Sven De Rijcke

Formation and evolution of dwarf elliptical galaxies – II. Spatially resolved star formation histories

We present optical Very Large Telescope spectroscopy of 16 dwarf elliptical galaxies (dEs) comparable in mass to NGC 205, and belonging to the Fornax cluster and to nearby groups of galaxies. Using full-spectrum fitting, we derive radial profiles of the SSP-equivalent ages and metallicities. We make a detailed analysis with ulyss and steckmap of the star formation history in the core of the galaxies and in an aperture of one effective radius. We resolved the history into one to four epochs. The statistical significance of these reconstructions was carefully tested; the two programs give remarkably consistent results. The old stellar population of the dEs, which dominates their mass, is likely coeval with that of massive ellipticals or bulges, but the star formation efficiency is lower. Important intermediate age (1–5 Gyr) populations and frequently tails of star formation until recent times are detected. These histories are reminiscent of their lower mass dwarf spheroidal counterparts of the Local Group. Most galaxies (10/16) show significant metallicity gradients, with metallicity declining by 0.5 dex over one half-light radius on average. These gradients are already present in the old population. The flattened (or discy), rotating objects (6/16) have flat metallicity profiles. This may be consistent with a distinct origin for these galaxies or it may be due to their geometry. The central single stellar population equivalent age varies between 1 and 6 Gyr, with the age slowly increasing with radius in the vast majority of objects. The group and cluster galaxies have similar radial gradients and star formation histories. The strong and old metallicity gradients place important constraints on the possible formation scenarios of dEs. Numerical simulations of the formation of spherical low-mass galaxies reproduce these gradients, but they require a longer time for them to build up. A gentle depletion of the gas, by ram pressure stripping or starvation, could drive the gas-rich, star-forming progenitors to the present dEs.

Full lensing analysis of Abell 1703: comparison of independent lens-modelling techniques

The inner mass-prole of the relaxed cluster Abell 1703 is analysed by two very dierent strong-lensing techniques applied to deep ACS and WFC3 imaging. Our parametric method has the accuracy required to reproduce the many sets of multiple images, based on the assumption that mass approximately traces light. We test this assumption with a fully non-parametric, adaptive grid method, with no knowledge of the galaxy distribution. Dierences between the methods are seen on

Hubble Space Telescope survey of the Perseus Cluster – I. The structure and dark matter content of cluster dwarf spheroidals

We present the results of a Hubble Space Telescope (HST) Advanced Camera for Surveys (ACS) study of dwarf galaxies in the core of the rich nearby Perseus Cluster, down to M-V = -12. We identify 29 dwarfs as cluster members, 17 of which are previously unstudied. All the dwarfs we examine are remarkably smooth in appearance, and lack internal features. This smoothness is quantified by parametric fitting of the two-dimensional light distribution, and through non-parametric structural parameters. The dwarfs in this study are found to be very smooth, and have very low asymmetry and clumpiness values ( = 0.03 +/- 0.04, = 0.02 +/- 0.09) throughout their entire structure, showing no evidence for internal features or star formation that could be the result of tidal processes or star formation induced by the cluster environment. Based on these observations, and the sizes of these dwarfs, we argue that some of the dwarfs in our sample must have a large dark matter content to prevent disruption by the cluster potential. We derive a new method, independent of kinematics, for measuring the dark matter content of dEs, based on the radius of the dwarf, the projected distance of the dwarf from the cluster centre, and the total mass of the cluster interior to it. We use this method to determine the mass-to-light ratios for 25 of the dwarfs in our sample, the others being too close to bright stars to accurately determine their radii. At their current positions in the cluster, 12 out of 25 of the dwarfs in our sample require dark matter to remain stable against the cluster potential. These 12 dwarfs are all fainter than M-V = -14, and nearly all dEs with M-V < -13 require dark matter to survive. We also consider the mass that the dwarfs would require to survive the tidal forces at the centre of the cluster, at a pericentric distance of 35 kpc from the cluster centre. We find that at this distance, the mass-to-light ratios of these dwarfs are comparable to those of the Local Group dSphs, ranging between M-circle dot/L-circle dot approximate to 1 and 120.

TOWARD A SOLUTION FOR THE Ca II TRIPLET PUZZLE : RESULTS FROM DWARF ELLIPTICAL GALAXIES

We present new estimates of ages and metallicities, based on VLT FORS optical (4400-5500 A) spectroscopy, of 16 dwarf elliptical galaxies (dEs) in the Fornax Cluster and in southern groups. These dEs are more metal-rich and younger than previous estimates based on narrowband photometry and low-resolution spectroscopy. For our sample we find a mean metallicity [Z/H] = -0.33 dex and mean age 3.5 Gyr, consistent with similar samples of dEs in other environments (Local Group, Virgo). Three dEs in our sample show emission lines and very young ages. This suggests that some dEs formed stars until a very recent epoch and were self-enriched by a long star formation history. Previous observations of large near-infrared (~8500 A) Ca II absorption strengths in these dEs are in good agreement with the new metallicity estimates, solving part of the so-called calcium puzzle.

The Binary Nucleus in VCC 128: A Candidate Supermassive Black Hole in a Dwarf Elliptical Galaxy

Hubble Space Telescope (HST) Wide Field Planetary Camera 2 (WFPC2) images of the Virgo Cluster dwarf elliptical galaxy VCC 128 reveal an apparently double nucleus. The two components, which are separated by ~32 pc in projection, have the same magnitude and color. We present a spectrum of this double nucleus and show that it is inconsistent with one or both components being emission-line background objects or foreground stars. The most likely interpretation is that, as suggested by Lauer et al. for the double nucleus in NGC 4486B, we are seeing a nuclear disk surrounding a supermassive black hole. This is only the second time an early-type dwarf (dE/dSph) galaxy has been suggested to host a supermassive black hole.

