Mining for normal galaxies in the first XMM-Newton Serendipitous Source Catalog

Abstract

This paper uses the first XMM–Newton Serendipitous Source Catalog compiled by the XMM–Newton Science Centre to identify low‐z X‐ray selected normal galaxy candidates. Our sample covers a total area of ≈6 deg2 to the 0.5–2 keV limit ≈10−15 erg s−1 cm−2. A total of 23 sources are selected on the basis of low X‐ray to optical flux ratio log fX/fopt < −2, soft X‐ray spectral properties and optical spectra, when available, consistent with stellar formation rather than active galactic nucleus (AGN) processes. This sample is combined with similarly selected systems from the Needles in the Haystack Survey to provide a total of 46 unique (z≲ 0.2) X‐ray detected normal galaxies, the largest low‐z sample yet available. This is first used to constrain the normal galaxy log N–log S at bright fluxes (10−15–10−13 erg s−1 cm−2). We estimate a slope of −1.46 ± 0.13 for the cumulative number counts consistent with the Euclidean prediction. We further combine our sample with 23 local (z≲ 0.2) galaxies from the Chandra Deep Field‐North and ‐South surveys to construct the local X‐ray luminosity function of normal galaxies. A Schechter form provides a good fit to the data with a break at log L★= 41.02+0.14−0.12 erg s−1 and a slope of α=−1.76 ± 0.10. Finally, for the sample of 46 systems, we explore the association between X‐ray luminosity and host galaxy properties, such as star formation rate (SFR) and stellar mass. We find that the LX of the emission‐line systems correlates with Hα luminosity and 1.4‐GHz radio power, both providing an estimate of the current SFR. In the case of early‐type galaxies with absorption‐line optical spectra, we use the K band as an approximation of stellar mass and find a correlation of the form LX∝L1.5K. This is flatter than the LX–LB relation for local ellipticals. This may be due to either LK providing a better approximation of galaxy mass or selection effects biasing our sample against very luminous early‐type galaxies, LX > 1042 erg s−1.

Normal galaxies in the XMM-Newton fields

“…Our data points are plotted as open circles in Fig. 4, while crosses represent the combined sample of the 1XMM survey of Georgakakis et al (2006) and the NHS survey (Georgakakis et al 2004). …”

“…Moreover, AGN are usually confined in the area −1 < log( f x / f o ) < 1 (Stocke et al 1991;Lehmann et al 2001). For the purposes of this study we selected as normal galaxy candidates sources having log( f x / f o ) < −2 (see also Georgakakis et al 2004;Georgantopoulos et al 2005;Georgakakis et al 2006). This is a conservative limit and it might introduce a bias against massive ellipticals and powerful starbursts (Tzanavaris et al 2006;Georgakakis et al 2007).…”

“…Searches for normal galaxies in broad (e.g. Georgakakis et al 2003Georgakakis et al , 2004Georgakakis et al , 2006Hornschemeier et al 2005;Tajer et al 2005;Tzanavaris et al 2006) and deep (e.g. Alexander et al 2002;Hornschemeier et al 2003;Norman et al 2004;Georgakakis et al 2007) surveys have provided a large number of X-ray selected galaxies.…”

“…Using these samples, the luminosity function of normal galaxies has been established, both locally (e.g. Georgantopoulos et al 2005;Georgakakis et al 2006) and at Article published by EDP Sciences higher redshifts (e.g. Norman et al 2004;Ptak et al 2007;Tzanavaris & Georgantopoulos 2008).…”

“…In this paper we expand the galaxy samples of Georgakakis et al (2006) and explore the relation between the X-ray and Hα luminosities for emission line systems. This provides an insight into the L x −SFR relation (see also Hornschemeier et al 2005).…”