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Register a new userStellar mass estimates in early-type galaxies from lensing+dynamical and photometric measurements
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In this paper we compare two different diagnostics for estimating stellar masses in early-type galaxies and we 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 of each lens, based on analyses of lensing and stellar dynamics combined and based on multiwavelength photometry spectral template fitting. 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. 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 lensing+dynamics and the photometric mass estimates. 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.
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The purpose of this study is to explore the relationship between galaxy stellar masses, based on multiwavelength photometry spectral template fitting and dynamical masses based on published velocity dispersion measurements, for a sample of 48 early-type galaxies at z ~ 1 with HST/ACS morphological information. We determine photometric-stellar masses and perform a quantitative morphological analysis of cluster and field galaxies at redshift 0.6 < z < 1.2, using ground- and space-based multiwavelegth data available on the GOODS-S field and on the field around the X-ray luminous cluster RDCS1252.9-2927 at z = 1.24. We use multi-band photometry over 0.4-8um from HST/ACS, VLT/ISAAC and Spitzer/IRAC to estimate photometric-stellar masses using Composite Stellar Population (CSP) templates computed with PEGASE.2 models. We compare stellar masses with those obtained using CSPs built with Bruzual & Charlot and Maraston models. We then compare photometric-stellar mass and dynamical mass estimates as a function of morphological parameters obtained from HST/ACS imaging. Based on our sample, which spans the mass range log(Mphot)=[10, 11.5], we find that 1) PEGASE.2, BC03, M05 yield consistent photometric-stellar masses for early-type galaxies at z ~ 1 with a small scatter (0.15 dex rms); 2) adopting a Kroupa IMF, photometric-stellar masses match dynamical mass estimates for early-type galaxies with an average offset of 0.27 dex; 3) assuming a costant IMF, increasing dark matter fraction with the increasing galaxy mass can explain the observed trend.
The purpose of this letter is to compare the quality of different methods for estimating stellar masses of galaxies. We compare the results of (a) fitting stellar population synthesis models to broad band colors from SDSS and 2MASS, (b) the analysis of spectroscopic features of SDSS galaxies (Kauffmann et al. 2003), and, (c) a simple dynamical mass estimate based on SDSS velocity dispersions and effective radii. Knowing that all three methods can have significant biases, a comparison can help to establish their (relative) reliability. In this way, one can also probe the quality of the observationally cheap broadband color mass estimators for galaxies at higher redshift. Generally, masses based on broad-band colors and spectroscopic features agree reasonably well, with a rms scatter of only ~ 0.25 dex over almost 4 decades in mass. However, as may be expected, systematic differences do exist and have an amplitude of ~ 0.15 dex, corrleting with Halpha emission strength. Interestingly, masses from broad-band color fitting are in better agreement with dynamical masses than masses based on the analysis of spectroscopic features. In addition, the differences between the latter and the dynamical masses correlate with Halpha equivalent width, while this much less the case for the broad-band masses. We conclude that broad band color mass estimators, provided they are based on a large enough wavelength coverage and use an appropriate range of ages, metallicities and dust extinctions, can yield fairly reliable stellar masses for galaxies. This is a very encouraging result as such mass estimates are very likely the only ones available at significant redshifts for some time to come.
We analyze the correlations between central dark matter (DM) content of early-type galaxies and their sizes and ages, using a sample of intermediate-redshift (z ~ 0.2) gravitational lenses from the SLACS survey, and by comparing them to a larger sample of z ~ 0 galaxies. We decompose the deprojected galaxy masses into DM and stellar components using combinations of strong lensing, stellar dynamics, and stellar populations modeling. For a given stellar mass, we find that for galaxies with larger sizes, the DM fraction increases and the mean DM density decreases, consistently with the cuspy halos expected in cosmological formation scenarios. The DM fraction also decreases with stellar age, which can be partially explained by the inverse correlation between size and age. The residual trend may point to systematic dependencies on formation epoch of halo contraction or stellar initial mass functions. These results are in agreement with recent findings based on local galaxies by Napolitano, Romanowsky & Tortora (2010) and suggest negligible evidence of galaxy evolution over the last ~ 2.5 Gyr other than passive stellar aging.
I review our understanding of classic dynamical scaling relations, relating luminosity, size and kinematics of early-type galaxies. Using unbiased determinations of galaxy mass profiles from stellar dynamical models, a simple picture has emerged in which scaling relations are driven by virial equilibrium, accompanied by a trend in the stellar mass-to-light ratio (M/L). This picture confirms the earliest insights. The trend is mainly due to the combined variation of age, metallicity and the stellar initial mass function (IMF). The systematic variations best correlate with the galaxy velocity dispersion, which traces the bulge mass fraction. This indicates a link between bulge growth and quenching of star formation. Dark matter is unimportant within the half-light radius, where the total mass profile is close to isothermal ($rhopropto r^{-2}$).
We measure the average mass properties of a sample of 41 strong gravitational lenses at moderate redshift (z ~ 0.4 - 0.9), and present the lens redshift for 6 of these galaxies for the first time. Using the techniques of strong and weak gravitational lensing on archival data obtained from the Hubble Space Telescope, we determine that the average mass overdensity profile of the lenses can be fit with a power-law profile (Delta_Sigma prop. to R^{-0.86 +/- 0.16}) that is within 1-sigma of an isothermal profile (Delta_Sigma prop. to R^{-1}) with velocity dispersion sigma_v = 260 +/- 20 km/s. Additionally, we use a two-component de Vaucouleurs+NFW model to disentangle the total mass profile into separate luminous and dark matter components, and determine the relative fraction of each component. We measure the average rest frame V-band stellar mass-to-light ratio (Upsilon_V = 4.0 +/- 0.6 h M_sol/L_sol) and virial mass-to-light ratio (tau_V = 300 +/- 90 h M_sol/L_sol) for our sample, resulting in a virial-to-stellar mass ratio of M_vir/M_* = 75 +/- 25. Finally, we compare our results to a previous study using low redshift lenses, to understand how galaxy mass profiles evolve over time. We investigate the evolution of M_vir/M_*(z) = alpha(1+z)^{beta}, and find best fit parameters of alpha = 51 +/- 36 and beta = 0.9 +/- 1.8, constraining the growth of virial to stellar mass ratio over the last ~7 Gigayears. We note that, by using a sample of strong lenses, we are able to constrain the growth of M_vir/M_*(z) without making any assumptions about the IMF of the stellar population.
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