Young stellar populations in early-type dwarf galaxies

Abstract

Aims. To understand the stellar population content of early-type dwarf galaxies (dEs) and their environmental dependence, we compare the slopes and intrinsic scatter of color–magnitude relations (CMRs) for three nearby clusters, Fornax, Virgo, and Coma. Additionally, we present and compare internal color profiles of these galaxies to identify central blue regions with younger stars. Methods. We use the imaging of the HST/ACS Fornax cluster in the magnitude range of −18.7 ≤ Mg′ ≤ −16.0 to derive magnitudes, colors, and color profiles, which we compare with literature measurements from the HST/ACS Virgo and Coma Cluster Survey. We take advantage of HST accuracy to investigate and parameterize the (g′−z′) color profiles of these dEs. Results. Based on analysis of the color profiles, we report on a large number of dEs with young stellar populations at their center in all three clusters. While for Virgo and Coma the number of blue-cored dEs is found to be 85%±2% and 53%±3%, respectively, for Fornax, we find that all galaxies have a blue core. We show that bluer cores reside in fainter dEs, similar to the trend seen in nucleated dEs. We find no correlation between the luminosity of the galaxy and the size of its blue core. Moreover, a comparison of the CMRs of the three clusters shows that the scatter in the CMR of Virgo is considerably larger than in the Fornax and Coma clusters. Presenting adaptive smoothing we show that the galaxies on the blue side of the CMR often show evidence for dust extinction, which strengthens the interpretation that the bluer colors are due to young stellar populations. We also find that outliers on the red side of the CMR are more compact than what is expected for their luminosity. We find several of these red outliers in Virgo, often close to more massive galaxies. No red outlying compact early-types are found in Fornax and Coma in this magnitude range while we find three in the Virgo cluster. We show that the CMR of the Fornax and Virgo clusters are slightly bluer than that of Coma. We suggest that the large number of outliers and larger scatter found for Virgo CMR are a result of the alternative assembly history of this cluster.

Early-type galaxies in the Antlia cluster: global properties

“…The colour dispersion at each magnitude from T 1 = 15 to 19 mag is σ (C−T 1) = 0.16 -0.17. The dispersion of the Virgo CMR, in the same luminosity regime, is σ (g −z ) = 0.144 mag (Hamraz et al 2019), which results to be larger than the dispersion obtained for Fornax and Coma. This larger scatter in the Antlia dEs colours and the most bluer galaxies that we found, could indicate that the forming star processes in the LSB population have no finished yet.…”

ELVES II: Globular Clusters and Nuclear Star Clusters of Dwarf Galaxies: the Importance of Environment

“…As a dwarf is being quenched via ram pressure stripping either in the cluster environment or in the halo of a MW-like host, the central regions would be the last to quench, as evinced by the blue cores of many dEs (e.g. Urich et al 2017;Hamraz et al 2019). Thus, it is not surprising that many NSCs have a subpopulation of young stars.…”

Galaxy populations in the Hydra I cluster from the VEGAS survey I. Optical properties of a large sample of dwarf galaxies

“…sequence defined by normal cluster members. However, usually these systems are at most only marginally redder than the brightest cluster galaxies (Hamraz et al 2019). We are therefore confident that our limits are sufficiently broad that none of the likely cluster members is excluded.…”