X-Ray and Ultraviolet Properties of AGNs in Nearby Dwarf Galaxies

Abstract

We present new Chandra X-ray Observatory and Hubble Space Telescope observations of eight optically selected broad-line active galactic nucleus (AGN) candidates in nearby dwarf galaxies (z < 0.055). Including archival Chandra observations of three additional sources, our sample contains all 10 galaxies from Reines et al. (2013) with both broad Ha emission and narrow-line AGN ratios (six AGNs, four composites), as well as one lowmetallicity dwarf galaxy with broad Ha and narrow- line ratios characteristic of star formation. All 11 galaxies are detected in X- rays. Nuclear X-ray luminosities range from L-0.5- (7keV). approximate to 5 x 10(39) to 1 Chi 10(42) ergs(-1). In all cases except for the star- forming galaxy, the nuclear X- ray luminosities are significantly higher than would be expected from X- ray binaries, providing strong confirmation that AGNs and composite dwarf galaxies do indeed host actively accreting black holes (BHs). Using our estimated BH masses (which range from similar to 7 x 10(4) to 1 x 10(6)M circle dot), we find inferred Eddington fractions ranging from similar to 0.1% to 50%, i.e., comparable to massive broad- line quasars at higher redshift. We use the HST imaging to determine the ratio of UV to X-ray emission for these AGNs, finding that they appear to be less X-ray luminous with respect to their UV emission than more massive quasars (i.e., alpha OX values an average of 0.36 lower than expected based on the relation between aOX and 2500 angstrom luminosity). Finally, we discuss our results in the context of different accretion models onto nuclear BHs.