Project Goal: Weighing Galaxies
We are using the radio telescope at the FCRAO to find the distribution of gas mass in other galaxies. We are mostly interested in mapping the emission from the molecule ¹³CO, an isotope of carbon monoxide (it has a heavier carbon nucleus). We compare the emission from ¹³CO and the more abundant ¹²CO (normal carbon monoxide - usually just written CO). The ratio of CO and ¹³CO intensities tells us about how well we are measuring mass, and with that information we can estimate how much gas a galaxy has.

Carbon Monoxide
Yes, this is the same noxious stuff from car exhaust pipes and cigarette smoke. However, CO is also the most abundant interstellar molecule easily seen with ground-based telescopes. In fact, practically anywhere there are molecules in space, we see the light from CO. So we can map the CO emission, and from its intensity, estimate how much gas there is.

From previous studies of CO in the Milky Way, people have found that there seems to be a simple relationship between CO intensity and the mass of gas. This conversion makes life great for astronomers since we can just observe a single spectral line and get a nice physical fact about some cloud - its mass. However, there are a few problems with this conversion (which most of us call the ``X factor''):

• We're not sure of its exact value. Many different kinds of studies in our galaxy seem to indicate a rather constant value for X(CO), but it probably varies with cloud conditions (temperature, density, etc.), and seems to be much lower in the Milky Way nucleus.
• CO is so abundant and easy to excite (to force it to emit light), that it is usually opaque. We say ``optically thick,'' but it means the same thing. Consider how on a foggy day, you can only see so far ahead of you. In the same way we can only look to a certain depth in a cloud. Beyond that point, its contents remain a mystery.
• We have no idea what the X factor is for other galaxies.

This uncertainty is a big problem since we need to know the gas mass in galaxies to understand a whole bunch of interesting things like how they rotate, evolve and form stars.

Why ¹³CO?
This less abundant isotope of CO is usually not optically thick, so it shows us an entire cloud (we see all the way through and can virtually ``count'' all the particles). By mapping ¹³CO, we can see where the mass truly is in a galaxy, and, in combination with CO maps, we can estimate how the X factor may vary in galaxies. For example:

• Do all galaxies have lots of mass in the nucleus like the Milky Way?
• How far out does the molecular gas go?
• Do gas mass and distribution depend on the type of galaxy?
• Does the X factor vary from place to place within a galaxy?
• Does it depend on galaxy properties like average temperature, star formation activity, etc.?
• Does it vary from galaxy to galaxy?

Plan
We are spending a few observing seasons mapping the CO and ¹³CO emission from 50 to 60 galaxies. The FCRAO is a great place for this project because of their new SEQUOIA array receiver; it allows us to make maps very neatly and efficiently. We're relying on a small army of undergraduate students to make the majority of the observations, data clean-up and analysis. With so many galaxies to observe, and time available nearly every clear day (we radio astronomers observe day and night), this work is easily shared by many, with each participant having a project in its own right as well as a piece of the larger scientific effort.