The location of an active nucleus and a shadow of a tidal tail in the ULIRG Mrk 273

¹ ICREA and Institut de Ciències del Cosmos, Universitat de Barcelona (IEEC-UB), Martí i Franquès, 1, 08028 Barcelona, Spain
e-mail: kazushi.iwasawa@icc.ub.edu
² IPAC, California Institute of Technology, Pasadena, CA 91125, USA
³ Spitzer Science Center, California Institute of Technology, Pasadena, CA 91125, USA
⁴ Institute for Astronomy, 2680 Woodlawn Drive, Honolulu, HI 96822-1839, USA
⁵ Department of Astronomy, University of Virginia, 530 McCormick Road, Charlottesville, VA 22904 and NRAO, 520 Edgemont Road, Charlottesville, VA 22903-2475, USA
⁶ Max Planck Institut für extraterrestrische Physik, Gießenbachstraße, 85748 Garching, Germany
⁷ Department of Astronomy, University of Maryland, College Park, MD 20742, USA

Received: 5 October 2010
Accepted: 17 January 2011

Abstract

Analysis of data from the Chandra X-ray Observatory for the double nucleus ULIRG Mrk 273 reveals an absorbed hard X-ray source coincident with the southwest nucleus, implying that this unresolved, near-infrared source is where an active nucleus resides, while the northern nuclear region contains a powerful starburst that dominates the far infrared luminosity. There is evidence of a slight image extension in the 6–7 keV band, where an Fe K line is present, towards the northern nucleus. A large-scale, diffuse emission nebula detected in soft X-rays contains a dark lane that spatially coincides with a high surface-brightness tidal tail extending  ~50 arcsec (40 kpc) to the south. The soft X-ray source is likely located behind the tidal tail, which absorbs X-ray photons along the line of sight. The estimated column density of cold gas in the tidal tail responsible for shadowing the soft X-rays is N_H ≥ 6 × 10²¹ cm ^-2, consistent with the tidal tail having an edge-on orientation.