Simon C.-C. Ho
Host galaxies of high redshift quasars (QSOs) are of interest. They provide us with a valuable opportunity to investigate the physics relevant to the starburst-active galactic nuclei (AGN), the energetic compact region at the center of a galaxy, connection at the earliest epoch of the Universe, with the most luminous black holes. Here we report an optical detection of an extended structure around a stacked QSO image. We have stacked 46 high-z (z_median = 6.13) QSOs images by data from Subaru telescope/Hyper Suprime-Cam. These QSOs are mainly identified by the Subaru High-z Exploration of Low-Luminosity Quasars (SHELLQs) survey. We have carefully subtracted a point spread function (PSF) constructed using nearby stars from the images. PSF appears on an optical image in a very similar way like QSO. After the PSF (QSO) subtraction from the stacked QSO image, a structure in the z-band extends for more than 4" on the sky (physical size = 11kpc). As the central part (QSO) is subtracted with a PSF. The remaining extended structure may be a forming giant galaxy, co-evolving with a supermassive black hole or emission at the earliest epoch of the Universe. More analysis will be carried out in the future to investigate the properties of this extended structure.
From 2016, the Subaru High-z Exploration of Low-Luminosity Quasars (SHELLQs) team has spectroscopically identified 93 QSOs at 5.7<z<7.0 from the Hyper Suprime-Cam (HSC) Subaru Strategic Program (SSP) survey (Matsuoka et al., 2016, 2017, 2018a,b, 2019). We have visually inspected each QSO and removed contaminated images. After applying the above selection criteria, we were left with a sample of 46 clean QSOs (zmedian= 6.13). This sample was contamination-free and was thus an ideal sample to study the QSO host properties. QSO host galaxies at redshift z~6 are very faint due to the absorption by the interstellar medium between us. By stacking the QSO images, we can increase the signal-to-noise ratio of the image. Thus, extending the limiting magnitude of the observing instruments. In the stacked QSO image, we use GALFIT (Peng et al., 2002) to subtract (1)Point Spread Function (PSF)+sky, (2)PSF+Sérsic+sky components from the stacked QSO image. PSF is a mathematical function that is very similar to QSO in the image and the Sérsic profile is a profile that most galaxy profiles can be fitted with.
After the subtraction of (1)PSF+sky, we detected for the first time, possible redshift 6 QSO host galaxies and its Lyman α emission! The QSO+host has an excess over the PSF in radii at 4-15 pixels in the radial profile. This implies there is an extended structure (host galaxies + Lyman α emission). The size of the extended structure is ∼8.75 pixels (1.49" or 9.3 kpc) in half-light radius. In the future, PSF subtraction in Y-band needs to be performed to check whether the extended structure includes Lyman α emission or not. Also, we will do SED fitting to the extended structure so we can calculate the stellar mass and do modeling (check the composition of the extended structure).