X-ray selected starbursts in the GOODS-North

We investigate claims that recent ultra-deep X-ray surveys are detecting starbursts at cosmologically interesting redshifts ( z = 0 − 1 ). We combine X-ray data from the 2 Ms Chandra Deep Field North and multi-wavelength observations obtained as part of the GOODS-North to build the Spectral Energy Distributions (UV, optical, infrared) of X-ray sources in this field. These are fit with model templates providing an estimate of the total infrared luminosity ( 3 − 1000 µ m ) of each source. We then exploit the tight correlation between infrared and X-ray luminosities for star-forming galaxies, established in the local Universe, to select sources that are dominated by star-formation rather than supermassive black hole accretion. This approach is efficient in discriminating normal galaxies from AGN over a wide range of star-formation rates, from quiescent systems to starbursts. The above methodology results in a sample of 45 X-ray selected star-forming systems at a median redshift z ≈ 0 . 5 , the majority of which (60%) are either Luminous or Ultra-Luminous Infrared Galaxies. This sample is least affected by incompleteness and residual AGN contamination and is therefore well suited for cosmological studies. We quantify the X-ray evolution of these sources by constructing their differential X-ray counts, dN/dS, and comparing them with evolving luminosity function models. The results are consistent with luminosity evolution of the form (1 + z ) p with p ≈ 2 . 4 . This is similar to the evolution rate of star-forming galaxies selected at other wavelengths, suggesting that the deep X-ray surveys, like the Chandra Deep Field North, are indeed finding the starburst galaxy population that drives the rapid evolution of the global star-formation rate density in the range z ≈ 0 − 1 . Our analysis also reveals a separate population of infrared-faint X-ray sources at moderate- z . These include old galaxies but also systems that are X-ray luminous for their stellar mass compared to local ellipticals. We argue that these may be post-starbursts that will, over time, become fainter at X-ray wavelengths and will eventually evolve into early-type systems (i.e E/S0).