[Paper Review] Spatially resolved spectroscopy of Coma cluster early -- type galaxies: III. The stellar population gradients

D. Mehlert, D. Thomas+3 more|ArXiv.org|Jun 11, 2003

Galaxies: Formation, Evolution, PhenomenaPhysics and Astronomy59 references162 citations

TL;DR

This study uses spatially resolved spectroscopy of 35 Coma cluster early-type galaxies and advanced stellar population models to measure radial gradients in age, metallicity, and α/Fe ratios. It finds that while metallicity gradients are significant and driven by gravitational potential, age and α/Fe gradients are negligible, implying uniform formation timescales across galaxies and global α/Fe enhancement, which reshapes the understanding of the Mg–σ relation’s origin.

ABSTRACT

We derive central values and logarithmic gradients for the Hbeta, Mg and Fe indices of 35 early-type galaxies in the Coma cluster. We find that pure elliptical galaxies have on average slightly higher velocity dispersions, lower Hbeta, and higher metallic line-strengths than galaxies with disks (S0). The gradients strongly correlate with the gradients of sigma, but only weakly with the central index values and galaxy velocity dispersion. Using stellar population models with variable element abundance ratios from Thomas, Maraston & Bender (2003a) we derive average ages, metallicities and [alpha/Fe] ratios in the center and at the effective radius. We find that the [alpha/Fe] ratio correlates with velocity dispersion and drives 30% of the Mg-sigma relation, the remaining 70% being caused by metallicity variations. We derive negative metallicity gradients (-0.16 dex per decade) that are significantly flatter than what is expected from gaseous monolithic collapse models, pointing to the importance of mergers in the galaxy formation history. The gradients in age are negligible, implying that no significant residual star formation has occurred either in the center or in the outer parts of the galaxies, and that the stellar populations at different radii must have formed at a common epoch. For the first time we derive the gradients of the [alpha/Fe] ratio and find them very small on the mean. Hence, [alpha/Fe] enhancement is not restricted to galaxy centers but it is a global phenomenon. Our results imply that the Mg-sigma local relation inside a galaxy, unlike the global Mg-sigma relation, must be primarily driven by metallicity variations alone.

Motivation & Objective

• To measure radial gradients in stellar population parameters (age, metallicity, α/Fe) in early-type galaxies of the Coma cluster using spatially resolved spectroscopy.
• To investigate how environmental density in the Coma cluster influences stellar population properties and their radial gradients.
• To disentangle the contributions of metallicity and α/Fe ratio to the local and global Mg–σ relations using α/Fe-sensitive stellar population models.
• To test whether recent star formation or formation timescale variations are responsible for observed line index gradients in S0 galaxies.
• To determine whether galaxy formation processes differ between cluster core and outer regions despite a 3 dex density range.

Proposed method

• Spatially resolved long-slit spectroscopy along the major axis (and minor axis for 10 galaxies) was used to measure radial profiles of Lick indices: Hβ, Mg b, Fe5270, and Fe5335.
• Stellar population models with variable α/Fe ratios (TMB 2003a) were applied to convert line indices into physical parameters: age, total metallicity [Z/H], and α/Fe abundance ratio.
• Logarithmic gradients of indices and velocity dispersion within one effective radius (1Re) were computed to quantify radial variations.
• Correlations between gradients and central properties (velocity dispersion, central index values) were analyzed to infer formation mechanisms.
• The intrinsic (local) Mg–σ relation was compared to the global Mg–σ relation to disentangle the roles of metallicity and α/Fe in driving the relation.
• Environmental effects were tested by comparing stellar population parameters and gradients across the cluster’s radial density profile (spanning ~3 dex).

Experimental results

Research questions

• RQ1 Do stellar population gradients in Coma cluster early-type galaxies vary with environmental density, despite a 3 dex range in cluster density?
• RQ2 What is the radial gradient in the α/Fe ratio, and does it vary significantly with radius, indicating formation timescale variations?
• RQ3 To what extent do metallicity gradients and velocity dispersion gradients correlate, and what does this imply about the role of gravitational potential in galaxy formation?
• RQ4 How do the global and intrinsic Mg–σ relations differ in their physical drivers, and what fraction of the global relation is due to α/Fe variations versus metallicity?
• RQ5 Do lenticular galaxies in the Coma cluster host young stellar populations, and if so, what does this imply about their formation history and environmental quenching?

Key findings

• The α/Fe ratio shows negligible radial gradients (mean gradient ≈ 0), indicating that α/Fe enhancement is a global phenomenon, not confined to galaxy centers.
• Metallicity gradients are significant and negative (≈ –0.16 dex per decade), with a strong correlation to velocity dispersion gradients, supporting the idea that deeper potential wells retain more metals.
• Age gradients are negligible (≈ 0), implying that stars formed at a common epoch across the galaxy, with no significant radial variation in formation timescale.
• The global Mg–σ relation is driven 70% by metallicity variations and 30% by α/Fe ratio variations, with the latter being the main driver of the intrinsic (local) Mg–σ relation.
• The intrinsic Mg–σ relation is driven solely by metallicity variations, while the global relation is driven by both metallicity and α/Fe, showing distinct physical origins.
• No significant correlations were found between stellar population parameters or their gradients and cluster-centric distance or environmental density, indicating that environmental effects did not significantly alter the evolution of early-type galaxies in Coma.