Archive for ‘Early universe’

February 8, 2012

How did some early black holes get so big so fast?

The supermassive black holes (SMBHs) found in the centers of large galaxies can be astonishingly large. The closest example to us is in the giant elliptical galaxy M87, and it’s estimated to be 6.6 billion solar masses (M). More distant examples can be even larger, more than 10 billion M (at distances ~300 million light-years).

Those are extremes. 1 or 2 billion M SMBHs are a little more common in our neighborhood, though still rare. Rather surprisingly, however, SMBHs that large can be found even in the very early universe. The largest yet discovered is about 2 billion M, and it’s 12.9 billion light-years away, at a redshift z=7.085. That SMBH reached its observed size only 765 million years after the big bang, i. e. perhaps 500 million years after the very first stars formed. It’s been a difficult problem to understand how SMBHs that large could have formed so quickly. A recently announced computer simulation of a large part of the very early universe may have come up with a good answer.

It is only barely possible to detect very bright objects (such as quasars or large galaxies) at redshifts z~7 with the best telescope technology today, and impossible to detect less bright objects (even the brightest stars) or objects at higher redshifts. So direct observation of the earliest stars – which may have begun to form as early as z~30, 100 million years after the big bang – is currently impossible, and computer simulations must be used to understand their properties and the process in which they formed.

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January 29, 2012

How large were the first stars in the universe?

Since it is currently, and for the foreseeable future, not possible to actually observe what the first stars in the universe were like when they formed, the only way to answer this question is by detailed calculations from first principles. In other words, by computer simulations. Until very recently, such simulations couldn’t be very conclusive, since they simply couldn’t handle the amount of detail required. But they indicated that the first stars may typically have been very massive – perhaps often 100 solar masses each.

However, the very latest simulations, which can take advantage of the most powerful supercomputers now available in order to do more detailed calculations, indicate that a typical first generation star may have been less than half as massive as previously indicated.

It’s much harder than one might suppose to simulate, in detail, the formation of stars in the very early universe. Ten years ago the best that could be done was to simulate the process just up to the point where a single clump of dense, hot gas only about 1% as massive as the Sun has formed. That’s really only the first part of the process, and what happens after that is crucial in order to figure out what the typical size of one of the first stars in the universe was.

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January 15, 2012

Primordial galaxy cluster is earliest ever seen

At what point did stars, galaxies, and galaxy clusters make their first appearance in the universe? There’s now good evidence that galaxy clusters were starting to form about 650 million years after the big bang. So galaxies must have begun forming earlier than that, and the first stars even earlier.

The evidence consists of the discovery that a known bright galaxy at z≈8 has 4 dimmer companions within a radius of about 10 million light-years. The known galaxy was originally found using the Wide Field Camera 3 (WFC3) of the Hubble Space Telescope, by means of the Lyman-break technique. The was done as part of a search for z≈8 galaxies, known as the Brightest of Reionizing Galaxies (BoRG) survey. The region in which the bright galaxy was found was designated BoRG58.

The Lyman-break technique is based on the fact that photons having wavelengths less than the Lyman-α length of 121.6 nm (medium ultraviolet) are easily absorbed by neutral hydrogen gas. Consequently, most of the light from hot young stars is extinguished by neutral hydrogen, since such stars are brightest at wavelengths shorter than Lyman-α. The wavelength of Lyman-α photons emitted at z≈8 is stretched by a factor of z+1 to 1094 nm, in the infrared range. So in order to identify bright galaxies at z≈8, images are made through a series of filters that pass photons on either side of 1094 nm. Objects that seem to disappear in images made through filters that pass only light with wavelengths below 1094 nm are probably at the desired redshift.

Four bright objects presumed to be at z≈8 were identified early in the survey process. Computer simulations have indicated that at that redshift the most massive galaxies (which are presumably the brightest) should have a number of less massive, less luminous companions. So the search was extended around each of the four candidates, using longer exposure times to detect fainter objects. 17 less luminous objects were detected with substantial confidence. Field BoRG58, which had the best original candidate, turned out to have 4 less luminous companions of the candidate at the best level of confidence.

It hasn’t yet been possible to obtain direct spectroscopic evidence for the actual redshift of the detected objects. This is probably because so much of the available light from those objects has been extinguished by neutral hydrogen. However, the detection of fainter z≈8 dropout objects near the original candidate provides additional evidence that the objects are not spurious dropouts (which can occur with low probability).

The computer simulations suggest that the halo mass (which includes dark matter) around the brightest object should be in the range 400 to 700 billion M. (The Milky Way’s halo is estimated to be more than 1 trillion M.) There should also be additional smaller halos nearby about 100 billion M, hosting the fainter dropouts. By the present time the system should have grown into a cluster with halo mass 200 trillion M, a fairly typical cluster size.

