Hubble Space Telescope observations of the serendipitous X-ray companion to Markarian 273:: Cluster at z=0.46?

We have used Hubble Space Telescope (HST) I-band images to identify Mrk 273X, the very unusual high-redshift X-ray-luminous Seyfert 2 galaxy found by ROSAT in the same held of view as Mrk 273. We have measured the photometric properties of Mrk 273X and have also analyzed the luminosity distribution of the faint galaxy population seen in the HST image. The luminosity of the galaxy and the properties of the surrounding environment suggest that Mrk 273X is the brightest galaxy in a relatively poor cluster at z approximate to 0.46. Its off-center location in the cluster and the presence of other galaxy groupings in the HST image may indicate that this is a dynamically young cluster on the verge of merging with its neighboring clusters. We find that Mrk 273X is a bright, featureless elliptical galaxy, with no evidence for a disk. It follows the de Vaucouleurs (r(1/4)) surface brightness law very well over a range of 8 mag. Although the surface brightness profile does not appear to be dominated by the active galactic nucleus (AGN), the galaxy has very blue colors that do appear to be produced by the AGN. Mrk 273X is most similar to the IC 5063 class of active galaxies-a hybrid Seyfert 2/powerful radio galaxy.