Galaxies' Formation

Knowledge of galaxies' early history is impeded due to time zones where they are dwelling, being at the limit of our astronomical tools. Two main theories now are accounting for galaxies' history: "co-evolution", which is linking galaxies and black holes; "Collisional Starburst Scenario" which thinks that galaxies are forming, merging smaller stellar groups. Two additional concepts -the role of dark matter, a further role of collisions and mergers- come along with these theories

This domain in astronomy is still largely in the making as the very beginnings of stars and galaxies is still hidden to astronomers' view or just at the limit of the tools of today. One acceptable scenario for the beginnings of the visible Universe may be induced from some views released at NASA (April 2005) at the occasion of the first of next generation tools, the James Webb Space Telescope, was to be presented as a model at the 21st National Space Symposium in Colorado Springs. Stars formed first from the collapse of hydrogen and helium gas clouds. Such early stars worked during a few million years only and quickly ended like supernovae, in turn producing a black hole. Such supernovae, on the other hand, enriched the medium with heavy elements like carbon, oxygen, and silicon, which were the seeds for next generation stars and planets. The accumulation of such first and second generation stars, black holes, and gas clouds, led then to the formation of the first proto-galaxies, at the center of which gas, stars, black holes and other stellar remnants accreted together, forming a massive black hole, featuring two energetic jets (when one of these jets is pointed to us, we see a quasar). It's the collisions and mergers between these proto-galaxies which formed at last the usual larger galaxies by a series of constructions. Spiral galaxies are thought to build when the inflow of material was -still is- smooth and is gas mainly, as elliptical galaxies were/are due to more violent, head-on events. Dark matter is thought now to be a necessary ingredient to the formation of any galaxy or quasar. This scenario does not contradict with the following details

The Latest Conception About Galaxies Formation!
NASA Galex mission recently, in 2007, came to the conclusion that the 'nurture' theory might be the last word about how galaxies are evolving along their life. According to those views, a typical young galaxy begins life as a spiral which is actively churning out stars, as it further merge with other spiral or irregular galaxies -leading to some bursts of stars formation- and, eventually, the galaxy settles into an elliptical through fuel exhaustion and perhaps suppression by black holes. A major consequence of such a finding is that astronomers today, tend to talk about galaxies by referring to them by their color (blue for small or irregular galaxies, red for large ellipticals), instead of their shape, like previously done. Those recent discoveries further proved that an intermediate-class, 'teen' galaxies fills the gap between young, and older galaxies. Some of those mid-age galaxies are ripening into old quickly, while others are lingering a long time in their state picture courtesy NASA/JPL-Caltech
from left to right: a young, blue, stars forming galaxy, a 'teen' one, and an older, elliptical, red galaxy

First Stars and Galaxies. ab. 200 Million Years After the Big Bang

It begins to be admitted that stars and galaxies are forming at an important rate very early in the Universe. This occurs as soon as before the end of the "Dark Ages" that is when Universe, beginning at 400 million years after the Big Bang, is mostly invisible to us due to neutral hydrogen blocking light of very first objects. First stars have been seen as far away as just 200 million years after the Universe began. First observed galaxy has been seen at 1 billion years after the Big Bang, that is 12.7 billion light-years from us! It's a group of one million stars, 1/20th our Galaxy in size, as more were found in 2007, small in size and composed of active, young, blue stars turning hydrogen and helium into heavier elements. Such primeval galaxies are the building blocks from which further galaxies form. It's there that first stars are forming. Cold dark matter is thought to play a role in these processes as these first galaxies are merging into larger galaxies inside dark matter halos. These mergers are yielding a higher rate of star formation. The "Central Quiescent theory", which thought that galaxies were forming by slow accretion had now been almost completely discarded. Stars successively formed in a star formation environment are getting progressively bluer at each generation due to that they benefit from heavier elements formed by the very first generation of them, mostly helium. Most recent studies are showing that those primeval galaxies may turn either into larger elliptical, or spiral, galaxies

