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Saturday, December 5, 2009

How Galaxies Lose Their Gas

As galaxies evolve, many lose their gas. But how they do this is a point of contention. One possibility is that it is used to form stars when the galaxies undergo intense periods of star formation known as starburst. Another is that when large galaxies collide, the stars pass through one another but the gas gets left behind. It's also possible that the gas is pulled out in close passes to other galaxies through tidal forces. Yet another possibility involves a wind blowing the gas out as galaxies plunge through the thin intergalactic medium in clusters through a process known as ram pressure.

A new paper lends fresh evidence to one of these hypotheses. In this paper, astronomers from the University of Arizona were interested in galaxies that displayed long gas tails, much like a comet. Earlier studies had found such galaxies, but it was unclear whether or not this gas tail was pulled out from tidal forces, or pushed out from ram pressure.

To help determine the cause of this the team used new observations from Spitzer to look for subtle differences in the causes of a tail following the galaxy ESO 137-001. In cases where tails are known to be pulled out tidally (such as in the M81/M82 system), there "is no physical reason why the gas would be preferentially stripped over stars." Stars from the galaxy are pulled out as well and often large amounts of new star formation are induced. Meanwhile, ram pressure tails should be largely free of stars although some new star formation may be expected if there is turbulence in the tail which causes regions of higher density (think like the wake of a boat).

Examining the tail spectroscopically, the team was unable to detect the presence of large numbers of stars suggesting tidal processes were not responsible. Furthermore, the disk of the galaxy seemed relatively undisturbed by gravitational interactions. To support this, the team calculated the relative strengths of the forces acting on the galaxy. They found that, between the tidal forces acting on the galaxy from its parent cluster, and its own centripetal forces, the internal forces where greater, which reaffirmed that tidal forces were an unlikely cause for the tail.

But to confirm that ram pressure was truly responsible, the astronomers looked at other parameters. First they estimated the gravitational force for the galaxy. In order to strip the gas, the force generated by the ram pressure would have to exceed the gravitational one. The energy imparted on the gas would then be measurable as a temperature in the gas tail which could be compared to the expected values. When this was observed, they found that the temperature was consistent with what would be necessary for ram stripping.

From this, they also set limits on how long gas could last in such a galaxy. They determined that in such circumstances, the gas would be entirely stripped from a galaxy in ~500 million to 1 billion years. However, because the density of the gas through which the galaxy would slowly become denser as it passed through the more central regions of the cluster, they suggest the timescale would be much simpler. While this timescale say seem long, it is still shorter than the time it takes such galaxies to make a full orbit in their cluster. As such, it is possible that even in one pass, a galaxy may lose its gas.

If the gas loss occurs on such short timescales, this would further predict that tails like the one observed for ESO 137-001 should be rare. The authors note that an “X-ray survey of 25 nearby hot clusters only discovered 2 galaxies with X-ray tails.”

Although this new study in no way rules out other methods of removing a galaxy's gas, this is one of the first galaxies for which the ram stripping method is conclusively demonstrated.

Source: universe today
Original source: A Warm Molecular Hydrogen Tail Due to Ram Pressure Stripping of a Cluster Galaxy

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