On the Origin of QSO Broad Emission Line Asymmetries

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Session 34 -- Spectroscopy of AGN II: Broad Lines and Spectral Evolution
Oral presentation, Monday, 9, 1995, 2:00pm - 3:30pm

[34.03] On the Origin of QSO Broad Emission Line Asymmetries

M.R.Corbin (KPNO/NOAO)

The broad optical and ultraviolet emission lines of QSOs and AGNs display both redward and blueward asymmetries. This result is particularly well established for H$\beta$ and C IV $\lambda 1549$, and it has been found that H$\beta$ becomes increasingly redward asymmetric with increasing soft X-ray luminosity. It has been proposed that these asymmetries arise from a difference between the velocity centroids of emission from the Intermediate Line region and the Very Broad Line Region. Numerical fitting of the H$\beta$ profile in a sample of 87 luminous low-redshift QSOs yields results consistent with this interpretation: the majority of profiles can be accurately fit by a combination of either a broad Gaussian base and a narrower logarithmic core or a Gaussian base and a Gaussian core, with mean core and base FWHM values of $\sim$ 3700 km s$^{-1}$ and $\sim$ 9500 km s$^{-1}$, respectively. The asymmetries can be interpreted as arising from a shift of the VBLR velocity centroid relative to systemic velocity over a range $\sim$ 6300 km s$^{-1}$. A similar model appears to apply to C IV $\lambda 1549$. The H$\beta$ asymmetry / X-ray correlation may in turn be interpreted as a progressive redshift of the VBLR velocity centroid to systemic velocity with increasing X-ray luminosity. This suggests that the underlying effect is gravitational redshift. Assuming the VBLR lies at a distance $\sim$ 0.1 pc from the central continuum source, the observed range in the VBLR centroid shift is consistent with the amount of gravitational redshift associated with a black hole of mass $\sim$ 7 \times 10$^{9}$ $M_{\sun}$, consistent with estimates from other methods. Similarly, the largest VBLR component FWHM values are $\sim$ 20,000 km s$^{-1}$, which yields a virial mass of the same order at the assumed distance. This model does not explain the blueshift of the VBLR centroid to systemic velocity among objects of low X-ray luminosity, which must be interpreted as arising from a competing effect such as electron scattering within the VBLR. If the gravitational redshift interpretation is correct, then the prevalence of redward asymmetries, wider H$\beta$ profiles, and stronger intermediate- and narrow-line emission among radio-loud objects in comparison to radio-quiet objects may signify larger black hole and host galaxy masses among the former.

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