The Origin of Line Emission in Massive z ~ 2.3 Galaxies: Evidence for Cosmic Downsizing of AGN Host Galaxies

Using the Gemini Near-Infrared Spectrograph (GNIRS), we have assembled a complete sample of 20 K-selected galaxies at 2.0<z<2.7 with high-quality near-infrared spectra. As described in a previous paper, 9 of these 20 galaxies have strongly suppressed star formation and no detected emission lines. The present paper concerns the 11 galaxies with detected Hα emission and studies the origin of the line emission using the GNIRS spectra and follow-up observations with SINFONI on the VLT. Based on their [N II]/Hα ratios, the spatial extent of the line emission, and several other diagnostics, we infer that 4 of the 11 emission-line galaxies host narrow-line active galactic nuclei (AGNs). The AGN host galaxies have stellar populations ranging from evolved to star-forming. Combining our sample with a UV-selected galaxy sample at the same redshift that spans a broader range in stellar mass, we find that black hole accretion is more effective at the high-mass end of the galaxy distribution (~2.9×10¹¹ M_solar) at z~2.3. Furthermore, by comparing our results with SDSS data, we show that the AGN activity in massive galaxies has decreased significantly between z~2.3 and 0. AGNs with similar normalized accretion rates as those detected in our K-selected galaxies reside in less massive galaxies (~4.0×10¹⁰ M_solar) at low redshift. This is direct evidence for downsizing of AGN host galaxies. Finally, we speculate that the typical stellar mass scale of the actively accreting AGN host galaxies, both at low and at high redshift, might be similar to the mass scale at which star-forming galaxies seem to transform into red, passive systems.

Based on observations collected at the European Southern Observatory, Paranal, Chile (076.A-0464 and 076.A-0718).

Based on observations obtained at the Gemini Observatory, which is operated by the Association of Universities for Research in Astronomy (AURA), Inc., under a cooperative agreement with the National Science Foundation (NSF) on behalf of the Gemini partnership.