The Faint Stellar Halos of Massive Red Galaxies from Stacks of More than 42,000 SDSS LRG Images

We study the properties of massive galaxies at an average redshift of z ~ 0.34 through stacking more than 42,000 images of luminous red galaxies (LRGs) from the Sloan Digital Sky Survey (SDSS). This is the largest data set ever used for such an analysis and it allows us to explore the outskirts of massive red galaxies at unprecedented physical scales. Our image stacks extend farther than 400 kpc, where the r-band profile surface brightness reaches 30 mag arcsec^-2. This analysis confirms that the stellar bodies of LRGs follow a simple Sérsic profile out to 100 kpc. At larger radii, the profiles deviate from the best-fit Sérsic models and exhibit extra light in the r-, i-, and z-band stacks. This excess light can probably be attributed to unresolved intragroup or intracluster light or a change in the light profile itself. We further show that standard analyses of SDSS-depth images typically miss 20% of the total stellar light and underestimate the size of LRGs by 10% compared to our best-fit r-band Sérsic model of n = 5.5 and r_e = 13.1 kpc. If the excess light at r > 100 kpc is considered to be part of the galaxy, the best-fit r-band Sérsic parameters are n = 5.8 and r_e = 13.6 kpc. In addition, we study the radially dependent stack ellipticity and find an increase with radius from epsilon = 0.25 at r = 10 kpc to epsilon = 0.3 at r = 100 kpc. This provides support that the stellar light that we trace out to at least 100 kpc is physically associated with the galaxies themselves and may confirm that the halos of individual LRGs have higher ellipticities than their central parts. Lastly, we show that the broadband color gradients of the stacked images are flat beyond roughly 40 kpc, suggesting that the stellar populations do not vary significantly with radius in the outer parts of massive ellipticals.