Using deep images and simulations to trace collisional debris around massive galaxies

Deep imaging programs, such as MATLAS which has just been completed at the CFHT, allows us to study with their diffuse light the outer stellar populations around large number of galaxies. We have carried out a systematic census of their fine structures, i.e. the collisional debris from past mergers. We have identified among them stellar streams from minor mergers, tidal tails from gas-rich major mergers, plumes from gas-poor major mergers, and shells from intermediate mass mergers. Having estimated the visibility and life time of each of these structures with numerical simulations, we can reconstruct the past mass assembly of the host galaxy. Preliminary statistical results based on a sample of 360 massive nearby galaxies are presented.Comment: 3 pages, 2 figures, proceedings of IAU Symposium 321, "Formation and evolution of galaxy outskirts

Probing the mass assembly of massive nearby galaxies with deep imaging

According to a popular scenario supported by numerical models, the mass assembly and growth of massive galaxies, in particular the Early-Type Galaxies (ETGs), is, below a redshift of 1, mainly due to the accretion of multiple gas--poor satellites. In order to get observational evidence of the role played by minor dry mergers, we are obtaining extremely deep optical images of a complete volume limited sample of nearby ETGs. These observations, done with the CFHT as part of the \AD, NGVS and MATLAS projects, reach a stunning 28.5)29 mag.arcsec-2 surface brightness limit in the g' band. They allow us to detect the relics of past collisions such as faint stellar tidal tails as well as the very extended stellar halos which keep the memory of the last episodes of galactic accretion. Images and preliminary results from this on-going survey are presented, in particular a possible correlation between the fine structure index (which parametrizes the amount of tidal perturbation) of the ETGs, their stellar mass, effective radius and gas content.Comment: 4 pages, to appear in the proceedings of the IAU S295: The intriguing life of massive galaxies, editors D. Thomas, A. Pasquali & I. Ferrera

Applying galactic archeology to massive galaxies using deep imaging surveys

Various programs aimed at exploring the still largely unknown low surface brightness Universe with deep imaging optical surveys have recently started. They open a new window for studies of galaxy evolution, pushing the technique of galactic archeology outside the Local Group (LG). The method, based on the detection and analysis of the diffuse light emitted by collisional debris or extended stellar halos (rather than on stellar counts as done for LG systems), faces however a number of technical difficulties, like the contamination of the images by reflection halos and Galactic cirrus. I review here the on-going efforts to address them and highlight the preliminary promising results obtained with a systematic survey with MegaCam on the CFHT of nearby massive early-type galaxies done as part of the Atlas3D, NGVS and MATLAS collaborations.Comment: 6 pages, 4 figures, invited talk to appear in in IAU Symp. 311, "Galaxy Masses as Constraints of Formation Models", ed. M. Cappellari & S. Courteau, (Cambridge: Cambridge Univ. Press

Tidal dwarf galaxies

We review the observational evidences for tidal dwarf galaxies, a class of small galaxies formed out of the tidal debris of collisions between massive galaxies. Tidal dwarfs are found far from the interacting parent galaxies, associated to massive clouds of atomic hydrogen located at the tip of long tidal tails. These newly formed galaxies are among the best cases for the study of galaxy formation in the nearby Universe.Comment: 10 pages including 7 figures; 2 figures in jpg format; to be published in Proceedings of IAU Symp. 186 (Kyoto); also available at http://www.eso.org/~pduc/articles/articles.htm

The dark side of star formation in galaxy clusters: spectroscopic follow-up of clusters observed with ISOCAM

The evolution of galaxies in cluster environments can be studied using mid-IR observations which trace star forming regions hidden by dust. The optical follow-up of A1689 (z=0.2), observed at 6.7 and 15um by ISOCAM, have revealed a systematic excess of the B-[15um] galaxy color distribution with respect that of Coma and Virgo clusters. This result suggests the existence of a dark side of the Butcher-Oemler effect measured in the optical. We present an analysis of the optical/mid-IR properties of the mid-IR emitters in A1689, comparing in particular the star formation rates based on mid-IR and optical data. Morover, we present preliminary result for J1888, a cluster at z=0.56 deeply observed with ISOCAM, based on recent VLT/FORS and NTT/SOFI observations.Comment: 4 pages, 2 figures, to appear in the proceedings of "Sesto 2001-Tracing cosmic evolution with galaxy clusters", PASP Conf. Se