Dwarf galaxies seen around major galaxies in the Universe today are thought now to be the building blocks of them. Much dwarf galaxies, that way, are lurking inside galaxy clusters. Such galaxies are of the size of the Small and Large Magellanic Clouds, those satellites to our own Milky Way Galaxy, and probably related to those first primeval galaxies as described above
A study in the summer of 2007, on the other hand, has found the most earliest galaxies ever seen, at 500 million years only after the Big Bang, likely being some of those objects which participated into re-ionizating the neutral hydrogen of the 'Dark Ages'

Galaxies Appearing in the Web Universe's Filaments
A recent, 2008, breathrough in the knowledge of how galaxies are forming occurred, with the astronomers now thinking that galaxies are forming into the filaments of the Web Universe, and then moving from there to the nodes of it, there where the galaxy clusters are found!
The Universe now is thought of like being an ensemble of vast filaments of hydrogen linking at nodes, making the Universe resemble like a vast sponge of sort

The Quasars Era. 1 Billion Years After the Big Bang

A quasar existing just 1 billion years after the Universe began has been seen by NASA's Chandra X-Ray telescope harbouring a supermassive black hole, like other, older, quasars are doing. This is of importance as this is the evidence that all quasars, whatever their age, are powered by such supermassive black holes. How does such massive black holes appeared so swiftly? It might that such black holes are due to the merger of thousands of stellar black holes, themselves due to massive stars' supernovae events. The black hole in SDSSp J1306 is a billion solar-mass worth! Such black holes have a similar shape. Infalling matter is producing a rapidly rotating "accretion disk", and a hot atmosphere (a "corona"). A part of the high-energy X-rays emanating from such events is due to low energy optical, UV, and X-ray photons coming from the accretion disk and bumping into hot electrons in the corona. The photons are then boosted up to the higher-energy X-ray range. picture courtesy X-ray: NASA/CXC/D.Schwartz & S.Virani; Illustration: CXC/M.Weiss

A recent study is showing that a galaxy formation bloom occurred around 900 million years after the Big Bang as, barely earlier in time, nothing similar is seen. It seems likely that such a bloom might be linked to the 'quasar era'. Galaxies, at about 1 billion years after the Big Bang are dwarf galaxies, as they are producing stars at a rate ten times higher than galaxies that come after that time

Quasars are very bright, faraway, objects thought to be supermassive black holes located in the center of galaxies and outshining them. According to the co-evolution theory, black holes and galaxies develop interacting as they compete for matter from the original gas clouds. Black holes might also appear in building-block galaxies of the previous period, as they would become active only during this second epoch. Like those of today, supermassive black holes are at 75 percent shrouded into thick envelopes of dust. A stronger correlation has been established between the size of black holes and the size of dark matter halos. During this period are found too "infrared" galaxies which are forming stars at an important rate. The average lifespan of stellar black holes' accretion disks are about 100 million years, with some local, special conditions, sometimes reducing that to 2,000 years only. This epoch too keeps having building-block galaxies, with some observed by the Hubble Space Telescope to be tiny, at about 100 or a 1,000 times smaller than our Milky Way Galaxy. A lot are seen slightly disrupted, possibly a sign that they are interacting. The study of faraway galaxies in the infrared, through the Spitzer Space Telescope, is showing that most galaxies are featuring a supermassive black hole, and that the number of quasars might be much higher than estimated now. Another recent discovery might be too that the quasars' supermassive black holes themselves might be at the origin of dust, and not only supernovae events

The Proto-Galaxies Period. 1.4-4.6 Billion Years After the Big Bang

A New Breed of Quasars? NASA's infrared Spitzer Telescope has recently allowed (the finding has been disclosed in March 2005) to find what might be a new breed of quasars existing at a distance of about 11 billion light-years as they shine like 10 trillions suns. They are enshrouded deep into very dusty galaxies. Galaxies comparable in dustiness -but not in brightness- might exist much nearer Earth

