Prof. Karen M. Leighly

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Research Summary

I study Active Galactic Nuclei (AGN).  AGN are the most luminous, persistently emitting individual objects in the Universe.  They can be seen at the largest distances, and provide a probe of the early Universe after structure formation.  Used as a background light, absorption lines in their spectra trace nonluminous matter. Their spectra are a potentially useful for understanding evolution of cosmic abundances and the onset of the first star formation.  They are powered by accretion onto black holes, and are key for understanding black hole demographics and the black hole mass function.

To use AGN effectively, we must understand their astrophysics.  My overall research goal is to understand how the primary physical parameters for black hole accretion, the black hole mass and accretion rate, manifest themselves in the broad band continuum and line emission from AGN. This is important; for example, if we want to use AGN X-ray emission as a gauge of the black hole activity, we must understand how much X-ray emission we can expect from a particular combination of black hole mass and accretion rate.

Before coming to OU, my research focused on the X-ray emission of Narrow-line Seyfert 1 Galaxies (NLS1s).  NLS1s are a subset of AGN that are identified by their optical line emission: they have narrow Hb lines, strong optical FeII emission and weak forbidden-line emission.  Their X-ray properties are also distinctive; as I showed in two papers published in 1999 on ASCA data from NLS1s, they have steeper X-ray spectra and higher amplitude X-ray variability than comparable Seyfert galaxies with broad optical lines.  These properties have led to the generally accepted view that NLS1s are objects accreting at a high rate with respect to their Eddington value.

Since coming to OU, the focus of my research has shifted to the study of the optical and UV line emission from AGN.  AGN emission lines are powered by photoionization. The line-emitting gas moves in the potential of the black hole, and is accelerated by resonance-line scattering and other processes.  From the emission-line strengths and ratios, and from the line profiles, we can constrain the physical conditions and dynamics of the emitting gas, and from those, we can potentially understand how much gas is present, the origin of its motion, its kinetic energy, the chemical abundances, and the broad-band continuum spectrum illuminating it, which is ultimately determined by the black hole mass and accretion rate.