Spectrophotometric Investigation of a Sample of Tidal Dwarf Galaxies

We define a Tidal Dwarf Galaxy (TDG) as a self-gravitating entity of dwarf-galaxy mass built from tidal material expelled during interactions. We then summarize our findings on broad-band imaging and spectroscopy of a sample of TDG candidates in a sequence of interacting systems. Evidence for decoupled kinematics in the ionized gas have been found in several objects. This could indicate that they are bound galaxies and therefore genuine TDGs. As a detailed example we analyze the system AM 1159-530, where surprisingly high velocity gradients have been measured.Comment: To be published in: ``Dwarf Galaxies and their Environment'', eds. K.S. de Boer, R.-J. Dettmar, U. Klei

What collisional debris can tell us about galaxies

I review what tidal tails in particular, collisional debris in general, might tell us about galaxies (their structure, current content and past mass assembly) about mergers in the nearby and distant Universe (major vs minor, wet vs dry, number evolution) and finally about the laws of gravity.Comment: 8 pages, 6 figures, review to appear in "Galaxy mergers in an evolving Universe", Hualien, ASP Conference Serie

A parsec-resolution simulation of the Antennae galaxies: Formation of star clusters during the merger

We present a hydrodynamical simulation of an Antennae-like galaxy merger at parsec resolution, including a multi-component model for stellar feedback and reaching numerical convergence in the global star formation rate for the first time. We analyse the properties of the dense stellar objects formed during the different stages of the interaction. Each galactic encounter triggers a starburst activity, but the varying physical conditions change the triggering mechanism of each starburst. During the first two pericenter passages, the starburst is spatially extended and forms many star clusters. However, the starburst associated to the third, final passage is more centrally concentrated: stars form almost exclusively in the galactic nucleus and no new star cluster is formed. The maximum mass of stars clusters in this merger is more than 30 times higher than those in a simulation of an isolated Milky Way-like galaxy. Antennae-like mergers are therefore a formation channel of young massive clusters possibly leading to globular clusters. Monitoring the evolution of a few clusters reveals the diversity of formation scenarios including the gathering and merger of gas clumps, the monolithic formation and the hierarchical formation in sub-structures inside a single cloud. Two stellar objects formed in the simulation yield the same properties as ultra-compact dwarf galaxies. They share the same formation scenario than the most massive clusters, but have a larger radius either since birth, or get it after a violent interaction with the galactic center. The diversity of environments across space and time in a galaxy merger can account for the diversity of the stellar objects formed, both in terms of mass and size.Comment: MNRAS accepted. Movies available here: http://personal.ph.surrey.ac.uk/~fr0005/movies.ph

Tidal Dwarf Galaxies: Their Present State and Future Evolution

Evolutionary synthesis models for Tidal Dwarf Galaxies (TDGs) are presented that allow to have varying proportions of young stars formed in the merger-induced starburst and of stars from the merging spirals' disks. Comparing model grids with observational data (see e.g. P.-A. Duc this conference for a review) we try to identify the present evolutionary state of TDGs. The influence of their specific metallicities as well as of the gaseous emission of actively star forming TDGs on their luminosity and colour evolution are studied.Comment: 5 pages Latex, 4 POSTSCRIPT figures, using psfig To appear in The Magellanic Clouds and other Dwarf Galaxies, eds. J. M. Braun, T. Richtler Proceedings of Workshop of the Graduiertenkolleg Bonn-Bochum, Bad Honnef (Jan. 18-22, 1998

VLA HI and OVRO CO Interferometry of a Tidal Dwarf Galaxy

We present high resolution interferometric observations of the cool atomic and cold molecular ISM of the TDG candidate Arp 245N, an object resembling a dwarf galaxy in the northern tidal tail of the interacting system NGC 2992/3. We observed the HI line with the NRAO VLA and the CO(1-->0) transition with the OVRO millimeter interferometer at 5''-6'' angular resolution (750 pc linear resolution). These datacubes offer the required spatial and velocity resolution to determine whether the mass concentration near the tip of the tail is a genuine feature, and hence a good TDG candidate, or an artefact caused by a fortuitous alignment of our line of sight with the direction of the tail. A preliminary analysis seems to confirm that Arp 245N is a self-gravitating entity.Comment: 6 pages 3 figures, to be published in: Proceedings of IAU Symposium 217: Recycling Intergalactic and Interstellar Matter (eds. Duc, Braine & Brinks