Thursday, 14 August 2008

Observations of the Gas Reservoir around a Star Forming Galaxy in the Early Universe

Frye et al.,

The figure shows a high-S/N spectrum of a z=4.9 starburst galaxy; this kind of S/N is only possible because of a long exposure time (14hrs on an 8m telescope) and because the flux is boosted by a factor of 10 due to gravitational lensing by a foreground cluster. The most interesting feature is the broad Gunn-Peterson trough blueward of the bright Lya emission line.

Since the optical depth in this trough is significantly larger than observed at the same wavelengths for random sight-lines through the IGM (the universe was already reionized at this redshift), the authors conclude that we are seeing direct evidence of a "cosmic web" of gas that surrounds galaxies and feeds their growth.

Another possibility is that the neutral gas is outflowing material from the galaxy itself, however the authors discount this explanation since stellar population modeling suggests that the galaxy is too young to have driven such large amounts of gas outwards. Also, typical outflow velocities are not large enough to explain the broad trough, even for outflows that are powered by AGN.

This is a neat observation, but I'm not sure how much we can infer from a single object. Unfortunately, a galaxy at this redshift has to be strongly lensed to be bright enough for this sort of analysis, so there isn't much hope of obtaining a large sample in the near future. Of course QSOs are also bright enough, but they tend to ionize most of the hydrogen in their immediate vicinity.

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