Star Formation in the Mountains

A view of the mountains surrounding Sesto, Italy. Photo by Dale Kocevski.

No two snowflakes are alike, and yet forecasters are pretty good at predicting snow. No two mountains are alike, and yet geologists can tell us quite a lot about how mountain ranges form and erode. Similarly, no two galaxies are alike and yet astronomers would like to understand how galaxies as a whole form and evolve. So what better place to talk about this topic than in a snowy mountain range! Last week a group of about 40 astronomers met in the small town of Sesto, Italy, nestled in the Dolomite mountains right near the Austrian border. The title of the workshop was "Star Formation Through Cosmic Time," and the focus was on trying to link together what we are learning about star-formation in very distant galaxies from Hubble observations like CANDELS to observations at infrared wavelengths from the Spitzer and Herschel observatories. This is important because more than half of the energy produced by stars in distant galaxies is absorbed by dust and re-emitted as infrared radiation. 

Most galaxies seem to form stars at a rate that is proportional 

to the number of stars that they already have. Some astronomers 

are calling this the main sequence of star-forming galaxies. 

Other  galaxies fall off the sequence. The red and dead ones

or quenched or quiescent ones aren't forming many stars at all. 

On the other hand there are some galaxies forming stars at much 

higher rates, which we call starbursts. Then there are a few galaxies 

that are still forming stars, but at lower rates than on the

main sequence. These populate the green valley,  although shutting 

down star formation isn't the only way to end up with greenish colors, 

so the green valley is sort of a hodgepodge of various

kinds of galaxies. 

A lot of the discussion at the meeting centered on the "Main Sequence of Star-Forming Galaxies" and on the galaxies that depart from that sequence. The "main sequence" is a term that was coined by CANDELS team-member Kai Noeske a few years ago and seems to have caught on. He noticed that most galaxies that are forming stars are forming them at a rate that is roughly proportional to their existing stellar mass. We don't understand in detail why this should be the case, so one item on the agenda was to discuss the evolution of this main sequence and the link between galaxies on the main sequence and galaxies that fall off it. The galaxies that fall off it fall into two classes: those that are forming stars at much higher rates ("starburst galaxies"), and those that have more-or-less stopped forming stars. Several people at the meeting talked about the starburst sequence. Depending a bit on how you define it, it looks like starbursts account for about 10-15% of all the cosmic star formation. I'm not sure the evidence that there are two separate sequences is all that compelling, but it is impressive that by assuming there are two sequences, it is possible to explain the evolution of the infrared luminosity function of galaxies, and to infer something about the evolution of the gas and the evolution of the heavy elements in galaxies. This is very handy because it can help inform us what to expect (and what to go look for) with two powerful radio telescopes that are just coming online, the JVLA and ALMA.

There were several talks about the ability of theoretical models to explain these two sequences. Currently, they seem to get the qualitative behavior right (there is a main sequence), but the quantitative behavior wrong (e.g. the proportion of stars forming in starbursts was about a factor of two too low in one of the models discussed). The failures of the model are almost certainly connected to the feedback of energy into the gas that is too cool to form the stars. This feedback can come from the stars themselves, particularly when they explode as supernovae, or from gas funneling into central massive black holes in the centers of galaxies. Supermassive black holes probably go through periods when they are not accreting a lot of gas, and other periods when they are. When they are actively accreting, they are called Active Galactic Nuclei (AGN) and emit a lot of high-energy radiation such as X-rays. However, if they are surrounded by dust, those X-rays can be absorbed and re-emitted as infrared radiation. There were discussions about new ways to identify AGN using infrared radiation as well as discussion about the properties of the host galaxies surrounding the AGN. CANDELS observations have revealed that distant AGN hosts don't really look any different than galaxies that are not hosting AGN, so that probably means that whatever is causing the gas to funnel into the black hole is not affecting the overall shape of the galaxy. That's a bit of a problem because it seemed quite likely that mergers between galaxies were a key way of getting the gas into the center.

