Uncovering Obscured Active Galactic Nuclei in Homogeneously Selected Samples of Seyfert 2 Galaxies

The Astrophysical Journal (Impact Factor: 5.99). 01/2011; 729(1). DOI: 10.1088/0004-637X/729/1/52
Source: arXiv


We have analyzed archival \textit{Chandra} and \textit{XMM-Newton} data for
two nearly complete homogeneously selected samples of type 2 Seyfert galaxies
(Sy2s). These samples were selected based on intrinsic Active Galactic Nuclei
(AGN) flux proxies: a mid-infrared (MIR) sample from the original IRAS 12$\mu$m
survey and an optical ([OIII]$\lambda$ 5007 \AA\ flux limited) sample from the
Sloan Digital Sky Survey (SDSS), providing a total of 45 Sy2s. As the MIR and
[OIII] fluxes are largely unaffected by AGN obscuration, these samples can
present an unbiased estimate of the Compton-thick (column density N$_H >
10^{24}$ cm$^{-2}$) subpopulation. We find that the majority of this combined
sample is likely heavily obscured, as evidenced by the 2-10 keV X-ray
attenuation (normalized by intrinsic flux diagnostics) and the large Fe
K$\alpha$ equivalent widths (several hundred eV to over 1 keV). A wide range of
these obscuration diagnostics is present, showing a continuum of column
densities, rather than a clear segregation into Compton-thick and Compton-thin
sub-populations. We find that in several instances, the fitted column densities
severely under-represent the attenuation implied by these obscuration
diagnostics, indicating that simple X-ray models may not always recover the
intrinsic absorption. We compared AGN and host galaxy properties, such as
intrinsic luminosity, central black hole mass, accretion rate, and star
formation rate with obscuration diagnostics. No convincing evidence exists to
link obscured sources with unique host galaxy populations from their less
absorbed counterparts. Finally, we estimate that a majority of these Seyfert 2s
will be detectable in the 10-40 keV range by the future NuSTAR mission, which
would confirm whether these heavily absorbed sources are indeed Compton-thick.

Download full-text


Available from: A. Ptak
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: We have used XMM-Newton to observe six Lyman break analogs (LBAs): members of the rare population of local galaxies that have properties that are very similar to distant Lyman break galaxies. Our six targets were specifically selected because they have optical emission-line properties that are intermediate between starbursts and Type 2 (obscured) active galactic nuclei (AGNs). Our new X-ray data provide an important diagnostic of the presence of an AGN. We find X-ray luminosities of order 1042 erg s–1 and ratios of X-ray to far-IR luminosities that are higher than values in pure starburst galaxies by factors ranging from ~3 to 30. This strongly suggests the presence of an AGN in at least some of the galaxies. The ratios of the luminosities of the hard (2-10 keV) X-ray to [O III]λ5007 emission line are low by about an order of magnitude compared with Type 1 AGN, but are consistent with the broad range seen in Type 2 AGN. Either the AGN hard X-rays are significantly obscured or the [O III] emission is dominated by the starburst. We searched for an iron emission line at ~6.4 keV, which is a key feature of obscured AGNs, but only detected emission at the ~2σ level. Finally, we find that the ratios of the mid-infrared (24 μm) continuum to [O III]λ5007 luminosities in these LBAs are higher than the values for Type 2 AGN by an average of 0.8 dex. Combining all these clues, we conclude that an AGN is likely to be present, but that the bolometric luminosity is produced primarily by an intense starburst. If these black holes are radiating at the Eddington limit, their masses would lie in the range of 105-106M ☉. These objects may offer ideal local laboratories to investigate the processes by which black holes grew in the early universe.
    Full-text · Article · Feb 2011 · The Astrophysical Journal
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: We report on our analysis of XMM-Newton observations of the Seyfert 2 galaxy ESO 138-G1 (z = 0.0091). These data reveal a complex spectrum in both its soft and hard portions. The 0.5-2 keV band is characterized by a strong 'soft-excess' component with several emission lines, as commonly observed in other narrow-line AGN. Above 3 keV, a power-law fit yields a very flat slope (Gamma ~0.35), along with the presence of a prominent line-like emission feature around ~6.4 keV. This indicates heavy obscuration along the line of sight to the nucleus. We find an excellent fit to the 3-10 keV continuum with a pure reflection model, which provides strong evidence of a Compton-thick screen, preventing direct detection of the intrinsic nuclear X-ray emission. Although a model consisting of a power law transmitted through an absorber with Nh ~2.5 x 10^{23} cm^-2 also provides a reasonable fit to the hard X-ray data, the equivalent width value of ~800 eV measured for the Fe Kalpha emission line is inconsistent with a primary continuum obscured by a Compton-thin column density. Furthermore, the ratio of 2-10 keV to de-reddened [OIII] fluxes for ESO 138-G1 agrees with the typical values reported for well-studied Compton-thick Seyfert galaxies. Finally, we also note that the upper limits to the 15-150 keV flux provided by Swift/BAT and INTEGRAL/IBIS seem to rule out the presence of a transmitted component of the nuclear continuum even in this very hard X-ray band, hence imply that the column density of the absorber could be as high as 10^{25} cm^-2. This makes ESO 138-G1 a very interesting, heavy Compton-thick AGN candidate for the next X-ray missions with spectroscopic and imaging capabilities above 10 keV.
    Full-text · Article · Aug 2011 · Astronomy and Astrophysics
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: We present Hubble Space Telescope Wide Field Camera 3 (WFC3) slitless grism spectroscopy of 28 emission-line galaxies at z ~ 2, in the GOODS-S region of the Cosmic Assembly Near-infrared Deep Extragalactic Legacy Survey. The high sensitivity of these grism observations, with >1σ detections of emission lines to f > 2.5 × 10–18 erg s–1 cm–2, means that the galaxies in the sample are typically ~7 times less massive (median M * = 109.5M ☉) than previously studied z ~ 2 emission-line galaxies. Despite their lower mass, the galaxies have [O III]/Hβ ratios which are very similar to previously studied z ~ 2 galaxies and much higher than the typical emission-line ratios of local galaxies. The WFC3 grism allows for unique studies of spatial gradients in emission lines, and we stack the two-dimensional spectra of the galaxies for this purpose. In the stacked data the [O III] emission line is more spatially concentrated than the Hβ emission line with 98.1% confidence. We additionally stack the X-ray data (all sources are individually undetected), and find that the average L [O III]/L 0.5-10 keV ratio is intermediate between typical z ~ 0 obscured active galaxies and star-forming galaxies. Together the compactness of the stacked [O III] spatial profile and the stacked X-ray data suggest that at least some of these low-mass, low-metallicity galaxies harbor weak active galactic nuclei.
    Full-text · Article · Dec 2011 · The Astrophysical Journal
Show more