Reflection in Seyfert Galaxies and the Unified Model of AGN

We present a deep study of the average hard X-ray spectra of Seyfert galaxies. We analyzed all public INTEGRAL IBIS/ISGRI data available on all the 165 Seyfert galaxies detected at z<0.2. Our final sample consists of 44 Seyfert 1's, 29 Seyfert 1.5's, 78 Seyfert 2's, and 14 Narrow Line Seyfert 1's. We derived the average hard X-ray spectrum of each subsample in the 17-250keV energy range. All classes of Seyfert galaxies show on average the same nuclear continuum, as foreseen by the zeroth order unified model, with a cut-off energy of Ec>200keV, and a photon index of Gamma ~1.8. Compton-thin Seyfert 2's show a reflection component stronger than Seyfert 1's and Seyfert 1.5's. Most of this reflection is due to mildly obscured (10^23 cm^-2 < NH < 10^24 cm^-2) Seyfert 2's, which have a significantly stronger reflection component (R=2.2^{+4.5}_{-1.1}) than Seyfert 1's (R<=0.4), Seyfert 1.5's (R<= 0.4) and lightly obscured (NH < 10^23 cm^-2) Seyfert 2's (R<=0.5). This cannot be explained easily by the unified model. The absorber/reflector in mildly obscured Seyfert 2's might cover a large fraction of the X-ray source, and have clumps of Compton-thick material. The large reflection found in the spectrum of mildly obscured Seyfert 2's reduces the amount of Compton-thick objects needed to explain the peak of the cosmic X-ray background. Our results are consistent with the fraction of Compton-thick sources being ~10%. The spectra of Seyfert 2's with and without polarized broad lines do not show significant differences, the only difference between the two samples being the higher hard X-ray and bolometric luminosity of Seyfert 2's with polarized broad lines. The average hard X-ray spectrum of Narrow line Seyfert 1's is steeper than those of Seyfert 1's and Seyfert 1.5's, probably due to a lower energy of the cutoff.Comment: 19 pages, accepted for publication in Astronomy and Astrophysics, final versio

Spitzer/IRS Observations of Seyfert 1.8s and 1.9s: A Comparison with Seyfert 1s and Seyfert 2s

We present Spitzer Space Telescope mid-infrared spectra of 12 Seyfert 1.8 and 1.9 galaxies over the 5-38 um region. We compare the spectral characteristics of this sample to those of 58 Seyfert 1 and Seyfert 2 galaxies from the Spitzer archives. An analysis of the spectral shapes, the silicate 10 um feature and the emission line fluxes have enabled us to characterize the mid-IR properties of Seyfert 1.8/1.9s. We find that the equivalent widths of the 10 um silicate feature are generally weak in all Seyfert galaxies, as previously reported by several studies. The few Seyfert galaxies in this sample that show deep 10 um silicate absorption features are highly inclined and/or merging galaxies. It is likely that these absorption features originate primarily in the dusty interstellar medium of the host galaxy rather than in a dusty torus on parsec scales close to the central engine. We find that the equivalent width of the polycyclic aromatic hydrocarbon (PAH) band at 6.2 um correlates strongly with the 20-30 um spectral index. Either of these quantities are good indicators of the amount of starburst contribution to the mid-IR spectra. The spectra of Seyfert 1.8 and 1.9s are dominated by these starburst features, similar to most Seyfert 2s. They show strong PAH bands and a strong red continuum toward 30 um. The strengths of the high-ionization forbidden narrow emission lines [O IV] 25.89 um, [Ne III] 15.56 um and [Ne V] 14.32 um relative to [Ne II] 12.81 um are weaker in Seyfert 1.8/1.9s and Seyfert 2s as compared to Seyfert 1s. The weakness of high-ionization lines in Seyfert 1.8-1.9s is suggestive of intrinsically weak active galactic nuclei (AGN) continua, and/or stronger star formation activity leading to enhanced [Ne II]. We discuss the implications of these observational results in the context of the Unified Model of AGN.Comment: 36 pages, 4 tables, 6 figures, Accepted for publication in The Astrophysical Journal, December 200

