Hubble Space Telescope Near Infrared Snapshot Survey of 3CR radio source counterparts III: Radio galaxies and quasars in context

TL;DR

This study compares the near-infrared properties of low-redshift 3CR radio galaxy hosts with other galaxy types, revealing similarities and differences that inform the understanding of active galactic nuclei and their host galaxies.

Contribution

It provides a detailed comparison of 3CR radio galaxy hosts with quasars and other galaxy types, highlighting differences in host galaxy luminosity and mass that impact AGN unification models.

Findings

Most 3CR FR II hosts are massive ellipticals similar to quasar hosts.

Approximately 20% of FR II hosts are under-luminous and often red or in groups.

RLQs require more massive hosts than radio galaxies and RQQs.

Abstract

We compare the near-infrared (NIR) H band photometric and morphological properties of low-redshift (z<0.3) 3CR radio galaxies with samples of BL Lac object and quasar host galaxies, merger remnants, quiescent elliptical galaxies, and brightest cluster galaxies drawn from the literature. In general the 3CR host galaxies are consistent with luminous (~L*) elliptical galaxies. The vast majority of FR II's (~80%) occupy the most massive ellipticals and form a homogeneous population that is comparable to the population of radio-loud quasar (RLQ) host galaxies in the literature. However, a significant minority (~20%) of the 3CR FR II's appears under-luminous with respect to quasar host galaxies. All FR II objects in this faint tail are either unusually red, or appear to be the brightest objects within a group. We discuss the apparent differences between the radio galaxy and RLQ host galaxy populations. RLQs appear to require >1E11 M_sun host galaxies (and ~1E9 M_sun black holes), whereas radio galaxies and RQQs can exist in galaxies down to 3E10 M_sun. This may be due to biases in the measured quasar host galaxy luminosities or populations studied, or due to a genuine difference in host galaxy. If due to a genuine difference, it would support the idea that radio and optical active galactic nucleii are two separate populations with a significant overlap.