Do some AGN lack X-ray emission?⋆
1 Instituto de Astrofísica and Centro de Astroingeniería, Facultad de Física, Pontificia Universidad Católica de Chile, Casilla 306, Santiago 22, Chile
2 Millennium Institute of Astrophysics (MAS), Nuncio Monseñor Sótero Sanz 100, Providencia, Santiago, Chile
3 Space Science Institute, 4750 Walnut Street, Suite 205, Boulder, Colorado 80301, USA
4 Astronomy Department, University of Virginia, PO Box 400325, Charlottesville, VA 22904-4325, USA
5 Main Astronomical Observatory, Ukrainian National Academy of Sciences, 27 Zabolotnoho Street, Kyiv 03680, Ukraine
6 Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA 91109, USA
7 Cahill Center for Astronomy and Astrophysics, California Institute of Technology, Pasadena, CA 91125, USA
Received: 13 July 2016
Accepted: 23 September 2016
Context. Intermediate-mass black holes (IMBHs) are thought to be the seeds of early supermassive black holes (SMBHs). While ≳100 IMBH and small SMBH candidates have been identified in recent years, few have been robustly confirmed to date, leaving their number density in considerable doubt. Placing firmer constraints both on the methods used to identify and confirm IMBHs/SMBHs, as well as characterizing the range of host environments that IMBHs/SMBHs likely inhabit is therefore of considerable interest and importance. Additionally, finding significant numbers of IMBHs in metal-poor systems would be particularly intriguing, since such systems may represent local analogs of primordial galaxies, and therefore could provide clues of early accretion processes.
Aims. Here we study in detail several candidate active galactic nuclei (AGN) found in metal-poor hosts.
Methods. We utilize new X-ray and optical observations to characterize these metal-poor AGN candidates and compare them against known AGN luminosity relations and well-characterized IMBH/SMBH samples.
Results. Despite having clear broad optical emission lines that are long-lived (≳10−13 yr), these candidate AGN appear to lack associated strong X-ray and hard UV emission, lying at least 1−2 dex off the known AGN correlations. If they are IMBHs/SMBHs, our constraints imply that they either are not actively accreting, their accretion disks are fully obscured along our line-of-sight, or their accretion disks are not producing characteristic high energy emission. Alternatively, if they are not AGN, then their luminous broad emission lines imply production by extreme stellar processes. The latter would have profound implications on the applicability of broad lines for mass estimates of massive black holes.
Key words: galaxies: active / galaxies: Seyfert / X-rays: galaxies / galaxies: dwarf
The reduced spectra (FITS files) are only available at the CDS via anonymous ftp to cdsarc.u-strasbg.fr (220.127.116.11) or via http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/596/A64
© ESO, 2016