The Seyfert Population in the Local Universe

The magnitude-limited catalog of the Southern Sky Redshift Survey (SSRS2), is used to characterize the properties of galaxies hosting Active Galactic Nuclei. Using emission-line ratios, we identify a total of 162 (3%) Seyfert galaxies out of the parent sample with 5399 galaxies. The sample contains 121 Seyfert 2 galaxies and 41 Seyfert 1. The SSRS2 Seyfert galaxies are predominantly in spirals of types Sb and earlier, or in galaxies with perturbed appearance as the result of strong interactions or mergers. Seyfert galaxies in this sample are twice as common in barred hosts than the non-Seyferts. By assigning galaxies to groups using a percolation algorithm we find that the Seyfert galaxies in the SSRS2 are more likely to be found in binary systems, when compared to galaxies in the SSRS2 parent sample. However, there is no statistically significant difference between the Seyfert and SSRS2 parent sample when systems with more than 2 galaxies are considered. The analysis of the present sample suggests that there is a stronger correlation between the presence of the AGN phenomenon with internal properties of galaxies (morphology, presence of bar, luminosity) than with environmental effects (local galaxy density, group velocity dispersion, nearest neighbor distance).Comment: 35 pages, 13 figures, Accepted to be publised in Astronomical Journa

Spitzer/IRS 5-35 um Low-Resolution Spectroscopy of the 12 um Seyfert Sample

We present low-resolution 5.5-35 um spectra for 103 galaxies from the 12 um Seyfert sample, a complete unbiased 12 um flux limited sample of local Seyfert galaxies selected from the IRAS Faint Source Catalog, obtained with the Infrared Spectrograph (IRS) on-board Spitzer Space Telescope. For 70 of the sources observed in the IRS mapping mode, uniformly extracted nuclear spectra are presented for the first time. We performed an analysis of the continuum emission, the strength of the Polycyclic Aromatic Hydrocarbon (PAH) and astronomical silicate features of the sources. We find that on average, the 15-30 um slope of the continuum is alpha_{15-30}=-0.85+-0.61 for Seyfert 1s and -1.53+-0.84 for Seyfert 2s, and there is substantial scatter in each type. Moreover, nearly 32% of Seyfert 1s, and 9% of Seyfert 2s, display a peak in the mid-infrared spectrum at 20 um, which is attributed to an additional hot dust component. The PAH equivalent width decreases with increasing dust temperature, asindicated by the global infrared color of the host galaxies. However, no statistical difference in PAH equivalent width is detected between the two Seyfert types, 1 and 2, of the same bolometric luminosity. The silicate features at 9.7 and 18um in Seyfert 1 galaxies are rather weak, while Seyfert 2s are more likely to display strong silicate absorption. Those Seyfert 2s with the highest silicate absorption also have high infrared luminosity and high absorption (hydrogen column density N_H>10^23 cm^-2 as measured from the X-rays. Finally, we propose a new method to estimate the AGN contribution to the integrated 12 um galaxy emission, by subtracting the "star formation" component in the Seyfert galaxies, making use of the tight correlation between PAH 11.2 um luminosity and 12 um luminosity for star forming galaxies.Comment: accepted for publication in Ap

