A ROSAT High Resolution Imager survey of bright nearby galaxies

Abstract

We use the extensive public archive of ROSAT High Resolution Imager (HRI) observations to carry out a statistical investigation of the X-ray properties of nearby galaxies. Specifically we focus on the sample of 486 bright () northern galaxies studied by Ho, Filippenko and Sargent (HFS) in the context of their exploration of the optical spectroscopic properties of nearby galactic nuclei. Over 20 per cent of HFS galaxies are encompassed in ROSAT HRI fields of reasonable (10 ks) exposure. The X-ray sources detected within the optical extent of each galaxy are categorized as either nuclear or non-nuclear, depending on whether the source is positioned within or outside a 25-arcsec radius circle centred on the optical nucleus. A nuclear X-ray source is detected in over 70 per cent of the galaxies harbouring either a Seyfert or LINER nucleus, compared with a detection rate of only ∼40 per cent in less active systems. The correlation of the Hα luminosity with nuclear X-ray luminosity previously observed in QSOs and bright Seyfert 1 galaxies appears to extend down into the regime of ultra-low luminosity (LX∼1038−1040 erg s−1) active galactic nuclei (AGN). The inferred accretion rates for this sample of low-luminosity AGN are significantly sub-Eddington. In total, 142 non-nuclear sources were detected. In combination with published data for M31, this leads to a luminosity distribution (normalized to an optical blue luminosity of 1010 L⊙) for the discrete X-ray source population in spiral galaxies of the form where L38 is the X-ray luminosity in units of 1038 erg s−1. The implied LXLB ratio is ∼1.1×1039 erg s−1(1010 L⊙)−1. The nature of the substantial number of ‘superluminous’ non-nuclear objects detected in the survey is discussed.