Formation and Evolution of Galaxies :: Articles :: Astronomy @ Stardust Sand ȬP

[2002-01-21]

This is an article appeared in the Annual Journal of Astronomy Club, Hong Kong University Students' Union, 2001. It outlined the introductory idea about galaxies formation and evolution. Note, some text are quoted from the book "After the First Three Minutes" by T. Padmanabhan and Encyclopedia of Astronomy and Astrophysics.


Formation and Evolution of Galaxies

In the "beginning," nothing; No time, no space, no matter.
No energy, no strings; nothing; not even a point, not even a void; nothing...
-- Don L. Anderson

Galaxies are family of stars, one typical galaxy contains a few million to ten trillion (1013) stars, and in diameter from a few hundred to over 600000 light-years. Galaxies constitute major components of the universe. How did galaxies form? Why galaxies have different shape, color and luminosity? Such questions are among the most important in the study of cosmology. So, let us first look at how galaxies were formed.

Gravitational Instability

At the very first beginning, there were no galaxies, the universe were filled with matter and radiation uniformly distributed throughout the space. In order to form structures like galaxies, it is important for the matter to be denser in some regions compared to some other regions. So, we need a mechanism to convert a completely uniform distribution of matter into a distribution punctuated by clumps of matter, while the key is gravitational force.

Infinitesimal density fluctuations were presented in the early universe, they originated from inflation at the very early universe epoch. Fluctuations grew in strength under the influence of self-gravity. Eventually, gravitational instability turns small clumps of matter into clouds of primordial gas, hydrogen and helium, and finally fragmented into stars.

Accretion, Cooling and Angular Momentum

The primordial gas formed spheroids of galaxies, normal and dark matter collapsed to form virialized halo. Accretion of matter and collision of gas increased the temperature of the cloud, and hence the pressure of the gas. If the pressure of gas was too high, it would balance the gravitational dynamical forces from crushing of clouds, preventing star formation, therefore cooling is an essential process for protogalaxies making.

As structure grow and collapse, they exert tidal torques on each other, providing them angular momentum. With the aid of angular momentum, the gas cloud rotated, and then further increased the rate of gas collisions, raised the temperature. Radiation of photons by charged particles and collision of electrons with neutral hydrogen are two examples of cooling process.

The relative magnitudes of the cooling and dynamical time scales are important for determining the mass range of galaxies, galaxies only form when the cooling time is smaller than the dynamical time.

Fragmentations

The cooling process prevents sudden enormous change of pressure, such that gas cloud fragmented into smaller pieces as a result of gravitational force. These small pieces will further break up into smaller pieces when gravity dominated thermal pressure again.

These processes go on until pressure forces are comparable to self-gravity. Each small piece enters a stage of very slow contraction and eventually forms stars.

Disk Galaxies and Ellipticals

When a rotating gas cloud contracts under its own gravity, it will have a tendency to get flattened in the direction perpendicular to the axis of rotation. Depending on the rate of the first generation of star formation, the protogalaxies will end-up in different morphology or shape.

For a slow star formation rate compared to the time taken for the gas to collapse, most of the gas will end up in a disk perpendicular to the direction of rotation before star formation takes place due to angular momentum. In such a case, most of the stars will form in the disk and the structure will end up as a disk galaxy. On the other hand, if the star formation rate is high, stars will form all over the place even before the gas can collapse to a disk. Once star formation has progressed significantly, each star will move in its own orbit in the common gravitational field of the system and further collapse will not take place. We end up with an elliptical galaxy.

Merging Galaxies - Another model for Galaxy Formation

There are currently two basic models for galaxy formation. One is monolithic collapse model as prescribed above, while another one is hierarchical merging model.

In the hierarchical merging scenario, galaxies are gradually assembled through multiple mergers of smaller subgalactic units, a process that continues from the early universe to the current epoch. The differences of the models extend to ideas about galaxy evolution.

Galaxy Morphology

Edwin Hubble classified galaxies into four principal types V ellipticals, lenticulars, spirals and irregulars. The classification was based on relative proportions of bulge and disk of the galaxies.

Galaxies in low-redshift (modern days) reveals well-developed morphology, such as prefect spirals, while observations of high-redshift galaxies (early galaxies) shows a greater proportions of irregular morphology, lacking well-developed galaxies in the early epoch of the universe.

In the Hubble Deep Field (HDF) images obtained by 10-day exposure of Hubble Space Telescope, the galaxies revealed were 2.5 to 10.5 billion light-years away from us. The galaxies in the images were therefore old compared to our Milky Way and neighborhood. HDF displayed an array of galaxies with unusual shapes and colors, many were much smaller than galaxies like our own. They have bright knots and condensations thousands of light-years across V features resembling huge star-forming regions in some nearby galaxies. Many have close companions, suggesting that they may come from merging of small galaxies or subgalactic fragments.

