Thursday, March 11, 2010

Galaxies are slowly running out of gas

Galaxies are made of stars, and stars are made of... gas. So a large part of understanding how galaxies evolve and grow is understanding how much "gas" (literally, not "gasoline") is present in galaxies – but has not yet been incorporated in stars – at different periods in the history of the universe.

What periods of the universe are most interesting in this regard? The answer is: periods somewhat less than the first half of the universe's existence since the time of the big bang, roughly the first 5.5 billion years, 40% of the total. That's because astronomers have good reason to believe that is the time when star formation, and hence galaxy growth, occurred most vigorously.

Assuming the best current estimate, that is has been about 13.7 billion years since the big bang, this means we're interested in observing the universe as it was more than 8.2 billion years ago. That's quite a long time ago, and until fairly recently observation of objects that far back in time has been infeasible. Technology is only now becoming available to study the details of such a remote time.

Astronomers find it convenient to represent distance (in either space or time) in terms of redshift. Because it takes light a finite amount of time to travel, any observable object is seen not as it looks "today", 13.7 billion years after the big bang, but instead as it looked a some time T<13.7, and so we see the object as it looked 13.7-T billion years ago. The light from such an object has taken 13.7-T billion years to reach us.

Due to the expansion of the universe, the wavelength of any photon of light has been increased by a factor of (z+1), where z is the observed redshift – z=0 corresponding to nearby objects for which the shift is negligible. z increases as a complicated function of the distance of the object, but it increases in a regular way as the distance increases. For objects at an age T=5.5 billion years, corresponding to a distance of 8.2 billion light years, the redshift would be about 1.1.

Astronomers have now done surveys of galaxies around z≈1.1. It's not easy, but there is plenty of data, even though only very large, bright galaxies can be observed in detail at that distance. It's even more difficult, though still feasible, to survey galaxies that are even more remote, say at z≈2.3, which corresponds to T≈2.9 billion years.

For the research under discussion here, the investigators relied on existing surveys to sample from, because of the difficulty of doing new surveys from scratch. There was a trade-off to be made. In order to be able to study a selected sample of galaxies in sufficient detail, it's desirable to pick the largest, brightest galaxies. On the other hand, it's also important to study galaxies that are representative of "typical" mature galaxies today, such as our Milky Way. Unfortunately, the most luminous objects at large z tend to be atypical things like quasars and merging galaxies. Those are "freaks", quite unlike typical nearby galaxies, and whatever we might learn about them might not tell us much about the typical case.

So the investigators had to select galaxies for study that were as large and bright as possible, but still "normal". In this case, they included only galaxies of estimated stellar mass (excluding dark matter) of ≥ 3×1010 M. (1 M is our Sun's mass.) Since we are inside the Milky Way and can't see all of it (because of thick dusty regions), it's hard to be sure of our galaxy's total stellar mass, but it's estimated to be about 5×1010 M. (Ref: here.)

An important objective of the research was to get a better understanding of galaxies in which new stars are actively being formed – unlike the Milky Way and other nearby large spirals, which are currently forming stars at the rate of about 5 M per year. Star formation rate is something else that's easier to determine from outside the galaxy, by measuring light flux in various parts of the spectrum (especially infrared and ultraviolet). For the present research, only galaxies with a star formation rate ≥ 40 M per year were selected.

For reasons we're coming to, the required observations are difficult and time-consuming, so for this kind of preliminary study it was necessary to work with small numbers. The net result is that the study was done with 11 galaxies selected from one survey, with z≈1.2, and 12 galaxies selected from another survey with z≈2.3.

Remember that the ultimate objective of the research is to determine how much gas is available for star formation in typical galaxies at the given values of z. That's what is so difficult that it had not been done before (for such distant galaxies).

It's relatively straightforward to determine how much of a galaxy's mass is in the form of stars. This "stellar mass" is proportional to the intrinsic luminosity of the galaxy (which is known since the galaxy distance is known), because most galaxies consist of stars with a predictable distribution of stars of given mass and luminosity ("initial mass function").

However, the total mass of a galaxy also includes non-baryonic dark matter, whose mass in the universe as a whole is known to be about 5 times as large as the mass of "ordinary" baryonic matter. The total mass of a galaxy can sometimes be inferred from measuring galaxy rotation curves. The baryonic matter of a galaxy consists of stars, gas, and (perhaps) massive nonluminous objects such as black holes. Even if one knew reliably the total mass of a galaxy, including dark matter, and one could neglect the contribution of nonluminous objects, one still could not estimate the mass of gas as the difference between the mass of a galaxy's stars and the roughly 17% of total mass that baryonic matter represents in the universe as a whole.

That's because there's no a priori reason to expect that a 1:5 ratio of baryonic matter to non-baryonic matter is present in any particular galaxy – it might well be either more or less. So there needs to be some way to measure fairly directly the amount of matter a galaxy contains in the form of gas that could form stars.

It is known that stars form only out of gas that's rather cold – with temperature less than 100 K. This is simply because hotter gas has a higher internal pressure that prevents the gravitational collapse that's necessary to form a star. Gas that cold is very hard to detect. Black body radiation at 100 K peaks at 29 μm, in the far infrared, and cooler gas emits at even longer wavelengths. Redshift stretches the wavelengths even more (by factors of 2 or 3 for z=1 or 2). Most of the radiation at such wavelengths is blocked by our atmosphere, so is observable only from space – and the necessary instruments don't exist yet.

