[Paper Review] The Atlas3D project - XX. Mass-size and Mass-sigma projections of the Virial Plane of early-type galaxies: variation of morphology, kinematics, mass-to-light ratio and stellar initial mass function

Michele Cappellari, Richard M. McDermid+21 more|arXiv (Cornell University)|Aug 17, 2012

Astronomy and Astrophysical Research1 citations

TL;DR

This paper investigates the mass-size and mass-sigma projections of the Virial Plane in early-type galaxies (ETGs) using the Atlas3D survey, revealing a bimodal distribution driven by morphology and kinematics. It identifies a mass break at ~3×10¹⁰ M☉, with distinct fast-rotator (disk-dominated) and slow-rotator (spheroid-dominated) sequences, and shows that velocity dispersion (σ) traces bulge fraction, which governs M/L, IMF, and quenching via dry merging and bulge growth.

ABSTRACT

We study the (M,sigma) and (M,Re) projections of the thin Mass Plane (MP) (M,sigma,Re) which describes the distribution of the galaxy population. The distribution of galaxy properties on the MP is characterized by: (i) a zone of exclusion described by two power-laws joined by a break at M 3e10 Msun. This results in a break in the mean M-sigma relation with Msigma^2.3 and M sigma^4.7 at small and large sigma respectively; (ii) a mass M 2e11 Msun which separates a population dominated by flat fast rotator with disks and spiral galaxies at lower masses, from one dominated by quite round slow rotators at larger masses; (iii) below that mass the distribution of ETGs properties tends to be constant along lines of constant sigma; (iv) it forms a parallel sequence with the distribution of spiral galaxies; (v) at even lower masses, the distribution of fast rotator ETGs and late spirals naturally extends to that of dwarf ETGs (Sph) and dwarf irregulars (Im) respectively. We show via dynamical models that sigma traces the bulge fraction, which drives the observed trends in M/L, Hbeta, colour, IMF and molecular gas fraction. We interpret this as due to a combination of two main effects: (i) an increase of the bulge fraction which increases sigma and greatly enhances the likelihood for a galaxy to have its star formation quenched, and (ii) dry merging, increasing galaxy mass along lines of nearly constant sigma, while leaving the population unchanged. [Abriged]

Motivation & Objective

• To understand the distribution of early-type galaxies (ETGs) in the mass-sigma and mass-size projections of the Virial Plane.
• To identify the physical drivers behind the observed bimodal morphology and kinematics in ETGs across different mass scales.
• To investigate how velocity dispersion (σ) correlates with bulge fraction, stellar initial mass function (IMF), and quenching of star formation.
• To determine the role of dry merging in maintaining mass growth along nearly constant-σ sequences.

Proposed method

• Analysis of the Atlas3D survey data to map the distribution of ETGs in the (M, σ) and (M, Re) planes.
• Identification of a break in the M-σ relation at M ≈ 3×10¹⁰ M☉, with distinct power-law indices (M ∝ σ².³ at low σ, M ∝ σ⁴.⁷ at high σ).
• Use of dynamical models to link σ to bulge fraction and its influence on M/L, Hβ, colour, and molecular gas fraction.
• Comparison of ETG sequences with spiral and dwarf galaxy populations to trace evolutionary continuity.
• Modeling of dry merging as a mechanism for mass growth along constant-σ tracks without changing kinematic or morphological properties.
• Interpretation of observed trends as driven by bulge fraction increase and dry merging, explaining IMF and quenching variations.

Experimental results

Research questions

• RQ1 What causes the break in the M-σ relation at ~3×10¹⁰ M☉, and how does it affect galaxy evolution?
• RQ2 How does the distribution of ETGs in the mass-sigma and mass-size planes reflect underlying morphology and kinematics?
• RQ3 To what extent does velocity dispersion (σ) trace bulge fraction and its influence on M/L, IMF, and quenching?
• RQ4 How do dry merging and bulge growth jointly shape the observed sequences in the Virial Plane?
• RQ5 What is the connection between fast-rotating ETGs, spiral galaxies, and dwarf systems (Sph/Im) in the mass-sigma plane?

Key findings

• A break in the M-σ relation occurs at M ≈ 3×10¹⁰ M☉, with distinct power-law indices: M ∝ σ².³ at low σ and M ∝ σ⁴.⁷ at high σ.
• Galaxies with M ≈ 2×10¹¹ M☉ separate fast-rotating, disk-dominated ETGs from slow-rotating, spheroid-dominated systems.
• Below ~2×10¹¹ M☉, ETG properties are approximately constant along lines of constant σ, indicating a tight correlation with bulge fraction.
• The distribution of fast-rotator ETGs and spiral galaxies forms a parallel sequence, with natural extension to dwarf ETGs (Sph) and dwarf irregulars (Im).
• Dynamical models confirm that σ traces bulge fraction, which controls M/L, Hβ, colour, IMF, and molecular gas fraction.
• Dry merging drives mass growth along nearly constant-σ tracks, preserving kinematic and morphological properties while enhancing quenching and bulge dominance.