Letter to the Editor

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

¹ European Southern Observatory, Karl-Schwarzschild-Str. 2, 85748 Garching bei München, Germany e-mail: cgrillo@eso.org
² Università degli Studi di Milano, Department of Physics, via Celoria 16, 20133 Milan, Italy

Received: 26 October 2007
Accepted: 16 November 2007

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 (–0.33). We examine here the correlation between the stellar mass values, enclosed inside the Einstein radius 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 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 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 , we find consistency between the values of and . 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.