Fortunately, black body radiation is not the only type of electromagnetic emission from cold gas. Vibrations and rotations of gas molecules also radiate at certain frequencies. Now, most of a galaxy's cold gas is in the form of atomic helium and molecular hydrogen. It would be convenient if hydrogen molecules, especially, had emissions at convenient wavelengths for ground-based observation, but no such luck. It turns out, however, that there is one molecule present in small amounts in interstellar gas which does have a convenient emission: CO (carbon monoxide). CO has a rotational emission at 870 μm (346 GHz). At the values of z of interest here (1.2 and 2.3) these fall into the 2 mm and 3 mm bands – which can be observed.

In a nutshell, then, what the research under discussion did was to measure the total flux from the selected galaxies at the appropriate wavelengths. This indicates that amount of cold CO gas present in the galaxies. Studies of nearby galaxies show that this accurately indicates the total amount of cold gas present. From this, and the estimated mass in the form of stars, one has fraction of total mass (stars + gas) represented by the cold gas available to form stars.

For the galaxies in the sample, this fraction was found to be 34% at z≈1.2 and 44% at z≈2.3. By contrast, contemporary large spiral galaxies have fractions in the 3% to 12% range – quite a difference.

For a summary of the results, here's the abstract:

High molecular gas fractions in normal massive star-forming galaxies in the young Universe
Stars form from cold molecular interstellar gas. As this is relatively rare in the local Universe, galaxies like the Milky Way form only a few new stars per year. Typical massive galaxies in the distant Universe formed stars an order of magnitude more rapidly. Unless star formation was significantly more efficient, this difference suggests that young galaxies were much more molecular-gas rich. Molecular gas observations in the distant Universe have so far largely been restricted to very luminous, rare objects, including mergers and quasars, and accordingly we do not yet have a clear idea about the gas content of more normal (albeit massive) galaxies. Here we report the results of a survey of molecular gas in samples of typical massive-star-forming galaxies at mean redshifts of about 1.2 and 2.3, when the Universe was respectively 40% and 24% of its current age. Our measurements reveal that distant star forming galaxies were indeed gas rich, and that the star formation efficiency is not strongly dependent on cosmic epoch. The average fraction of cold gas relative to total galaxy baryonic mass at z = 2.3 and z = 1.2 is respectively about 44% and 34%, three to ten times higher than in today’s massive spiral galaxies. The slow decrease between z ≈ 2 and z ≈ 1 probably requires a mechanism of semi-continuous replenishment of fresh gas to the young galaxies.

The results from this research about star formation rates (SFR) are especially interesting. From other research involving much larger samples, it's known that when SFR is plotted against galaxy stellar mass, the distribution can be fit by a power law:
SFR (M/year) = 150 (M*/1011M)0.8×([1+z]/3.2)2.7
In this equation, M* is the galactic stellar mass. Thus SFR depends on total stellar mass of a galaxy, which makes sense, because the larger the galaxy, the more cold molecular gas is available to make stars. Further, because of the factor involving 1+z (where z is redshift), the SFR curve is shifted upwards at larger z – the rate of star formation is greater, in a regular way, at earlier times in the universe.

This equation is pretty close even in the nearby universe, where z=0. For a galaxy the size of the Milky Way (which is not among the largest of spirals), M* is estimated as 5×1010 M, predicting SFR of about 3.7 M/year – which is surprisingly accurate. So SFR has continued to decline in a fairly regular way.

Interestingly enough, however, in the galaxies sampled in the present research, the percentage of cold gas in a galaxy does not appear to have any clear relationship to either the SFR or the total stellar mass of a galaxy. So almost all of the variation in SFR is related to the total stellar mass. This is what it means to say that the "efficiency" of star formation is not very dependent on percentage of cold gas or cosmic epoch. Instead, SFR is probably largely dependent on total available cold gas, which is proportional (at a given z) to a galaxy's stellar mass.

One additional interesting conclusion can be drawn from the research. Namely, given the SFR in sampled galaxies at z≈2.3, there ought to be much less cold gas in equivalent galaxies at the later time (about 2.3 billion years later) corresponding to z≈1.2 than is actually observed. Much of that cold gas should have been incorporated into stars. Yet the amount of cold gas actually observed at the later time is more than the original amount less what was converted to stars. And so there is apparently more cold gas added over time, even though, as a whole, galaxies really are "runnng out of gas".
Tacconi, L., Genzel, R., Neri, R., Cox, P., Cooper, M., Shapiro, K., Bolatto, A., Bouché, N., Bournaud, F., Burkert, A., Combes, F., Comerford, J., Davis, M., Schreiber, N., Garcia-Burillo, S., Gracia-Carpio, J., Lutz, D., Naab, T., Omont, A., Shapley, A., Sternberg, A., & Weiner, B. (2010). High molecular gas fractions in normal massive star-forming galaxies in the young Universe Nature, 463 (7282), 781-784 DOI: 10.1038/nature08773

Further reading:

Young galaxies gorge on gas (2/10/10)

Why Today's Galaxies Don't Make As Many Stars As They Once Did (2/11/10)

Early Galaxies Formed Stars Fast Because They Had More Gas (2/10/10)

Stellar Baby Boom of Early Universe Explained (2/11/10)

Ancient Galaxies Packed More Raw Material for Stellar Formation (2/10/10)

In the News this month: the molecular content of early galaxies (3/4/10)

Astrophysics: Less greedy galaxies gulp gas (2/11/10)

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Blogger jill said...

Charles Daney, I just want to say Thank You.

I've been reading your blogs for a while and should have taken a few seconds to thank you before. I might not understand everything that you write, yet, but I do very much enjoy reading it. You spark that fascination with anything science and make me want to learn more. I'm going to try and learn everything about redshift now. :)

3/12/2010 08:30:00 AM  
Blogger Charles Daney said...

Hi, Jill. Thanks for the note. In case you haven't come across it, I wrote a whole article on redshift - here. Feel free to leave a comment there if you have questions about it.

3/12/2010 06:10:00 PM  

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