Galaxies

Formation and Evolution of Galaxy

		The most accepted view on the formation and evolution of large scale structure is that it was formed as a consequence of the growth of primordial fluctuations by gravitational instability. Galaxies can form in a "bottom up" process in which smaller units merge and form larger units. It is referred to as the "Inside-out Theory" or "Merger" as shown in the upper half of Figure 05-08a. In the present epoch, large concentrations of galaxies (clusters of galaxies) are still in the process of assembling. The opposing view is the "top down" process in which large clump breaks up into smaller units. It is referred to as the "Outside-in Theory" as shown in the lower half of Figure 05-08a. The figure also shows the kind of objects the NGST (Next Generation Space Telescope) will detect according to the two opposing theories.
Figure 05-08a BH, Initial Formation [view large image]	Figure 05-08b BH, Supermassive

The "bottom up" theory have been given a boost in 2008 by the first ever detection of the infant protogalaxies with an unprecedented 92-hour session on the European Southern Observatory's Very Large Telescope. These protogalaxies were irregularly shaped and with low star-formtion rates, but the stars that did form were massive and consequently exploded as supernovae. The image on the left shows a group of protogalaxies in the process of merging. (See a movie of "Galaxy Formation")

The difference between the "bottom up" (inside-out) and "top down" (outside-in) point of view is related to whether the universe is composed with cold dark matter (CDM, slow moving) or hot dark matter (HDM, fast moving). In the former scenario there is fluctuation in the power spectrum over a wide range of physical scales as shown in Figure 05-08c. Smaller size have larger fluctuation, therefore structure formed first with small objects, which then merge to form ever larger structures. This is called ``bottom up'' structure formation. The observations strongly favour this scenario over its competitor: "top down" structure formation. The proto-typical ``top down'' scenario is structure formation in a universe dominated by hot dark matter. Hot dark matter cannot support fluctuations on small length scales - they are washed out with the rapid motion of the particles. Thus only large scale fluctuations survive to the present epoch. Structure forms first large scale objects which fragment into smaller objects.

Figure 05-08c Density Fluctuations in Three Models

prodigious rate that they lived for only about 3 million years before exploding. Those explosions spewed elements like carbon, oxygen and iron into the void at tremendous speeds. By the remarkably young age of 275 million years, the universe was substantially seeded with metals thrown off by exploding stars. That seeding process was aided by the structure of the infant universe, where small protogalaxies less than one-millionth the mass of the Milky Way clustered together into vast filamentary structures. Giant stars form at the intersections of these great filaments of primordial hydrogen, forming the nuclei of the first galaxies - the protogalaxies (Figure 05-08d). The small sizes and distances between those protogalaxies allowed an individual supernova to rapidly seed a significant volume of star forming space. New simulations show that the first, "greatest generation" of stars spread incredible amounts of such heavy elements like carbon, oxygen and iron across thousands of light-years of space, thereby seeding the cosmos with the stuff of life.

Figure 05-08d Protogalaxy Evolution [view large image]

Galaxies used to be considered as "island universes" that formed in the distant past and have since evolved in isolation from their surroundings. Such concept was replaced by the hierarchical growth scenario, which enrols galaxy interactions, collisions and mergers to shape the mature galaxies in the current epoch. Lately in the 2010s, observations reveal that the decline in galaxy formation is not matched by the decrease of atomic hydrogen - only half of the latter has been consumed by the galaxies. The latest explanation proposes that the hydrogen fuel is supplied by ionized hydrogen in the intergalactic environment. Such novel idea is supported by the recent (2013) detection of neutral hydrogen in space between the Andromeda and Triangulum galaxies. The detection of 21 cm radio emission from neutral hydrogen is much easier although the neutral variety is only about 1% of the ionized one. The new discovery adds one more element into the process of galaxy assembly.

Figure 05-08e Galactic Mass Assembly [view large image]

Figure 05-08e is the numerical simulation of galaxy formation and evolution, in which green denotes higher ionized hydrogen density and black is lower (a-c are zoom in views).

After the initial phase of galaxy formation, there was an era of cosmic fireworks: galaxies collided and merged (see Figure 05-08f), powerful black holes in quasars sucked in huge whirlpools of gas, and stars were born in unrivaled profusion. The activity of star formation peaked about four to six billion years. Since then galactic mergers became much less common, the gargantuan black holes were replaced by numerous moderate ones, star formation continued but mostly in the low mass variety. In other words, the contents of the universe have transitted from a small number of bright objects to a large number of dimmer ones. Computer simulations suggest that such shift may be a direct consequence of cosmic expansion.

