At the end of March I had the opportunity to visit Palomar Observatory, near San Diego, with Shan, a younger student in the ALFALFA group. We were there to “follow up” on some detections of dwarf galaxies that we had made with ALFALFA. We can learn a lot about galaxies by observing their neutral gas, which Arecibo helps us find, but when we want to get some information on the stars we need to use an optical observatory, and that’s what took us to Palomar. In particular, we were interested in looking at the star formation histories of these dwarfs and the amount of metals that they contain. Astronomers have this weird habit of referring to pretty much any element that isn’t hydrogen or helium as a “metal,” even though we all know from chemistry that that’s not accurate. What’s important, though, is that these things astronomers call “metals” are the heavier elements that can only be made in stars or, in some cases, in catastrophic stellar death throes called supernovae. These are the important elements that make up your everyday life, like oxygen, carbon, iron, even gold. Since metals are made in stars, looking for metals in galaxies tells you something about the formation of stars in those galaxies over their lifetimes.
What, specifically, were we looking for?
There are some well-known relationships between the brightness of galaxies and the abundance of metals within them, but what’s not very clear is whether these trends remain true for the tiniest dwarf galaxies of all. This is a pretty difficult thing to investigate, since dwarf galaxies are dim and are therefore only easy to find very nearby. Shan and I were observing a subset of dwarf galaxies that have been found in ALFALFA, looking for the amount of metals in their star-forming regions in order to learn about the relationship between gas, stars, and metals in this special set of objects. The galaxies that we find in ALFALFA have lots of neutral hydrogen (HI) gas, which basically means that these galaxies are sitting in a big reservoir of star ingredients that they haven’t yet turned into stars. It’s kind of like dumping a batch of chocolate chip cookies into a bag of flour. Why is that? What are the star formation histories of galaxies that are very small and very dim but have a lot of gas left? And do the answers to these questions depend on the environment that the galaxy is in?
In order to do this, we wanted to get spectra of star forming regions in our dwarfs. That means we needed to find the star formation first! This we were able to do only through the help of our many collaborators, who were able to provide us with images of our galaxies in the optical (specifically, the red part of the spectrum, ‘R’ band) and with a special filter (called an H-alpha filter) that targets a particular line emitted by hot hydrogen in star forming regions. By subtracting the red image from the H-alpha image, we find the “knots” of star formation in the galaxy.
We start by identifying a galaxy in ALFALFA, based on its low mass as well as its proximity to us. We then check it out in the Sloan Digital Sky Survey, aka SDSS to see whether we can visually identify any regions of star formation. Here’s an example, UGC9540:
We could see immediately that this was a promising galaxy. Newly-formed stars are hot and therefore shine bright blue, and you can clearly see several distinct, bright regions matching that description. When we identify a galaxy like this that we would like to look at using an optical observatory like Palomar, we then obtain R-band images, in this case from the Vatican Observatory:
The galaxy looks great here, and those knots that we could identify from the SDSS image are coming through loud and clear. And finally, we’re able to subtract the R band from the H-alpha image (not shown) in order to identify regions of star formation — and this one’s a doozy!
Ignore the big black splotches — those are artifacts left over from trying to subtract the stars in both images, and I’m pretty new at this so there’s a little junk left behind when I’m done. Instead, focus on the bright white spots in the same location as the galaxy in the R band image. Those are the star forming regions in this galaxy.
Armed with this information, Shan and I headed off to Palomar. Now that we knew where the star forming regions were in this galaxy, we could take aim and try to uncover the metals hiding in those knots. But of course, the weather had other plans . . .