Optical and Near-Infrared Imaging of Ultra–Steep-Spectrum Radio Sources: The [ITAL]K[/ITAL]-[CLC][ITAL]z[/ITAL][/CLC] Diagram of Radio-selected and Optically Selected Galaxies

Abstract

We present optical and/or near-IR images of 128 ultra-steep-spectrum radio sources. Roughly half of the objects are identified in the optical images (R ≲ 24), while in the near-IR images, more than 94% are detected at K ≲ 22. The mean K magnitude is K = 19.26 within a 2″ diameter aperture. The distribution of R-K colors indicates that at least one-third of the objects observed have very red colors (R-K > 5). The major axes of the identifications in the AT band are preferentially oriented along the radio axes, with half of them having compact morphologies. The 22 sources with spectroscopic redshifts and K-band magnitudes follow the K-z relation found from previous radio samples, but with a larger scatter. We argue that this may be due to a dependence of K magnitude on the radio power, with the most luminous radio sources inhabiting the most massive host galaxies. We present a composite K-z diagram of radio-loud and radio-quiet galaxies, selected from the Hubble Deep Field (North) and the Hawaii surveys. Out to z ≲ 1, the radio-loud galaxies trace the bright envelope of the radio quiet galaxies, while at z ≳ 1, the radio-loud galaxies are ≳2 mag brighter. We argue that this is not due to a contribution from the active galactic nucleus or emission lines. This difference strongly suggests that radio galaxies pinpoint the most massive systems out to the highest known redshifts, probably as a result of the mutual correlation of the mass of the galaxy and the radio power on the mass of the central black hole.