Veronique Cayatte

A Population of Compact Elliptical Galaxies Detected with the Virtual Observatory

Elliptical Galaxy Evolution Our closest elliptical galaxy, M32, represents a rare class of elliptical galaxies that are too compact for their luminosities. Only a handful of elliptical galaxies with luminosities and sizes comparable to M32 have been found, making it difficult to understand how they evolved. Chilingarian et al. (p. 1379, published online 1 October) present a sample of 21 compact elliptical galaxies gathered through automated data mining of the Hubble Space Telescope Legacy Archive and other databases by means of virtual observatory tools. The results suggest that tidal stripping of more massive progenitor galaxies produces compact elliptical galaxies. A sample of elliptical systems provides evidence that disruption of galaxies plays an important role in their evolution. Compact elliptical galaxies are characterized by small sizes and high stellar densities. They are thought to form through tidal stripping of massive progenitors. However, only a handful of them were known, preventing us from understanding the role played by this mechanism in galaxy evolution. We present a population of 21 compact elliptical galaxies gathered with the Virtual Observatory. Follow-up spectroscopy and data mining, using high-resolution images and large databases, show that all the galaxies exhibit old metal-rich stellar populations different from those of dwarf elliptical galaxies of similar masses but similar to those of more massive early-type galaxies, supporting the tidal stripping scenario. Their internal properties are reproduced by numerical simulations, which result in compact, dynamically hot remnants resembling the galaxies in our sample.

Kinematics and stellar populations of low-luminosity early-type galaxies in the Abell 496 cluster

The morphology and stellar populations of low-luminosity early-type galaxies in clusters have until now been limited to a few relatively nearby clusters such as Virgo or Fornax. Scenarii for the formation and evolution of dwarf galaxies in clusters are therefore not well constrained. We investigate here the morphology and stellar populations of low-luminosity galaxies in the relaxed cluster Abell 496 (z=0.0330). Deep multiband imaging obtained with the CFHT Megacam allowed us to select a sample of faint galaxies (-18.8

Discovery of a new M 32-like Compact Elliptical galaxy in the halo of the Abell 496 cD galaxy

Aims: ``Compact ellipticals are so rare that a search for M32 analogs is needed to ensure the very existence of this class. Methods: We report here the discovery of A496cE, a M32 twin in the cluster Abell 496, located in the halo of the central cD. Results: Based on CFHT and HST imaging we show that the light profile of A496cE requires a two component fit: a Sersic bulge and an exponential disc. The spectrum of A496cE obtained with the ESO-VLT FLAMES/Giraffe spectrograph can be fit by a stellar synthesis spectrum dominated by old stars, with high values of [Mg/Fe] and velocity dispersion. Conclusions: The capture of A496cE by the cD galaxy and tidal stripping of most of its disc are briefly discussed.

Stellar population constraints on the dark matter content and origin of ultra-compact dwarf galaxies

We analyse intermediate-resolution Very Large Telescope (VLT) FLAMES/Giraffe spectra of six ultra-compact dwarf (UCD) galaxies in the Fornax cluster. We obtained velocity dispersions and stellar population properties by full spectral fitting against pegase.hr models. Objects span a large range of metallicities (−0.95 to −0.23 dex), four of them are older than 8 Gyr. Comparison of the stellar and dynamical masses suggests that UCDs have little dark matter at best. For one object, UCD3, the Salpeter initial mass function (IMF) results in the stellar mass significantly exceeding the dynamical one, whereas for the Kroupa IMF the values coincide. Although, this object may have peculiar dynamics or/and stellar populations, the Kroupa IMF seems more realistic. We find that UCDs lie well above the metallicity–luminosity relation of early-type galaxies. The same behaviour is demonstrated by some of the massive Milky Way globular clusters, known to contain composite stellar populations. Our results support the two following UCD formation scenario: (i) tidal stripping of nucleated dwarf elliptical galaxies and (ii) formation of tidal superclusters in galaxy mergers. We also discuss some of the alternative channels of the UCD formation binding them to globular clusters.

NGC 6340: an old S0 galaxy with a young polar disc. Clues from morphology, internal kinematics and stellar populations

Context. Lenticular galaxies are believed to form by a combination of environmental effects and secular evolution. Aims. We study the nearby disc-dominated S0 galaxy NGC 6340 photometrically and spectroscopically to understand the mechanisms of S0 formation and evolution in groups. Methods. We use SDSS images to build colour maps and the light profile of NGC 6340, which we decompose using a three-component model including Sersic and two exponential profiles. We also use Spitzer Space Telescope archival near-infrared images to study the morphology of regions containing warm interstellar medium and dust. Then, we re-process and re-analyse deep long-slit spectroscopic data for NGC 6340, applying a novel sky subtraction technique and recover its stellar and gas kinematics, distribution of age and metallicity with the NBursts full spectral fitting. Results. We obtain the profiles of internal kinematics, age, and metallicity out to >2 half-light radii. The three structural components of NGC 6340 are found to have distinct kinematic and stellar population properties. We see a kinematic misalignment between inner and outer regions of the galaxy. We confirm the old metal-rich centre and a wrapped inner gaseous polar disc (r ∼ 1 kpc) having weak ongoing star formation, counter-rotating in projection with respect to the stars. The central compact pseudo-bulge of NGC 6340 looks very similar to compact elliptical galaxies. Conclusions. In accordance with the results of numerical simulations, we conclude that the properties of NGC 6340 can be explained as the result of a major merger of an early-type galaxy and a spiral galaxy that occurred about 12 Gyr ago. The intermediate exponential structure might be a triaxial pseudo-bulge formed by a past bar structure. The inner compact bulge could be the result of a nuclear starburst triggered by the merger. The inner polar disc appeared recently, 1/3-1/2 Gyr ago, as a result of another minor merger or cold gas accretion.

Toward accurate radial velocities with the fiber-fed GIRAFFE multi-object VLT spectrograph

We describe briefly the Data-Reduction of the VLT fiber-fed multi-object GIRAFFE spectrograph - part of the VLT FLAMES facility. We focus on specific features of GIRAFFE - the simultaneous wavelength calibration - and their impact on the data-reduction strategy. We describe the implementation of the global physical model and we compare the results obtained with the simulated, laboratory and preliminary data. We discuss the influence of critical parameters, the overall accuracy of the wavelength solution, and the stability and the robustness of the global model approach. We address the accuracy of radial velocity measurements illustrated by solar spectra obtained during the Preliminary Acceptance in Europe.

Non-confirmation of reported H I clouds without optical counterparts in the Hercules cluster

21 cm HI line observations were made with the Arecibo Gregorian telescope of 9 HI clouds in the Hercules Cluster which were reported as tenative detections in a VLA HI study of the cluster (Dickey 1997) and for which our deep CCD imaging failed to find any optical counterparts. No sensitive observations could be made of one of these (sw-174) due to the presence of a close-by strong continuum source. The other 8 tentative HI detections were not reconfirmed by the Arecibo HI measurements. The CCD images did reveal faint, low surface brightness counterparts near the centres of two other VLA HI sources invisible on the Palomar Sky Survey, sw-103 and sw-194.

Compact elliptical galaxies – compact bulges of stripped lenticulars/spirals?

We present discovery of compact elliptical galaxies in the centres of clusters and briefly discuss their evolution.

A Tidal Dwarf Galaxy in the Hercules Cluster

A candidate Tidal Dwarf Galaxy, ce-61, was identified in the merger system IC 1182 in the Hercules supercluster. The multi-wavelength data we obtained so far do not prove, however, that it is kinematically detached from the IC 1182 system and gravitationally bound.