The origin of the cosmic soft X-ray background: optical identification of an extremely deep ROSAT survey

Abstract

We present the results of the deepest optically identified X-ray survey yet made. The X-ray survey was obtained with the ROSAT position-sensitive proportional counter (PSPC) and reaches a flux limit of 1.6 x 1O_15 erg cm 2 s -1 (0.5-2.0 keV). Above a flux limit of 2 x 10 -15 erg cm 2 s -1 we define a complete sample of 70 sources, of which 59 are identified. For a further five sources we have tentative identifications and for a further four the X-ray error boxes are blank to R = 23 mag. At brighter fluxes ( >_ 10 -14 erg cm 2 s -1 ) we confirm the results of previous less deep X-ray surveys, with 84 per cent of our sources being QSOs. However, at the faint flux limit the survey is dominated by a population of galaxies with narrow emission lines (NELGs). In addition, at intermediate fluxes we find a small number of groups and clusters of galaxies at redshifts generally > 0.3. Most of these groups are poor systems of low X-ray luminosity and the number that we find is consistent with a zero evolutionary scenario, unlike the situation for high-luminosity clusters at the same redshift. To a flux limit of 2 x 1O_15 erg cm -2 s -1 , QSOs contribute > 31 per cent of the cosmic soft X-ray background (XRB), groups/clusters contribute '' 10 per cent and NELGs contribute ~ 8 per cent. However, the QSO differential source count slope below 1O_14 erg cm -2 s -1 is -1.4, severely sub-Euclidean, as is the (poorly defined) group/cluster slope, whereas the differential NELG slope is close to Euclidean (^, -2.4). If the NELG source counts continue to rise at that slope, all of the remaining cosmic soft XRB will be explained by a flux limit of ~ 1-2 x 1O_16 erg cm 2 s -1 , with NELGs contributing about one quarter of the XRB. The average NELG X-ray spectrum is harder than that of the QSOs, and similar to that of the remaining unresolved cosmic XRB, suggesting that NELGs will also be substantial contributors to the XRB at higher energies. The observed NELGs lie in the redshift range 0.1-0.6 and have MR = -20 to -23, approximately 3 mag more luminous than typical field galaxies. They have predominantly blue colours, and some are definitely spirals, but the presence of some ellipticals cannot yet be ruled out. Many are in interacting or disturbed systems. The NELGs have optical spectra similar to © 1998 RAS at North Dakota State University on July 2, 2015 http://mnras.oxfordjournals.org/ Downloaded from 642 I. M. M'Hardy et al.those of the majority of the field galaxy population at a similar redshift and may simply be the more luminous members of the emission-line field galaxy population. Based on optical line ratios and X-ray/optical flux ratios, the NELGs, both as a sample and within individual galaxies, appear to be a mixture of starburst galaxies and true active galactic nuclei (AGN).

Deep Extragalactic X-Ray Surveys

“…Deep 0.5-2 keV surveys with ROSAT to limiting fluxes of ≈ 10 −15 erg cm −2 s −1 were for the first time able to resolve the majority (≈ 75%) of the soft CXRB into discrete sources (e.g., Hasinger et al 1993Hasinger et al , 1998. Extensive optical follow-up spectroscopy identified the bulk of these sources as AGN (e.g., McHardy et al 1998;Schmidt et al 1998;Zamorani et al 1999;Lehmann et al 2001), demonstrating that at least the 0.5-2 keV CXRB is predominantly due to accretion onto SMBH, integrated over cosmic time. The deep ROSAT surveys detected an AGN sky density (≈ 780-870 deg −2 ) larger than at any other wavelength and found evidence for luminosity-dependent density evolution of AGN (Miyaji, Hasinger & Schmidt 2000), contrary to the pure luminosity evolution which had been proposed for both optically and X-ray selected AGN (e.g., Boyle et al 1993).…”

Untitled

“…It has been suggested by several authors (e.g., Bookbinder et al 1980;Griffiths & Padovani 1990;McHardy et al 1998) that X-ray luminous (L X > 10 41−42 erg s −1 in the 0.5 − 2.5 keV band for H 0 = 50 km s −1 Mpc −1 ) starburst galaxies may make a significant contribution to the extragalactic X-ray background (XRB). However there is still a considerable debate as to whether significant numbers of these objects exist (e.g., Moran et al 1994Moran et al , 1996Wisotzki & Bade 1997).…”

“…In recent deep ROSAT X-ray surveys (e.g., Georgantopoulos et al 1996;McHardy et al 1998), a population of faint soft X-ray sources (f X ∼ 10 −14 − 10 −15 erg cm −2 s −1 ) with relatively high X-ray luminosities has been identified with narrow emission line galaxies (NELGs). The typical X-ray luminosities of these objects are about 10 42 erg s −1 or even higher.…”

“…The typical X-ray luminosities of these objects are about 10 42 erg s −1 or even higher. Modeling of the various classes of objects (QSOs, clusters, NELGs) which contribute to the XRB (Pearson et al 1997;McHardy et al 1998) has shown that depending on their evolution, NELGs may contribute up to 50% in the 0.5 − 2.0 keV range at fainter fluxes (f X < 10 −15 erg cm −2 s −1 ). Optical spectroscopy of these objects has shown that there is a significant fraction of bona-fide starburst galaxies together with a number of objects lying on the borderline between starbursts and AGN McHardy et al 1998).…”

“…Modeling of the various classes of objects (QSOs, clusters, NELGs) which contribute to the XRB (Pearson et al 1997;McHardy et al 1998) has shown that depending on their evolution, NELGs may contribute up to 50% in the 0.5 − 2.0 keV range at fainter fluxes (f X < 10 −15 erg cm −2 s −1 ). Optical spectroscopy of these objects has shown that there is a significant fraction of bona-fide starburst galaxies together with a number of objects lying on the borderline between starbursts and AGN McHardy et al 1998). However, there has been a poor record in the past, with many misclassified starbursts, which after a careful spectroscopic classification turned out to be AGN (e.g., Halpern et al 1995;Moran et al 1996).…”