Quadruple-peaked spectral line profiles as a tool to constrain gravitational potential of shell galaxies

Abstract

Context. Stellar shells observed in many giant elliptical and lenticular as well as a few spiral and dwarf galaxies presumably result from galaxy mergers. Line-of-sight velocity distributions of the shells could, in principle, if measured with a sufficiently high signalto-noise ratio, constitute a method to constrain the gravitational potential of the host galaxy. Aims. Merrifield & Kuijken (1998, MNRAS, 297, 1292) predicted a double-peaked line profile for stationary shells resulting from a nearly radial minor merger. In this paper, we aim at extending their analysis to a more realistic case of expanding shells, inherent to the merging process, whereas we assume the same type of merger and the same orbital geometry. Methods. We used an analytical approach as well as test particle simulations to predict the line-of-sight velocity profile across the shell structure. Simulated line profiles were convolved with spectral PSFs to estimate peak detectability. Results. The resulting line-of-sight velocity distributions are more complex than previously predicted due to nonzero phase velocity of the shells. In principle, each of the Merrifield & Kuijken (1998) peaks splits into two, giving a quadruple-peaked line profile, which allows more precise determination of the potential of the host galaxy and contains additional information. We find simple analytical expressions that connect the positions of the four peaks of the line profile and the mass distribution of the galaxy, namely, the circular velocity at the given shell radius and the propagation velocity of the shell. The analytical expressions were applied to a test-particle simulation of a radial minor merger, and the potential of the simulated host galaxy was successfully recovered. Shell kinematics can thus become an independent tool to determine the content and distribution of the dark matter in shell galaxies up to ∼100 kpc from the center of the host galaxy.

Tidal debris morphology and the orbits of satellite galaxies

“…This is in contrast to previous work (Merrifield & Kuijken 1998;Ebrová et al 2012;Sanderson & Helmi 2013) that uses shells to constrain host halo parameters. Those analyses concentrated on modeling shell systems around individual galaxies in detail through density and line-of-sight velocity matching, while here we are interested in the general conditions that lead to shells and streams (without assuming any spectral information) in order to examine the implications of observed merger morphologies for the distribution of satellite orbital properties around a large sample of galaxies.…”

Testing MOND gravity in the shell galaxy NGC 3923

“…To take this property into account, we will hereafter take r S,n as positive for even n and as negative for odd n. Self-consistent simulations show that formation of shells in several generations is common (Cooper et al 2011;Bartošková et al 2011;Seguin & Dupraz 1996). For the minor merger model, this manner of formation is actually needed to explain the high radial ranges observed in shell galaxies (the radial range is the ratio of radii of the innermost and the outermost shell); for example, in our test-particle simulations (Ebrová et al 2012) and that of Dupraz & Combes (1986), where the shells are forced to form in a single generation, more than ten shells are never visible. If the secondary core survives a passage through the primary center, it loses part of its kinetic energy by dynamical friction, so the shells formed in the subsequent generation lie in lower range of radii.…”

“…We do not offer the analytical expression for v S,n . However, it can be approximated by the phase velocity of apocenters v A,n because the two velocities are very similar (see Ebrová et al 2012, Table 1). The derivative of Eq.…”

Deep imaging of the shell elliptical galaxy NGC 3923 with MegaCam

“…Stellar shells in elliptical galaxies are likely remnants of galactic interactions (see Dupraz & Combes 1986;Ebrová 2013;or Bílek et al 2015 for a review). The most accepted formation scenario is the accretion of a small satellite along an almost radial trajectory (Quinn 1983) where the stars forming the shells are released from the progenitor when it passes through the pericenter.…”