A consensus is emerging that interacting galaxies show depressed nuclear gas metallicities compared to isolated star-forming galaxies. Simulations suggest that this nuclear underabundance is caused by interaction-induced inflow of metal-poor gas, and that this inflow concurrently flattens the radial metallicity gradients in strongly interacting galaxies. We present metallicities of over 300 H II regions in a sample of 16 spirals that are members of strongly interacting galaxy pairs with mass ratio near unity. The deprojected radial gradients in these galaxies are about half of those in a control sample of isolated, late-type spirals. Detailed comparison of the gradients with simulations show remarkable agreement in gradient distributions, the relationship between gradients and nuclear underabundances, and the shape of profile deviations from a straight line. Taken together, this evidence conclusively demonstrates that strongly interacting galaxies at the present day undergo nuclear metal dilution due to gas inflow, as well as significant flattening of their gas-phase metallicity gradients, and that current simulations can robustly reproduce this behavior at a statistical level.

We present new dynamical models of the merger remnant NGC 7252 which include star formation simulated according to various phenomenological rules. By using interactive software to match our model with the observed morphology and gas velocity field, we obtain a consistent dynamical model for NGC 7252. In our models, this proto‐elliptical galaxy formed by the merger of two similar gas‐rich disc galaxies which fell together with an initial pericentric separation of ∼2 disc scalelengths approximately 620 Myr ago. Results from two different star formation rules – density‐dependent and shock‐induced – show significant differences in star formation during and after the first passage. Shock‐induced star formation yields a prompt and wide‐spread starburst at the time of first passage, while density‐dependent star formation predicts a more slowly rising and centrally concentrated starburst. A comparison of the distributions and ages of observed clusters with results of our simulations favours shock‐induced mechanism of star formation in NGC 7252. We also present simulated colour images of our model of NGC 7252, constructed by incorporating population synthesis with radiative transfer and dust attenuation. Overall, the predicted magnitudes and colours of the models are consistent with observations, although the simulated tails are fainter and redder than observed. We suggest that a lack of star formation in the tails, reflected by the redder colours, is due to an incomplete description of star formation in our models rather than insufficient gas in the tails.

We present the results of a Hubble Space Telescope ACS/HRC FUV, ACS/WFC optical study into the cluster populations of a sample of 22 Luminous Infrared Galaxies in the Great Observatories All-Sky LIRG Survey. Through integrated broadband photometry we have derived ages and masses for a total of 484 star clusters contained within these systems. This allows us to examine the properties of star clusters found in the extreme environments of LIRGs relative to lower luminosity star-forming galaxies in the local Universe. We find that by adopting a Bruzual & Charlot simple stellar population (SSP) model and Salpeter initial mass function, the age distribution of clusters declines as dN/dτ = τ −0.9+/−0.3 , consistent with the age distribution derived for the Antennae Galaxies, and interpreted as evidence for rapid cluster disruption occuring in the strong tidal fields of merging galaxies. The large number of 10 6 M young clusters identified in the sample also suggests that LIRGs are capable of producing more high-mass clusters than what is observed to date in any lower luminosity star-forming galaxy in the local Universe. The observed cluster mass distribution of dN/dM = M −1.95+/−0.11 is consistent with the canonical -2 power law used to describe the underlying initial cluster mass function (ICMF) for a wide range of galactic environments. We interpret this as evidence against mass-dependent cluster disruption, which would flatten the observed CMF relative to the underlying ICMF distribution.

Galaxy interactions are known to trigger starbursts. The young star clusters formed in mergers may be young globular clusters. The ages of these young star clusters yield the timing of interaction-triggered star formation and provide an important way to reconstruct the history of merging galaxies. Here we present the first results from our investigation into age and metallicity of 12 young clusters in the merging galaxy pair NGC 4676, using spectra from the multiobject Low Resolution Imaging Spectrometer (LRIS) on Keck. For 10 clusters, comparison of the Balmer emission lines with model equivalent widths (EWs) yields ages less than 10 Myr. Two spectra display Balmer absorption lines typical of star clusters dominated by A-type stars, with estimated ages of about 170 Myr. These ages are comparable to the dynamical age of the tidal tails and are consistent with star formation triggered during the first passage of the pair. The locations of these two clusters in the tidal tails are generally consistent with predictions of shock-induced star formation models. One of these older objects appears unresolved on the image and is luminous enough to qualify as a young globular cluster. Using EWs of the diagnostic lines [O ii] and [O iii], we obtain oxygen abundances in the range . These values show a 7.3 ! 12 ϩ log (O/H) ! 9.0 nearly flat distribution along the northern tail, suggesting efficient gas mixing in the tail.