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 Physics , 2000, DOI: 10.1086/320355 Abstract: We analyze X-ray spectra and images of a sample of Seyfert 2 galaxies that unambiguously contain starbursts, based on their optical and UV characteristics. Although all sample members contain active galactic nuclei (AGNs), supermassive black holes or other related processes at the galactic centers alone cannot account for the total X-ray emission in all instances. Eleven of the twelve observed galaxies are significantly resolved with the ROSAT HRI, while six of the eight sources observed with the lower-resolution PSPC also appear extended on larger scales. The X-ray emission is extended on physical scales of 10 kpc and greater, which we attribute to starburst-driven outflows and supernova-heating of the interstellar medium. Spectrally, a physically-motivated composite model of the X-ray emission that includes a heavily absorbed (N_H > 10^{23} cm^{-2}) nuclear component (the AGN), power-law like scattered AGN flux, and a thermal starburst describes this sample well. Half the sample exhibit iron K alpha lines, which are typical of AGNs.
 Physics , 2000, DOI: 10.1086/318295 Abstract: We report the results of a spectroscopic investigation of a sample of 20 of the brightest type 2 Seyfert nuclei. Our goal is to search for the direct spectroscopic signature of massive stars, and thereby probe the role of circumnuclear starbursts in the Seyfert phenomenon. The method used is based on the detection of the higher order Balmer lines and HeI lines in absorption and the Wolf-Rayet feature at $\sim$4680 \AA in emission. These lines are strong indicators of the presence of young (a few Myrs) and intermediate-age (a few 100 Myrs) stellar populations. In over half the sample, we have detected HeI and/or strong stellar absorption features in the high-order (near-UV) Balmer series together with relatively weak lines from an old stellar population. In three others we detect a broad emission feature near 4680 \AA that is most plausibly ascribed to a population of Wolf-Rayet stars (the evolved descendants of the most massive stars). We therefore conclude that the blue and near-UV light of over half of the sample is dominated by young and/or intermediate age stars. The young'' Seyfert 2's have have larger far-IR luminosities, cooler mid/far-IR colors, and smaller [OIII]/H$\beta$ flux ratios than the old'' ones. These differences are consistent with a starburst playing a significant energetic role in the former class. We consider the possibility that there may be two distinct sub-classes of Seyfert 2 nuclei (starbursts'' and hidden BLR''). However, the fact that hidden BLRs have been found in three of the young'' nuclei argues against this, and suggests that nuclear starbursts may be a more general part of the Seyfert phenomenon.
 Physics , 1998, DOI: 10.1086/306154 Abstract: We present and discuss HST (WFPC2 and FOC) images and UV GHRS spectra plus ground-based near UV through to near IR spectra of three Seyfert 2 nuclei (NGC 7130, NGC 5135 and IC 3639). These galaxies, together to Mrk 477, were selected from a bigger sample that comprises the 20 brightest Seyfert 2 nuclei, with the goal to study the origin of the UV-optical-near IR featureless continuum in Seyfert 2 nuclei. These four galaxies have bolometric luminosities, as computed with the four IRAS bands, of 10^11 Lsol. They are close enough to be resolved with HST the nuclear zone. This makes these Seyfert 2 galaxies benchmarks to study the Starburst-AGN connection in more distant galaxies. The data provide direct evidence of the existence of a central nuclear starburst that dominates the UV light, and that seem to be responsible for the origin of the so called featureless continuum. These starbursts are dusty and compact. They have sizes (from less than 100 pc to a few hundred pc) much smaller and closer to the nucleus than that seen in the prototype Seyfert 2 galaxy NGC 1068. The bolometric luminosity of these starbursts is similar to the estimated bolometric luminosities of their obscured Seyfert 1 nuclei, and thus they contribute in the same amount to the overall energetics of these galaxies.
 Rosa M. Gonzalez Delgado Physics , 2001, Abstract: Observations at ultraviolet, optical and near-infrared wavelengths have shown the existence of recent star formation in the nuclear regions of Seyfert 2 (Sy2) galaxies that suggest a connection between the Starburst and the Seyfert phenomenon. According with the standard unified models of AGN circumnuclear starbursts also have to be present (and in the same numbers) in Sy1 as in Sy2 galaxies. This review discuss evidence in favor of the Starburst-AGN connection, as well as possible differences in terms of star formation activity between Sy1 and Sy2, that suggest an alternative interpretation of the Seyfert classification to that proposed by the standard unification model.
 Physics , 2000, DOI: 10.1086/317792 Abstract: We use the CO band at 2.3 micron to constrain the populations of young stars in the central regions of Seyfert galaxies. We report new CO band spectroscopy of 46 Seyfert galaxies. In most cases, the observed CO indices appear diluted by the presence of a non-stellar component (most likely, warm dust surrounding the active nucleus). We used JHKL aperture photometry to estimate the non-stellar contribution at 2.3 micron. We successfully corrected the CO band for the dilution for 16 galaxies which were not dominated by the non-stellar component. Comparing with CO indices measured in elliptical and purely starbursting galaxies, we find no evidence for strong starbursts in the majority of these galaxies.
