Discovery of a deep Seyfert-2 galaxy at z = 0.222 behind NGC 300

Abstract

We report on the unveiling of the nature of the unidentified X-ray source 3XMM J005450.3-373849 as a Seyfert-2 galaxy located behind the spiral galaxy NGC 300 using Hubble Space Telescope data, new spectroscopic Gemini observations and available XMM-Newton and Chandra data. We show that the X-ray source is positionally coincident with an extended optical source, composed by a marginally resolved nucleus/bulge, surrounded by an elliptical disc-like feature and two symmetrical outer rings. The optical spectrum is typical of a Seyfert-2 galaxy redshifted to z=0.222 +/- 0.001, which confirms that the source is not physically related to NGC 300. At this redshift the source would be located at 909+/-4 Mpc (comoving distance in the standard model). The X-ray spectra of the source are well-fitted by an absorbed power-law model. By tying $N_{\mathrm{H}}$ between the six available spectra, we found a variable index $\Gamma$ running from ~2 in 2000-2001 years, to 1.4-1.6 in the 2005-2014 period. Alternatively, by tying $\Gamma$, we found variable absorption columns of N_H ~ 0.34 x $10^{-22}$ cm${}^{-2}$ in 2000-2001 years, and 0.54-0.75 x $10^{-22}$ cm${}^{-2}$ in the 2005-2014 period. Although we cannot distinguish between an spectral or absorption origin, from the derived unabsorbed X-ray fluxes, we are able to assure the presence of long-term X-ray variability. Furthermore, the unabsorbed X-ray luminosities of 0.8-2 x 10${}^{43}$ erg s${}^{-1}$ derived in the X-ray band are in agreement with a weakly obscured Seyfert-2 AGN at $z\approx 0.22$.

arXiv commentMNRAS, accepted

References in corpus (16)

• The European Photon Imaging Camera on XMM-Newton: The MOS Cameras
• A Cosmology Calculator for the World Wide Web
• WFPC2 Stellar Photometry with HSTphot
• Toward the Standard Population Synthesis Model of the X-Ray Background: Evolution of X-Ray Luminosity and Absorption Functions of Active Galactic Nuclei Including Compton-Thick Populations
• The ACS Nearby Galaxy Survey Treasury
• X-ray source populations in galaxies
• The XMM-Newton serendipitous survey. VII. The third XMM-Newton serendipitous source catalogue
• The incidence of obscuration in active galactic nuclei
• Morphology of the 12-micron Seyfert Galaxies: I. Hubble Types, Axial Ratios, Bars, and Rings
• GaBoDS: The Garching-Bonn Deep Survey -- I. Anatomy of galaxy clusters in the background of NGC 300
• The Araucaria Project: The effect of blending on the Cepheid distance to NGC 300 from Advanced Camera for Surveys images
• The Araucaria Project. An improved distance to the Sculptor spiral galaxy NGC 300 from its Cepheid variables
• The role of bars in AGN fueling in disk galaxies over the last seven billion years
• X-ray emission from the Sculptor galaxy NGC 300
• X-ray properties of NGC 300 I. Global properties of X-ray point sources and their optical counterparts
• Ultraviolet Signposts of Resonant Dynamics in the Starburst-Ringed Sab Galaxy, M94 (NGC 4736)