Rennan Barkana and Avi Loeb: Nature Article

First Milky Way Galaxies Discovered

Rennan Barkana (Tel Aviv University) and Avi Loeb (Institute for Advanced Study; on sabbatical leave from Harvard University) , Nature, Jan. 23'rd 2003 issue
Technical paper available here: PDF format or PS format

Brief summary of the findings

The formation of galaxies is part of our cosmic history. Our own Sun is a typical star formed within a mature galaxy, and astronomers are trying to figure out how such galaxies were able to form in the universe. The leading model is the hierarchical model of galaxy formation, in which small galaxies form first and then merge or accrete gas to form larger galaxies. Thus, galaxies observed in the distant universe, seen as they were in the distant past, may be similar to the progenitors of our own Milky Way galaxy. However, distant galaxies are very faint and hard to study. Our findings offer the first method for weighing distant galaxies.

Background and basic setup

We were interested in probing the distribution of gas in the distant universe. Most of the gas in the universe is in the form of hydrogen. When we observe the distant, early universe, galaxies are still rare because gravity has managed to pull only a small fraction of the total cosmic gas into galaxies. Thus, the early universe is still filled with tenuous intergalactic hydrogen that is very hard to detect directly. It is instead detected indirectly through a shadowing effect: the gas absorbs part of the light of any sources lying in the background.

Thanks to the incredible tenuousness of gas in the vast spaces between galaxies, astronomers can see sources of light from billions of light-years away. However, these sources are very faint because they are so far away. If we want to study how the gas absorbs light, it makes sense to study the very brightest sources, namely quasars. Quasars lie at the centers of galaxies that are thought to harbor supermassive black holes weighing several billion times the mass of the Sun. As gas falls in toward the black hole, violent processes in the gas cause the quasar to emit intense light over a broad range of wavelengths (colors).

Observing distant quasars, and using the shadowing effect to study the intergalactic hydrogen lying between us and these quasars, is now a standard cosmological tool. Usually the absorption is studied only at large distances from the quasar, but we realized that the absorption had not been studied carefully in the region right near the quasar. Thus, we set out to construct a theoretical model in order to predict what could be learned about the quasar environment from such absorption.

At early times, large galaxies are still very rare and form only in the regions of highest density in the universe. As the matter that eventually forms the galaxy collapses toward these density peaks, a large surrounding region feels the strong gravitational pull of the accumulating mass. So when the galaxy finally forms along with the quasar inside it, gas from all around is in the process of falling toward the galaxy, and this gas is already much denser than typical intergalactic gas. This gas infall pattern should be reflected in the absorption pattern that the gas produces on background light.

In analyzing gas infall due to gravity, we must remember that the mass that produces this gravity is thought to consist mostly of "dark matter". Most of the matter in the universe is dark, with only about one-fifth in the form of the familiar chemical elements. In today's universe, galaxies are created at the centers of much more massive halos of this dark matter - an unknown substance that makes its presence felt by its strong gravitational effects. Just as the velocity and radius of the Earth's orbit allow us to measure the mass of the Sun, so the motion of stars and gas around the centers of nearby galaxies allow us to weigh those galaxies. On larger scales, the aggregate gravitational pull of dark matter affects the distribution of galaxies and the expansion rate of the universe.

How was the discovery made?

After calculating the expected absorption pattern for typical parameters of quasars, we wanted to compare to observations of quasars. We predicted that the gravitational infall effect would be strongest for quasars that are both very bright and formed very early (i.e., are very distant). A very high quality observation is required to test our prediction, and there are only a few published examples that fit all the criteria. We looked first at the most distant known quasar (it has since become the second-most distant known quasar), and immediately noticed that it seemed to show absorption similar to our predicted pattern. We found a similar pattern in a second, somewhat less distant quasar, that was particularly bright. These quasar spectra had been published in 2001 but it seemed that no-one had noticed this absorption pattern before. As we thought about this further, we realized that if this detection of gas infall is correct, it has many important consequences.

What are the key consequences?

1) Infalling gas from a large surrounding region towards a galaxy has not been seen before, since the gas at large distances is so tenuous. Our detection verifies this fundamental prediction of the paradigm by which galaxies form gravitationally.

2) The pattern of infall is determined by the total mass of the dark matter halo surrounding the quasar. Dark matter halos have been discovered and studied in the nearby universe, but it is much harder to study them in the early universe (which we can observe only at large distances). For example, in the local universe people study the motion of gas at the outskirts of galaxies, in order to infer the mass of the dark matter which is producing the motion, but the outskirts of distant galaxies are too faint to see. In fact, no-one has previously measured the mass of a dark matter halo (whether one containing a quasar or not), except in the nearby universe. Our detection of infall yields an estimate of the masses of halos when the universe was only around one billion years old (compared to its current age of about 14 billion years).

3) Since we did detect this effect around quasars, this also has important consequences for the nature of quasars. In the local universe, supermassive black holes (which shine as quasars if large amounts of gas fall onto them) are known to lie in the centers of galaxies, which in turn lie in the centers of dark matter halos. Typically, the dark halo contains around 5-10 times more mass than the galaxy, and the black hole contains only a fraction of one percent of the mass of the galaxy. Remember, all that was seen directly is a bright source of light corresponding to the quasar. We used the absorption effect to detect the large mass concentration around it. This suggests that the most distant quasars form similarly to those in the nearby universe.

What's the next step?

The next step will be to try to demonstrate more definitively that the absorption patterns we are seeing are indeed due to infalling gas, and to apply this method more widely. There exists data on a large number of quasars that is of very high quality, but it has not yet been published. We are contacting observers in order to try to analyze this data. Real life is always more complicated than any model, so any particular quasar could show a somewhat different absorption pattern. The most promising way to verify our prediction is to study a large number of quasars, and to search for general trends that we predict. For example, we predict that gravitational infall should be stronger around quasars that are brighter, and around quasars that formed earlier on.

What's the ultimate goal?

The ultimate goal of this particular research is to develop it into a tool for measuring the masses of dark matter halos around quasars. The current demonstation of the method yields a rough estimate of the masses of the dark matter halos around two quasars. The masses come out a bit above that of our own Milky Way halo. Such halos should be very rare in the early universe. According to standard models, they should be the most massive dark matter halos in the universe at that time. This is consistent with the fact that only a few quasars have been found that are so bright and distant. If the method can be applied to a larger number of quasars, it will begin to test the models much more strongly.

It's important to note that gravitational effects are particularly interesting, since gravity is very well understood. Complicated gas processes often make it difficult to compare between theoretical models and observed data. On large scales, however, gravity dominates, potentially allowing a more direct test of the models.

More generally, the goal is to test models of how galaxies form, and look for surprises. For example, very little is known about dark matter other than its gravitational effects. If the dark matter interacted with itself through some other process, this would affect the number of dark matter halos that form. If we succeed in measuring many halo masses, they could come out either too high or too low, compared to the standard model, and any disagreement would provide a crucial clue about the nature of dark matter.

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