 Physics , 2001, DOI: 10.1086/323958 Abstract: Far-infrared flux densities are newly extracted from the IRAS database for the RSA and CfA complete samples of Seyfert galaxies. These data are used to classify the Seyfert galaxies into those where the far-infrared continuum emission is dominated by the active galactic nucleus (AGN), circumnuclear starburst, or host galaxy. While AGN-dominant objects consist of comparable numbers of Seyfert 1 and 2 galaxies, starburst- and host-dominant objects consist preferentially of Seyfert 2 galaxies. Thus, in addition to the dusty torus, the circumnuclear starburst region and host galaxy are important in hiding the broad-line region. Morphologically, starburst-dominant Seyfert galaxies are of later types and more strongly interacting than AGN-dominant Seyfert galaxies. In a later-type galaxy, the AGN central engine has a lower Eddington luminosity, and the gaseous content is higher. The gas is efficiently supplied to the starburst via a galaxy-galaxy interaction. Morphologies of host-dominant Seyfert galaxies are of various types. Since starbursts in Seyfert galaxies are older than those in classical starburst galaxies, we propose an evolution from starburst to starburst-dominant Seyfert to host-dominant Seyfert for a late-type galaxy. An evolution from AGN-dominant Seyfert to host-dominant Seyfert is proposed for an early-type galaxy. These sequences have durations of a few x 10^8 yr and occur repeatedly within a galaxy during its evolution from a late type to an early type.
 Physics , 2001, Abstract: We present a simple population synthesis scheme which recognizes composite starburst+Seyfert 2 nuclei from a few easy-to-obtain optical measurements. Composite systems seem to evolve towards less luminous Seyfert 2's which do not harbor detectable circum-nuclear starbursts. We encourage applications of this cheap diagnostic tool to large samples of Seyfert 2's, as well as its extension to other activity classes, in order to test and refine this evolutionary scenario.
 Physics , 2009, DOI: 10.1038/nature08557 Abstract: Although Galactic cosmic rays (protons and nuclei) are widely believed to be dominantly accelerated by the winds and supernovae of massive stars, definitive evidence of this origin remains elusive nearly a century after their discovery [1]. The active regions of starburst galaxies have exceptionally high rates of star formation, and their large size, more than 50 times the diameter of similar Galactic regions, uniquely enables reliable calorimetric measurements of their potentially high cosmic-ray density [2]. The cosmic rays produced in the formation, life, and death of their massive stars are expected to eventually produce diffuse gamma-ray emission via their interactions with interstellar gas and radiation. M 82, the prototype small starburst galaxy, is predicted to be the brightest starburst galaxy in gamma rays [3, 4]. Here we report the detection of >700 GeV gamma rays from M 82. From these data we determine a cosmic-ray density of 250 eV cm-3 in the starburst core of M 82, or about 500 times the average Galactic density. This result strongly supports that cosmic-ray acceleration is tied to star formation activity, and that supernovae and massive-star winds are the dominant accelerators.
 Physics , 2001, DOI: 10.1086/322398 Abstract: We explore the formation of dusty gas walls induced by a circumnuclear starburst around an AGN. We concentrate our attention on the role of the radiation force by a starburst as well as an AGN, where the effects of optical depth of dusty gas are taken into consideration. In two-dimensional axisymmetric space, we analyze the configuration and the stability of geometrically thin walls which are in balance between radiation pressure and gravity. As a result, it is shown that the radiation force by the circumnuclear starburst works to stabilize optically thick walls surrounding the nucleus. In the case of a brighter starburst with a fainter AGN, there form double walls, an inner one of which is located between the nucleus and the circumnuclear starburst, and an outer one of which enshrouds both the starburst regions and the nucleus. The total extinction (Av) of both walls turns out to be ~10 for a brighter starburst. As a consequence, double walls could heavily obscure the nucleus to make the AGN type 2. The outer wall may provide an explanation for the recent indications for large-scale obscuring materials in Seyfert 2's. Also, it is predicted that the AGN type is shifts from type 2 to type 1 in several times 10^7 yr according to the stellar evolution in the starburst. In contrast, if the AGN itself is much brighter than the starburst as a quasar is, then neither wall forms regardless of the starburst activity and the nucleus is likely to be identified as type 1. To conclude, the radiatively-supported gas walls could be responsible for the putative correlation between AGN type and the starbursts, whereby Seyfert 2 galaxies are more frequently associated with circumnuclear starbursts than type 1, whereas quasars are mostly observed as type 1 regardless of star-forming activity in the host galaxies.
 Physics , 2010, DOI: 10.1051/0004-6361/201015644 Abstract: The Fermi/LAT collaboration recently reported the detection of starburt galaxies in the high energy gamma-ray domain, as well as radio-loud narrow-line Seyfert 1 objects. Motivated by the presence of sources close to the location of composite starburst/Seyfert 2 galaxies in the first year Fermi/LAT catalogue, we aim at studying high energy gamma-ray emission from such objects, and at disentangling the emission of starburst and Seyfert activity. We analysed 1.6 years of Fermi/LAT data from NGC 1068 and NGC 4945, which count among the brightest Seyfert 2 galaxies. We search for potential variability of the high energy signal, and derive a spectrum of these sources. We also analyse public INTEGRAL IBIS/ISGRI data over the last seven years to derive their hard X-ray spectrum. We find an excess of high energy gamma-rays of 8.3 sigma and 9.2 sigma for 1FGL J0242.7+0007 and 1FGL J1305.4-4928, which are found to be consistent with the position of the Seyfert 2 galaxies NGC 1068 and NGC 4945, respectively. The energy spectrum of the sources can be described by a power law with a photon index of Gamma=2.31 \pm 0.13 for NGC 1068, while for NGC 4945, we obtain a photon index of Gamma=2.31 \pm 0.10. For both sources, we detect no significant variability nor any indication of a curvature of the spectrum. We discuss the origin of the high energy emission of these objects in the context of Seyfert or starburst activity. While the emission of NGC 4945 is consistent with starburst activity, that of NGC 1068 is an order of magnitude above expectations, suggesting dominant emission from the active nucleus. We show that a leptonic scenario can account for the multi-wavelength spectral energy distribution of NGC 1068.
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