Further reading:

Hubble Pinpoints Furthest Protocluster of Galaxies Ever Seen

CU-led study pinpoints farthest developing galaxy cluster ever found

CU-Boulder finds farthest galaxy cluster ever seen

Hubble spies earliest galaxy cluster ever seen

Hubble shows images from record-breaking 13.1 billion light-years

Overdensities of Y-dropout Galaxies from the Brightest-of-Reionizing Galaxies Survey: A Candidate Protocluster at Redshift z≈8

November 23, 2011

Most stars in dwarf galaxies formed early in the universe

A study based on a survey of very distant galaxies selected 69 individual objects that had exceptionally bright emissions lines, suggesting that they were galaxies very actively forming new stars. Out of that number, four objects for which the necessary measurements could be made proved to have strong lines due to ionized oxygen. In these cases it was possible to conclude that the objects were relatively small galaxies whose mass was about 100 million solar masses (M) – and they were forming stars so rapidly that the mass due to stars would double in just 10 million years – an average of 10 M per year. That compares to a rate of about 3 M per year at present in the Milky Way – even though it is 1000 times as massive as the early galaxies studied.

The galaxies studied were at a redshift z~1.7, which corresponds to a light-travel distance of 11.5 billion light-years, so the galaxies are seen as they were about 2.2 billion years after the big bang. Other surveys have suggested that this era was when the rate of star formation everywhere was near its peak. From the observed numbers of small galaxies of ~108 M and the star formation rates at z~1.7 it can be estimated that most of the stars in present-day dwarf galaxies (from 108 M to 109 M) formed in a few short bursts from 1 to 5 billion years after the big bang.

Simulations of star formation in dwarf galaxies suggests that rapid star formation activity occurs in bursts. This is because when star formation occurs most rapidly the combined energy emitted by the hot young stars quickly either heats gas inside the galaxy to a temperature too high for new stars to form or expels the gas entirely. Only after enough time has passed following the burst does the gas cool off enough and fall back into the galaxy to restart the cycle. So the 4 galaxies most carefully studied are probably not representative, since they were more likely to have been noticed than similar galaxies not in the middle of a burst of star formation.

According to the research paper:

Extreme Emission Line Galaxies in CANDELS: Broad-Band Selected, Star-Bursting Dwarf Galaxies at z>1

Our discovery of an abundant galaxy population at z ~ 1.7 with extremely high emission line equivalent widths implies that many high-redshift, low-mass galaxies form many of their stars in extreme starbursts. We propose that we have observed an important formation mode for dwarf galaxies: a small number of strong starbursts that occur at early epochs (z > 1) each form ~108 M in stars in a very short time span (~30 Myr) to build up the bulk of the stellar components of present-day dwarf galaxies. This is in quantitative agreement with ’archaeological’ studies of present-day dwarf galaxies, which have shown that their star formation histories are burst-like and that the ages of their stellar populations suggest formation redshifts z > 1. Under the reasonable assumption based on ΛCDM predictions for galaxy growth that the observed galaxies grow in mass by less than an order of magnitude up to the present day, our observations provide direct evidence for such an early formation epoch and, in particular, that short-lived bursts contribute much or even the majority of star formation in dwarf galaxies.

Further reading:

Hubble Uncovers Tiny Galaxies Bursting with Starbirth in Early Universe

Population explosion in dwarf galaxies

Tiny galaxies bursting with stars

Extreme Emission Line Galaxies in CANDELS: Broad-Band Selected, Star-Bursting Dwarf Galaxies at z>1

November 9, 2011

Observations of gamma-ray burst reveal surprising ingredients of early galaxies

According to observations of a very distant gamma-ray burst (GRB) recently reported, it appears that a couple of galaxies in the early universe, only 1.8 billion years after the big bang, contain a higher concentration of some elements heavier than hydrogen and helium than the Sun does. This is rather surprising, since the Sun is about 4.5 billion years old, and formed out of gas and dust in which heavy elements had been accumulating for about 9 billion years.

Isaac Asimov supposedly remarked, “The most exciting phrase to hear in science, the one that heralds new discoveries, is not ‘Eureka!’, but ‘That’s funny …'” That may be appropriate in this case. How did those galaxies have all those heavy elements so early?

The trigger for the observation was a GRB detected in March 2009 and designated GRB 090323. This was the type of GRB known as a “long” GRB, because the brightest phase lasts more than two seconds, and there is a diminishing afterglow that may last days. (The “short” type lasts less than two seconds and shows no afterglow.) Long GRBs are thought to result from supernova events in which a jet of relativistic particles is aimed in our direction.

