We analyse the stellar populations of 75 red-sequence dwarf galaxies in the Coma cluster , based on high signal-to-noise spectroscopy from the 6.5m MMT .
The sample covers a luminosity range 3–4 magnitudes below M ^ { \star } , in the cluster core and in a field centred 1 deg 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 ( \sim 2.7 \sigma significance ) .
The light element ( Mg , C , N , Ca ) abundances are almost independent of radius .
At radius of 0.4–1.3 degree ( \sim 0.3–1.0 \times the virial radius ) , dwarf galaxies have ages \sim 3.8 Gyr on average , compared to \sim 6 Gyr near the cluster centre .
The outer dwarfs are also \sim 50 % 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 exceptionally strong trends found here suggest that dwarf galaxies are especially susceptible to environmental “ quenching ” , and/or that the south-west part of Coma is a particularly clear example of recent quenching in an infalling subcluster .