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

Abstract

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)≃[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.