in the southern hemisphere. The participating Observatories are the Hubble Space Telescope and the Chandra X-ray Observatory (together with some data from the Compton X-Ray Observatory). They will be joined by the Space Infrared Telescope Facility (SIRTF), scheduled for launch in August of 2003. The spots in the sky segments surveyed that show distant galaxies in each Observatory panorama will be co-registered to synergize their interpretations. An overview of this program is accessed on the Net at GOODS. This GOODS view, made by the ACS on HST, shows a much wider field of view in which most of the small "points of light" are Deep Field galaxies lying from 10 to 12 billion light years from Earth. Chandra, not to be outdone by its Hubble partner among the Great Observatories, has produced its own Deep Field view. Here then is the x-ray version of very distant galaxies in the outer regions of the Universe:
In both these images, larger galaxies are closer; the distant ones are mostly Deep Field candidates. In the next illustration a part of the Hubble Deep Field is shown. Distant galaxies within it are circled and their counterpart view by Chandra appears in the upper right image. Chandra has provided some detailed images that further our insights into galaxy formation. Consider this view of Galaxy 3C294, located about 12 billion l.y. away, and hence seen now as it was some 2.5 billion years after the Big Bang. An example of the current ambiguities (perhaps solvable only with the NGST and other more powerful and sensitive telescopes of the future) appears in this quartet of red objects extracted from HST Deep Field images. Astronomer Kenneth Lanzetta believes these to be now faint galaxies as far away as 12.8 billion light years, with their color to be that expected from the redshifts at that distance. But an alternative explanation is that they are closer and are composed of stars now dominated by those with luminosities that shine in the visible red. In March of 2004, the the Space Telescope Institute released an Ultra Deep Field image (HST; very long composite exposure involving integration of about 400 observations over a year's time) in which the faint light spots are galaxies put at 13 to 13.3 billion years from Earth (redshifts from 7 to 12). These, then, have the characteristics of galaxies that had already formed very early in Universe time - we see them now as they appear at an age of 400 to 700 million years after the Big Bang. The top image shows the general field of view, with the smallest, faintest objects being the distant galaxies. The larger, better formed galaxies are much closer to us - not in the Deep Field. The image below it consists of 6 panels which are enlargements of parts of the upper image (try to locate each panel in the parent image), with the smaller spots (distant; early in time) being the ancient galaxies. These have a variety of shapes (mostly irregular, some in strings, others pencil-like; only a few seem spiral- or elliptical-like). The HST has now gathered enlargements of individual distant galaxies. Here are 20 that all lie about 11 billion light years from Earth. Note their general similarity in shapes, with some variations. The HST is approaching its limits of outward looking. Tantallizing small objects are discernible. Astronomers are legitimately upset that the HST is at present not going to be serviced again by the much reduced Space Shuttle program (designed to concentrate on finishing the International Space Station). Unless there is a reprieve, we must await the launch of the James Web Space Telescope to improve the image quality of Deep Space objects. Lets now shift topics. Some of the subject matter remaining to be considered on this, and several other Cosmology pages, has been gleaned from a highly recommended review article entitlted "The Life Cycle of Galaxies", by G. Kauffmann and F. van den Bosch, in the June 2002 issue of Scientific American. They describe a model, growing in acceptance but still provisional and unproved, that gives Cold Dark Matter (CDM)
a key role in the evolution of galaxies. We will summarize it here, but consult their article for details:
At the Big Bang and for a time thereafter, the rapidly expanding Universe consisted of baryons and other ordinary matter and CDM, undifferentiated in space (i.e., mixed). Initially, expansion overcame the effects of gravity but as will be shown on page 20-9 there were local slight fluctuations in density of both kinds of matter. These thinned out less that the more uniform general distribution of matter and radiation during expansion. The denser patches (initially irregular in shape) attracted nearby matter until their gravity exceeded the effects of that expansion and they began to collapse. The CDM and ordinary matter (baryon-rich) attained a mutual equilibrium, with the density of both kinds maximum in the center and decreasing outwards. Most cosmologists now favor the concept that clumping of the mysterious Dark Matter (see page 20-9 and 20-10) that makes up at least 15% of the contents of the Universe leads to the beginnings of eventual galaxy formation. As the collapsing continues, the dark matter separates from the baryons and remains primarily in the halo region. Ordinary matter, primarily as gas, even before it starts to organize into stars, radiates energy outwards as its particles interact by collision; in doing so it loses some of the energy that earlier counteracted gravity and begins to collect in small regions overly dense compared with their surroundings. The weakly interactive CDM, however, does not collide and does not radiate its energy as photons into space but does retain its gravitational influence on the patch.
