The slowing down of galaxy disks in dissipationless minor mergers

Abstract

We have investigated the impact of dissipationless minor galaxy mergers on the angular momentum of the remnant. Our simulations cover a range of initial orbital characteristics, and the system consists of a massive galaxy with a bulge and disk merging with a much less massive (one-tenth or one-twentieth) gasless companion that has a variety of morphologies (disk-or elliptical-like) and central baryonic mass concentrations. During the process of merging, the orbital angular momentum is redistributed into the internal angular momentum of the final system; the internal angular momentum of the primary galaxy can increase or decrease depending on the relative orientation of the orbital spin vectors (direct or retrograde), while the initially nonrotating dark matter halo always gains angular momentum. The specific angular momentum of the stellar component always decreases independently of the orbital parameters or morphology of the satellite, the decrease in the rotation velocity of the primary galaxy is accompanied by a change in the anisotropy of the orbits, and the ratio of rotation speed to velocity dispersion of the merger remnant is lower than the initial value, not only because of an increase in the dispersion but also of the slowing-down of the disk rotation. We briefly discuss several astrophysical implications of these results, suggesting that minor mergers do not cause a "random walk" process of the angular momentum of the stellar disk component of galaxies, but rather a steady decrease. Minor mergers may play a role in producing the large scatter observed in the Tully-Fisher relation for S0 galaxies, as well as in the increase of the velocity dispersion and the decrease in v/σ at large radii as observed in S0 galaxies.

The Fossil Record of Two-Phase Galaxy Assembly: Kinematics and Metallicities in the Nearest S0 Galaxy

“…In this case, inner bulges form at high redshift while subsequent outer bulge and halo growth is driven primarily by dry minor-merger accretion events. The satellites fall in from many different directions and provide little net rotational support (Vitvitska et al 2002;Abadi et al 2006;Bournaud et al 2007;Qu et al 2010). The radial decline in rotation of the MRGC system could then represent a transition from an inner bulge formed in violent, dissipative processes at high redshift, to an outer spheroid (around one-third of the bulge mass in the case of NGC 3115) built largely from accreted material over a more protracted period.…”

Characteristics of thick disks formed through minor mergers: stellar excesses and scale lengths

“…A natural outcome of minor mergers is therefore that both the significance and scale height of the thick disk should be related to whether a galaxy is a late-or early-type disk type. Moreover, since mergers can have a wide range of initial orbital parameters, we would expect to find counter-rotating thick disk stars if the interaction are violent enough or that the some satellites haves sufficiently high density to begin to contribute significantly to the kinematics of the thick disk (Qu et al 2011). Observations have suggested that the thick disk scale height is related to Hubble type (de Grijs & Peletier 1997), that some disk dominated late-type galaxies do not have significant thick disks, and that some thick disks counter-rotate (Morrison et al 1994;Gilmore et al 2002;Yoachim & Dalcanton 2005).…”

“…It consists of a spherical stellar and a dark matter component, both modeled with initially non-rotating Plummer profiles, whose parameters are also given in Table 1. As we have done in Qu et al (2010), all galaxy models are evolved in isolation for 1 Gyr before the interaction starts. The orbital parameters for these minor interactions have been described in Chilingarian et al (2010 , Table 9) and Qu et al (2010, Table 2) and we refer the reader to these papers for a detailed description.…”

“…As we have done in Qu et al (2010), all galaxy models are evolved in isolation for 1 Gyr before the interaction starts. The orbital parameters for these minor interactions have been described in Chilingarian et al (2010 , Table 9) and Qu et al (2010, Table 2) and we refer the reader to these papers for a detailed description. In this paper, we follow the nomenclature adopted in Qu et al (2010), i.e., using a six-character string to indicate the morphology of the interacting galaxies: the first three describe the type of the primary galaxy, "gS0", "gSa" or "gSb", depending on its gas fraction, and the following three describe the satellite galaxy, dE0 or sE0.…”

“…A detailed study of various kinematic properties of minor merger remnants will be made separately (Qu et al 2011), and here we will discuss only one of the kinematical characteristics of the stellar excess: its rotational lag with respect to thick disk stars. This rotational lag is clear in Fig.…”