G2C2 - IV. A novel approach to study the radial distributions of multiple populations in Galactic globular clusters

We use the horizontal branch (HB) morphology of 48 Galactic globular clusters (GCs) to study the radial distributions of the different stellar populations known to exist in GCs. Assuming that the (extremely) blue HB stars correspond to stars enriched in helium and light elements, we compare the radial distributions of stars selected according to colour on the HB to trace the distribution of the secondary stellar populations in GCs. Unlike other cases, our data show that the populations are well mixed in 80 per cent of the cases studied. This provides some constraints on the mechanisms proposed to pollute the interstellar medium in young GCs.

The Counterstreaming Instability in Dwarf Elliptical Galaxies with Off-Center Nuclei

In many nucleated dwarf elliptical galaxies (dE,Ns), the nucleus is offset by a significant fraction of the scale radius with respect to the center of the outer isophotes. Using a high-resolution N-body simulation, we demonstrate that the nucleus can be driven off-center by the m = 1 counterstreaming instability, which is strong in flattened stellar systems with zero rotation. The model develops a nuclear offset on the order of 30% of the exponential scale length. We compare our numerical results with the photometry and kinematics of FCC 046, a Fornax Cluster dE,N with a nucleus offset by 12; we find good agreement between the model and FCC 046. We also discuss mechanisms that may cause counterrotation in dE,Ns and conclude that the destruction of box orbits in an initially triaxial galaxy is the most promising.

Constraining the subgrid physics in simulations of isolated dwarf galaxies

Simulating dwarf galaxy haloes in a reionizing Universe puts severe constraints on the subgrid model employed in the simulations. Using the same subgrid model that works for simulations without a UV-background (UVB) results in gas-poor galaxies that stop forming stars very early on, except for haloes with high masses. This is in strong disagreement with observed galaxies, which are gas rich and star forming down to a much lower mass range. To resolve this discrepancy, we ran a large suite of isolated dwarf galaxy simulations to explore a wide variety of subgrid models and parameters, including timing and strength of the UVB, strength of the stellar feedback and metallicity-dependent Pop III feedback. We compared these simulations to observed dwarf galaxies by means of the baryonic Tully–Fisher relation (BTFR), which links the baryonic content of a galaxy to the observationally determined strength of its gravitational potential. We found that the results are robust to changes in the UVB. The strength of the stellar feedback shifts the results on the BTFR, but does not help to form gas-rich galaxies at late redshifts. Only by including Pop III feedback are we able to produce galaxies that lie on the observational BTFR and that have neutral gas and ongoing star formation at redshift zero.

EA01A/B: High-Resolution H I Imaging of an Interacting Pair of Poststarburst (E+A) Galaxies

We present high spatial resolution 21 cm H I observations of EA01A and EA01B, a pair of interacting poststarburst, or E+A, galaxies at z = 0.0746. Based on optical HST/WFPC2 images, both galaxies are known to display disturbed morphologies. They also appear to be linked by a bridge of stars. Previous H I observations by Chang et al. in 2001 had already uncovered sizable quantities of neutral gas in or near these galaxies, but they lacked the spatial resolution to locate the gas with any precision within this galactic binary system. We have analyzed deep, high-resolution archival VLA observations of the couple. We find evidence for three gaseous tidal tails: one connected to EA01A and two emanating from EA01B. These findings confirm, independently from the optical imaging, that (i) EA01A and EA01B are actively interacting, and that, as a consequence, the starbursts that occurred in these galaxies were most likely triggered by this interaction, and that (ii) 6.6 ± 0.9 × 109 M☉ of neutral gas are still present in the immediate vicinity of the optical bodies of both galaxies. The H I column density is lowest at the optical positions of the galaxies, suggesting that most of the neutral gas that is visible in our maps is associated with the tidal arms and not with the galaxies themselves. This might provide an explanation for the apparent lack of ongoing star formation in these galaxies.

G2C2 – III. Structural parameters for Galactic globular clusters in SDSS passbands

We use our Galactic Globular Cluster Catalog (G2C2) photometry for 111 Galactic globular clusters (GCs) in g and z ,a s well asr and i photometry for a subset of 60 GCs and u photometry for 22 GCs, to determine the structural parameters assuming King models. In general, the resulting core radii are in good comparison with the current literature values. However, our half-light radii are slightly lower than the literature. The concentrations (and therefore also the tidal radii) are poorly constrained mostly because of the limited radial extent of our imaging. Therefore, we extensively discuss the effects of a limited field of view on the derived parameters using mosaicked Sloan Digital Sky Survey data, which do not suffer from this restriction. We also illustrate how red giant branch (RGB) stars in cluster cores can stochastically induce artificial peaks in the surface brightness profiles. The issues related to these bright stars are scrutinized based on both our photometry and simulated clusters. We also examine colour gradients and find that the strongest central colour gradients are caused by central RGB stars and thus not representative for the cluster light or colour distribution. We recover the known relation between the half-light radius and the Galactocentric distance in the g band, but find a lower slope for redder filters. We did not find a correlation between the scatter on this relation and other cluster properties. We find tentative evidence for a correlation between the half-light radii and the [Fe/H], with metal-poor GCs being larger than metal-rich GCs. However, we conclude that this trend is caused by the position of the clusters in the Galaxy, with metal-rich clusters being more centrally located.