A Multi-Wavelength Census of Star Formation at Redshift z~2 - Tapa blanda

Sinopsis

We examine the census of star-forming galaxies and their extinction properties at redshift z~2, when a large fraction of the stellar mass in the universe formed. We find a good agreement between the X-ray, radio, and de-reddened UV estimates of the average star formation rate (SFR) for our sample of z~2 galaxies of ~50 Msun/yr, indicating that the locally calibrated SFR relations appear to be statistically valid from redshifts 1.5<3.0. Spitzer MIPS data are used to assess the extinction properties of individual star-forming galaxies, and we find that the rest-frame UV slope of most galaxies at z~2 can be used to infer their attenuation factors, L(bol)/L(UV). As in the local universe, the obscuration, L(FIR)/L(UV), is strongly dependent on bolometric luminosity, and ranges in value from <1 to ~1000 within the sample considered. However, the obscuration is ~10 times smaller at a given L(bol) (or, equivalently, a similar level of obscuration occurs at luminosities ~10 times larger) at z~2 than at z=0. This trend is expected as galaxies age and their gas becomes enriched with dust. Specific SFRs indicate a wide range in the evolutionary state of galaxies at z~2, from galaxies that have just begun to form stars to those that have already accumulated most of their stellar mass and are about to become, or already are, passively evolving. Finally, we examine two techniques for assessing the census of galaxies at z~2. In the first, we select galaxies using optical, near-IR, and sub-mm criteria, and find a large overlap between optical and near-IR selected samples of galaxies at z~2. The second technique involves reconstructing the luminosity function of z~2 galaxies as determined from Monte Carlo simulations. We find that the SFR density increases between z=4 to z=2 by a factor of ~1.2. However, this increase is mild compared to the much steeper increase from z~6 to z~4, suggesting that the processes responsible for the increase in SFR density at early epochs saturate around z~2.

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