X-ray properties of radio-selected star forming galaxies in the Chandra-COSMOS survey

Ranalli, P. and Comastri, A. and Zamorani, G. and Cappelluti, N. and Civano, F. and Georgantopoulos, I. and Gilli, R. and Schinnerer, E. and Smolčić, Vernesa and Vignali, C. (2012) X-ray properties of radio-selected star forming galaxies in the Chandra-COSMOS survey. Astronomy and Astrophysics, 542. Art. No. A16. ISSN 0004-6361

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Abstract

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.

Item Type: Article
Keywords: galaxies: active, galaxies: distances and redshifts, galaxies: high-redshift, galaxies: starburst, submillimeter: galaxies
Date: 2012
Subjects: NATURAL SCIENCES > Physics > Astronomy and Astrophysics
Additional Information: © 2012 ESO. Received 22 December 2011. Accepted 20 April 2012. Published online 25 May 2012.
Divisions: Faculty of Science > Department of Physics
Publisher: EDP Sciences
Depositing User: Vernesa Smolčić
Date Deposited: 15 Apr 2015 08:50
Last Modified: 19 Apr 2016 10:43
URI: http://digre.pmf.unizg.hr/id/eprint/3807

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