Deep Field studies have found a good example of dark matter in a cluster of protogalaxies, one of which appears as a yellow starlike object with a red halo that is shaped by local dark matter, as shown in this image:
The dark matter remains in its roughly spherical concentration around a galaxy, becoming the dominant material in the Dark Halo (containing both CDM and some ordinary matter) that many astronomers now consider to be the controlling part (through gravitational influence) of protogalaxies as well as subsequent normal ones. Some light had been shed on at least part of the composition of this dark matter - it is ordinary matter that doesn't give off detectable radiation at most wavelengths. When our own Milky Way was surveyed by searching for neutral hydrogen at 21 cm using radio telescopy (see page 20-3), clouds of this gas were found outside the central plane of the M.W., as shown in this illustration:
These hydrogen-rich clouds are the source of new hydrogen that is attracted into the Milky Way over time to resupply intragalactic space with material to form new stars. Ordinary matter settles toward the central interior of the halo sphere until it configures through self-gravitation into a rotating disk. This rotation is generated by transfer of angular momentum as neighboring (and in the early Universe, closer-spaced) protogalaxies exert forces on each other. Individual protogalaxies influence others nearby, exerting torque forces that induce spinning. Some of the protogalaxies (in particular the smallest ones that have the highest densities) meet each other and merge their star populations into elliptical galaxies. Those that don't interact directly, but grow large enough, tend to flatten into disks that become the spirals; in these, there develops an equilibrium between centrifugal forces from the spin and inward-acting gravitational forces that permits maintenance of the spiral arms. Others that do not enlarge significantly make up the dwarf galaxies or the irregular galaxies described above.Some galaxies probably experienced minimal (or no) collisions. Within each halo patch giving rise to a protogalaxy, gas and dust condense into individual stars. Where gas densities are high enough, the stars form at a 'frenzied' rate (these comprise the "starburst" galaxies). Comments are in order at this point about the acquisition of angular momentum from rotational processes that appear to develop in most (all?) galaxies. As a giant gas-dust cloud destined to form a galaxy reaches a certain density, it begins to contract. The cloud also starts to rotate owing initially to torque developed from gravitational interactions with other, nearby galaxies (in the early Universe, galaxies were closer, and dark matter also seemed to be distributed in pockets of greater density). For many clouds - those that led to spiral galaxies - matter dragged inward by the cloud's own gravitational contraction moved faster from directions near parallel with the axis of rotation. Contraction is slower in the plane perpendicular to the axis. Thus, in time a disk shape evolves, with the stars that form moving faster near the disk center (commonly, determined by presence of a black hole) and slower further out. Interior matter tends over time to move inward while angular momentum is transferred outward. The spiral arms of a disk galaxy develop their distinct curvature because of differences in rotational speed and because of effects developed by density waves. If densities in the cloud are high during contraction, stars form early and inhibit the development of a disk; at least some elliptical galaxies may have formed this way. If the gas-dust cloud attains a disk shape before most of the galaxy stars begin to form, the result is a spiral galaxy. (Note: the above mechanisms for disk formation also generally apply to other astronomical bodies such as protoplanetary disks and accretion disks [material taken from one of a binary star pair and accumulating around the other]). A good general review of disk processes is found in the article "A Universe of Disks" by Omer Blaes in Scientific American, October, 2004. In that article, the author points out that turbulence is a hallmark of disks, probably induced in part by a process called magnetorotational instability; interactions between matter involved in this turbulence produce at least some of the energy radiated at different wavelengths from the galaxy's material constituents. The end result of the organization process which gives rise to a galaxy and its halo is the attainment of two kinds of equilibrium: hydrostatic, for the galaxies, which maintains a balance between gravitational and thermal plus radiative forces (however gravity varies inwardly towards the center); virial, for the surrounding halo, in which (time-averaged) kinetic energy K is balanced with respect to gravitational potential energy according to the virial theorem (K = -1/2G) (gravitational field associated with dark matter does not fluctuate. As interpreted by Andrew Fabian of Cambridge University and his colleagues, most galaxies form by huge clouds of gas organizing into many small galaxies that