A deep AAOmega survey of low-luminosity galaxies in the Shapley supercluster: Stellar population trends

Abstract

We present new optical spectroscopy for 342 R < 18 galaxies in the Shapley supercluster (and 198 supplementary galaxies), obtained from 8-h integrations with the AAOmega facility at the Anglo-Australian Telescope. We describe the observations and measurements of central velocity dispersion σ, emission-line equivalent widths and absorption-line indices. The distinguishing characteristic of the survey is its coverage of a very wide baseline in velocity dispersion (90 per cent range σ = 40-230 km s -1), while achieving high signal-to-noise ratio throughout (median 60 Å-1 at 5000 Å). The data quality will enable estimates of Balmer-line ages to better than 20 per cent precision even for the faintest galaxies in the sample. Significant emission at Hα was detected in ∼30 per cent of the supercluster galaxies, including ∼20 per cent of red-sequence members. Using line-ratio diagnostics, we find that the emission is LINER (low ionization nuclear emission region) like at high luminosity, but driven by star formation in low-luminosity galaxies. To characterize the absorption lines, we use the classical Lick indices in the spectral range 4000-5200 Å. We introduce a new method for applying resolution corrections to the line-strength indices. We define a subset of galaxies with very low emission contamination, based on the Hα line, and fit the index-σ relations for this subset. The relations show the continuation of the familiar trends for giant galaxies into the low-luminosity regime, with little change in slope for most indices. Comparing the index-σ slopes against predictions from single-burst stellar population models, we infer the scaling relations of age, total metallicity, [Z/H], and α-element abundance ratio, [α/Fe]. To reproduce the observed index-σ slopes, all three parameters must increase significantly with increasing velocity dispersion. Specifically, we recover Age ∝ σ 0.52±0.06±0.10, Z/H ∝ σ 0.34±0.04±0.07 and α/Fe ∝ σ0.23±0.04±0.06 (where the second error reflects systematic effects), derived over a decade baseline in velocity dispersion, σ = 30-300 km s-1. The equivalent slopes for the subset of galaxies with σ > 100 km s-1 are similar for age and Z/H. For α/Fe, a steeper slope is recovered for the high-σ subset, α/Fe ∝ σ0.36±0.07. The recovered age-σ relation is shown to be consistent with the observed evolution in the giant-to-dwarf galaxy ratio in clusters at redshifts z = 0.4-0.8. In a companion paper, we determine the age, [Z/H] and [α/Fe] for individual galaxies, and investigate in detail the distribution of galaxy properties at fixed velocity dispersion. © 2007 RAS.