Daryl Haggard (CIERA/Northwestern University)

Local X-ray Bright Optically Normal Galaxies: Fossil Groups, X-ray Variables, and AGN

Why do some luminous X-ray sources (log Lx [erg/s] > 42, rest-frame 2-8 keV) lack any detectable optical emission lines? Such high X-ray luminosities typically suggest power from accretion onto supermassive black holes, which produce copious UV photons to photoionize the line-emitting circumnuclear gas, or a large amount of dark matter, which can bind more hot gas for a given optical luminosity. Competing explanations for these high X-ray luminosity, line-less sources include: (1) optical dilution by the host galaxy starlight, (2) obscuration, (3) radiatively inefficient accretion flows, (4) X-ray variability, or (5) extended hot gas. Resolving this puzzle is crucial for understanding the AGN phenomenon and its cosmic evolution, and the completeness of optically-selected samples. We present surprising new Chandra observations of eight candidate Optically Dull X-ray Bright Galaxies at low redshift (z ~ 0.3).