Monday 09/17/2012

On the Lack of Evolution in Galaxy Star Formation Efficiency
Peter S. Behroozi (1), Risa H. Wechsler (1), Charlie Conroy (2) ((1) KIPAC, Stanford University, (2) UC Santa Cruz)
Comments: 6 pages, 4 figures; submitted to ApJ Letters
Subjects: Cosmology and Extragalactic Astrophysics (astro-ph.CO)

Using reconstructed galaxy star formation histories, we calculate the instantaneous efficiency of galaxy star formation (i.e., the star formation rate divided by the baryon accretion rate) from z=8 to the present day. This efficiency exhibits a clear peak near a characteristic halo mass of 10^11.7 Msun, which coincides with longstanding theoretical predictions for the mass scale relevant to virial shock heating of accreted gas. Above the characteristic halo mass, the efficiency falls off as the mass to the minus four-thirds power; below the characteristic mass, the efficiency falls off at an average scaling of mass to the two-thirds power. By comparison, the shape and normalization of the efficiency change very little since z=4. We show that a time-independent star formation efficiency simply explains the shape of the cosmic star formation rate since z=4 in terms of dark matter accretion rates. The rise in the cosmic star formation from early times until z=2 is especially sensitive to galaxy formation efficiency. The mass dependence of the efficiency strongly limits where most star formation occurs, with the result that two-thirds of all star formation has occurred inside halos within a factor of three of the characteristic mass, a range that includes the mass of the Milky Way.

The Chemical Effects of Mutual Shielding in Photon Dominated Regions
Richard P. Rollins, Jonathan M. C. Rawlings
Comments: Accepted to MNRAS, 9 Pages, 2 Figures
Subjects: Galaxy Astrophysics (astro-ph.GA)

We investigate the importance of the shielding of chemical photorates by molecular hydrogen photodissociation lines and the carbon photoionization continuum deep within models of photon dominated regions. In particular, the photodissociation of N2 and CN are significantly shielded by the H2 photodissociation line spectrum. We model this by switching off the photodissociation channels for these species behind the HI to H2 transition. We also model the shielding effect of the carbon photoionization continuum as an attenuation of the incident radiation field shortwards of 1102\AA. Using recent line and continuum cross section data, we present calculations of the direct and cosmic ray induced photorates for a range of species, as well as optically thick shielding factors for the carbon continuum. Applying these to a time dependent PDR model we see enrichments in the abundances of N2, N2H+, NH3 and CN by factors of roughly 3-100 in the extinction band Av=2.0 to Av=4.0 for a range of environments. While the precise quantitative results of this study are limited by the simplicity of our model, they highlight the importance of these mutual shielding effects, neither of which has been discussed in recent models.

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