Archive for ‘Galaxy formation and evolution’

February 27, 2012

The dwarf satellite galaxy problem

Simulations of galaxy formation based on the Lambda-Cold Dark Matter (ΛCDM) cosmological model predict that a large galaxy such as the Milky Way should have many dwarf satellite galaxies, perhaps thousands. However, only about 20 or 30 have been identified. Where are the rest? Are they really there? That question alludes to the “dwarf galaxy problem“.

Astrophysicists suspect that most satellite galaxies are much smaller than the galaxies they orbit. And, in addition, such dwarf galaxies may consist mainly of dark matter, with few visible stars, so they should be very difficult to detect, even if there are a lot of them. Since dwarf galaxies consisting mainly of dark matter are so difficult to find by visible light, there could be enough of them to reconcile the large number of dwarf galaxies that simulations predict to exist with the small number actually observed.

Surprisingly, recent research (Vegetti, et al) has been able to detect a very distant dwarf satellite galaxy by gravitational lensing effects – and from that it is possible to infer that a large number should exist.

The image shows an Einstein ring, which consists of a foreground galaxy (JVAS B1938+666) in the middle, and the distorted image of a more distant galaxy making up most of the ring. A detailed mathematical analysis of the image has confirmed that a minor irregularity in the ring is caused by the presence of a dwarf satellite galaxy of the lens galaxy.

read more »

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

January 4, 2012

Some supermassive black holes are much more super than others

Supermassive black holes (SMBHs) can get to be pretty large. Astrophysicists don’t really know what the upper limit is, if any. But before some recent research, the mass of the largest SMBH yet determined was 6.3×109 M (solar masses). That value is known fairly precisely, since the SMBH is in the nearby giant elliptical galaxy M87, which is a mere 53 million light-years away.

The latest research has identified two substantially larger SMBHs, but the masses are known less precisely, since the objects are a lot farther away. One SMBH is in NGC 3842 and has estimated mass of 9.7×109 M, at a distance of 320 million light-years. The other is in NGC 4889. Its mass is known considerably less precisely but may be more than twice that of the SMBH in NGC 3842. (2.1×1010 M is the midpoint of the possible range.) It’s 336 million light years away. Both of these galaxies are also giant ellipticals. The uncertainty in the SMBH mass is much larger for NGC 4889 than for NGC 3842 because the mass estimates are based on the velocities of stars very close to the SMBH, and the uncertainties of velocity measurements in the former case were more than in the latter.

The establishment of new records for directly measured SMBH masses is actually not the most interesting aspect of the new research. (Although the amount of media attention to the results might lead one to think it was.) One thing that’s more interesting is that a fairly straightforward method of estimating SMBH mass can be used out to a distance of several hundred million light-years with present technology.

read more »

December 11, 2011

Active star-forming galaxies have substantial halos

Detailed new research shows that there is a distinct correlation between galaxies with large, oxygen-rich gas halos and active ongoing star formation. Although active star formation requires large amounts of available gas, what is surprising is that much, or perhaps even most, of the gas may be in the halo region outside of where most stars are found. Galaxies without substantial halos evidently do not have sufficient gas in the inner regions alone, where most stars exist, to sustain star formation.

Using the Hubble Space Telescope, a survey of 42 galaxies collected information about the distribution of gas in galactic halos that extend far beyond galaxies’ visible stars. Although the gas itself is not visible, its characteristics can be inferred from traces imprinted on the spectra of light from distant quasars. Two other large ground-based telescopes collected information for the same galaxies on their distances, masses due to visible stars, and rates of star formation.

When the two sets of data were compared, some surprises emerged. In galaxies where stars are actively forming, the halos are full of large quantities of oxygen-enriched gas, which may have at least as much mass as is present in the visible stars. However, galaxies not actively forming stars did not have such massive halos.

Keck, Magellan & Hubble Telescopes Find Galactic Recyclers

Among the key findings of the work is that the color and shape of a galaxy is largely controlled by gas flowing through an extended halo around it. All modern simulations of galaxy formation find that they cannot explain the observed properties of galaxies without modeling the complex accretion and “feedback” processes by which galaxies acquire gas and then later expel it after processing by stars. The three studies investigated different aspects of the gas recycling phenomenon.

“Our results confirm a theoretical suspicion that galaxies expel and can recycle their gas, but they also present a fresh challenge to theoretical models to understand these gas flows and integrate them with the overall picture of galaxy formation,” said Jason Tumlinson of the Space Telescope Science Institute in Baltimore, Maryland; a coauthor of one of the Science papers.

The implication is that there must be an efficient process of exchange between gas in a galaxy’s halo, the “circumgalactic medium” (CGM), and the inner region of the galaxy where most stars form and reside: the “interstellar medium” (ISM). Since stars need cold, relatively dense gas in order to form, they cannot form in the warmer, more diffuse CGM, so the gas from there must migrate inward, cool down, and become more dense.

read more »

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 11, 2011

Galaxy interactions accelerate the growth of supermassive black holes

It’s now well-known that there’s a rough correlation between a galaxy’s size and the size of its central supermassive black hole (SMBH). The correlation is even better (for spiral galaxies) between the black hole size and the size of the central bulge of the galaxy. It’s been found that the mass of a SMBH is generally close to 1/1000th of the mass of the central bulge. So in some way or other, it seems that galaxies and their black holes grow together, although it’s still unclear whether the galaxy or the black hole takes the lead.

