[Paper Review] Nearby early-type galaxies with ionized gas. III. Analysis of line-strength indices with new stellar population models

F. Annibali, A. Bressan+3 more|Phaidra (Universität Wien)|Sep 6, 2006

Galaxies: Formation, Evolution, PhenomenaPhysics and Astronomy89 references92 citations

TL;DR

This study analyzes stellar population properties in 65 nearby early-type galaxies using new simple stellar population (SSP) models that include alpha-element enhancement, deriving age, metallicity, and [α/Fe] from Lick line-strength indices at multiple radii. Key results show that metallicity and [α/Fe] increase with central velocity dispersion, indicating more efficient and shorter-duration star formation in massive galaxies, while young galaxies in low-density environments likely result from recent rejuvenation episodes involving no more than 25% of total mass.

ABSTRACT

In this paper we study the underlying stellar population of a sample of 65 nearby early-type galaxies predominantly located in low density environments. Ages, metallicities and [alpha/Fe] ratios have been derived through the comparison of Lick indices measured at different galacto-centric distances with new SSP models which account for the presence of alpha/Fe enhancement. The SSPs cover a wide range of ages, metallicities and [alpha/Fe] ratios. To derive the stellar population parameters we have devised an algorithm based on the probability density function. We derive a large spread in age ((1-15) Gyrs). Age does not show any significant trend with central velocity dispersion sigma_c but E galaxies appear on average older than S0. On the contrary, an increasing trend of metallicity and [alpha/Fe] with sigma_c is observed, testifying that the chemical enrichment was more efficient and the duration of the star formation shorter in more massive galaxies. We have also sought for possible correlations with the local galaxy density but neither metallicity nor alpha-enhancement show clear trends. However we find that while low density environments (LDE) contain very young objects (from 1 to 4 Gyr), none of the galaxies in the higher density environments (HDE) is younger than 5 Gyrs. Considering the lack of environmental effect on the [alpha/Fe] ratio and the high value of [alpha/Fe] in some young massive objects, we argue that young galaxies in LDE are more likely due to recent rejuvenation episodes. By comparing the number of rejuvenated objects with the total number of galaxies in our sample, and by means of simple two-SSP component models, we estimate that, on average, the rejuvenation episodes do not involve more than 25 % of the total galaxy mass.

Motivation & Objective

• To determine the underlying stellar population properties—age, metallicity, and [α/Fe]—in a sample of 65 nearby early-type galaxies with ionized gas.
• To investigate how these stellar population parameters correlate with galaxy potential (via central velocity dispersion σc) and local environmental density (ρxyz).
• To assess the role of environmental conditions in shaping the formation and evolution of early-type galaxies, particularly through rejuvenation episodes.
• To analyze spatial gradients in stellar populations to infer the radial evolution of star formation and chemical enrichment processes.

Proposed method

• Measured Lick line-strength indices at 7 apertures and 4 radial gradients across each galaxy to probe stellar populations at different galactocentric radii.
• Compared observed indices with new simple stellar population (SSP) models that include variable [α/Fe] ratios over a wide range of ages (1–16 Gyr), metallicities (0.0004 ≤ Z ≤ 0.05), and [α/Fe] (0–0.8).
• Applied an algorithm that computes the full probability density function in (age, Z, [α/Fe]) space, resolving the age-metallicity degeneracy within 1σ confidence intervals.
• Used two-SSP component models to estimate the mass fraction involved in rejuvenation episodes in young galaxies.
• Analyzed radial gradients of metallicity, age, and [α/Fe] to infer the spatial distribution of star formation and enrichment efficiency.
• Correlated stellar population parameters with both central velocity dispersion (σc) and local galaxy density (ρxyz) to assess environmental and dynamical influences.

Experimental results

Research questions

• RQ1 How do age, metallicity, and [α/Fe] vary across early-type galaxies, and what do they reveal about their formation timescales?
• RQ2 What is the relationship between stellar population parameters and central velocity dispersion (σc), and what does it imply about the role of gravitational potential in galaxy formation?
• RQ3 Is there a detectable environmental dependence of stellar population properties on local galaxy density (ρxyz), particularly for young galaxies?
• RQ4 To what extent do rejuvenation episodes—such as major mergers—contribute to the mass assembly of early-type galaxies?
• RQ5 How do radial gradients in metallicity, age, and [α/Fe] reflect the spatial and temporal evolution of star formation and chemical enrichment?

Key findings

• Stellar population ages span a wide range, from a few Gyr to 15 Gyr, with ellipticals (average 8.7 Gyr) older than lenticulars (average 6.3 Gyr), and no significant trend with central velocity dispersion.
• Metallicity and [α/Fe] show strong positive correlations with central velocity dispersion (σc), indicating more efficient chemical enrichment and shorter star formation timescales in more massive galaxies.
• The [α/Fe] ratio peaks at ∼0.22, with a narrower distribution than age or metallicity, supporting the role of α-enhancement in constraining formation timescales.
• Galaxies in low-density environments (ρxyz ≤ 0.4) host very young objects (1–4 Gyr), while none in high-density environments (HDE) are younger than 5 Gyr, suggesting environmental quenching of recent star formation.
• The lack of environmental dependence on [α/Fe] in young galaxies implies that rejuvenation episodes—likely from major mergers—rather than prolonged star formation, explain recent star formation in low-density regions.
• Radial gradients show a firm negative metallicity gradient of Δlog Z/Δlog(r/re) ∼ -0.21, while age and [α/Fe] remain relatively flat within re/2, indicating uniform star formation timescales across the central region with increasing central efficiency.