As Betsey has mentioned, ALFALFA is a great way to find hydrogen-rich galaxies regardless of their stellar content. With ALFALFA, we can find lots of galaxies, including a lot that have very few stars and are therefore incredibly dim. This is just another way to learn about the incredible diversity and structure in the Universe around us, and when we’re done we’ll have a big pile of about 30,000 galaxies to work with. There’s a lot that we can learn about the Universe in this way, but I’d like to tell you about just one: the HI Mass Function. We call it the HIMF for short.
The HI mass function (HIMF) is a way of measuring “how much hydrogen there is out there,” and, more precisely, how much hydrogen is contained in the galaxies we find with ALFALFA. We measure the amount of hydrogen in solar masses; that tells us how many suns worth of hydrogen there is in a particular galaxy. ALFALFA finds galaxies that have 1 million solar masses of hydrogen all the way up to galaxies that have a bit less than 100 billion solar masses of hydrogen. That’s quite a range, from pretty tiny galaxies to really big guys. Those tiny galaxies may sound insignificant, but actually, there’s a lot of them and only a few of the truly massive ones.
Astronomers who study the stars in galaxies worry about the “luminosity function”: how bright are galaxies, and how many galaxies are there in each brightness category? The luminosity function and the HI mass functions are just different ways of trying to figure out the same thing; we’re wondering how mass is distributed in the Universe today, and how that might relate to the history of galaxy formation. First, we pick some category: how many galaxies did ALFALFA find with hydrogen masses between 1 million and 10 million solar masses? Then, we divide that number by the total volume we were able to look in. That gives us a density, or a number of galaxies with a given mass in a certain volume. This is a handy unit for us to use, because then we can easily compare the ALFALFA result to any previous or future results that we’re interested in, and we can use such a unit to describe the Universe in general: “If you look in a cube of Universe that’s 1 megaparsec long on each side, you will find this many galaxies with this much hydrogen.” We want to be able to describe the Universe in this way along the whole range of hydrogen masses, from the dwarfiest dwarfs to the richest, biggest spirals.
There’s a problem, though, and that’s where ALFALFA comes in. Because extremely low-mass galaxies are only detectable nearby, blind surveys have only found them in small numbers; previously published HIMFs include only a couple dozen objects with less than 10 million solar masses of hydrogen. The HIMF of the first-generation blind HI survey, HIPASS, is shown below. Underneath the plot, you can see a histogram, a bar graph showing you how many galaxies there are for each hydrogen mass. This is a log-log plot, but the key thing is that anything falling below “8” is a low-mass galaxy with less than 10 million suns worth of hydrogen. This is the most interesting part of the HIMF, since there are so many galaxies in this category but they are so hard to see. And the histogram shows you that astronomy’s best knowledge of the HI mass function in this range is based on only a few galaxies. That means we have a lot of “error” — we can’t really be sure how many galaxies fall into that category in the Universe at large.
ALFALFA hopes to improve upon this result, since we have more than 300 galaxies falling into this category so far, and lots more throughout the rest of the range of galaxy masses. More information means we can have more confidence in our results once we’ve measured an HI mass function (which is something I’m working on currently). The plot below will give you an idea of how well previous studies in this field have been able to agree:
The plot above shows a comparison of the HIMFs measured by the ADBS survey (green; Rosenberg and Schneider 2002) and the HIPASS survey (blue; Zwaan et al. 2005 ); an optically-selected sample of galaxies contributed to the Springob et al. HIMF (red; Springob et al. 2005). These might look pretty similar, but the devil is in the details. Look in the region at the extreme left, above the “6” which stands for 1 million solar masses of hydrogen. The disagreement means that, when you look in a volume of the Universe outlined by a cube that is 1 Megaparsec on each side, you don’t know whether you should find one such galaxy in that volume, or whether you should have to look in ten such cubes before you find that kind of galaxy. The improvements made by ALFALFA should help us settle this question!