  X-ray surveys contain sizable numbers of star forming galaxies, beyond the
AGN which usually make the majority of detections. Many methods to separate the
two populations are used in the literature, based on X-ray and multiwavelength
properties. We aim at a detailed test of the classification schemes and to
study the X-ray properties of the resulting samples. We build on a sample of
galaxies selected at 1.4 GHz in the VLA-COSMOS survey, classified by Smolcic et
al. (2008) according to their optical colours and observed with Chandra. A
similarly selected control sample of AGN is also used for comparison. We review
some X-ray based classification criteria and check how they affect the sample
composition. The efficiency of the classification scheme devised by Smolcic et
al. (2008) is such that ~30% of composite/misclassified objects are expected
because of the higher X-ray brightness of AGN with respect to galaxies. The
latter fraction is actually 50% in the X-ray detected sources, while it is
expected to be much lower among X-ray undetected sources. Indeed, the analysis
of the stacked spectrum of undetected sources shows, consistently, strongly
different properties between the AGN and galaxy samples. X-ray based selection
criteria are then used to refine both samples. The radio/X-ray luminosity
correlation for star forming galaxies is found to hold with the same
X-ray/radio ratio valid for nearby galaxies. Some evolution of the ratio may be
possible for sources at high redshift or high luminosity, tough it is likely
explained by a bias arising from the radio selection. Finally, we discuss the
X-ray number counts of star forming galaxies from the VLA- and C-COSMOS surveys
according to different selection criteria, and compare them to the similar
determination from the Chandra Deep Fields. The classification scheme proposed
here may find application in future works and surveys.
