HST Observations of the Serendipitous X-ray Companion to Mrk 273: Cluster at z=0.46?

TL;DR

This study uses HST imaging to analyze Mrk 273X, an X-ray luminous Seyfert 2 galaxy at z=0.46, revealing it as the brightest galaxy in a young, possibly merging cluster with unique spectral and environmental properties.

Contribution

First detailed optical analysis of Mrk 273X, identifying its cluster environment and morphological features, and linking its properties to a hybrid Seyfert 2/radio galaxy class.

Findings

Mrk 273X is a bright elliptical galaxy with a de Vaucouleurs profile.

It resides in a poor, possibly merging galaxy cluster at z=0.46.

The galaxy exhibits blue colors likely influenced by the AGN.

Abstract

We have used 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 field-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 a redshift near 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 magnitudes. Though the surface brightness profile does not appear to be dominated by the 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 Sy 2 / powerful radio galaxy.