Dynamically-Driven Star Formation In Models Of NGC 7252
  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 disk galaxies which fell together with an initial pericentric
separation of ~2 disk scale lengths 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
favors shock-induced mechanism of star formation in NGC 7252. We also present
simulated color images of our model of NGC 7252, constructed by incorporating
population synthesis with radiative transfer and dust attenuation. Overall the
predicted magnitudes and colors 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 colors, is
due to an incomplete description of star formation in our models rather than
insufficient gas in the tails.
