Star formation and black hole growth at z 4.8

Abstract

We report Herschel/SPIRE, Spitzer and Wide-field Infrared Survey Explorer observations of 44 z 4.8 optically selected active galactic nuclei (AGNs). This flux-limited sample contains the highest mass black holes (BHs) at this redshift. Ten of the objects were detected by Herschel and five show emission that is not clearly associated with the AGNs. The star formation (SF) luminosity (LSF) obtained by fitting the spectral energy distribution (SED) with standard SF templates, taking into account AGN contribution, is in the range 1046.62–1047.21 erg s−1 corresponding to SF rates of 1090–4240 M yr−1. Fitting with very luminous submillimeter galaxy SEDs gives SF rates that are smaller by 0.05 dex when using all bands and 0.1 dex when ignoring the 250 μm band. A 40 K graybody fits to only the 500 μm fluxes reduce LSF by about a factor of two. A stacking analysis of 29 undetected sources gives significant signals in all three bands. A SF template fit indicates LSF = 1046.19–46.23 erg s−1 depending on the assumed AGN contribution. A 40 K fit to the stacked 500 μm flux gives LSF = 1045.95 erg s−1. The mean BH mass (MBH) and AGN luminosity (LAGN) of the detected sources are significantly higher than those of the undetected ones. The spectral differences are seen all the way from UV to far infrared wavelengths. The mean optical–UV spectra are similar to those predicted for thin accretion disks around BHs with similar masses and accretion rates. We suggest two alternative explanations to the correlation of LSF, LAGN and MBH, one involving no AGN feedback and the second involving moderate feedback that affects, but does not totally quench, SF in three-quarters of the sources. We compare our LSF and LAGN to lower redshift samples and show a new correlation between LSF and MBH. We also examine several rather speculative ideas about the host galaxy properties including the possibility that the detected sources are above the SF mass sequence (MS) at z 4.8, perhaps in mergers, and most of the undetected sources are on the MS.