January 2015
Adam Muzzin
Why Do Galaxies Stop Forming
Stars in Massive Clusters?
Why galaxies in massive clusters stop forming stars has remained an
unsolved problem for decades. Now, results from the Gemini CLuster
Astrophysics Spectroscopic Survey (GCLASS) is providing a clearer picture
of the events leading up to the quenching process.
It’s not a complete mystery that several processes can occur to “quench” star formation in
cluster galaxies. We know that galaxy clusters are filled with very hot, dense, X-ray emitting
gas, and that as galaxies orbit the cluster, they pass through this medium. We also know that
the pressure created can strip out the galaxies’ own gas, which is needed to fuel their star
formation; hence, if it is stripped, star formation ceases. We can even directly observe this
process in action in some nearby cluster galaxies. But the details of exactly how this happens
are sketchy at best.
Galaxies themselves are filled with hot diffuse gas throughout their dark matter halos. This
gas is continually cooling to ultimately provide the cold molecular gas that ends up in their
spiral disks to form stars. How this gas is stripped in galaxy clusters largely remains a mystery, however. The process may be strong enough to remove only the loosely bound gas in a
galaxy’s hot diffuse halo. If so, it will very slowly truncate the galaxy’s gas supply; most disks
contain enough cold gas to continue forming stars for an additional ~ 1 - 2 billion years (Gyr),
if not replenished at all. Then again, the gas stripping process could be more violent and
able to re-move both the diffuse halo as well as the disk’s dense cold gas, and truncate star
formation nearly instantaneously. At some level both are likely to happen, but it has been
very challenging to prove convincingly that one process is more common than the other.
January 2015
2014 Year in Review
GeminiFocus
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