Radio and Millimeter Observations of z ~ 2 Luminous QSOs

Abstract

We present Very Large Array observations at 1.4 and 5 GHz of a sample of 16 quasi-stellar objects (QSOs) at z=1.78-2.71. Half of the chosen quasars are bright at millimeter wavelengths (250 or 350 GHz), while the other half were not detected at millimeter wavelengths; the former QSOs were detected at 1.4 GHz, in most cases at high significance (signal-to-noise ratio S/N>=7), but only three of the latter sources were detected at radio frequencies, and only at lower significance (S/N~3). The data are consistent with a correlation between the millimeter and radio fluxes, indicating a physical connection between the mechanisms responsible for the radio and millimeter emission. However, this conclusion is based on data including many upper limits, and deeper data are clearly needed to verify this correlation. All eight millimeter-detected QSOs are detected in the radio continuum, with radio flux densities consistent with the radio-to-far-IR correlation for low-z star-forming galaxies. However, four of these have flatter spectral indices than is typical for star-forming galaxies (i.e., greater than -0.5), suggesting that radiation from the central active galactic nucleus (AGN) dominates the observed radio emission. All the sources detected at 1.4 GHz are spatially unresolved, with size limits typically <1''=6 kpc. High star formation rate galaxies at low redshift are typically nuclear starbursts, with sizes <1 kpc. Hence, the current radio size limits are insufficient to constrain the emission model (AGN or starburst).