Details

Autor(en) / Beteiligte

• Graves, Genevieve J
• Faber, S. M

Titel

Dissecting the Red sequence. III. Mass-to-Light Variations in Three-dimensional Fundamental Plane Space

Ist Teil von

• The Astrophysical journal, 2010-07, Vol.717 (2), p.803-824

Ort / Verlag

Bristol: IOP Publishing

Erscheinungsjahr

2010

Quelle

EZB Electronic Journals Library

Beschreibungen/Notizen

• The fundamental plane (FP) of early-type galaxies is observed to have finite thickness and to be tilted from the virial relation. Both of these represent departures from the simple assumption that dynamical mass-to-light ratios (M{sub dyn}/L) are constant for all early-type galaxies. We use a sample of 16,000 quiescent galaxies from the Sloan Digital Sky Survey to map out the variations in M {sub dyn}/L throughout the three-dimensional FP space defined by velocity dispersion ({sigma}), effective radius (R{sub e}), and effective surface brightness (I{sub e}). Dividing M{sub dyn}/L into multiple components allows us to separately consider the contribution to the observed M{sub dyn}/L variation due to stellar population effects, initial mass function (IMF) variations, and variations in the dark matter fraction within one R{sub e} . Along the FP, we find that the stellar population contribution given some constant IMF (M{sub *,IMF}/L) scales with {sigma} such that M{sub *,IMF}/L {proportional_to} f({sigma}). Meanwhile, the dark matter and/or IMF contribution (M{sub dyn}/M {sub *,IMF}) scales with M{sub dyn} such that M{sub dyn}/M {sub *,IMF} {proportional_to} g(M{sub dyn}). This means that the two contributions to the tilt of the FP rotate the plane around different axes in the three-dimensional space. The observed tilt of the FP requires contributions from both, with dark matter/IMF variations likely comprising the dominant contribution. Looking at M {sub dyn}/L variations through the thickness of the FP, we find that M{sub dyn}/L variations must be dominated either by IMF variations or by real differences in the dark matter fraction with R{sub e} . This means that the finite thickness of the FP is due to variations in the stellar mass surface density within R{sub e} ({Sigma}{sub *,IMF}), not the fading of passive stellar populations. It therefore represents genuine structural differences between early-type galaxies. These structural variations are correlated with galaxy star formation histories such that galaxies with higher M{sub dyn}/M {sub *,IMF} have higher [Mg/Fe], lower metallicities, and older mean stellar ages. We discuss several physical mechanisms that might explain the observed co-variation between M {sub dyn}/M {sub *,IMF} and galaxy star formation histories. It is difficult to explain the observed enhancement of {alpha}-elements in lower-surface-brightness galaxies by allowing the IMF to vary. Differences in dark matter fraction can be produced by variations in the 'conversion efficiency' of baryons into stars or by the redistribution of stars and dark matter through dissipational merging. The former explanation, specifically a model in which some galaxies experience low conversion efficiencies due to premature truncation of star formation, provides a more natural explanation for the co-variation of M{sub dyn}/M {sub *,IMF} and the observed stellar population properties.