Kinematics and stellar populations of dwarf elliptical galaxies

Abstract

Galaxy clusters serve as ideal laboratories to address the fundamental question of how environmental influence governs galaxy formation and evolution. They predominantely host elliptical and lenticular (early-type) galaxies, and are in fact dominated in number by early-type dwarfs. Since low-mass galaxies are much more vulnerable to external mechanisms due to their shallow gravitational potential, they are much more prone to be harmed by a cluster’s tidal forces and the ram pressure of its intra-cluster medium than their larger counterparts. Furthermore, from analyzing low-mass subhaloes in cosmological simulations, it has been shown that many of the early-type dwarfs in clusters have been exposed to a group or cluster environment for most of their lifetime, even before entering their present-day cluster. So the question of the origin of low surface brightness early-type dwarfs in clusters is key to determining the role of the environment in the formation of galaxies over cosmic time. Deep observations of the dwarf elliptical (dE) galaxy NGC 1396 (MV = −16.60, Mass 4 × 108 M ), located in the Fornax cluster, have been performed with the VLT/ MUSE spectrograph in the wavelength region from 4750 − 9350 °A. We present a stellar population analysis studying chemical abundances, the star formation history (SFH) and the stellar initial mass function (IMF) as a function of galacto-centric distance. Different, independent ways to analyse the stellar populations result in a luminosity-weighted age of 6 Gyr and a metallicity [Fe/H] −0.4, similar to other dEs of similar mass. We find unusually overabundant values of [Ca/Fe] +0.1, and under-abundant Sodium, with [Na/Fe] values around −0.1, while [Mg/Fe] is over-abundant at all radii, increasing from +0.1 in the centre to +0.2 dex. We notice a significant metallicity and age gradient within this dwarf galaxy. To constrain the stellar IMF of NGC 1396, we find that the IMF of NGC 1396 is consistent with either a Kroupa-like or a top-heavy distribution, while a bottom-heavy IMF is firmly ruled out. An analysis of the abundance ratios, and a comparison with galaxies in the Local Group, shows that the chemical enrichment history of NGC 1396 is similar to the Galactic disc, with an extended star formation history. This would be the case if the galaxy originated from a LMC-sized dwarf galaxy progenitor, which would lose its gas while falling into the Fornax cluster. We present stellar kinematics of a sample of ten dwarf elliptical galaxies, located in the Fornax cluster. The sample covers a large spatial area in the cluster and was observed with the Visible Multi-Object Spectrograph (VIMOS) integral field unit at the VLT. We analyse the kinematics and present velocity and velocity dispersion maps, and analyse the rotational support with the use of the specific stellar angular momentum parameter R. We compare results with some data taken with the SAMI IFU instrument and also compare properties with more massive early-type galaxies (ETGs) and place our sample on the Fundamental plane. We notice a range in rotational velocities and also different kinematic signatures which include kinematically decoupled cores (KDCs), offsets between the kinematic and photometric major axis, a prolate rotator, and also disc- and bar structures. We also notice a small offset on the FP compared to massive ETGs which could be described by different mass-to-light ratios caused by different star formation histories in dEs. Investigation into these properties suggest that late-type progenitors of dEs could be shaped during encounters in groups before entering a more dense cluster environment, where the environment is responsible for the final transformation and quenching of star formation. We present a stellar population analysis of a sample of ten dwarf elliptical galaxies, located in the Fornax cluster. The sample covers a large spatial area in the cluster and was observed with the Visible Multi-Object Spectrograph (VIMOS) integral field unit at the VLT. The high signal to noise (S/N), Integral field unit (IFU), data allows us to derive spatially resolve spectra for our sample of dwarfs. We derive velocity and velocity dispersion fields. We also analyse the stellar populations by using the full-spectrum fitting method in comparison with the more conventional line-strength analysis. With the fullspectral fitting we compare different population scenarios for each galaxy which includes fitting a single stellar population (SSP), a combination of two SSPs of which the old population is fixed and also a weighted combination of all possible populations. In the sample of 10 dEs, we find a wide range in SSPages with a average metallicity around -0.4. We present star-formation histories of all galaxies. We compare our results with some independent data from the SAMI IFU instrument (Sydney-AAO Multi object Integral-field spectrograph) and also compare properties with more massive early-type galaxies (ETGs).The Fornax cluster is a compact and rich cluster making it an ideal environment to study the environmental effect on dwarf galaxy formation.