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.
| Original language | English (US) |
|---|---|
| Article number | A16 |
| Journal | Astronomy and Astrophysics |
| Volume | 542 |
| DOIs | |
| State | Published - Jan 1 2012 |
| Externally published | Yes |
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Keywords
- Active - galaxies
- Fundamental parameters - galaxies
- Galaxies - galaxies
- Galaxies - radio continuum
- High-redshift
- Star formation - galaxies
- X-rays
ASJC Scopus subject areas
- Astronomy and Astrophysics
- Space and Planetary Science
Cite this
X-ray properties of radio-selected star forming galaxies in the Chandra-COSMOS survey. / Ranalli, P.; Comastri, A.; Zamorani, G.; Cappelluti, Nico; Civano, F.; Georgantopoulos, I.; Gilli, R.; Schinnerer, E.; Smolčić, V.; Vignali, C.
In: Astronomy and Astrophysics, Vol. 542, A16, 01.01.2012.Research output: Contribution to journal › Article
}
TY - JOUR
T1 - X-ray properties of radio-selected star forming galaxies in the Chandra-COSMOS survey
AU - Ranalli, P.
AU - Comastri, A.
AU - Zamorani, G.
AU - Cappelluti, Nico
AU - Civano, F.
AU - Georgantopoulos, I.
AU - Gilli, R.
AU - Schinnerer, E.
AU - Smolčić, V.
AU - Vignali, C.
PY - 2012/1/1
Y1 - 2012/1/1
N2 - 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.
AB - 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.
KW - Active - galaxies
KW - Fundamental parameters - galaxies
KW - Galaxies - galaxies
KW - Galaxies - radio continuum
KW - High-redshift
KW - Star formation - galaxies
KW - X-rays
UR - http://www.scopus.com/inward/record.url?scp=84865701016&partnerID=8YFLogxK
UR - http://www.scopus.com/inward/citedby.url?scp=84865701016&partnerID=8YFLogxK
U2 - 10.1051/0004-6361/201118723
DO - 10.1051/0004-6361/201118723
M3 - Article
AN - SCOPUS:84865701016
VL - 542
JO - Astronomy and Astrophysics
JF - Astronomy and Astrophysics
SN - 0004-6361
M1 - A16
ER -