Journal Club Talk: Star Formation Rates

One of the "neat" things done here to help graduate students become accostomed to talking in front of a group of people is a weekly journal club. The idea is that every Friday at 12:30 (usually people bring their lunch) a grad student gives a half hour talk (assuming no interruptions, of which there are always plenty, so it's usually more like forty-five mintues or an hour) on some recent paper. Usually, this paper is completely unrelated to their own research, so that instead of the student "defending" their own work, they are learning new science.

Journal club is one of the aspects of the program here that I gladly tell prospective students about, but personally try to weasel my way out of. In principle, it's a great thing to force grad students to learn how to make effective slides, to become comfortable standing in front of a room full of people, to be able to easily field unsolicited questions. In practice, it's fantastic, as long as I'm not the student in question. I succeeded in not giving a talk last year, but when, on our way to the conference in Japan in June, Paul (who is in charge of scheduling and running journal club this year) found out I hadn't yet given one, I found myself first up on the list for this year, unable to sneak out.

Thus I found myself standing in front of the department today talking about "Radial variation of attenuation and star formation in the largest late-type disks observed with GALEX." I had resolved to not be one of those students who waits until the last minute to prepare, but well, there's this paper I've been procrastinating on for a while, and I didn't get around to mailing it out to the co-authors until Wednesday night... which meant that I didn't start, uhm, reading the paper until yesterday morning.

I felt like I was in Junior Lab all over again, only I actually found the material interesting, wasn't being graded (per se), and didn't have a lab partner to ask questions of. It took me all day to actually read the paper, and I didn't start even making slides until about 5p.m., when I realized that I haven't actually given a talk in over two years. Everyone these days seems to be using PowerPoint... which I haven't used since, I think, 1997? So I dallied for about half an hour, finally deciding that, in fact, LaTeX would be easier.

I finished the slides this morning, and then Paul suggested that I might want to practice. That was about when I started trying to crawl out of my skin. Normally, I don't get the slightest bit nervous when talking in front of a group, but then, normally, I actually know what the hell I'm talking about. This... this, I couldn't have given you a solid definition of "attenuation" or told you how astronomers ("we") estimate star formation rates 48 hours ago. So, I tried practicing, and it was disastrous: I couldn't stand still, I couldn't find any words not etymogically related to "uhm," and I definitely couldn't remember half of what I actually wanted to say.

But the talk wound up going fine, I think. Have you ever been so tired you can't fall asleep? It was like that: I think I was so nervous I forgot to be nervous. There was a plethora of interuptions (before I could even get off the title slide!), which, as I discovered, actually helped. If your audience keeps interupting you, it makes the "talk" more like a conversation than an information dump. Some people get really nervous and upset if their audience can't keep quiet, asks really difficult questions ("But isn't that just bogus?"), or talks amongst themselves. I find that not only do I not mind, but I actually kind of like it: it takes the pressure off me. Furthermore, if need be, I know I can always regain control of the room (since I am standing in front of it).

The paper itself is fairly interesting, and, from what I can tell, important. The most compelling result "concerns the notion of a threshold" of star formation. The rate at which galaxies, or more specifically, regions within galaxies, make stars has lots of implications for galaxy evolution and other areas of astronomy. Unfortunately, we can't just take a yearly (or million-years-ly) census of a galaxy and count up how many new stars we see. Instead, people model what the spectra of stars of different mass look like (how much light they put out at different wavelengths) and the relative numbers and ages of stars of different masses, which can then be turned into "we expect to see this much of this kind of light when stars are forming at this rate." One star formation tracer is quite popular because it involves looking at an emission line from hydrogen, called Hα, which is a very strong feature in red light, which we can see from the ground. People have noticed that in most galaxies, at some large radius, Hα just quits being seen. Since this radius corresponds to a surface mass density, people have also talked about a critical "threshold" density below which no star formation can occur. The star formation tracer used in this paper, on the other hand, is ultraviolet (UV) light, which cannot be seen from the ground (at least while we still have an ozone layer in the atmosphere!). So, the data are from a satellite, GALEX. The main problem with the notion of a star formation threshold is that it's not actually a star formation threshold; it's an Hα threshold. This paper shows pretty convincingly that UV radiation is still seen beyond the threshold at which Hα is observed, which, presumably, means that star formation is occuring. They argue that no more Hα is seen at large radii because really massive stars (more than about ten solar masses) are needed in order to have Hα, and these stars are very rare and die very quickly. When it becomes very improbable to have even one of these massive stars within a large region, then no more Hα will be seen. The UV light, on the other hand, is sensitive to slightly less massive (and therefore more common) stars, and can therefore be seen at larger radii.