continue to grow. Their history may be controlled by a supermassive black hole that helps to organize these galaxies into a larger unit. The blue areas in the above image are galactic gas concentrations giving off strong x-rays (and radio waves). Jets are involved in maintaining high energy levels which help to keep galactic matter from disappearing into the black hole. Over time some of this inflow happens and the galactic growth reaches an equilibrium, slowing the galaxy formation process to a near halt. Eventually, as more galactic gas is heated up the organizational process repeats and galactic growth recycles one or more times until the current (final?) galactic unit is stablized. This process may be a controlling factor in the development of clusters of galaxies. This notion that large galaxies develop by the merger over time of ten to perhaps 100 small satellite galaxies has been gaining acceptance in recent years. Researchers are designing a test, RAVE (RAdial Velocity Experiment), to study the proper motions of small galaxies around a central galaxy to determine if this process is going on and is common. Initially, the Milky Way will play that central role as small clusters approaching galaxy-size in the halo are examined for their motions with respect to the M.W. Recent renewed interest in our own Milky Way has led to further insight in which it (and by inference most galaxies) has grown over time by "gobbling up" small galaxies that enter its halo. The Milky Way is one component of the Local Cluster, within which the Large and Small Magellanic Clouds (the latter 190000 l.y. away) are major members. The Sagittarius galaxy (described above) is a much smaller grouping of stars located about 75000 light years away. It has come under the gravitational influence of the M.W. itself to the extent that the larger galaxy has been pulling stars from Sagittarius and capturing these within the M.W. This implies that many of our galaxy's stars are late comers into the M.W.'s spiral arms. This capture is depicted in this artist's conception: In November of 2003 an even more startling discovery of a galaxy interacting with the M.W. was announced. A ring of stars has been determined to be a part of a dwarf galaxy (part of which is visible in the Canus Major constellation grouping) that lies about 25000 l.y. beyond the plane of the M.W. This disrupting mini-galaxy is even now interacting with our galaxy as stars are drawn in and merge. It has yet to be imaged in its entirety but here is an artist's drawing based on observational data: Thus, galaxy growth has an element of "cannibalism" except that captured stars can remain intact. This process of small galaxy capture is now thought to be a major means by which galaxies grow over time. The University of Chicago Cosmological Physics group has produced a web site with an illustration sequence that shows the progressive growth of a galaxy cluster, starting with a acoustical or gravity wave structure at the early stage of the Universe's expansion (high z value, meaning distant and old) followed by gradual clumping and cannibalism through lower z values up to the present. In the January 2004 issue of Scientific American is an article, Our Growing, Breathing Galaxy by B.P. Wakker and P. Richter which summarizes much of what has been learned about the Milky Way (and by extrapolation, probably typical of most spiral galaxies) in the last 20 years. Their main points are summarized in this diagram: Some of the items shown in this diagram have been talked about before. But several new ideas need further explanation. First is the Corona, a term which is related to the Halo (stars beyond the M.W. disk region). The Corona, however, describes the presence of very hot ionized gaseous material influenced by the Milky Way. This includes not only hydrogen but other elements (one is oxygen, which is ionized to O-5 [5 electrons stripped away], that has a thermal motion temperature of 300000°K). Another feature are concentrations of hydrogen gas that form clumps within the Corona (Halo) region that move in and out of the central disc. HVC's or High Velocity Clouds move about at velocities of ~400 km/sec faster than the speed of the Sun (200 km/sec) and its neighbors within the disk. These clouds contain no stars but have masses up to 10 million solar masses and they extend to dimensions around 10000 light years. Their temperatures range from about 100°K in their interior to 8000°K in their outer edges (heated and ionized by radiation). These HVC clumps exist both within the Corona and evidence points to their existence also in the Local Group; by inference they may be pervasive throughout the Universe where concentrations occur in the intergalactic gas. As individual HVC's move into the Milky Way itself, they replenish the supply of hydrogen gas that becomes available for new star formation; this explains why many of the M.W.'s stars appear young and depleted of "metals" (elements produced by star burning and subsequent dispersal through supernova explosions). As supernovae explode, they produce "super bubbles" of gas some of which is expelled as gas "fountains" beyond