X-ray spectral properties of Seyfert galaxies and the unification scheme

Aims: The unification scheme of Seyfert galaxies predicts that the observed differences between type 1 and type 2 Seyfert galaxies are solely due to the differing orientations of the toroidal-shaped obscuring material around AGN. The observed X-ray spectra of Seyfert type 2s compared to type 1s are expected to be affected by higher absorbing column density due to the edge-on view of the obscuring torus. We study the 0.5 - 10 keV X-ray spectral properties of Seyfert type 1s and type 2s with the aim to test the predictions of Seyfert unification scheme in the X-ray regime. Methods: We use an optically selected Seyfert sample in which type 1s and type 2s have matched distributions in the orientation independent parameters of AGN and host galaxy. Results: The 0.5 - 10 keV XMM-Newton pn X-ray spectra of Seyfert galaxies are in general best fitted with a model consists of an absorbed power-law, a narrow Gaussian fitted to the Fe K{\alpha} emission line and an often seen soft excess component characterized by either a thermal plasma model with temperature kT \sim 0.1 - 1.0 keV and/or a steep power-law. The 2.0 - 10 keV hard X-ray continuum emission in several Seyfert type 2s is reflection dominated and suggests the Compton-thick obscuration. Results on the statistical comparison of the distributions of the observed X-ray luminosities in the soft (0.5 - 2.0 keV) and hard (2.0 - 10.0 keV) bands, the X-ray absorbing column densities, the equivalent widths of Fe K{\alpha} line and the flux ratios of hard X-ray to [OIII] {\lambda}5007{\AA} for the two Seyfert subtypes are consistent with the obscuration and orientation based unification scheme.Comment: 15 pages, 3 figures, 6 tables, accepted for publication in A&

Neutral Gas Outflows and Inflows in Infrared-Faint Seyfert Galaxies

Previous studies of the Na I D interstellar absorption line doublet have shown that galactic winds occur in most galaxies with high infrared luminosities. However, in infrared-bright composite systems where a starburst coexists with an active galactic nucleus (AGN), it is unclear whether the starburst, the AGN, or both are driving the outflows. The present paper describes the results from a search for outflows in 35 infrared-faint Seyferts with 10^9.9 < L_IR/L_sun < 10^11, or, equivalently, star formation rates (SFR) of ~0.4 -- 9 solar masses per year, to attempt to isolate the source of the outflow. We find that the outflow detection rates for the infrared-faint Seyfert 1s (6%) and Seyfert 2s (18%) are lower than previously reported for infrared-luminous Seyfert 1s (50%) and Seyfert 2s (45%). The outflow kinematics of infrared-faint and infrared-bright Seyfert 2 galaxies resemble those of starburst galaxies, while the outflow velocities in Seyfert 1 galaxies are significantly larger. Taken together, these results suggest that the AGN does not play a significant role in driving the outflows in most infrared-faint and infrared-bright systems, except the high-velocity outflows seen in Seyfert 1 galaxies. Another striking result of this study is the high rate of detection of inflows in infrared-faint galaxies (39% of Seyfert 1s, 35% of Seyfert 2s), significantly larger than in infrared-luminous Seyferts (15%). This inflow may be contributing to the feeding of the AGN in these galaxies, and potentially provides more than enough material to power the observed nuclear activity over typical AGN lifetimes.Comment: 17 pages, 12 figures, published in ApJ (article updated 12/30/09

The First INTEGRAL AGN Catalog

We present the first INTEGRAL AGN catalog, based on observations performed from launch of the mission in October 2002 until January 2004. The catalog includes 42 AGN, of which 10 are Seyfert 1, 17 are Seyfert 2, and 9 are intermediate Seyfert 1.5. The fraction of blazars is rather small with 5 detected objects, and only one galaxy cluster and no star-burst galaxies have been detected so far. A complete subset consists of 32 AGN with a significance limit of 7 sigma in the INTEGRAL/ISGRI 20-40 keV data. Although the sample is not flux limited, the distribution of sources shows a ratio of obscured to unobscured AGN of 1.5 - 2.0, consistent with luminosity dependent unified models for AGN. Only four Compton-thick AGN are found in the sample. Based on the INTEGRAL data presented here, the Seyfert 2 spectra are slightly harder (Gamma = 1.95 +- 0.01) than Seyfert 1.5 (Gamma = 2.10 +- 0.02) and Seyfert 1 (Gamma = 2.11 +- 0.05).Comment: 17 pages, 12 figures, accepted for publication in Ap