Near-Infrared Imaging of a z=6.42 Quasar Host Galaxy With the Hubble Space Telescope Wide Field Camera 3

TL;DR

This study used Hubble's WFC3 to set upper limits on the near-ultraviolet emission from a z=6.42 quasar host galaxy, revealing it is more similar to local luminous infrared galaxies than typical star-forming galaxies at that epoch.

Contribution

First deep near-infrared imaging of a z=6.42 quasar host galaxy with Hubble WFC3, establishing upper limits on its UV emission and infrared excess.

Findings

Set upper limits on host galaxy brightness (m_J>22.8, m_H>23.0 AB mag).

Derived surface brightness and integrated magnitude limits consistent with infrared excess.

Host galaxy's properties resemble local luminous infrared galaxies more than typical high-redshift star-forming galaxies.

Abstract

We report on deep near-infrared F125W (J) and F160W (H) Hubble Space Telescope Wide Field Camera 3 images of the z=6.42 quasar J1148+5251 to attempt to detect rest-frame near-ultraviolet emission from the host galaxy. These observations included contemporaneous observations of a nearby star of similar near-infrared colors to measure temporal variations in the telescope and instrument point spread function (PSF). We subtract the quasar point source using both this direct PSF and a model PSF. Using direct subtraction, we measure an upper limit for the quasar host galaxy of m_J>22.8, m_H>23.0 AB mag (2 sigma). After subtracting our best model PSF, we measure a limiting surface brightness from 0.3"-0.5" radius of mu_J > 23.5, mu_H > 23.7 AB magarc (2 sigma). We test the ability of the model subtraction method to recover the host galaxy flux by simulating host galaxies with varying integrated magnitude, effective radius, and S\'ersic index, and conducting the same analysis. These models indicate that the surface brightness limit (mu_J > 23.5 AB magarc) corresponds to an integrated upper limit of m_J > 22 - 23 AB mag, consistent with the direct subtraction method. Combined with existing far-infrared observations, this gives an infrared excess log(IRX) > 1.0 and corresponding ultraviolet spectral slope beta > -1.2\pm0.2. These values match those of most local luminous infrared galaxies, but are redder than those of almost all local star-forming galaxies and z~6 Lyman break galaxies.