Stellar mass estimates in early-type galaxies from lensing+dynamical and photometric measurements

Aims. Our purpose is to compare two different diagnostics for estimating stellar masses in early-type galaxies and to establish their
level of reliability. In particular, we consider the well-studied sample of 15 field elliptical galaxies selected from the Sloan Lens ACS
(SLACS) Survey (z = 0.06–0.33). We examine here the correlation between the stellar mass values, enclosed inside the Einstein
radius (REin) of each lens, based on analyses of lensing and stellar dynamics combined and based on multiwavelength photometry
spectral template fitting.
Methods. The lensing+dynamics stellar mass M∗len+dyn(≤REin) is obtained from the published SLACS Survey results, assuming a twocomponent
density distribution model and a prior from the fundamental plane on the mass-to-light ratio for the lens galaxies. The
photometric stellar mass M∗phot(≤REin) is measured by fitting the observed spectral energy distribution of the galaxies (from the SDSS
multi-band photometry over 354–913 nm) with composite stellar population templates, under the assumption that light traces stellar
mass.
Results. The two methods are completely independent. They rely on several different assumptions, and so, in principle, both can have
significant biases. Based on our sample of massive galaxies (log M∗phot(≤REin) [10.3, 11.5]), we find consistency between the values
of M∗len+dyn(≤REin) and M∗phot(≤REin).We obtain a Pearson linear correlation coefficient of 0.94 and a median value of the ratio between
the former and the latter mass measurements of 1.1 ± 0.1. This suggests that both methods can separately yield reliable stellar masses
of early-type galaxies, and confirms that photometric mass estimates are accurate, as long as optical/near-IR rest frame photometry is
available.