Searching for X-ray luminous “normal” galaxies in 2dFGRS

Free access article

Issue		A&A Volume 454, Number 2, August I 2006 APEX Special Booklet


Page(s)		447 - 452
Section		Extragalactic astronomy
DOI		http://dx.doi.org/10.1051/0004-6361:20054630

A&A 454, 447-452 (2006)
DOI: 10.1051/0004-6361:20054630

Searching for X-ray luminous "normal" galaxies in 2dFGRS

P. Tzanavaris¹, I. Georgantopoulos¹ and A. Georgakakis²

¹ Institute of Astronomy & Astrophysics, National Observatory of Athens, I. Metaxa & V. Pavlou, Penteli 15236, Greece
e-mail: pana@astro.noa.gr
² Astrophysics Group, Imperial College London, Blackett Laboratory, Prince Consort Rd., London, SW7 2AW, UK

(Received 2 December 2005 / Accepted 26 February 2006 )

Abstract
Aims.We cross-correlated the Chandra XASSIST and XMM-Newton Serendipitous Source Catalogues with the 2 degree Field Galaxy Redshift Survey (2dFGRS) database. Our aim was to identify the most X-ray luminous ( $L_{\rm X} > 10^{42}$ erg s^-1) examples of galaxies in the local Universe whose X-ray emission is dominated by stellar processes rather than AGN activity ("normal" galaxies), as well as to test the empirical criterion $\log (f_{\rm X}/f_{\rm O})$ < -2 for separating AGN from NGs. We also performed a similar search in two nearby-galaxy samples from the literature.
Methods.With XMM-Newton (Chandra) we covered an area of $\sim$ 8.2 ( $\sim$ 5.8) deg² down to a flux limit of ${\sim} 10^{-15}$ ( ${\sim} 1.6 \times 10^{-15}$ ) erg cm^-2 s^-1. We classified 2dFGRS spectra using emission-line intensity ratios.
Results.We found 18 (20) 2dFGRS galaxies with XMM-Newton (Chandra), which constitute our XMM-Newton-2dFGRS (Chandra-2dFGRS) correlation sample. We classified 6 2dFGRS spectra as star-forming, $\ion{H}{ii}$ nuclei, and 2 spectra as possible $\ion{H}{ii}$ nuclei. The rest of the objects are absorption-line galaxies and AGN, including 3 possible LINERs. No luminous "normal" galaxies have been found, but out of 19 "normal" galaxies in this sample, 5 $\ion{H}{ii}$ and 3 absorption-line galaxies have $\log (f_{\rm X}/f_{\rm O})$ > -2. Furthermore, all 44 galaxies in the first literature sample have $\log (f_{\rm X}/f_{\rm O}) < -2$ and $L_{\rm X} < 10^{42}$ erg s^-1. In the second literature sample, out of a total of 170 "normal" galaxies, we found 16 galaxies with $\log (f_{\rm X}/f_{\rm O})$ >-2, the majority of which are massive ellipticals. Three of these have $L_{\rm X} > 10^{42}$ erg s^-1.
Conclusions.We found no luminous "normal" galaxies in our XMM-Newton-2dFGRS and Chandra-2dFGRS correlation samples. We found three such galaxies in the second literature sample. Considering all samples, we find that the $\log (f_{\rm X}/f_{\rm O}) < -2$ criterion seems to select primarily against the brightest, massive ellipticals.

Key words: X-rays: galaxies -- galaxies: starburst -- galaxies: active

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