Then comes what might be called the proto-galaxies period. Proto-galaxies are compact objects which are forming stars at the most important rate of all Universe's history. These galaxies are grouping into into clusters -or, better, "proto-clusters". Energetic "infant galaxies" are gathered around a massive radio pre-galaxy emitting large jets from a supermassive black hole. Our own Milky Way Galaxy appeared at that time. It is 10 billion years old. During this period, previous infrared galaxies have come out of age and though not producing stars anymore they are accounting then for up to two-thirds of stars. One of their mystery is that they did not grow according to the merging model, but only as they had a lot of time to form stars. Quasars are continuing to be active and it's even at that time that they are are most numerous (about 3 billion years after the Big Bang). This might mean that galaxies endure several quasar phases due to frequent collisions. On another hand, first galaxies are thought too to be blown out in about less than 2 billion years due to the supernovae explosions of their first generation stars. It might that galaxy clusters begin to build as soon as 3 billion years after the Big Bang as they reach maturity 2 billion years later. Their galaxies then are elliptical, contain red stars, as they are several billion years old already. It's obvious that galaxies' life is mostly determined by the interactions between themselves inside galaxy clusters, and by interaction between themselves and the gas in the clusters. Both events are triggering star formation. It might that 'starburst galaxies', producing stars at the high rate of 4,000 a year, instead of 10 for our own Milky Way Galaxy, are seen too during those epochs

The galaxies' merger are an obvious actor in the history of the early Universe, 10 to 12 billion years ago. Such collisions are driving a lot of gas towards the center of the event, triggering both a burst of star formation (1 star is created each day, 100 times the current rate in our Milky Way Galaxy!) and the fusion of the galactic black holes, contributing to the increase of their number in the Universe. The continuing flow of gas keeps triggering more energy, and... a quasar is born! This further, is in line with the observation, at the current epoch, between the mass total of stars in the central bulges and the mass of the central black holes. The quasars thus formed are then cleaning the center of the galaxies' fusion of the infalling gas, through their superwinds, expelling the gas up to tens of thousands of light-years away! picture courtesy NASA/CXC/IoA/D.Alexander et al.

Black Holes and Host Galaxies Really Compete For Life
A study in 2006 found that the interaction between a galaxy and its black hole has as a consequence that when the black hole reaches a critical size relative to its host galaxy, the latter's development comes to a halt, as does the formation of stars in it. This is obviously due to that a large black hole creates unsuitable environment for stellar birth, either that the black hole powerfuls jets blows away the gas and matter stars would feed upon, or that the gas in the galaxy is stirred down towards the black hole, hence heated and inappropriate to stars formation

About the Large Magellanic Cloud
Both the Large and the Small Magellanic Clouds (LMC, SMC) are satellites to our Milky Way Galaxy. As far as the LMC is concerned, astronomers believe that approximately six billion years ago, not long before our solar system formed, this dwarf galaxy was shaken up via a close encounter with the Milky Way. The resulting chaos triggered bursts of massive star formation similar to what is thought to occur in more primitive galaxies billions of light-years away

Ultradense Baby Galaxies Numerous at that Epoch of the Universe
Strong-weighted baby galaxies have been pinpointed at distances of 11 billion years, with a size of about 5,000 light-years and a mass of 200 billion Sun, that is about the weigh of a current spiral galaxy! Those ultradense galaxies might be due to interaction in their formation with dark matter, as they might represent up to the half of all the galaxies of such a mass at that epoch of the Universe and might be a kind of standard them, as other galaxies would have been more massive

Usual Galaxies. Starting at 4.5 Billion Years After the Big Bang

Then comes an usual evolution of sort. More irregular galaxies than today are found. Galaxies appears to be a lot more numerous than today -between 3 and 10 times more. They are becoming mature and sort into present categories: spirals, ellipticals, irregulars. All of them

Galactic Supermassive Black Holes the Results of Mergers! It appears now that the supermassive black holes found at the center of most galaxies do not grow by themselves nor appear big all a sudden. Such galactic black holes are the result of galaxies' mergers. As galaxies are enduring several merges in their lifetime, their respective black holes merge together too, leading to the construction of the supermassive variety. Such merging galaxies are so much enshrouded in dust that it takes hundreds of million to 1 billion of years after the fusion, for the astronomers to be able to have a look at the center of such galaxies!

