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 <i>Hubble Space Telescope</i> data, new spectroscopic Gemini observations and available <i>XMM–Newton</i> and <i>Chandra</i> data. We show that the X-ray source is positionally coincident with an extended optical source, composed of 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_H between the six available spectra, we found a variable index Γ running from ∼2 in 2000–2001 to 1.4–1.6 in the 2005–2014 period. Alternatively, by tying Γ, we found variable absorption columns of N_H ∼ 0.34 × 10⁻²² cm⁻² in 2000–2001, and 0.54−0.75 × 10⁻²² cm⁻² in the 2005–2014 period. Although we cannot distinguish between a 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 × 10⁴³ erg s⁻¹ derived in the X-ray band are in agreement with a weakly obscured Seyfert-2 AGN at z ≈ 0.22.