Nature vs. nurture in the low-density environment: structure and evolution of early-type dwarf galaxies in poor groups

Abstract

We present the stellar population properties of 13 dwarf galaxies residing in poor groups (low-density environment, LDE) observed with VIMOS at VLT. Ages, metallicities, and [α/Fe] ratios were derived within an r < r e /2 aperture from the Lick indices Hβ, Mgb, Fe5270, and Fe5335 through comparison with our simple stellar population (SSP) models that account for variable [α/Fe] ratios. For a fiducial subsample of 10 early-type dwarfs, we derived median values and scatters around the medians of 5.7 ± 4.4 Gyr, −0.26 ± 0.28, and −0.04 ± 0.33 for age, log Z/Z , and [α/Fe], respectively. For a selection of bright early-type galaxies (ETGs) from an earlier sample residing in a comparable environment, we derive median values of 9.8 ± 4.1 Gyr, 0.06 ± 0.16, and 0.18 ± 0.13 for the same stellar population parameters. It follows that dwarfs are on average younger, less metal rich, and less enhanced in the α-elements than giants, in agreement with the extrapolation to the low-mass regime of the scaling relations derived for giant ETGs. From the total (dwarf+giant) sample, we find that age ∝σ 0.39±0.22 , Z ∝ σ 0.80±0.16 , and α/Fe ∝ σ 0.42±0.22 . We also find correlations with morphology, in the sense that the metallicity and the [α/Fe] ratio increase with the Sersic index n or with the bulge-to-total light fraction B/T . The presence of a strong morphology-[α/Fe] relation appears to contradict the possible evolution along the Hubble sequence from low B/T (low n) to high B/T (high n) galaxies. We also investigate the role played by environment by comparing the properties of our LDE dwarfs with those of Coma red passive dwarfs from the literature. We find possible evidence that LDE dwarfs experienced more prolonged star formations than Coma dwarfs, however larger data samples are needed to draw firmer conclusions.

The Influence of Environment on the Chemical Evolution in Low-Mass Galaxies

“…To eliminate the systematic offsets between these two models, we apply the following correction from Smith et al (2009) to adjust the values of Schiavon (2007;Sch) to the scale of Thomas et al (2003;TMB) The models of Proctor & Sansom (2002) and Sansom & Northeast (2008) are the same as that of Thomas et al (2003), except that they did not account for the [α/Fe] bias in the solar neighborhood; this, however, has negligible effects on our results. The models of Annibali et al (2007Annibali et al ( , 2011 are consistent with those of Thomas et al (2003). A more detail discussion is given in Section 3.3.…”

“…Previous studies all had relatively large uncertainties. Sansom & Northeast (2008), Smith et al (2009), andAnnibali et al (2011) obtained different slopes for this relation. Using a sample of low-mass ETGs from the Virgo Cluster, Liu et al (2016) found a [α/Fe]-σ relation with larger scatter and, based on a correlation between [α/Fe] and distance from the cluster center, concluded that low-mass ETGs are more governed by environment instead of mass.…”

Nearby early-type galaxies with ionized gas

“…accretion of pure stellar systems. The spectral analysis of faint companions of ETGs in low density environments, where our ETGs are mostly located, performed by Grützbauch et al (2009) showed that the vast majority of these latter do not show emission lines (see also the population analysis in Annibali et al 2011).…”

Metal-poor galaxies in the local universe

“…One of the intriguing questions in this context, particularly with respect to dwarf galaxies, is the importance of a galaxy's intrinsic properties ("nature") vs. the importance of external influences ("nurture"). Clearly, both play a role in shaping dwarf galaxies in groups and clusters (e.g., [5,25,28,29,33,42,99,100,68,69,70,71,72,126,128,162]). …”