Thursday, March 12, 2009

What fuels star formation in galaxies? (part 2)

How do Galaxies fuel Star Formation? Where do they get the gas that forms new stars?

Back when I started this blog in 2006 I had a short post discussing the issue of where galaxies got the gas they use to fuel ongoing star formation from, which was meant to lead onto the discussion of a paper by Keres et al. (I never got around to discussing that paper.)

Star formation requires interstellar gas (specifically cold dense molecular gas), ... (a) where does the gas come from, and (b) what controls the star formation rate in a galaxy with a given amount of gas?
The old view (say in the 1980's era) was that galaxies accumulated the baryonic matter (in the form of neutral gas or an ionized plasma) into the center of the gravitational potential well (dominated by Dark Matter) they were forming within via dissipative processes (collision and radiation), and this essentially comprised their total gas budget for the rest of the life of the galaxy. Rapid collapse of the gas to the center of the potential well (within a dynamical time) created elliptical galaxies, whereas slower (less dissipative) accretion preserved angular momentum and led to the formation of rotation-support disk galaxies (i.e. spiral galaxies). Star formation rates in galaxies decreased with time exponentially (e.g. if you convert a fixed fraction eSF of your total gas budget to stars in every galactic rotation you get an exponential decrease in gas mass with a e-folding time constant 1/eSF).

So galaxies are just running down in this view, sooner or later running out of gas and becoming "red and dead", i.e. like ellipticals which presumably ran out of gas first.

The more modern viewpoint is to account for continued accretion of fresh intergalactic gas into the gravitational potential wells of galaxies. Differences between red (dead) and blue (actively star forming) galaxies may be in the character of the ongoing accretion process. If that accretion process changes your galaxy can change its type.

Gas falling into the potential well of massive galaxies gains enough gravitational potential energy to reach a velocity where the shock wave it encounters when hitting other gas heats it to X-ray emitting temperatures. At such temperatures the cooling time is long, so it has trouble losing enough energy to fall to the center of the potential well and end up in the galaxy. Instead it forms a hot but tenuous halo of gas surrounding the galaxy. So with decreased dissipation the gas has trouble fueling any star formation in the massive galaxy. Conversely the gas falling into the potential well of a lower mass galaxy is either never shocked, or if shock heated it only achieves temperatures where cooling is rapid (T < 106 K). So it dissipates its energy easily and falls into the galaxy at the center of the gravitational potential well, and is available to fuel additional star formation in this lower mass galaxy. Massive galaxies are predominantly ellipticals or low SF rate spirals, while actively star forming galaxies are typically lower mass galaxies, so the distinction between the different models of star formation in spiral versus elliptical galaxies is preserved but explained in a different manner (nature, rather than initial nurture, as it were).

Indeed, some would argue that the current star formation rate of a galaxy must closely matches its current accretion rate. This raises the question of whether enough gas is falling onto our own Galaxy to account for our star formation rate of 1-3 Solar masses per year. Again this is very different from the old picture of SF in galaxies where you started off with a fixed mass of gas and just consumed it until you run out.

The preceding discussion is only a crude cartoon view of the subject, but serves to introduce the subject. I brought this subject up again because I noticed one of the Astro2010 Decadal Survey White Papers is on the subject of where galaxies get their gas and how do they turn that gas into stars.

If you are interested in seeing what the latest state-of-the-art in this subject is, and where people who actually work on this subject would like to see it go, then please read

How do Galaxies Accrete Gas and Form Stars? by M.E. Putman, et al, arXiv:0902.4717 [ps, pdf, other].

It is written for an audience of professional astrophysicists, but their is content that an educated and interested layperson can benefit from.

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