A deep ROSAT survey -- V. The extragalactic populations at faint fluxes

Abstract

In the central regions of five deep (21 –49 ks) ROSAT Position Sensitive Proportional Counter (PSPC) pointings we have detected 194 soft X-ray sources down to a limiting flux of ∼3 × 10⁻¹⁵ erg cm⁻¹ s⁻¹ (0.5–2 keV). Spectroscopic follow-up observations at the Anglo-Australian Telescope (AAT) show that the majority of these sources are active galactic nuclei (AGN) (QSOs and Seyfert 1 galaxies); some X-ray luminous galaxies (L_x∼ 10⁴² erg s⁻¹) are also identified at faint fluxes; these may have active nuclei or they may be star-forming galaxies. The intrinsic spectrum of the total X-ray background, including sources, is well fitted above 1 keV by a power-law model with a photon index of Г ∼ 1.6, flatter than the average AGN spectrum in our fields. The normalization of this component at 1 keV is some 50 per cent higher than an extrapolation of the X-ray background (XRB) spectrum measured above 3 keV. This extends the XRB spectral paradox to lower energies than has previously been noted, and suggests that some population other than QSOs and Seyfert 1 galaxies may contribute substantially to the XRB at faint fluxes. This is supported by an analysis of the source number count distribution (log Nlog S): the differential log N-log S of AGN and unidentified objects becomes a flat power law (γ ≈ 1.1 ± 0.9, 90 per cent error) at S ∼ 10⁻¹⁴ erg cm⁻² s⁻¹, yielding a contribution of only 45 per cent at the 1-keV background when integrated down to 10⁻¹⁶ erg cm⁻² s⁻¹; despite the large log Nlog S uncertainty at faint fluxes the hypothesis that AGN produce all the soft XRB intensity can be ruled out at the 90 per cent confidence level. Galaxies such as those we have detected will contribute 20 per cent of the 1-keV background if they follow a Euclidean log N-log S distribution.