Individual Stellar Halos of Massive Galaxies Measured to 100 kpc at $0.3<z<0.5$ using Hyper Suprime-Cam

TL;DR

This study uses deep wide-field imaging from Hyper Suprime-Cam to analyze the stellar halos of over 3000 massive galaxies at redshifts 0.3 to 0.5, revealing detailed surface mass density profiles and ellipticity gradients that support the two-phase galaxy formation model.

Contribution

First to measure individual stellar halo profiles out to 100 kpc for a large sample of massive galaxies at intermediate redshift using high-quality imaging.

Findings

More massive galaxies have more extended outer profiles.

Outer ellipticity increases with radius and steepens with galaxy mass.

No significant mass dependence in outer color gradients.

Abstract

Massive galaxies display extended light profiles that can reach several hundreds of kilo parsecs. These stellar halos provide a fossil record of galaxy assembly histories. Using data that is both wide (~100 square degree) and deep (i>28.5 mag/arcsec^2 in i-band), we present a systematic study of the stellar halos of a sample of more than 3000 galaxies at 0.3 < z < 0.5 with $\log M_{\star}/M_{\odot} > 11.4$. Our study is based on high-quality (0.6 arcsec seeing) imaging data from the Hyper Suprime-Cam (HSC) Subaru Strategic Program (SSP), which enables us to individually estimate surface mass density profiles to 100 kpc without stacking. As in previous work, we find that more massive galaxies exhibit more extended outer profiles. When this extended light is not properly accounted for as a result of shallow imaging or inadequate profile modeling, the derived stellar mass function can be significantly underestimated at the highest masses. Across our sample, the ellipticity of outer light profiles increases substantially as we probe larger radii. We show for the first time that these ellipticity gradients steepen dramatically as a function of galaxy mass, but we detect no mass-dependence in outer color gradients. Our results support the two-phase formation scenario for massive galaxies in which outer envelopes are built up at late times from a series of merging events. We provide surface mass surface mass density profiles in a convenient tabulated format to facilitate comparisons with predictions from numerical simulations of galaxy formation.