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N. A. Levenson, K. A. Weaver, T. M. Heckman (JHU)
We analyze X-ray spectra of a selection of Seyfert 2 galaxies that unambiguously contain starbursts, based on their optical and UV characteristics. In all cases, soft (kT < 1 keV) thermal emission in addition to an absorbed power law contributes significantly to the X-ray emission. The total flux of the soft component, which dominates below 2 keV, is comparable to that of the power law, which is the primary source above 3 keV. The ratios of hard to soft X-ray luminosity and X-ray to far-infrared luminosity together help to distinguish Seyfert galaxies that contain starbursts. The template we establish with these known Seyfert/starburst examples will enable identification of similar composite galaxies through X-ray spectroscopy alone, which will be particularly useful at higher redshift.
Understanding the relationship between Seyfert galaxies and starbursts is important in the context of the unified model. Starbursts are common in Seyfert 2 galaxies and in some cases account for the featureless continuum at optical and UV wavelengths that buried Seyfert 1 nuclei alone cannot create. Identifying the starburst component of Seyfert galaxies is also significant if they are part of a common evolutionary sequence.
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