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Nereid
2007-Feb-25, 03:06 PM
IGM = inter-galactic medium.

One of the many fascinating discoveries made by x-ray astronomers is that rich clusters of galaxies are dominated by hot gas (http://antwrp.gsfc.nasa.gov/apod/ap991217.html), in the sense that this gas is the biggest component of the baryonic mass of such clusters (measured by mass); it turns out that galaxies are mere bit players.

How do astronomers work out the mass of such hot, x-ray emitting gas?

How do they estimate the (baryonic) mass of the stars, gas, and dust in the constituent galaxies of rich clusters?

StupendousMan
2007-Feb-25, 04:04 PM
IGM = inter-galactic medium.

One of the many fascinating discoveries made by x-ray astronomers is that rich clusters of galaxies are dominated by hot gas (http://antwrp.gsfc.nasa.gov/apod/ap991217.html), in the sense that this gas is the biggest component of the baryonic mass of such clusters (measured by mass); it turns out that galaxies are mere bit players.

How do astronomers work out the mass of such hot, x-ray emitting gas?

How do they estimate the (baryonic) mass of the stars, gas, and dust in the constituent galaxies of rich clusters?

Good questions. The answers are simple in theory, complicated in practice. I'll start at the end and work backwards.

To estimate the mass of stars, we simply measure the amount of all the starlight -- which means in most case, measure all the light in the visible part of the spectrum -- and take a spectrum of all that light as a whole. The spectrum gives us a rough idea of the mixture of stars making up the galaxy, so we can make a model of the population. With that model, and the total amount of light, we can estimate the total number of stars (breaking it down by spectral type at some rough level). Then, we multiply the number of each type of star by the mass of that type of star, and we're done.

To estimate the mass of gas, one can turn to radio observations. The 21-cm line of hydrogen and/or the CO line may be optically thin. If we know the intensity of the radiation and the distance to the galaxy, we can compute the mass of the gas. Yes, this is a simplification.

To estimate the mass of dust, I think measurements in the IR are most useful. Starlight heats us small grains of dust, which radiate in the thermal IR, at temperatures of a few tens to a few hundreds of Kelvin. If one makes some reasonable assumptions about the distribution of grain sizes and location of dust relative to stars, one can estimate the amount of dust.

Now, the X-rays. Our X-ray telescopes provide measures of both the total intensity of X-rays, and a rough spectrum of the radiation. One can in some cases assume that the gas contained within a cluster has reached hydrostatic equilibrium: the pressure within the gas balances exactly the gravitational force pulling it in towards the center of the cluster. With some additional assumptions, it is possible to build a model for the distribution of the mass of the hot gas (as a function of distance from the center), and the temperature of the hot gas (again, as a function of distance from the center). By comparing the model to the X-ray measurements, one can place constraints on the mass of the gas.

In real life, of course, some of the simplifying assumptions I mentioned above aren't valid. But this may give you a basic idea for the process.

A good reference for these questions -- and many others! -- is the book "An Introduction to Modern Astrophysics", by Carroll and Ostlie. It is aimed at upper-level undergraduates and starting graduate students. It helps me a lot when I'm teaching classes.