The Seyfert-starburst Connection in X-rays. Ii. Results and Implications

We present the results of X-ray imaging and spectroscopic analysis of a sample of Seyfert 2 galaxies that contain starbursts, based on their optical and UV characteristics. These composite galaxies exhibit extended, soft, thermal X-ray emission, which we attribute to their starburst components. Comparing their X-ray and far-infrared properties with ordinary Seyfert and starburst galaxies, we identify the spectral characteristics of their various intrinsic emission sources. The observed far-infrared emission of the composite galaxies may be associated almost exclusively with star formation, rather than the active nucleus. The ratio of the hard X-ray luminosity to the far-infrared and [O III] λ5007 luminosity distinguishes most of these composite galaxies from “pure” Seyfert 2 galaxies, while their total observed hard X-ray luminosity distinguishes them from “pure” starbursts. The hard nuclear X-ray source is generally heavily absorbed (NH > 10 23 cm) in the composite galaxies. Based on these results, we suggest that the interstellar medium of the nuclear starburst is a significant source of absorption. The majority of the sample are located in groups or are interacting with other galaxies, which may trigger the starburst or allow rapid mass infall to the central black hole, or both. We conclude that starbursts are energetically important in a significant fraction of active galaxies, and starbursts and active galactic nuclei may be part of a common evolutionary sequence. Subject headings: galaxies: Seyfert — X-rays: galaxies 1. seyfert and starburst galaxies One of the major goals of astrophysics is to elucidate the physical processes that drove the strong cosmic evolution of the active galactic nucleus (AGN) and galaxy populations. The apparent ubiquity of supermassive black holes in the nuclei of present-day galaxies and the strong correlation between the mass of the black hole and the velocity dispersion of its “host” stellar spheroid (Ferrarese & Merritt 2000; Gebhardt et al. 2000) implies that the creation of supermassive black holes (presumably corresponding to a luminous QSO phase) was an integral part of the formation of ellipticals and galactic bulges. Thus, understanding the “starburst-AGN connection” is a crucial component of modern cosmogony. Directly studying the starburst-AGN connection at high redshift is hard. The limitations on spatial resolution and signal-to-noise make it very difficult to use imaging or spectroscopy to disentangle processes due to the starburst or AGN, or to try to separate cause from effect. Thus, very little is known about the possible role of starbursts in typical high-redshift QSOs (cf. Ridgway et al. 2000), about AGNs in the “Lyman Break” population of star-forming galaxies (e.g., Adelberger & Steidel 2000), about the relative energetic importance of starformation and AGN in high-redshift sub-mm sources (e.g., Barger, Cowie, & Richards 2000), or about the nature of the optically-faint (presumably distant) contributors to the cosmic X-ray background that Chandra has recently discovered (Mushotzky et al. 2000; Hornschemeier et al. 2000; Giacconi et al. 2000). Fortunately, we have some excellent local laboratories in which the starburst-AGN connection can be probed in considerably more detail. The most powerful AGNs near enough to study in such detail are the Seyfert galaxies. The type 2 Seyferts are particularly well-suited to such investigations because the bright glare from the central engine has been providentially blocked by the high gas and dust column density of an “obscuring torus”. Indeed, while a standard AGN model of accretion onto supermassive black holes generally describes the central engines of many Seyfert galaxies successfully (e.g., Miyoshi et al. 1995; Tanaka et al. 1995, and Nandra et al. 1997), starbursts are also significant, often in the same galaxies. To address these issues, we are specifically examining the X-ray properties of a sample of Sy 2 galaxies that definitely contain starbursts, the Sy2/SB composite galaxies. We present the data and detailed analysis separately (Levenson, Weaver, & Heckman 2000; Paper I) and in this work concentrate on the relationship of these results to the broader questions of the Seyfert-starburst connection. We summarize the physical characteristics derived from the spatial and spectral modelling of the sample in §2 and present general results in §3. We discuss the X-ray properties of this sample and compare them with other samples in §4, and we summarize our conclusions in §5.