The Kinematics of 3 : 1 Merger Remnants and the Formation of Low‐Luminosity Elliptical Galaxies

Abstract

We test the formation of low-luminosity elliptical galaxies through collisionless mergers of unequal-mass disk galaxies. The kinematic properties of a small survey of simulated merger end products with initial disk mass ratios of 3 : 1 are compared to a sample of seven low-luminosity galaxies by H.-W. Rix and coworkers that were chosen photometrically to be ellipticals. In this paper, we go beyond a comparison in terms of global properties (using one number to characterize a model or a galaxy, e.g., a4, ellipticity at some fixed radius, or central velocity dispersion) and examine the detailed kinematics as a function of galactocentric distance. The merger remnants are observed through a slit along the major and minor axes, using a pixel binning and slit width similar to the ones used during the spectroscopic observations. Inside each bin, we determine the line-of-sight velocity distributions and parameterize them with Gauss-Hermite functions. We compare the rotational support of the merger remnants, i.e., the ratio of the mean line-of-sight velocity v to the velocity dispersion σ along the major axis, the normalized rotation on the minor axis, and the major-axis Gauss-Hermite moments h3, to those of the galaxies. The N-body remnants are very flattened when viewed edge-on ( ~ 0.6) and should be inclined before making a fair comparison with the data set of H.-W. Rix and coworkers (which has ~ 0.3). When the merger remnants are appropriately inclined, their v/σ profiles rise slower than the ones: v/σmerger is in the range [0.1, 0.8] at 1Reff and [0.2, 1.6] at 2Reff, whereas v/σobserved spans the intervals [0.8, 2.0] at 1Reff and [1.4, 3.5] at 2Reff. Note that even when the remnants are viewed edge-on, the v/σ profiles do not match the observations. The detailed comparison of the observations with our set of purely collisionless 3 : 1 merger remnants shows that these objects and low-luminosity ellipticals do not have similar kinematic profiles. This suggests that this kind of dissipationless merger (or mergers with more even masses, e.g., mass ratios of 2 : 1 or 1 : 1) is not likely to be the dominant formation channel for low-luminosity elliptical galaxies.