Although the burst was determined to be quite distant (redshift z=3.57, corresponding to a light travel time of about 11.9 billion years), there was nothing about it at first that seemed especially unusual. However, an analysis of the spectrum of the afterglow indicated that the light from the GRB had passed through two galaxies relatively close to each other, one of which may have been the host of the GRB. What was especially odd was that the spectrum showed the presence of higher concentrations of zinc and sulfur than occur in the Sun.

Observations of gamma-ray burst reveal surprising ingredients of early galaxies

An international team of astronomers led by the Max Planck Institute for Extraterrestrial Physics has used the brief but brilliant light of a distant gamma-ray burst as a probe to study the make-up of very distant galaxies. Surprisingly the new observations revealed two galaxies in the young Universe that are richer in the heavier chemical elements than the Sun. The two galaxies may be in the process of merging. Such events in the early Universe will drive the formation of many new stars and may be the trigger for gamma-ray bursts.

Gamma-ray bursts are the brightest explosions in the Universe. They are first spotted by orbiting observatories that detect the initial short burst of gamma rays. After their positions have been pinned down, they are then immediately studied using large ground-based telescopes that can detect the visible-light and infrared afterglows that the bursts emit over the succeeding hours and days. One such burst, called GRB 090323, was first spotted by the NASA Fermi Gamma-ray Space Telescope. Very soon afterwards it was picked up by the X-ray detector on NASA’s Swift satellite and with the GROND system at the MPG/ESO 2.2-metre telescope in Chile. From the GROND observations, the astronomers estimated the minimum rate of star formation, which has to be several times higher than the one in our Galaxy. They could, however, only determine a minimum value because the detected emission could be heavily affected (i.e. absorbed) by the presence of dust in the galaxies. The real rate of star formation, once the (unknown) dust absorption has been taken into account, could easily be 50 times higher than in the Milky Way.

The authors of the research paper point out that “These are the highest metallicities ever measured in galaxies at z > 3.” (“Metallicity” refers to the relative abundance of heavy elements.) But how unusual is this, really?

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October 20, 2011

Distant Galaxies Reveal The Clearing of the Cosmic Fog

The first billion years after the big bang (out of about 13.7 billion years total since then) were among the most interesting in terms of giving birth to the kind of objects that still dominate the scene today. Mostly that means stars and galaxies, plus a few exotica such as quasars. Unfortunately, it’s very difficult for astronomers to actually see what was going on back then.

There are three reasons for this difficulty. First, astronomers can detect objects at very early times only at very large distances from us, due to the light travel time. So such objects are likely to be very dim, if detectable at all. Second, due to the expansion of the universe, light emitted by objects in the very early universe will be shifted in wavelength to much higher values. This places a large portion of the light into the infrared part of the spectrum, which is difficult or impossible to observe with ground-based telescopes.

However, the third reason that very early objects are difficult to observe is that conditions in the early universe, namely the presence of a great deal of neutral hydrogen gas in the space between galaxies, obscures light from the galaxies just like atmospheric fog. This is not only unfortunate but also ironic, since a good determination of just how much “fog” was present is one of the key pieces of information that astronomers need in order to understand what objects in the early universe were really like. Astronomers need to know how much “fog” there was in order to correct for it so the visual characteristics of early galaxies can be determined. Yet lack of understanding when and how the “fog” cleared makes this effort rather frustrating.

Research that’s just been published gives new information that helps clarify things a little. It’s based on observations of just 5 very early galaxies, which are among the earliest, most distant galaxies known. What the research is telling us is that the “fog” was clearing rapidly at the time the light we now see from these 5 galaxies was actually emitted.

Distant Galaxies Reveal The Clearing of the Cosmic Fog

An international team of astronomers used the VLT as a time machine, to look back into the early Universe and observe several of the most distant galaxies ever detected. They have been able to measure their distances accurately and find that we are seeing them as they were between 780 million and a billion years after the Big Bang.

The new observations have allowed astronomers to establish a timeline for what is known as the age of reionisation for the first time. During this phase the fog of hydrogen gas in the early Universe was clearing, allowing ultraviolet light to pass unhindered for the first time.

The new results, which will appear in the Astrophysical Journal, build on a long and systematic search for distant galaxies that the team has carried out with the VLT over the last three years.

Astronomers are sure that they know what the “fog” consisted of: ordinary atomic hydrogen gas that is not ionized. A neutral (not ionized) hydrogen atom consists of an electron and a single proton. High-energy photons interact strongly with neutral hydrogen but not with ionized hydrogen. (As will be explained below.) In the early universe about 75% of the mass was in the form of hydrogen, and the rest was helium, with only a small trace of a few other elements. It’s known that this hydrogen “fog” dissipated as most hydrogen atoms became ionized. But it’s not known just when the process of ionization began or when it ended. Even less is it known exactly what caused the ionization. The new research, however, does indicate that the process was occurring rapidly at a specific point in the universe’s history.

Let’s take a closer look at the details of the process as they are currently understood.

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