Friday, July 19, 2013

1307.4758 (M. Shirazi et al.)

Stars were born in significantly denser regions in the early Universe    [PDF]

M. Shirazi, J. Brinchmann, A. Rahmati
The density of the warm ionized gas in high-redshift galaxies is known to be higher than what is typical in local galaxies on similar scales. At the same time, the mean global properties of the high- and low-redshift galaxies are quite different. Here, we present a detailed differential analysis of the ionization parameters of 14 star forming galaxies at redshift 2.6-3.4, compiled from the literature. For each of those high-redshift galaxies, we construct a comparison sample of low-redshift galaxies closely matched in specific star formation rate and stellar mass, thus ensuring that their global physical conditions are similar to the high-redshift galaxy. We find that the median log [OII] 3727/[OIII] 5007 line ratio of the high-redshift galaxies is 0.5 dex higher than their local counterparts. We construct a new calibration between the [OII] 3727/[OIII] 5007 emission line ratio and ionization parameter to estimate the difference between the ionization parameters in the high and low-redshift samples. Using this, we show that the typical density of the warm ionized gas in star-forming regions decreases by a median factor of 8 from z~ 3.3 to z~0 0 at fixed mass and specific star formation rate. We show that metallicity differences can not explain the observed density differences. Because the high- and low-redshift samples are comparable in size, we infer that the relationship between star formation rate density and gas density must have been significantly less efficient at z~2-3 than what is observed in nearby galaxies with similar levels of star formation activity.
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