except irregulars are surrounded by a dark matter halo and vast clouds of gas. Spirals form by mergers of proto-galaxies and star clusters. They already come importantly under their barred form. Elliptical form by mergers of spirals (forming in high or low-density clusters, this is surely occurring based on low-velocity mergers). Clusters continue to form. They are less numerous and less tight than today's but they emit more X-rays. Clusters are forming by many sub-clusters mergers. These young clusters are embedded into vast gas clouds emitting in X-rays. Hence their member galaxies are growing fast. Inside those clusters, galaxies are colliding, gaining or losing dark matter, and forming stars due to the pressure of the cluster's gas against their own gas. In such a context stars are going supernovae in about 10 million years. Star and galaxies formation is continuing until 7 billion years have elapsed since the Big Bang. The pace brakes then and formation declines until now, thus featuring a striking time similarity with the time at when the pace of the expansion of the Universe is accelerated. The star formation inside galaxies is following the hydrogen track, meaning that stars forms, at the outreaches of galaxies included, there where there is hydrogen gas. Active Galactic Nuclei (AGNs) are seen keeping being frequent inside galaxies until the Universe reaches 60 percent of its age as such objects become rarer after that time. Today Universe is mainly composed of dwarf spheroidals, dwarf and giant ellipticals. Ellipticals are twice as numerous today than 9 billion years ago. Black holes are still active and growing in most galaxies except in ellipticals. They are the relics of the ancient quasars. Young building-block galaxies still appears, being as young as 500 million years. One of them is located 45 million light-years from our Milky Way Galaxy. Half of the spiral galaxies today are barred spirals as the galaxies which have moved down to the center of their cluster have been stripped of their gas, hence can not participate anymore into any star formation process. Some have even been completely disrupted, spilling their stars and gas into the intergalactic space. Clusters nowaday are containing between hundreds and thousands galaxies, with temperatures in the cluster ranging 10-100 million degrees. Galaxy mergers are keeping now, even between mature galaxies, ending up in galaxies which may reach as big as ten times the size of our Milky Way Galaxy. Mergers may be of the type gas-rich, with stars formation, or gas-poor, with none. A last point: dwarf galaxies as seen in the Virgo Cluster might find their origin 6 billion years ago

It's the Interactions in the Galaxy Clusters Which has the Spirals Turn Ellipticals! The half of the galaxies in the Universe today are gas-poor, few forming new stars ellipticals, with the other half keeping being spirals and irregulars with much gas and a high formation rate. It looks like it's likely the interaction in the galaxy clusters which lead to that. 7 billion years ago, one to 5 galaxy only was gas-poor. As the spirals are interacted by their cluster's gravity, this lead to them stripped of their gas -and even stars, which end 'homeless' and scattered in the cluster. The process is thought to last about 1 billion years with the elliptical eventually settling near the clusters' center

Recentest Data

• a recent study published in 'Nature' is showing that star formation peaked about 8.7 billion year after Big Bang with a disparity between low-mass (peak at 11.7 billion years after Big Bang only) and high-mass (peak at 3.7 billion years after Big Bang) galaxies. All this is pointing to a formation further in time than previously thought (peak was thought at Big Bang plus 5.7 billion years). Rate is downplayed too: "Nature" study sees it at 10 times today's rate as previous studies were at 15-20 times
• giant and dwarf ellipticals are now seen to be equivalent as far as their stars distribution and their formation is concerned
• ellipticals seem to have less dark matter halos; an explanation could be that they often gather in clusters hence frequent collisions might strip halos
• evidence of accretion unto ellipticals is better provided in X-rays than in optical as the latter quickly fades into the stars background of the galaxy
• accretion process is disrupting cannibalized galaxies by tidal forces. This yields long gas trails which in turn are producing massive, short-lived, stars swiftly turning supernovae, black holes, and neutron stars
• a thoroughful understanding of clusters history from earlier times to today is underway. This will be made possible by combining different wavelengths using e.g. Hubble for visible, Chandra for X-rays, VLT for distances and chemical composition accurate measurements, or near-infrared for assessment of star formation rate. An example of clusters impact about galaxy history is they are colliding together bringing a decline in the number of spiral galaxies. As spirals are very numerous in the early Universe, they are taking the brunt of such collisions