Another interesting question is whether AGN prefer to be in star-forming galaxies or in galaxies that are shutting down their star formation. If feedback from AGN is important for quenching star formation, than one might expect that the galaxies that host AGN might look like they are starting to shut down. You might expect the "green valley" of galaxies in the diagram above to be populated by galaxies with AGN in their centers. The jury is out on this. Dale Kocevski showed evidence that the AGN hosts in CANDELS span the full range of star-forming activity that is seen in galaxies of the same mass. On the other hand another CANDELS member, David Rosario, showed evidence from far-infrared data that AGN hosts are drawn from a population of normal actively star-forming galaxies, and tend to avoid weakly star-forming, quenched or quiescent galaxies. So the observations are giving us somewhat contradictory information, and it is going to take some work to see how to reconcile these results.

One of the things that CANDELS provides is a good way to find and study quenched or quiescent galaxies at great distances. Several talks focused on the numbers of these galaxies. We are now finding massive quiescent galaxies when the universe was just a few billion years old. These galaxies are much more compact than massive non-star-forming galaxies today, so a couple of questions arise: (1) can they grow into their high-mass cousins by just acquiring stars in their outskirts by merging with surrounding galaxies and (2) can we find galaxies on the star-forming sequence that have enough stars jammed into their centers to be the likely progenitors of the quenched galaxies. The answers to these questions are tentatively yes: the densities of stars in the centers of the very distant quenched galaxies are pretty comparable to the central densities today, so adding stars to the outskirts probably works. And there appear to be enough compact galaxies on the star-forming sequence to form the galaxies on the quenched sequence if the star-formation shuts down on a reasonable timescale. On the other hand, CANDELS observations are finding fewer quenched low-mass galaxies than theory predicts, so that may be a problem. 

Harry Ferguson talked about some of the difficulties of inferring the
star-formation histories of high-redshift galaxies. Photo by Dale Kocevski.

There was also a lot of discussion about the star-forming histories of galaxies. We can estimate the stellar masses of galaxies in a variety of ways, and lots of checking suggests that these measurements are pretty robust; for an individual galaxy the estimates based on existing data are probably within a factor of two of the true value. Estimating star formation rates is much more difficult, but if you have information from the far-infrared together with infrared from the ultraviolet part of the spectrum, then it is also possible to make pretty good estimates. So putting those together, astronomers can estimate the total number of stars forming per year, and can do this at various "lookback times" from the present day to about 12 billion years in the past. Astronomers can also estimate the amount of stellar mass present at each of these lookback times. Now the stellar mass at later times ought to agree with what we infer from the rate of star formation at earlier times. This has been a problem in the past, but it now looks like the problem has been resolved with better estimates of star formation rates and stellar masses. So that's good news. On the other hand, the very fact that these estimates agree means that there can't be a lot of galaxies missing from the census of either star-forming or non-star-forming galaxies. That's a bit weird because galaxies can disappear from the census pretty easily if they become very dusty, or fade enough between bursts of star formation. Some theoretical models predict a lot of bursting and a lot of dusty galaxies, so these models might need to be revisited. 

We can also look in detail at the measurements of galaxy colors and spectra and try to infer a bit more about their individual histories of star formation. A lot of discussion at the meeting was about the difficulties involved in doing this. Unfortunately, the current state-of-the art is that when you use all of the information provided in the spectrum to try to estimate the star formation rate, you probably get a worse estimate than if you ignore the optical and near-infrared portion of the spectrum and just use the information from the ultraviolet or the far-infrared (or better yet, both). This is probably because we don't have the correct star formation histories in our models, but we need to find a way to introduce more realistic star-forming histories without "over fitting" the data. 

The useful thing about small workshops is that people are more willing to admit what they don't understand. That tends to make for very fruitful discussion and provides the fodder for new projects. On that score, the meeting was very successful.