The faint stellar halos of massive red galaxies from stacks of more than
  42000 SDSS LRG images
  We study the properties of massive galaxies at an average redshift of z~0.34
through stacking more than 42000 images of Luminous Red Galaxies from the Sloan
Digital Sky Survey. This is the largest dataset 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 luminous red galaxies follow a
simple Sersic profile out to 100 kpc. At larger radii the profiles deviate from
the best-fit Sersic models and exhibit extra light in the g, 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 Sersic 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 Sersic
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 e=0.25 at
r=10 kpc to e=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.
