THE IMPRINT OF DISSIPATION ON THE SHAPES OF MERGER REMNANT LOSVDs

The properties of elliptical galaxies are broadly consistent with simulated remnants of gas-rich mergers between spirals, motivating more detailed studies of the imprint of this formation mechanism on the remnant distribution function. Gas has a strong impact on the non-Gaussian shapes of the line-of-sight velocity distributions (LOSVDs) of the merger remnant, owing to the embedded disk that forms out of the gas that retains its angular momentum during the merger, and the strong central mass concentration from the gas that falls to the center. The deviations from Gaussianity are effectively parameterized by the Gauss-Hermite moments h(3) and h(4), which are related to the skewness and kurtosis of the LOSVDs. We quantify the dependence of the (h(3), h(4))-v/sigma relations on the initial gas fraction f(gas) of the progenitor disks in 1:1 mergers, using Gadget-2 simulations including star formation, radiative cooling, and feedback from supernovae and active galactic nuclei. For f(gas) less than or similar to 15%, the overall correlation between h(3) and v/sigma is weak, consisting of a flat negatively correlated component arising from edge-on viewing angles plus a steep positively correlated part from more face-on projections. The spread in v/sigma values decreases toward high positive h(4), and there is a trend toward lower h(4) values as fgas increases from 0% to 15%. For f(gas) greater than or similar to 20%, the (h(3), h(4))-v/sigma distributions look quite different-there is a tight negative h(3)-v/sigma correlation, and a wide spread in v/sigma values at all h(4), in much better agreement with observations. Re-mergers of the high-f(gas) remnants (representing dry mergers) produce slowly rotating systems with near-Gaussian LOSVDs. We explain all of these trends in terms of the underlying orbit structure of the remnants, as molded by their dissipative formation histories.