Constraints on the Redshift Evolution of the L_X-SFR Relation from the Cosmic X-Ray Backgrounds

Astronomy Department, Harvard University, 02138, Cambridge, MA, USA
Monthly Notices of the Royal Astronomical Society (Impact Factor: 5.11). 08/2011; 421(1). DOI: 10.1111/j.1365-2966.2011.20292.x
Source: arXiv


Observations of local star forming galaxies have revealed a correlation
between the rate at which galaxies form stars and their X-Ray luminosity. We
combine this correlation with the most recent observational constraints on the
integrated star formation rate density, and find that star forming galaxies
account for 5-20% of the total soft and hard X-ray backgrounds, where the
precise number depends on the energy band and the assumed average X-ray
spectral energy distribution of the galaxies below ~20 keV. If we combine the
L_X-SFR relation with recently derived star formation rate function, then we
find that star forming galaxies whose X-ray flux falls well (more than a factor
of 10) below the detection thresholds of the Chandra Deep Fields, can fully
account for the unresolved soft X-ray background, which corresponds to ~6% of
its total. Motivated by this result, we put limits on the allowed redshift
evolution of the parameter c_X \equiv L_X/SFR, and/or its evolution towards
lower and higher star formation rates. If we parametrize the redshift evolution
of c_X ~ (1+z)^b, then we find that b \leq 1.3 (95% CL). On the other hand, the
observed X-ray luminosity functions (XLFs) of star forming galaxies indicate
that c_X may be increasing towards higher redshifts and/or higher star
formation rates at levels that are consistent with the X-ray background, but
possibly at odds with the locally observed L_X-SFR relation.

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