Thursday, September 13, 2007

A galactic wind from Messier 100?

Jiménez-Vicente et al (2007) appear to have discovered evidence for a galactic wind emanating from the nuclear region of the roughly face-on spiral galaxy Messier 100
(Astronomy Picture Of the Data image of M100 here). Blue-shifted sodium absorption lines (arising in neutral hydrogen gas at several thousand degrees Kelvin) and blue-shifted hydrogen and nitrogen emission lines (emitted from ionized hydrogen gas at about 10000 Kelvin) are a classic observational signature of a starburst-driven outflow, and their data is pretty persuasive.

These absorption and emission lines arise in gaseous "clouds" that are initially at rest in the host galaxy, but now have been entrained into (and accelerated by the ram pressure of) a much-hotter enveloping wind of merged supernova ejecta (at a temperature of order 10 000 000 Kelvin). In the standard theoretical model for galactic winds the clouds with the lowest column density (i.e. thinnest, lowest mass clouds) are accelerated up to higher velocity than the higher column density clouds, so that the flow ends up multiple clouds covering a broad range of outflow.

It is important to realize that the material seen using optical absorption or emission lines (the clouds) might not necessarily ever achieve the same velocity as the hot, metal-enriched gas that drives the wind, so that the velocity of the clouds is not the same as the true wind outflow velocity.

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