[Paper Review] Early-type galaxies in the SDSS

Mariangela Bernardi, Sheth, R. K.+29 more|arXiv (Cornell University)|Oct 15, 2001

Astronomy and Astrophysical Research3 citations

TL;DR

This paper analyzes nearly 9,000 early-type galaxies from the Sloan Digital Sky Survey (SDSS) to characterize their scaling relations, evolution, and stellar populations. Using morphological and spectral selection, it finds that luminosity, size, velocity dispersion, and color correlate strongly with one another, with the Fundamental Plane and stellar population models indicating passive evolution since ~9 Gyr ago, consistent across multiple bands and environments.

ABSTRACT

A sample of nearly 9000 early-type galaxies, in the redshift range 0.01

Motivation & Objective

• To characterize the scaling relations (luminosity, size, velocity dispersion, color) of early-type galaxies in the SDSS.
• To quantify the evolution of these galaxies over cosmic time, distinguishing between true physical evolution and observational selection effects.
• To investigate the dependence of chemical abundances and spectral features on velocity dispersion and redshift using co-added spectra.
• To assess whether environmental density influences the structural, photometric, or stellar population properties of early-type galaxies.
• To test whether observed trends are consistent with single-burst stellar population models and passive evolution since ~9 Gyr ago.

Proposed method

• Selected early-type galaxies from the SDSS using morphological and spectral criteria within 0.01 ≤ z ≤ 0.3.
• Measured luminosity (L), effective radius (R), surface brightness (I), velocity dispersion (σ), and colors (g*, r*, i*, z*) in SDSS filters.
• Used mock catalogs to model and correct for measurement biases in scaling relations and fitting techniques.
• Co-added spectra of galaxies with similar σ, luminosity, size, redshift, and environment to create high signal-to-noise composite spectra.
• Fitted the Fundamental Plane: R ∝ σ^1.49 ± 0.05 × I^−0.75 ± 0.01, and derived L ∝ σ^3.91 ± 0.20 and L ∝ R^1.58 ± 0.06.
• Compared observed line-strength indices (Hβ, Mg2, Mg b, ⟨Fe⟩) with single-burst stellar population models (e.g., Worthey 1994) to infer age and metallicity evolution.

Experimental results

Research questions

• RQ1 How do luminosity, size, velocity dispersion, and color correlate in early-type galaxies across the SDSS redshift range?
• RQ2 To what extent do observed trends in the Fundamental Plane and scaling relations reflect true physical evolution versus selection effects?
• RQ3 How do chemical abundances (Mg2, Mg b, ⟨Fe⟩, Hβ) correlate with velocity dispersion and redshift, and what do they imply about formation times?
• RQ4 Is there a significant dependence of galaxy properties on environment (field vs. cluster), and if so, how does it affect the observed relations?
• RQ5 Is the observed evolution in luminosity and color consistent with a passively evolving population formed ~9 Gyr ago?

Key findings

• The Fundamental Plane relation is R ∝ σ^1.49 ± 0.05 × I^−0.75 ± 0.01, with consistent scaling in g*, r*, i*, and z* bands.
• Luminosity scales with velocity dispersion as L ∝ σ^3.91 ± 0.20, and with size as L ∝ R^1.58 ± 0.06, indicating strong structural and dynamical correlations.
• The mass-to-light ratio scales as M/L ∝ L^0.14 ± 0.02 at fixed luminosity or M/L ∝ M^0.22 ± 0.05 at fixed mass, suggesting mild mass-to-light evolution.
• At fixed velocity dispersion, the high-redshift population (z ≈ 0.2) is bluer and has weaker Mg2 and stronger Hβ absorption than the local population, indicating ~2 Gyr younger age.
• Chemical abundance trends show Hβ ∝ σ^−0.25 ± 0.02, Mg2 ∝ σ^0.18 ± 0.02, Mg b ∝ σ^0.28 ± 0.02, and ⟨Fe⟩ ∝ σ^0.10 ± 0.03, indicating higher metallicities in more massive galaxies.
• Evolution in color and spectral features is consistent with a passively evolving population that formed the bulk of its stars ~9 Gyr ago, with no significant environmental dependence in spectra or color–magnitude relations.