X-ray and Ultraviolet Properties of AGN in Nearby Dwarf Galaxies

Abstract

We present new Chandra X-ray Observatory and Hubble Space Telescope observations of eight optically selected broad-line AGN candidates in nearby dwarf galaxies ($z<0.055$). Including archival Chandra observations of three additional sources, our sample contains all ten galaxies from Reines et al. (2013) with both broad H$\alpha$ emission and narrow-line AGN ratios (6 AGNs, 4 Composites), as well as one low-metallicity dwarf galaxy with broad H$\alpha$ and narrow-line ratios characteristic of star formation. All eleven galaxies are detected in X-rays. Nuclear X-ray luminosities range from $L_{0.5-7\mathrm{keV}}\approx 5\times 10^{39}$ to $1\times 10^{42}$ $\mathrm{erg}{\mathrm{s}}^{-1}$. 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 AGN and composite dwarf galaxies do indeed host actively accreting BHs. Using our estimated BH masses (which range from $\sim 7\times 10^4-1\times 10^6{M}_{\odot }$), we find inferred Eddington fractions ranging from $\sim 0.1-50\%$, i.e. comparable to massive broad-line quasars at higher redshift. We use the HST imaging to determine the ratio of ultraviolet to X-ray emission for these AGN, finding that they appear to be less X-ray luminous with respect to their UV emission than more massive quasars (i.e. ${\alpha }_{\mathrm{OX}}$ values an average of 0.36 lower than expected based on the relation between ${\alpha }_{\mathrm{OX}}$ and $2500\mathring{\mathrm{A}}$ luminosity). Finally, we discuss our results in the context of different accretion models onto nuclear BHs.

15 pages, 15 figures, 4 tables. Submitted to ApJ

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