Figure 05-08f Simulation of Galactic Merger [view large image]

		smaller galaxies that are active today will have consumed much of their fuel, and the total cosmic output of radiation will decline drametically. As the cosmic expansion continues, the dwarf galaxies - which hold only a few million stars each but are the most numerous type of galaxy in the universe - will become the primary hot spots of star formation. Inevitably, though, the universe will darken, and its only contents will be the fossils of galaxies from its past. Figure 05-08g shows the evolution subsequent to the initial phase. Figure 05-08h shows another evolutuon simulation with cold dark matter.
Figure 05-08g Evolution History [view large image]	Figure 05-08h Evolution Simulation

	galaxy as shown in Figure 05-08i, where the dwarf galaxies are in the lowest mass range of 107 - 108 Msun, the mass of irregular galaxies is in between 108 - 1010 Msun, the range for spiral galaxies is 1010 - 1012 Msun, while the giant elliptical galaxies is in the range 1012 - 1013 Msun. The mass ranges form a
Figure 05-08i Mass Range of Galaxies	continuous sequence without overlapping. Thus, if the mass of the galaxy is known, the type would follow according to Figure 05-08i. It is not that straight forward conversely, for if we know the type of the galaxy there is a range of mass within that type.

mass and effective radius re for nearby galaxies (Figure 05-08j). The picture on the left of the Figure shows very compact galaxies evolved to bigger, more diffuse objects and became more abundant. Diagram on the right depicts the scaling relationship for nearby galaxies (the black dots) and the high redshift galaxy 1255-0 (at z = 2.186 in red symbol, dynamical mass refers to mass obtained from velocity dispersion of the stars). Current theory predicts that galaxies evolve according to the scaling law perhaps by merger (the blue arrow). The new observation implies another path as shown by the red arrow.

Figure 05-08j Evolution of Elliptical Galaxy

Further research is required to confirm the atypical observation. It remains to be seen whether we need conventional or novel explanations for the evolution of elliptical galaxies.

Table 05-01 Evolution of Galaxies

Figure 05-08k shows the evolution of galaxies in the form of various astronomical objects as we look back in time:

Nearby individual galaxies (triangles). Clusters of galaxies such as the Coma cluster. SDSS (Sloan Digital Sky Survey) galaxies. Superclusters (blank strips: planes of the Milkyway blocking light from outside). GRBs (gamma-ray bursts) with known distances (asterisks). Boundary between cosmic deceleration and acceleration at z = 0.7554. SDSS quasars. SN 1997ff, the most distant type Ia supernova at z = 1.7. GRB 050904, most distant GRB at z = 6.295. Most distant SDSS quasar at z = 6.42 (T 13.0 billion years). Most distant SDF (Subaru Deep Field) galaxy at z = 6.578 (to z = 6.96 in 2006). Epoch of first star formation with z ~ 20. Cosmic microwave background at z ~ 1100.

Figure 05-08k Evolution of Galaxies [view large image]

The lookback time T (in billions of years) is computed from the red shift z by the formula: T 13.7 x [1 - (1 + z)-3/2].    Figure 05-08l shows an updated cosmic timeline as appears in the September 2012 issue of Astronomy.

Figure 05-08l Cosmic Timeline [view large image]


Near the year end of 2004, it is reported that the NASA orbiting telescope Galaxy Evolution Explorer has discovered about three dozens bright and compact galaxies (Baby Galaxies) within one billion light years from the Milky Way.

These objects emit strong ultraviolet light (from newborn stars and exploding supernovae), have low metal content, and are in the form of amorphous blob. The galaxy's gas contains just 2% of the Sun's abundance of heavy elements, or metals - the most pristine galactic gas seen since the big bang (star-forming regions in the Milky Way contain 100 to 200 more of these elements than the baby galaxy). This kind of astronomical objects is thought to exist more than 10 billion years ago (a few billion years after the Big Bang). Such nearby baby galaxies probably started out as a small gas cloud in a relatively empty region of space. It grew very slowly until, after nearly 13 billion years, it had enough density to form stars. The images on top of Figure 05-09a compares the mature and newborn galaxies in visible and ultraviolet lights. The lower image is the baby galaxy I Zwicky 18, at a distance of only 45 million light years. The galaxy's proximity allowed Hubble's eagle-eyed Advanced Camera for Surveys to resolve a few thousand of its estimated 20,000 stars. The stars' colour and brightness suggest that none are more than 500 million years old.

Figure 05-09a Baby Galaxy [view large image]

More HST observations in 2008 indicate that star formation in I Zwicky 18 began at least one billion - and perhaps up to 10 billion - years ago, making the galaxy no more remarkable than most of its neighbors.

