2005/02/21 by Russell J. Smith · 1 citation
Physics and Astronomy · #Astrophysics #Cluster (spacecraft) #Degeneracy (biology) #Demography #Galaxies: Formation, Evolution, Phenomena #Galaxy #Metallicity #Physics #Population #Scaling #Star formation #Stellar population #Stellar, planetary, and galactic studies #Velocity dispersion #astro-ph
paper · pdf · doi:10.1111/j.1365-2966.2005.08952.x
published as Mon.Not.Roy.Astron.Soc.359:975-984,2005 · 11 pages, 9 figures, accepted by MNRAS
arxiv created 2005/02/21 · openalex publication_date 2005/05/01 · arxiv updated 2014/10/13 · openalex created_date 2025/10/10 · openalex updated_date 2026/08/05
We compute population synthesis models for the variation of Hα absorption indices (HαA and HαF), as a function of age and metallicity in old stellar systems. The models are based on the STELIB spectral library of Le Borgne et al., and defined at a resolution of 3 Å full width at half-maximum. Errors in the age and metallicity responses are derived by bootstrap resampling the input measurements on the stellar library. The indices are found to be highly sensitive to age variation, with only moderate response to metallicity. For galaxies uncontaminated by nebular emission, our HαA index is more powerful in breaking the age–metallicity degeneracy than Hβ or HγF. Using a sample of red cluster galaxies from Nelan et al., carefully selected to exclude objects with emission, we find a steep decline of HαA with velocity dispersion (slope −0.75 ± 0.07 Å dex−1). The slope can be translated to constraints on age and metallicity scaling relations, incorporating measurement errors and also the model errors determined from the bootstrap method. If the HαA–σ slope is due only to age, we obtain Age ∝σ0.95±0.12. Because HαA depends quite weakly on [Fe/H], a metallicity interpretation would require Fe/H ∝σ1.2 or steeper. The HαA–σ slope is consistent with the combined age and metallicity scaling relations reported by Nelan et al. from classical Lick indices. The relations obtained by Thomas et al. significantly underpredict the observed slope. The discrepancy could arise from differences in the sample selection. In particular, our sample probes a lower mass range, is not explicitly selected on morphological criteria, and excludes objects significantly bluer than the red sequence. We discuss in detail the impact of emission contamination on the results, and conclude that this effect is unlikely to yield the observed behaviour in the Hα–σ relations. Indeed, similar results are obtained using HαF, despite its different sensitivity to Hα and [N ii] emission lines. The steep age–mass relation supports a ‘downsizing’ formation scenario: fainter red-sequence galaxies became quiescent at lower redshifts, z≲ 0.5. This picture accords with recent observations of truncated red sequences in clusters at z∼ 0.7.