When I left you last, Shan and I were headed to Palomar Observatory near sunny San Diego. We were going to look for signs of metals in star forming regions of gas-rich dwarf galaxies. That’s a mouthful, but what it comes down to is that hydrogen gas is what galaxies turn into stars, and stars fuse and burn hydrogen and helium to make the other elements we call metals (some elements can only be formed in supernovae, the final death stages of massive stars, but it comes down to the same thing: star formation sets the whole process in motion).
We had used R-band and H-alpha filtered optical images, provided by our collaborators, to identify star formation regions in some galaxies that we found with ALFALFA. An example is below, and shows some pretty fantastic knots, which is where the galaxy is creating a brand new generation of stars:
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 . . .
We were scheduled for four nights on the telescope, and we had images of 20 galaxies. Each image provided a “map” to the star forming regions in that galaxy, and with any luck, we were hoping to have four nights of beautiful weather so that we could take a look at each of these galaxies. Instead, we got two really great nights and two fairly awful nights. On our last two nights, the dome could barely be opened. The telescope’s mirror needs to be protected from the elements, like dust (which can be carried on a strong wind) and moisture (which can roll in with a thick, dense fog). We had both dust and moisture, so the last two nights were pretty much a lost cause. In the end, though, we were able to obtain spectra for about 14 galaxies, so it certainly wasn’t all bad news.
What do our results look like? We’re taking a spectrum of each galaxy, looking for the spectral signatures of certain elements, such as oxygen and neon. Each element has its own special spectral pattern, which works exactly like a fingerprint, and astronomers use these well-known patterns to identify elements. This is how we know what far-off planets, stars, and galaxies are made of, and it’s the most powerful tool in astronomy as far as I’m concerned. An example of a spectrum taken at Palomar is shown below:
When you look up and down along the spectrum, you’re looking at different wavelengths of light emitted by the galaxy. When you look from left to right, though, you’re basically looking from side to side on the sky — the galaxy is the thick, cloudy line running down the center. Around the galaxy, we’re just looking at the empty sky, although since the Earth’s atmosphere is full of atoms and molecules you can see a lot of bright lines that run all the way from the left edge to the right edge. When the data is eventually analyzed by a colleague in our group, these “night sky lines” will have to be subtracted out so that she can focus on the light from the galaxy alone. The galaxy is bright because of all the light from the many, many stars within it. But we already knew that the galaxy contained stars. We’re interested in some specific spectral lines that indicate star formation and metals.
As your eye traces the central galaxy from the top to the bottom, you might be able to notice several bright “nubs” — there’s one about a third of the way down that should be fairly obvious. And that is what Shan and I went to California to look for. Those “nubs” are spectral lines from the star forming region we’re pointed at, and once this data is cleaned up it will be those nubs that teach us something about the star formation history of this particular dwarf galaxy.