Uta Fritze-v. Alvensleben

Stellar populations and star cluster formation in interacting galaxies with the Advanced Camera for Surveys

Abstract Pixel-by-pixel colour–magnitude and colour–colour diagrams—based on a subset of the Hubble Space Telescope Advanced Camera for Surveys Early Release Observations—provide a powerful technique to explore and deduce the star and star cluster formation histories of the Mice and the Tadpole interacting galaxies. In each interacting system we find some 40 bright young star clusters (20≲F606W(mag)≲25, with a characteristic mass of ∼3×106 M⊙), which are spatially coincident with blue regions of active star formation in their tidal tails and spiral arms. We estimate that the main events triggering the formation of these clusters occurred ∼(1.5–2.0)×108 yr ago. We show that star cluster formation is a major mode of star formation in galaxy interactions, with ≳35% of the active star formation in encounters occurring in star clusters. This is the first time that young star clusters have been detected along the tidal tails in interacting galaxies. The tidal tail of the Tadpole system is dominated by blue star forming regions, which occupy some 60% of the total area covered by the tail and contribute ∼70% of the total flux in the F475W filter (decreasing to ∼40% in F814W). The remaining pixels in the tail have colours consistent with those of the main disk. The tidally triggered burst of star formation in the Mice is of similar strength in both interacting galaxies, but it has affected only relatively small, spatially coherent areas.

Large Velocity Gradients in the Tidal Tails of the Interacting Galaxy AM 1353–272 (“The Dentist’s Chair”)*

We present Very Large Telescope observations of the interacting system AM 1353-272. Using the FORS2 instrument, we studied the kinematics of the ionized gas along its prominent tidal tails and discovered strikingly large velocity gradients associated with seven luminous tidal knots. These kinematical structures cannot be caused by streaming motion and most likely do not result from projection effects. More probably, instabilities in the tidal tails have led to the formation of kinematically decoupled objects that could be the progenitors of self-gravitating tidal dwarf galaxies.

On the Origin of S0 Galaxies

I will review the basic properties of S0 galaxies in the local Universe in relation to both elliptical and spiral galaxies, their neighbours on the Hubble sequence, and also in relation to dwarf spheroidal (dSph) galaxies. This will include colours, luminosities, spectral features, information about the age and metallicity composition of their stellar populations and globular clusters, about their ISM content, as well as kinematic signatures and their implications for central black hole masses and past interaction events, and the number ratios of S0s to other galaxy types in relation to environmental galaxy density.

The Globular Cluster System of NGC 5128: Combining Broad-Band Color and Lick Index Analysis

We present a mathematically advanced method for the determination of age and metallicity of individual members of globular cluster systems (CGSs) in galaxies by combining all the information inherent in broad-band color and Lick index measurements, and we present first results of our analysis of the GCS of the early type galaxy NGC 5128.

Star Formation Efficiencies and Star Cluster Formation

Starbursts produce large numbers of Young Star Clusters (YSCs). Multi-color photometry, in combination with a dedicated SED analysis tool, allows to derive ages, metallicities, E(B − V ), and masses, including 1σ uncertainties, for individual clusters and, hence, mass functions for YSC systems. The mass function, known to be Gaussian for old Globular Cluster (GC) systems, is still controversial for YSC systems. GC formation is expected in massive gas-rich spiral–spiral mergers because of their high global star-formation efficiencies, and observed in ≥ 1 Gyr-old merger remnants. Yet, it has not been possible to identify young GCs among YSC populations. We suggest a compactness parameter involving masses and half-light radii of YSCs to investigate if young GCs are formed in starbursts and if the ratio of young GCs to more loosely bound star clusters depends on galaxy type, mass, burst strength, etc. 1. Star Formation Efficiencies & Star Cluster Formation Both burst strengths, b, defined by the relative increase of the stellar mass in the course of starbursts, b ≡ ∆Sburst Stotal , and Star Formation Efficiencies (SFEs), defined as the total stellar mass formed from the available mass of gas, SFE ≡ ∆Sburst G , are difficult to determine. Reasonable estimates are only possible in young post-starbursts. As long as a burst is active, only lower limits can be given. Once a burst is over, or if a burst lasts longer than the lifetimes of the most massive stars, the amount of stars already died needs to be accounted for. The stellar and gaseous masses before the burst can only be estimated on the basis of Hubble types, HI observations, etc. The strongest bursts are reported in mergers of massive gas-rich galaxies, with total burst durations on the order of a few 100 Myr. Bursts in massive interacting galaxies are much stronger and last much longer than those in isolated dwarf galaxies. Blue Compact Dwarf galaxies (BCDs), for instance, feature bursts with durations of the order of a few Myr, b 0.1, SFE ≤ 0.01, and a trend of decreasing burst strengths for increasing total galaxy masses (including HI) (Kruger et al. 1995). Massive interacting galaxies feature bursts that are stronger and more efficient by one toStarbursts produce large numbers of Young Star Clusters (YSCs). Multi-color photometry, in combination with a dedicated SED analysis tool, allows to derive ages, metallicities, E(B − V), and masses, including 1σ uncertainties, for individual clusters and, hence, mass functions for YSC systems. The mass function, known to be Gaussian for old Globular Cluster (GC) systems, is still controversial for YSC systems. GC formation is expected in massive gas-rich spiral-spiral mergers because of their high global star-formation efficiencies, and observed in ≥ 1 Gyr-old merger remnants. Yet, it has not been possible to identify young GCs among YSC populations. We suggest a compactness parameter involving masses and half-light radii of YSCs to investigate if young GCs are formed in starbursts and if the ratio of young GCs to more loosely bound star clusters depends on galaxy type, mass, burst strength, etc.

The stochastic effects of Ly

We investigate the variations in the broadband luminosities of low and intermediate redshift galaxies due to the stochastic nature of the neutral hydrogen distribution present in the form of Ly

\alpha

\alpha

absorbers on the broadband luminosities of galaxies

absorbers in the intergalactic medium. This effect is caused by variations in the distribution and properties (redshift, column density, Doppler parameter) of the absorbers along different lines-of-sight out to a given redshift. Using a set of observationally constrained redshift-, column density-, and Doppler parameter distributions we perform Monte Carlo simulations for a large number of lines-of-sight towards galaxies at a given redshift

Star cluster analyses from multi-band photometry: the key advantage of SALT's U -band sensitivity

z_{em}

Galaxy transformation processes in clusters.

and calculate attenuated FUV/NUV magnitudes and corresponding 1-, 2-, and 3