ChandraObservations of Markarian 273: Unveiling the Central Active Galactic Nucleus and the Extended Hot Gas Halo

Abstract

We report X-ray observations of the field containing the ultraluminous IRAS galaxy Mrk 273. The data were obtained using the Advanced CCD Imaging Spectrometer S3 instrument on board Chandra. The high-resolution X-ray image for the first time reveals a compact hard X-ray nucleus in Mrk 273. Its position is coincident with the northern nucleus identified in the optical, infrared, and radio and in molecular CO maps. Its X-ray energy distribution is well described by a heavily obscured active galactic nucleus (AGN) spectrum with an absorbed power law plus a narrow Fe Kα emission line at 6.4 keV. The neutral hydrogen column density is about 4 × 1023 cm-2, implying an absorption-corrected X-ray luminosity (0.1-10 keV) for the nucleus of LX ≈ 6.5 × 1043 ergs s-1 for H0 = 50 km s-1 Mpc-1. The X-ray properties therefore firmly establish the northern nucleus of Mrk 273 as a Seyfert 2 AGN. There are also bright soft X-ray clumps and diffuse soft X-ray emissions surrounding the central hard X-ray nucleus within the 10'' of the nuclear region. Its spectrum can be fitted by a MEKAL thermal model with a temperature of about 0.8 keV and high metallicity (Z ~ 1.5 Z☉) plus emission lines from α elements and ions. We find that a soft X-ray clump, about 4'' (projected separation of about 4 kpc) southwest of the northern hard X-ray nucleus, is coincident with a nebula with strong [O III] λ5007 emissions. Further outside the central region, the Chandra observations reveal a very extended hot halo in Mrk 273. The X-ray halo encompasses the entire optical tidal tail and plume, with a projected diameter of about 108 × 68 kpc. The total soft X-ray luminosity (0.1-2.4 keV) of the hot halo is LX ≈ 1.9 × 1041 ergs s-1, in the range of the soft X-ray luminosity of bright elliptical galaxies. The temperature of the hot gas is about 0.62 keV with a low metallicity (Z ~ 0.1 Z☉). We discuss the nature of the AGN in Mrk 273 and the implications of our results on the origin of X-ray halos in elliptical galaxies. We also discuss the properties of Mrk 273x, a background AGN at redshift z = 0.46 in the Mrk 273 field. The AGN has an X-ray luminosity of LX ≈ 2.43 × 1044 ergs s-1 in the 0.5-10 keV band. Its X-ray properties resemble those of Seyfert 1 galaxies, while its optical properties are similar to Seyfert 2 galaxies. Such mixed classifications in the optical and X-ray may be a challenge for the unification scheme of AGNs.