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

COMBI, J. A.; GARCÍA, F.; RODRIGUEZ, M. J.; GAMEN, R.; CELLONE, S. A.

MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY

WILEY-BLACKWELL PUBLISHING, INC

Lugar: Londres; Año: 2016 vol. 460 p. 1640 - 1644

0035-8711

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 NH 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 NH ∼ 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 - 2x10^{43} erg s^{-1} derived in the X-ray band are in agreement with a weakly obscured Seyfert-2 AGN at z = 0.22.