but near the disc. This gas then condenses and returns as Intermediate Velocity Clouds (IVCs). Thus both HVCs and IVCs are entering the galaxy, and some gas is returned to the Corona (hence, the implication of "breathing" in the dynamic behavior of the Milky Way as materials are changed and exchanged). Other gas within the Corona is derived by stripping from such features as the Magellanic Clouds, and is drawn out in the Magellanic Stream. Strong evidence for these HVC clouds of neutral hydrogen has been reported for our neighbor galaxy, Andromeda. This radio telescope image (Green Bank, West Virginia) shows the central disk gases (ionized hydrogen, helium, others) in blue. Patches of neutral hydrogen, in red, lie outside the disk and over time provide new material for nascent stars. There are two competing views as to which type of large galaxy forms first. There is general agreement that protogalaxies preceded Spiral and Elliptical Galaxies; some may still persist and may comprise some of the Irregular types. Dwarf galaxies and Globular clusters also were early members of the hierarchy of galactic types. Many hold that the dominant first galaxies were spirals. In this view, elliptical galaxies were developed mainly from collisions of spirals. Other opinions favor elliptical galaxies (with their pre-eminent old stars) were the primordial type, at least those which may constitute AGN's in spirals, but most of these subsequently captured gases that organized in the stars occupying the arms that characterize Spiral Galaxies. Radio astronomy (see next page) has shed further light on galaxy formation and types during the first few million years. Supermassive galaxies (1000 billion stars) have been detected at distances of 12 billion light years (when the Universe was less than 3 billion years). Careful analysis has determined that this group consists of Giant Elliptical galaxies that may have formed by coalescence of smaller, generally irregular-shaped galaxies. Here are two typical views of these large elliptical forms: This question of which came first (if indeed it proves out that one type preceded the other) was in a sense answered by images (that include an IR band) taken by the European Southern Observatory (ESO) telescope that has acquired the best yet images of galaxies determined (by distance measurements) to have formed in the observed state only about 2 billion years after the Big Bang. The majority of the few so far imaged by the ESO telescope are apparently elliptical. But among the group is at least one that appears to be evolving into a spiral. Thus: In terms of evolutionary trends, in the first half of cosmic time the morphology of galaxies was less well developed (larger number of irregular/peculiar types), with spiral arms less organized and barred spirals quite uncommon. The appearance and maturity of the various types in the Hubble classification of larger galaxies may have been reached only in the last 4 billon years. Deep field observations and other sources of information (much derived from have led astronomers to conclude that galaxy formaton rates were much higher in the first 6 billion years of Universe time and since than have been slowing to a present estimated 10% of the maximum in the first few billion years. To supplement the above interpretive remarks, here is the gist of a press release made when the first major results of the GOODS survey were presented in June of 2003: Galaxies began to form in the first half of the first billion years after the Big Bang. It is hypothesized that they developed in regions where dark matter (see page 20-9) was denser (mature galaxies also contain an excess of dark matter) and thus preferentially attracted the hydrogen gases formed soon after the Big Bang. Those galaxies that started in these early eons tended to be irregular in shape and to contain a high number of massive stars. These stars burned out rapidly and exploded by the supernova/hypernova processes (page 20-6) that expelled materials throughout the protogalaxies, thus assuring that the galaxy shapes remained distorted by the high velocity ejections of matter that inhibited galactic dvelopment into regular types. Since the Universe's size was still small during this first billion years, these "wispy" galaxies stood a higher chance of enlarging by collisions and accretion. In time, then, the rate of new galaxy formation started to decrease and the number of galaxies may have actually diminished by "consolidation". As disruptive explosions and their attendant shock waves became less frequent with the decrease in giant star formation, the galaxies stabilized into the more orderly spiral types and ellipticals that now dominate the galactic Universe. But, lest there be a misconception that all galaxies came into being early, there is little doubt that galaxies can form well after the principal period of development in the first few billions of years after the B.B. This view made by the Hubble Space Telescope shows POX189, a galaxy 68 million l.y. away, seemingly in the throws of organizing its gas and dust into what appears will become a spiral type. Only 900 light years in