The question can be asked in a different way. Which factors contribute to galaxy growth and which to black hole growth? The problem is that black holes usually can’t be observed, or even detected, directly. About the only way to find SMBHs is when they are part of an “active galaxy”, in the form of an active galactic nucleus (AGN). AGNs represent a stage in which large amounts of gas and dust are being swept into the vicinity of a black hole, before actually falling into the black hole. This is a fairly unusual state, which occupies perhaps only about 1% of a galaxy’s life time, if it occurs at all. Nevertheless, AGN are commonly used as indicators of a growing SMBH. It’s certainly true that SMBHs grow while its galaxy is “active”.

One obvious mechanism for the simultaneous growth of galaxies and their SMBHs is merger between two (or more) galaxies. This has been the favored theory for some time, and a number of studies provide some evidence for its occurrence. Recent examples: here, here, here, here, here, here.

However, more recently, other studies suggest that mergers may not be the most common cause of AGNs: here, here.

Cleary, this is still an issue that’s as yet very much unresolved. The latest research to appear adds more evidence that galaxy mergers – and probably even close encounters – do contribute to the growth of SMBHs and stimulate flare-ups of AGNs.

read more »

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?

read more »

October 30, 2011

Astronomers Pin Down Galaxy Collision Rate

Galaxies have been growing over most of the 13.7 billion year history of the universe. Some of the growth is due to intergalactic gas gradually swept up by an existing galaxy and then driving star formation in the galaxy. But another growth mechanism is the merger of two (and sometimes more) existing galaxies into one. In this case, star formation in the merged galaxies increases as the gas within the galaxies is stirred up during the merger.

Bursts of star formation are important, since in most cases our only indication of the size of a galaxy is its brightness resulting from visible stars. In general, there’s no good way to determine the mass of a galaxy that is due to dark matter and the presence of gas outside stars.

Astrophysicists generally assume that large, symmetrical spiral galaxies (such as our Milky Way) have not merged with other galaxies of similar size, though they may have incorporated several smaller galaxies. However, a large elliptical galaxy is expected to result from the merger of a large spiral and another galaxy of similar size.

For a large elliptical galaxy, it’s natural to wonder about the relative importance of the two possible growth mechanisms in the galaxy’s history – growth by merger and growth by accretion of intergalactic gas. That in turn naturally raises the question of how often mergers occur between galaxies of varying relative sizes.

New research that is soon to be published gives much better estimates of merger rates.

NASA – Astronomers Pin Down Galaxy Collision Rate

A new analysis of Hubble surveys, combined with simulations of galaxy interactions, reveals that the merger rate of galaxies over the last 8 billion to 9 billion years falls between the previous estimates.

The galaxy merger rate is one of the fundamental measures of galaxy evolution, yielding clues to how galaxies bulked up over time through encounters with other galaxies. And yet, a huge discrepancy exists over how often galaxies coalesced in the past. Measurements of galaxies in deep-field surveys made by NASA’s Hubble Space Telescope generated a broad range of results: anywhere from 5 percent to 25 percent of the galaxies were merging.

The study, led by Jennifer Lotz of the Space Telescope Science Institute in Baltimore, Md., analyzed galaxy interactions at different distances, allowing the astronomers to compare mergers over time. Lotz’s team found that galaxies gained quite a bit of mass through collisions with other galaxies. Large galaxies merged with each other on average once over the past 9 billion years. Small galaxies were coalescing with large galaxies more frequently. In one of the first measurements of smashups between dwarf and massive galaxies in the distant universe, Lotz’s team found these mergers happened three times more often than encounters between two hefty galaxies.

It has not been easy to get good estimates of these merger rates. The problem is that all we can observe, for any given possible merger, is a snapshot of the action.

read more »

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.

read more »

October 14, 2011

Suspects in the quenching of star formation exonerated

Active galactic nuclei (AGN) – produced by matter swept violently into the vortex around a supermassive black hole that may have billions of times as much mass as our Sun – can put on some of the most spectacular fireworks in the universe, over periods as long as 100 million years.

Astrophysicists have sometimes speculated that AGN may be so energetically active that they diminish or even extinguish formation of new stars in the host galaxy.

Recently published research, making use of the PRIMUS faint galaxy spectroscopic redshift survey, suggests that the speculations are wrong, and that AGN can be found even in galaxies in which very active star formation is occurring. The research gives some answers to the larger question of what special characteristics, if any, the host galaxies of AGN may have.

Suspects in the quenching of star formation exonerated

Because astronomers had seen these objects primarily in the oldest, most massive galaxies that glow with the red light of aging stars, many thought active galactic nuclei might help to bring an end to the formation of new stars, though the evidence was always circumstantial.

That idea has now been overturned by a new survey of the sky that found active galactic nuclei in all kinds and sizes of galaxies, including young, blue, star-making factories.

“The misconception was simply due to observational biases in the data,” said Alison Coil, assistant professor of physics at the University of California, San Diego and an author of the new report, which will be published in The Astrophysical Journal.

“Before this study, people found active galactic nuclei predominantly at the centers of the most massive galaxies, which are also the oldest and are making no new stars,” said James Aird, a postdoc at the University of California, San Diego’s Center for Astrophysics and Space Sciences, who led the study.

read more »