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

Abstract

We report X-ray observations of the Ðeld 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 Ðrst time reveals a compact hard X-ray nucleus in Mrk 273. Its position is coincident with the northern nucleus identiÐed 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 Ka emission line at 6.4 keV. The neutral hydrogen column density is about 4 ] 1023 cm~2, implying an absorption-corrected X-ray luminosity (0.1È10 keV) for the nucleus of ergs s~1 for km s~1 Mpc~1. The L X B 6.5 ] 1043 H 0 \ 50 X-ray properties therefore Ðrmly establish the northern nucleus of Mrk 273 as a Seyfert 2 AGN. There are also bright soft X-ray clumps and di use soft X-ray emissions surrounding the central hard X-ray nucleus within the 10A of the nuclear region. Its spectrum can be Ðtted by a MEKAL thermal model with a temperature of about 0.8 keV and high metallicity (Z D 1.5 plus emission lines from a ele-Z _) ments and ions. We Ðnd that a soft X-ray clump, about 4A (projected separation of about 4 kpc) southwest of the northern hard X-ray nucleus, is coincident with a nebula with strong [O III] j5007 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 diam-eter of about 108 ] 68 kpc. The total soft X-ray luminosity (0.1È2.4 keV) of the hot halo is L X B 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 D 0.1 We discuss the nature of the AGN Z _). 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 Ðeld. The AGN has an X-ray luminosity of ergs s~1 in the 0.5È10 keV band. Its X-ray properties L X B 2.43 ] 1044 resemble those of Seyfert 1 galaxies, while its optical properties are similar to Seyfert 2 galaxies. Such mixed classiÐcations in the optical and X-ray may be a challenge for the uniÐcation scheme of AGNs.