Red and Blue

The luminosity and color of the galaxies are closely related to the stellar content and star-formation activity of its own. The old and evolved stars are characterized by red colors, while young stars are blue.

Star formation regions require a reservoir of gas and dust to supply raw material for making protostars. The Bulge contains no appreciable amount of gas and dust and therefore no activity of star formation, it contains only old and evolved stars. The disk is composed of a mixture of old and young stars and by gas and dust, and it is frequently site of star formation activity. Its color is generally bluer than those of the bulge. The most visually striking feature of the disk is spiral arms, density waves that generate in the disk and which contain active regions of star formation.

As we have seen above, ellipticals formed their stars very efficiently in a dramatic burst during the initial collapse, while spirals probably formed only ten percent of their stars in the initial phase. Most of the stars in disk galaxies formed in a leisurely fashion spread over the past ten billion years after the gas had already formed the disk. As a consequence, most of the light in an elliptical is from older, redder stars, while the spirals shine mostly due to younger, bluer stars.

Star Formation History

The evolution of galaxies is based on the evolution of stellar contents of its own. Imagine that a galaxy is experiencing an episode of star formation, and that it is generating new stars at a steady rate per year. The stellar masses of freshly produced stars are not all equal. The relative proportions of stars of a given mass produced during an episode of star formation are, to first approximation, the same everywhere and are so-called initial mass function (IMF). In simple words to explain IMF, given a total amount of stellar mass produced in a galaxy, there is a greater proportion of number of low-mass stars formed.

Although low-mass stars carry the bulk of the new stellar mass produced during an episode of star formation, they contribute to very little to the luminosity of the event. This is because high-mass stars are much more luminous than low-mass ones. Therefore the luminosity of the galaxy mainly contributed by massive stars. After the first million years of continuously star formation, say at a rate of 10 solar masses per year, the galaxy would increase its luminosity by 20%, and the color would become bluer due to the massive young stars.

Although low-mass stars were fainter, they live much longer than high-mass ones. Thus, after the end of star formation, when all massive stars died out, the luminosity of the galaxy eventually dominated by low-mass stars, both the recently created new ones and the ones that existed before the episode of star formation. Once star formation ended, new generation of stars can be formed only when massive stars died such that new stars formed from stellar recycling process. However, the star formation rate would be gradually decreased.

The luminosity of the galaxy during starburst phase depends only on the rate at which stars are being produced because this is what determines the amount of massive stars that can be observed at a given time. Also, after star formation ends, the color of the galaxy also return to be dominated by the light of the low-mass stars, which is redder.

Apart from changes of luminosity, stellar mass and color, the chemical composition of the galaxy is also changed. Heavy elements were made during nucleosynthesis in stellar evolution and supernova explosion.

Extremely Red Objects

Elliptical galaxies are typically with little or without dust, gas, and star formation activity. Furthermore, they are mostly composed of an old stellar population, about as old as the universe. Therefore ellipticals are the reddest galaxies in the local universe and are typical Extremely Red Objects (EROs).

Future study on the abundance of EROs in high redshift would indicate whether the monolithic collapse model or the hierarchical merger scenario is right for galaxies formation and evolution.

Faint Blue Galaxies

Surveys of very faint galaxies in the 1970s found an excess number of fainter galaxies at bluer wavelength, and therefore so-called faint blue galaxies.

Most of the faint blue galaxies observed in the imaging surveys are relatively small systems located at moderate to intermediate redshift, which are undergoing a robust activity of star formation. The luminosity of the blue galaxies was found to evolve very rapidly from the current epoch up to the earliest epoch studied, implying that the luminosity and abundance of these galaxies were progressively higher in the past.

By Hubble Space Telescope observations, faint blue galaxies were clarified as they consist of irregular galaxies, and much more abundant in the past then they are today, forming stars at a higher pace. These galaxies, in effect, have been the drivers of the evolution of the overall luminosity function and of the star formation history of the universe during the last 8 billion years of cosmic age.

Prospect

The study of galaxies over the two-thirds of the cosmic age has shown clear patterns in the evolution of galaxies. Further study on EROs, faint blue galaxies and a massive population of star-forming galaxies called Lyman-break galaxies would reveal the story happened at the early epoch of the universe.

However, we are still lack of observations of most distant galaxies to provide clues and insight for us to investigate and model early-type galaxy morphology and their evolution. Awaiting for the launch of Next Generation Space Telescope, a powerful new eye will be opened in the universe, we will be able to directly observe the early stage of galaxies formation.

Recent studies on galactic center found that most galaxies harbor supermassive black hole in their core, does supermassive black holes play an important role in galaxy formation model? How do quasars and active galactic nuclei related to the current day galaxies? There are still a lot of questions in the theoretical framework of cosmology remain to be solved.

However, one thing is very clear - the fact that the universe turns nothing into variety.


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