The X‐Ray–derived Cosmological Star Formation History and the Galaxy X‐Ray Luminosity Functions in the Chandra Deep Fields North and South

The cosmological star formation rate in the combined Chandra Deep FieldsNorth and South is derived from our X-Ray Luminosity Function for Galaxies inthese Deep Fields. Mild evolution is seen up to redshift order unity with SFR ~(1 + z)^{2.7}. This is the first directly observed normal star-forming galaxyX-ray luminosity function (XLF) at cosmologically interesting redshifts (z>0).This provides the most direct measure yet of the X-ray derived cosmicstar-formation history of the Universe. We make use of Bayesian statisticalmethods to classify the galaxies and the two types of AGN, finding the mostuseful discriminators to be the X-ray luminosity, X-ray hardness ratio, andX-ray to optical flux ratio. There is some residual AGN contamination in thesample at the bright end of the luminosity function. Incompleteness slightlyflattens the XLF at the faint end of the luminosity function. The XLF has alognormal distribution and agrees well with the radio and infrared luminosityfunctions. However, the XLF does not agree with the Schechter luminosityfunction for the H-alpha LF indicating that additional and different physicalprocesses may be involved in the establishment of the lognormal form of theXLF. The agreement of our star formation history points with the other starformation determinations in different wavebands (IR, Radio, H-alpha) gives aninteresting constraint on the IMF, and X-rays may be measuring directly thebinary star formation history of the Universe. X-ray studies will continue tobe useful for probing the star formation history of the universe by avoidingproblems of obscuration. Star formation may therefore be measured in moredetail by deep surveys with future x-ray missions.