Abstract
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 parametrized 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 of the progenitor disks in 1:1 mergers, using Gadget-2 simulations including star formation, radiative cooling, and feedback from supernovae and AGN. For gas fractions f_gas < ~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 face-on projections. The spread in v/sigma values decreases toward high positive h_4, and there is a trend toward lower h_4 as the gas fraction increases from 0 to 15%. For f_gas > ~20% the (h_3,4)- 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 better agreement with observations. Re-mergers of the high-f_gas remnants (dry mergers) produce slowly rotating systems with nearly 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.
Highlights
Many of the observed properties of elliptical galaxies indicate a violent formation history
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 parametrized by the Gauss-Hermite moments h3 and h4, which are related to the skewness and kurtosis of the LOSVDs
Summary
Many of the observed properties of elliptical galaxies indicate a violent formation history. For example the remnant may tend to rotate in the sense of the initial orbital angular momentum or disk spins (Barnes 1992; Hernquist 1992, 1993), and an initial stellar metallicity gradient will be blurred but not erased during the merger (White 1978) This incompleteness is enhanced by the presence of gas, which can form cold features in the stellar distribution relatively late in the merger process, which subsequently experience less violent relaxation than features present at the beginning of the merger. Dehnen & Gerhard 1994), but can be accounted for by the superposition of a hot stellar spheroid with a cold embedded disk This type of distribution function arises naturally from a combination of violent relaxation and dissipation in gas-rich mergers of spiral galaxies (Hopkins et al 2009b). Our primary goals are: (a) to see whether this diagnostic points to the same typical gas fraction as other indicators, such as the FP tilt and peak quasar redshift; (b) to locate “wet” (gas-rich disk-disk) and “dry” (re-mergers of gas-poor spheroid) mergers in (h3,4 − v/σ) space, with the aim of distinguishing these two populations in IFS observations; and (c) to connect the observed trends in the non-Gaussian moments with the underlying orbit structure typical of merger remnants in an intuitive way
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