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