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 (R_Ein) 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(≤R_Ein) is obtained from the published SLACS Survey results, assuming a two-component 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(≤R_Ein) 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(≤R_Ein) $sime; [10.3, 11.5]), we find consistency between the values of M _Len-dyn(≤R_Ein) and M_phot(≤R _Ein). 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.