Limits to Rest-Frame Ultraviolet Emission From Far-Infrared-Luminous z~6 Quasar Hosts

Abstract

We report on a Hubble Space Telescope search for rest-frame ultraviolet emission from the host galaxies of five far-infrared-luminous $z\simeq 6$ quasars and the $z=5.85$ hot-dust free quasar SDSS J0005-0006. We perform 2D surface brightness modeling for each quasar using a Markov-Chain Monte-Carlo estimator, to simultaneously fit and subtract the quasar point source in order to constrain the underlying host galaxy emission. We measure upper limits for the quasar host galaxies of $m_J>22.7$ mag and $m_H>22.4$ mag, corresponding to stellar masses of $M_{\ast }<2\times 10^{11}{M}_{\odot }$. These stellar mass limits are consistent with the local $M_{\text{BH}}$-$M_{\ast }$ relation. Our flux limits are consistent with those predicted for the UV stellar populations of $z\simeq 6$ host galaxies, but likely in the presence of significant dust ($⟨A_{\mathrm{UV}}⟩\simeq 2.6$ mag). We also detect a total of up to 9 potential $z\simeq 6$ quasar companion galaxies surrounding five of the six quasars, separated from the quasars by 1.4''-3.2'', or 8.4-19.4 kpc, which may be interacting with the quasar hosts. These nearby companion galaxies have UV absolute magnitudes of -22.1 to -19.9 mag, and UV spectral slopes ${\hat{I}}^2$ of -2.0 to -0.2, consistent with luminous star-forming galaxies at $z\simeq 6$. These results suggest that the quasars are in dense environments typical of luminous $z\simeq 6$ galaxies. However, we cannot rule out the possibility that some of these companions are foreground interlopers. Infrared observations with the James Webb Space Telescope will be needed to detect the $z\simeq 6$ quasar host galaxies and better constrain their stellar mass and dust content.

22 pages, 13 figures. Accepted for publication in ApJ