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

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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 = 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 K 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.
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