Drivers of asymmetry in synthetic H i emission-line profiles of galaxies in the eagle simulation

Abstract

We study the shapes of spatially integrated $\rm{H}\, \small {{I}}$ emission line profiles of galaxies in the eagle simulation using three separate measures of the profile’s asymmetry. We show that the subset of eagle galaxies whose gas fractions and stellar masses are consistent with those in the xGASS survey also have similar $\rm{H}\, \small {{I}}$ line asymmetries. Central galaxies with symmetric $\rm{H}\, \small {{I}}$ line profiles typically correspond to rotationally supported $\rm{H}\, \small {{I}}$ and stellar disks, but those with asymmetric line profiles may or may not correspond to dispersion-dominated systems. Galaxies with symmetric $\rm{H}\, \small {{I}}$ emission lines are, on average, more gas rich than those with asymmetric lines, and also exhibit systematic differences in their specific star formation rates, suggesting that turbulence generated by stellar or AGN feedback may be one factor contributing to $\rm{H}\, \small {{I}}$ line asymmetry. The line asymmetry also correlates strongly with the dynamical state of a galaxy’s host dark matter halo: older, more relaxed haloes host more-symmetric galaxies than those hosted by unrelaxed ones. At fixed halo mass, asymmetric centrals tend to be surrounded by a larger number of massive subhaloes than their symmetric counterparts, and also experience higher rates of gas accretion and outflow. At fixed stellar mass, central galaxies have, on average, more symmetric $\rm{H}\, \small {{I}}$ emission lines than satellites; for the latter, ram pressure and tidal stripping are significant sources of asymmetry.

Ram Pressure Stripping in the EAGLE Simulation

“…Because we would like to trace the kinematics of individual gas particles, as we will describe in Section 3.2, we do not divide the particles into neutral and ionized components, but rather simply label each gas particle as either "neutral" or "ionized" based on whether its neutral hydrogen fraction is above or below 50%. The distribution of the neutral hydrogen fraction of gas particles in EAGLE galaxies tends to be strongly bimodal (Manuwal et al 2022), and thus the results should not depend significantly on the choice of 50% as the separation criterion. Neutral gas particles generally have temperature T ∼ 10 4 K, as EAGLE has an imposed temperature floor due its finite resolution, while ionized gas has T ∼ 10 5 -10 6 K.…”

Morphological Evolution of Disk Galaxies and Their Concentration, Asymmetry, and Clumpiness (CAS) Properties in Simulations across Toomre’s Q Parameter

“…Other studies attributed the formation of a lopsided bar to tidal interactions (Yozin & Bekki 2014;Łokas 2022;Varela-Lavin et al 2023). There were hypotheses that other astrophysical processes such as gas accretion (Bournaud et al 2005;Dupuy et al 2019) and stellar feedback (Manuwal et al 2022) could generate the lopsidedness. Our important finding is that the established bisymmetry can spontaneously be broken and the disk becomes lopsided even when the initial disk is constructed from a circularly symmetric profile if the gravitationally unstable fluctuations are taken into account.…”