Young stellar populations in early-type dwarf galaxies

Occurrence, radial extent, and scaling relations^⋆

¹ Kapteyn Astronomical Institute, University of Groningen, Postbus 800 9700 AV Groningen, The Netherlands
e-mail: hamraz@astro.rug.nl
² School of Astronomy, Institute for Research in Fundamental Sciences (IPM), Tehran 19395-5746, Iran
³ Leibniz-Institut für Astrophysik Potsdam, An der Sternwarte 16, 14482 Potsdam, Germany
⁴ Astronomy Research Unit, University of Oulu, Oulu 90014, Finland

Received: 17 January 2019
Accepted: 28 March 2019

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 ≤ M_g′ ≤ −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.