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 \mu 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 \approx 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 \approx 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 \approx 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 ) .