X-ray properties of radio-selected star forming galaxies in the Chandra-COSMOS survey⋆
P. Ranalli1,2,3, A. Comastri3, G. Zamorani3, N. Cappelluti3, F. Civano4, I. Georgantopoulos2,3, R. Gilli3, E. Schinnerer5, V. Smolčić6,7,8⋆⋆ and C. Vignali1
1 Università di Bologna, Dipartimento di Astronomia, via Ranzani 1, 40127 Bologna, Italy
2 Institute of Astronomy and Astrophysics, National Observatory of Athens, Palaia Penteli, 15236 Athens, Greece
3 INAF – Osservatorio Astronomico di Bologna, via Ranzani 1, 40127 Bologna, Italy
4 Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA, 02138, USA
5 Max-Planck Institut für Astronomie, Königstuhl 17, 69117 Heidelberg, Germany
6 Argelander Institut for Astronomy, Auf dem Hügel 71, 53121 Bonn, Germany
7 European Southern Observatory, Karl-Schwartzschild-Straße 2, 85748 Garching b. München, Germany
8 University of Zagreb, Physics Department, Bijenička cesta 32, 10002 Zagreb, Croatia
Received: 22 December 2011
Accepted: 20 April 2012
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 Smolčić et al. (2008, ApJS, 177, 14) 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 Smolčić et al. 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.
Key words: X-rays: galaxies / radio continuum: galaxies / galaxies: fundamental parameters / galaxies: star formation / galaxies: active / galaxies: high-redshift
Tables 2 and 3 are only available at the CDS via anonymous ftp to cdsarc.u-strasbg.fr (126.96.36.199) or via http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/542/A16
© ESO, 2012