diameter, but with about 10 million years stars, this proto-galaxy is estimated to have begun organizing about 100 million years ago, perhaps through a collision of two nebular masses. One of the youngest well-formed galaxies is I Zwicky18, about 45 million light years from Earth, seen in this Hubble image at the lower left (an older, less developed galaxy appears in the upper right. Its age (since first star formation) ranges from 500,000 to 1,000,000 billion years. "Youngish" giant galaxies like those that formed in the early history of the Universe are much less likely to develop in later history but nevertheless some still do. The illustration below, made in ultraviolet light sensed by the Galaxy Evolution Explorer, is three views of a large galaxy about 2 billion l.y. distant that shows evidence of being less than 3 billion years old; others found in this survey may even be less than a billion years old. Astronomers have long suspected that another type of galaxy exists through the Universe - probably billions in number. This is the so-called dark galaxy. Such a galaxy does not emit radiation in the shorter wavelengths (through the Infrared) but may still emit microwave radiation. The first discovery of a Dark Galaxy was reported by the radio telescope operated by the Lowell Observatory. Its presence is defined by a peak at 21 cm; the signal is displayed below. That wavelength denotes hydrogen but in a state where it does not emit detectable light. The galaxy, which is devoid of stars, slowly rotates as a coherent clump. Such galaxies, if they proved to be plentiful, can account for much of the missing (dark) matter in the Universe needed to hold galaxies together and to regulate expansion. More on dark matter is found on pages 20-9 and 20-10. To recapitulate, the overall trend of galaxy formation since the Big Bang has been this: At the beginning (first billion years), many small galaxies formed, quickly coalesced (because they were closer in the then much smaller Universe) to yield ever bigger (more massive) galaxies (that were fewer in number) which had many giant stars that burned out and exploded as supernovae dispersing heavier elements that were incorporated in new star formation from remaining hydrogen; these early galaxies have largely survived; galaxy formation continued into mid-Universe time (8 to 6 billion years ago) but thereafter the rate has been slowing so that at present only a few large galaxies are coming into existence and even smaller ones are also developing at slower rates, with the trend for the future favoring mainly new dwarf galaxies (at this time dwarf galaxies are probably the most abundant type in the Universe but their number is hard to assess because their small sizes allow them to escape detection by the most powerful telescopes); in the long run the continued expansion of the Universe will constrain galaxy development as hydrogen is both used up and is made more tenuous (lower density). Most, if not all, galaxies contain a central supermassive black hole which, despite it seldom containing more than 2% of the total galactic mass, helps to hold each one together; black holes sweep in matter from the stars and gases in the galaxy (which upon reaching the Black Hole react to give off extreme energy which we perceive as quasars [page 20-6] until these inputs are depleted and the Black Holes become undetected). In recent years, the concept that spiral galaxies undergo cyclic changes - between barred and non-barred types - and that the spiral arms experience continual modifications has grown in favor. A reminder: barred spirals have an elongated inner region with denser star populations, as shown for NGC1300: Central to the idea of a dynamic spiral galaxy evolving through repeated episodes of barred and non-barred stages is the notion of galactic waves, which are also known as gravitational or density waves. These result from perturbations of gas/dust density that lead to alternating regions of higher densities of stars (the arms) with lower densities (much fewer stars). As stars form they tend to be attracted to the gravitational stronger arms; but some stars can leave the arms as well. The rotation of the gas/dust ingredients of the entire galactic environment, is responsible for the stars moving the spirals around the galactic center, as though being driven by traveling wave trains. Because the inner stars have moved about the nucleus many times more than those farther out, the spiral and bar structures are not fixed but undergo patterns of change that result in synchronization of stellar orbits in the galaxy. Before a wave develops, star paths have traced elliptical orbits around the center. The galactic wave begins when most ellipses rotate at the same or closely similar rates. The non-barred spiral develops when the ellipses, now moving in unison, are not yet aligned (i.e., skewed relative to neighboring elliptical paths). As orbits of inner ellipses become aligned, that region of the galaxy becomes aligned, leading to a elongated densification of stars within the wave patterns. A spiral pattern in the arms results from progressive outward misalignment.
