Extreme kinematic misalignment in IllustrisTNG galaxies: the origin, structure, and internal dynamics of galaxies with a large-scale counterrotation

Abstract

Modern galaxy formation theory suggests that the misalignment between stellar and gaseous components usually results from an external gas accretion and/or interaction with other galaxies. The extreme case of the kinematic misalignment is demonstrated by so-called galaxies with counterrotation that possess two distinct components rotating in opposite directions with respect to each other. We provide an in-deep analysis of galaxies with counterrotation from IllustrisTNG100 cosmological simulations. We have found 25 galaxies with substantial stellar counterrotation in the stellar mass range of 2 × 109 − 3 × 1010 M⊙ . In our sample the stellar counterrotation is a result of an external gas infall happened ≈2 − 8 Gyr ago. The infall leads to the initial removal of pre-existing gas, which is captured and mixed together with the infalling component. The gas mixture ends up in the counterrotating gaseous disc. We show that $\approx 90\%$ of the stellar counterrotation formed in-situ, in the counterrotating gas. During the early phases of the infall, gas can be found in inclined extended and rather thin disc-like structures, and in some galaxies they are similar to (nearly-) polar disc or ring-like structures. We discuss a possible link between the gas infall, AGN activity and the formation of misaligned components. In particular, we suggest that the AGN activity does not cause the counterrotation, although it is efficiently triggered by the retrograde gas infall, and it correlates well with the misaligned component appearance. We also find evidence of the stellar disc heating visible as an increase of the vertical-to-radial velocity dispersion ratio above unity in both co- and counterrotating components, which implies the importance of the kinematical misalignment in shaping the velocity ellipsoids in disc galaxies.

Star formation in outer rings of S0 galaxies. IV. NGC 254 -- a double-ringed S0 with gas counter-rotation

“…A retrograde merger with a gas-rich galaxy in the past (Thakar & Ryden 1996, 1998 could naturally explain the large-scale counter-rotating gaseous disk, the disturbed morphology, the signs of counter-rotating stellar components in the central region R < 15 as a relic of an accreted stellar body, and the dynamically heated outer part of the disk. We cannot completely rule out retrograde accretion from cosmological filaments as the main source of counter-rotating gas (Algorry et al 2014;Khoperskov et al 2021); however, in the case of NGC 254 it seems less likely, because it does not explain the morphological disturbed appearance of NGC 254. Khoperskov et al (2021) studied galaxies with stellar counter-rotation in the IllustrisTNG simulations and showed how existing gas in their disks can initially be replaced by external accretion from retrograde orbits, accompanied by an efficient inflow to the center causing rejuvenation of the stellar population and AGN activity.…”

SDSS-IV MaNGA : spatial resolved properties of kinematically misaligned galaxies

“…These studies include morphologies/environments (Davis et al 2011;Barrera-Ballesteros et al 2014Jin et al 2016;Bassett et al 2017;Yu et al 2021), stellar populations (Chen et al 2016;Jin et al 2016;Bevacqua et al 2021), gas-phase metallicities (Chen et al 2016;Jin et al 2016), kinematics (Katkov et al 2014;Naab et al 2014;Bevacqua et al 2021). Following the development in the observations, gas-star misaligned galaxies are also found in cosmological simulation (Osman & Bekki 2017;Bassett et al 2017;Taylor et al 2018;Duckworth et al 2019;Starkenburg et al 2019;Duckworth et al 2020a,b;Khoperskov et al 2020;Khim et al 2021), in which we can trace back the formation histories of the misaligned galaxies, searching for the physical mechanisms responsible for misaligned gas-star kinematics. Although several different mechanisms, such as gas precession and AGN feedback, have been proposed to produce or make it easier to produce misaligned phenomena, all of these works share a common sense that external processes, e.g.…”

Modeling of Spiral Structure in a Multi-Component Milky Way-Like Galaxy

“…In our simulations, the gas surface density in the solar neighborhood was kept within the observed estimates of (H 2 , H I and H I I) σ g = 13.7 ± 1.6 M pc −2 [42]. The observational data for distant galaxies indicate a significant role of gas accretion, minor and major mergers, which makes the stellar-gas disk a non-conservative system, exerting a strong influence on the kinematics and morphology of galaxies [84].…”