	Another baby galaxy in the early universe has been detected in 2005. It is located at a point about 800 million years after the Big Bang. It has a mass eight times that in the Milky Way. The discovery was surprising, since astronomers have long theorized that galaxies form when stars gradually cluster together, with small galaxies preceding bigger galaxies. Now the new evidence suggests that the process of galaxy formation started really very early on. This galaxy, known as HUDF-JD2 was found by researchers using NASA's Hubble and Spitzer space telescopes. It is the smaller red object in Figure 05-09b. Other survey found massive galaxies originated 700 million years after the Big Bang, whereas the supposedly old dwarf galaxies showed the most recent bouts of star formation just
Figure 05-09b HUDF-JD2	4 billion years ago. Theoretical consideration also suggests that the proto-galaxies in the early universe may have fizzled out by supernova (as shown in Figure 05-08b) and thus unable to support the "merger". These new evidences have turned the "Inside-out Theory" cherished by astronomers in the 1990's upside down. It is at least incomplete, if not entirely wrong.


In 2010, ESA's Herschel Space Observatory obtained conclusive evidence for dramatic star birth surge within a newly discovered population of massive galaxies in the early universe. The measurements confirm the idea that stars formed most rapidly about 11 billion years ago, or about 3 billion years after the Big Bang, and that the rate of star formation is much faster than anticipated. The Herschel Space Observatory is an infrared telescope with a mirror 3.5 m in diameter, launched in 2009. It is used to study distant objects in detail with the Spectral and Photometric Imaging Receiver (SPIRE) camera.



Report from Hubble Ultra Deep Field in 2011 shows the oldest galaxy (to date) 13.2 billion light years from Earth or 480 million after the Big Bang at a redshift of about 8 (Figure 05-09c). In contrast to the 60 or so galaxies detected in a period roughly 650 million years after the Big Bang, this lone object indicates large galaxies rapidly built up from smaller ones in a short span fewer than 200 million years, and the rate of star formation increased tenfold. This discovery raises a puzzle about not enough sources to re-ionize the neutral hydrogen in this epoch. Further observations with Hubble and the JWST will be needed to clear up the discrepancy. An article in Nature Online 13 April 2012 reports that another very old galaxy was

Figure 05-09c Oldest Galaxy

found at a red shift of 9.6 - about 490 million years after the Big Bang. Its age is no more than 200 million years and has only 1% the mass of the Milkway. By lucky coincidence, it locates right behind a galactic cluster, which amplifies the image 15 times, and will enable JWST to make close examination in the future (~ 2018?).

N.B. According to a computer program on cosmological evolution (also see Theory of Cosmic Inflation and Acceleration), for _m = 0.26 and = 0.74, the age of the universe corresponding to z = 9.6 is about 450 million years, while it is about 600 million years for z = 8. These values are at variance with the ages quoted above. Those numbers are also contradictory with each others.

Since the 1990's, astronomers suspect that beside the dark energy and dark matter, a large chunk of baryonic matter is also missing.

		According to WMAP data, baryonic matter accounts for about 5% of the total mass-energy in the observable universe, i.e., about 2x1054 gm. In the early epoch, this much of the baryonic matter were mostly neutral hydrogen atoms, which can be measured in the absorption spectrum known as "Lyman alpha forest". It seems that around 5 billion years after the Big Bang, the balance in the book keeping of baryonic mass had gone awry with the full-blown formation of galaxies. Auditing of the various kinds of galactic components
Figure 05-09d WHIM Detection	Figure 05-09e WHIM Replenishment	(such as stars, stellar remnants, neutral gas, ionized gas, dust, planets, ...) through measuring the different forms of electromagnetic radiation reveals that the total amounts to only 1/10 of the initial inventory. Where is the rest?

It is suggested that the missing baryonic matter is in the form of ionized gas not hot enough to blaze in X-rays and thus avoids detection. Using the technique similar to the detection of "Lyman alpha forest" but instead targeting absorption in the X-rays spectrum, the Chandra X-ray observatory has obtained data pointing to the presence of highly ionized oxygen in the Sculptor Wall (Figure 05-09d). An updated theory of galactic evolution proposes that stars and black holes feed matter back into the interstellar and intergalactic medium (Figure 05-09e) in a cycle of drawing in cold matter and spitting out hot gas to form the warm-hot intergalactic medium (WHIM). Thus, the baryonic universe is predominately gaseous, not galactic.



In 2013, computer study of dwarf galaxies reveals that cold dark matter such as WIMP would produce a simulation too dense for the real things. Hot dark matter such as neutrinos move too fast to settle down into compact structures like galaxies. Only the warm dark matter such as the sterile neutrinos plus disturbance from supernova explosions would generate galaxies of just the right density (Figure 05-09f). This work seems to be in good agreement with the detection of WHIM as mentioned above.

Figure 05-09f Warm Dark Matter

The above narrations on finding different kinds of galaxies at various distances seems to be very confusing as they are collected from the news at different times. Perhaps it may be helpful to read a coherent presentation by checking out a 73 pages reference on "GALAXY FORMATION AND EVOLUTION".

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