Waves become self-reenforcing because of gravitational effects from regions of increased density. Over time, stars from outside an arm are "sucked" in by gravity, only to be carried beyond the arm when variable speeds reduce the gravitational hold. Over time scales of several billion years, a galaxy (which continues to attract extragalactic gas and dust) will modify from a simple spiral to a barred spiral; the bar concentration will eventually break down as more gas is moved inward. But a later bar likely forms as more gas enters the system. Over the history of our Universe, multiple bar formation is the norm, so that a large number of galaxies at any one time are of the bar type. For a fuller treatment of galactic waves read Ripples in the Galactic Pond by Francoise Combes, in Scientific American, October, 2005.
In the other direction of time - namely, towards the future, the fate of galaxies is still speculative but a clue is evident from projecting the eventual demise of the Milky Way. We saw above that the M.W. has been growing through capture of stars from small nearby galaxies, and by inflow of material (including stars intact) from globular clusters in its halo. Motion studies of the M.W. and the neighboring Andromeda galaxy suggest that over time they may become closer, proximate enough to affect each other gravitationally until they finally merge into an elliptical galaxy. Estimates of when this might occur range from 1 to 3 billion years to as much as 6 billion years into the future. We will see later that the expansion of the Universe, now said to be accelerating, will ultimately drive galaxies far apart from one another even as these disperse within and burn out as hydrogen fuel is consumed throughout the Universe. As expansion continues gas cloud matter that can form new galaxies and stars will be further dispersed, lowering densities, so that fewer galaxies will come into being. Those that do organize will be predominantly dwarf types. Our knowledge of galaxies has expanded enormously since Hubble first recognized that "stars" beyond the Milky Way were mostly other galaxies. He dubbed each such assemblage of stars "an Island Universe", meaning that, much like our earlier notions of the Milky Way, each galaxy seemed to consist of a completely contained but isolated collection of billions of stars with empty space between neighboring galaxies. While that still has a strong semblance of veracity, our understanding of how galaxies develop, how they can actually interact (collisions), and how they maintain themselves has increased significantly in the last 80 years. The "sphere of influence" of a galaxy has enlarged with the recognition of haloes containing star clusters, dark matter/energy, and other radiation sources. But, perhaps surprising, the so-called "intergalactic space" has been found not to be "empty space", as it contains much of the dark matter/energy, virtual particles, minute but meaningful numbers of atoms, protons and neutrons, huge concentrations of neutrinos and other items that mean the "vacuum" of space is actually replete with cosmic substances. Now, there is evidence for strong intergalactic "winds", somewhat analogous to the "solar wind" of high speed particles of varied nature. These seem to emanate from galaxies and travel far enough to pass into other galaxies beyond. In a sense, the Universe of space is seething with a great deal of activity that intrudes into all available volume.
Finally, an informative review of galaxy formation and subsequent history, which ties nicely into the above treatment of this topic, can be found at this PhysicsWorld Web site.
The
closest (to the Milky Way) neighboring spiral galaxy, Andromeda, located about 225,000 l.y.
distant from Earth. Here is an HST view of this galaxy which was thought to be slightly smaller than the Milky Way (77,000 l.y. across):
Courtesy Scientic American and Slim Films, Inc.
