“Hidden” Seyfert 2 Galaxies and the X-Ray Background

By E. Moran, A. Filippenko, Ryan Chornock. 2002

149 citations recorded by the source corpus.

Obscured active galactic nuclei, which are classified optically as type 2 (narrow line) Seyfert galaxies in the local universe, are by far the most promising candidates for the origin of the hard (2-10 keV) X-ray background radiation. However, optical follow-up observations of faint X-ray sources in deep Chandra images have revealed surprising numbers of apparently normal galaxies at modest redshift. Such objects represent ~40%-60% of the sources classified in deep Chandra surveys, raising the possibility that the X-ray galaxy population has evolved with cosmic time. Alternatively, most of the faint X-ray galaxies in question are so distant that their angular diameters are comparable to the slit widths used in ground-based spectroscopic observations; thus, their nuclear spectral features may be overwhelmed ("hidden") by host galaxy light. To test this hypothesis, we have obtained integrated spectra of a sample of nearby, well-studied Seyfert 2 galaxies. The data, which accurately simulate observations of distant Chandra sources, demonstrate convincingly that the defining spectral signatures of Seyfert 2s can be hidden by light from their host galaxies. In fact, 60% of the observed objects would not be classified as Seyfert 2s on the basis of their integrated spectra, similar to the fraction of faint X-ray sources identified with "normal" galaxies. Thus, the numbers of narrow-line active galaxies in deep Chandra surveys (and perhaps all ground-based spectroscopic surveys of distant galaxies) are likely to have been underestimated.

Subfields: Astronomy and Astrophysics, Nuclear and High Energy Physics

View Paper: https://www.semanticscholar.org/paper/5c3362624f56bc6a032beb1e7eb8b7694b53fdde

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In Astronomy and Astrophysics

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Citations in the corpus

• X-RAY SPECTRAL PROPERTIES OF THE BAT AGN SAMPLE (2008, background): “Another possibility is that, at lower luminosities, the host galaxy light is so much brighter than the emission from the broad line region that it is completely masked (Moran et al. 2002).”
• The Hard X-Ray Spectrum as a Probe for Black Hole Growth in Radio-Quiet Active Galactic Nuclei (2008, background): “…more rewarding in cases where a source is either optically faint and/or the broad-emission lines are too weak to measure if, for example, these lines are overwhelmed by host-galaxy continuum (e.g., Moran et al. 2002; provided the source is not radio loud and that the redshift can be determined).”
• BAT AGN Spectroscopic Survey. I. Spectral Measurements, Derived Quantities, and AGN Demographics (2017, background): “An additional complication is that in some AGN, bursts of star formation can overwhelm the AGN photoionization signature (e.g., Moran et al. 2002; Trump et al. 2015).”
• DEEP EXTRAGALACTIC X-RAY SURVEYS (2005, background): “Many and perhaps most may just be normal Seyfert galaxies where dilution by host-galaxy light hinders optical detection of the AGN (e.g., Moran et al. 2002; Severgnini et al. 2003), but some have high-quality follow-up observations and appear to be truly remarkable objects (e.g., Comastri et al.…”
• Luminosity-dependent evolution of soft X-ray selected AGN : New Chandra and XMM-Newton surveys (2005, background): “…as Szokoly et al show, at low X–ray luminosities and intermediate redshifts the optical AGN classification often breaks down because of the dilution of the AGN excess light by the stars in the host galaxy (see e.g. Moran et al. (2002), so that only an X–ray classification scheme can be utilized.”
• The Cosmic Evolution of Hard X-Ray-selected Active Galactic Nuclei (2004, background): “…region of the CDF-N shown for three X-ray luminosity intervals (triangles, LX > 10 44 ergs s 1; diamonds, LX ¼ 1043 1044 ergs s 1; squares, LX ¼ 1042 1043 ergs s 1). narrow AGNs were being swamped by the host galaxy light, thereby washing out the broad-line signal (Moran et al. 2002; Moran 2004).”
• The Relationship of Hard X-Ray and Optical Line Emission in Low-Redshift Active Galactic Nuclei (2005, background): “This makes it difficult to recognize the presence of an AGN (Moran et al. 2002).”
• Spectral Energy Distributions of Hard X-Ray Selected Active Galactic Nuclei in the XMM-Newton Medium Deep Survey (2007, unknown): “…can be applied to sources that are too faint at optical wavelengths for optical spectroscopic observations, or with emission lines that are diluted by the host-galaxy light or affected by obscuration (Moran et al. 2002; Barger et al. 2001; Rigby et al. 2006), and to sources in the redshift desert.”
• The Chandra Deep Field-South: Optical Spectroscopy. I. (2003, background): “The study of local Seyfert galaxies confirms that about 60% of the local Seyfert type 2 galaxies would not be classified as Seyfert 2, if only the total emission were available (Moran et al. 2002).”
• IDENTIFYING LUMINOUS ACTIVE GALACTIC NUCLEI IN DEEP SURVEYS: REVISED IRAC SELECTION CRITERIA (2012, background): “…those with narrow optical emission lines, those with no emission lines yet X-ray luminosities exceeding 2× 1042 ergs s−1 (e.g., optically dull AGN, Moran et al. 2002; Comastri et al. 2002), and those lacking optical spectra yet best-fit by a galaxy-dominated template are classified as “optical…”
• MID-INFRARED SELECTION OF ACTIVE GALACTIC NUCLEI WITH THE WIDE-FIELD INFRARED SURVEY EXPLORER. II. PROPERTIES OF WISE-SELECTED ACTIVE GALACTIC NUCLEI IN THE NDWFS BOÖTES FIELD (2012, background): “Several explanations have been offered to explain such sources, ranging from systematic effects that dilute the AGN signature for the wide slit widths typically used for these distant sources (e.g., Moran et al. 2002), to radiatively inefficient accretion flows (e.g., Trump et al. 2011).”
• Active galactic nuclei flicker: an observational estimate of the duration of black hole growth phases of ∼105 yr (2015, background): “Such AGN host galaxies have been reported repeatedly in the literature in a variety of surveys (Comastri et al. 2002; Moran et al. 2002; Maiolino et al. 2003; Caccianiga et al. 2007; Georgantopoulos & Georgakakis 2005; Rigby et al. 2006; Cocchia et al. 2007; Smith et al. 2014).”
• THE XMM-NEWTON WIDE-FIELD SURVEY IN THE COSMOS FIELD (XMM-COSMOS): DEMOGRAPHY AND MULTIWAVELENGTH PROPERTIES OF OBSCURED AND UNOBSCURED LUMINOUS ACTIVE GALACTIC NUCLEI (2010, background): “…motivated by the fact that both obscured and unobscured AGN can be misclassified in spectroscopic studies, given that the host galaxy light may over-shine the nuclear emission (as in the cases of the XBONG; Moran et al. 2002, Comastri et al. 2002, Severgnini et al. 2003, Civano et al. 2007).”
• The Fall of Active Galactic Nuclei and the Rise of Star-forming Galaxies: A Close Look at the Chandra Deep Field X-Ray Number Counts (2004, background): “…caution that spectral misclassifications are likely for some sources, since broad lines may be apparent in certain parts of the optical spectrum but not others or may be missed because of host galaxy contamination or poor signal-tonoise ratio data (e.g., see discussion in Moran et al. 2002).”
• Cosmic X-ray surveys of distant active galaxies (2015, background): “For weak or highly obscured AGNs, such dilution by host starlight can make AGNs difficult to separate from galaxies in the optical/UV regime (e.g., Moran et al 2002; Hopkins et al 2009).”