Large population of recently quenched red-sequence dwarf galaxies in the outskirts of the Coma cluster

We analyse the stellar populations of 75 red- sequence dwarf galaxies in the Coma cluster, based on high signal- to- noise ratio spectroscopy from the 6.5- m MMT. The sample covers a luminosity range 3 - 4 mag below M*, in the cluster core and in a field centred 1 degrees to the south-west. We find a strong dependence of the absorption-line strengths with location in the cluster. Galaxies further from the cluster centre have stronger Balmer lines than inner-field galaxies of the same luminosity. The magnesium lines are weaker at large radius, while the iron lines are not correlated with radius. Converting the line strengths into estimates of stellar age, metallicity and abundance ratios, we find the gradients are driven by variations in age (> 6 sigma significance) and in the iron abundance Fe/ H (similar to 2.7 sigma significance). The light element ( Mg, C, N, Ca) abundances are almost independent of radius. At radius of 0 degrees.4 - 1 degrees.3 (similar to 0.3 - 1.0 times the virial radius), dwarf galaxies have ages similar to 3.8 Gyr on average, compared to similar to 6 Gyr near the cluster centre. The outer dwarfs are also similar to 50 per cent more iron-enriched, at given luminosity. Our results confirm earlier indications that the ages of red- sequence galaxies depend on location within clusters, and in Coma in particular. The very strong trends found here suggest that dwarf galaxies are especially susceptible to environmental 'quenching', and/or that the south-west part of Coma is particularly a clear example of recent quenching in an infalling subcluster.