Investigating the Disk – Halo Connection using Numerical Simulations

Abstract

Investigating stellar structures such as stellar bars within the central regions of galaxies holds promise as a means to examine the properties of dark matter halos associated with those galaxies. Cosmological simulations extensively explore dark matter halo properties, whereas estimating these properties from observations poses significant challenges. Various probes tracing the dark matter potential are employed to model the dark matter halo density profile of observed galaxies. Nonetheless, the distribution of angular momentum within dark matter halos remains a topic of ongoing debate. We investigate a hybrid method for estimating the halo spin of a low surface brightness (LSB), gasrich dwarf barred galaxy UGC 5288, by forward modelling disk properties derived from observations (stellar and gas surface densities, disk scale length, HI rotation curve, bar length and bar ellipticity) into N-body simulations. We combine semi-analytical techniques, N-body/SPH and cosmological simulations to model the dark matter halo of UGC 5288 with both a cuspy Hernquist profile and a flat-core pseudo-isothermal profile. We find that the best match with observations is a pseudo-isothermal halo model with a core radius of rc = 0.23 kpc, and halo spin of λ= 0.08 at the virial radius. Although our findings are consistent with previous core radius estimates of the halo density profile of UGC 5288, as well as with the halo spin profiles of similar mass analogues of UGC5288 in the high-resolution cosmological-magneto-hydrodynamical simulation TNG50, there still remain some uncertainties as we are limited in our knowledge of the formation history of the galaxy. Furthermore, we investigate the connection of halo spin λ and galaxy properties in the presence/absence of stellar bars, using the cosmological magneto-hydrodynamic TNG50 simulations at three redshifts zr = 0, 0.1 and 1. We estimate the halo spin for barred and unbarred galaxies (bar strength: 0 < A2/A0 < 0.7) at the central regions of the dark matter halo close to the galaxy disk and far from the disk, close to halo virial radius. At zr = 0 and 0.1, strongly barred galaxies (A2/A0 > 0.4) reside in dark matter halos having low spin and low specific angular momentum, while unbarred and weakly barred galaxies (A2/A0 < 0.2) are hosted in high spin and high specific angular momentum halos. The inverse correlation between bar strength and halo spin at low redshifts changes to a more complex form at higher redshift (zr = 1) with higher halo spin for all galaxies than that at zr = 0. Using galaxy samples across various dark matter halo mass ranges, we highlight the importance of sample selection in obtaining meaningful results. We delve deeper into the physical mechanisms responsible for bar formation and destruction in galaxies. While we have gained valuable insight into how bars form and evolve from isolated idealized simulations, in the cosmological domain, galactic bars evolve in complex environments with mergers, gas accretion events, in presence of turbulent Inter Stellar Medium with multiple star formation episodes, in addition to coupling to their host galaxies’ dark matter halos. We investigate bar formation in 13 Milky Way-mass galaxies from the FIRE-2 (Feedback in Realistic Environments) cosmological zoom-in simulations. 8 of the 13 simulated galaxies form bars at some point during their history: three from tidal interactions and five from internal evolution of the disk. We find that bar formation in FIRE-2 galaxies is influenced by satellite interactions and the stellar-to-dark matter mass ratio in the inner galaxy, but neither is a sufficient condition for bar formation. Bar formation is more likely to occur, and the bars formed are stronger and longer-lived, if the disks are kinematically cold; galaxies with high central gas fractions and/or vigorous star formation, on the other hand, tend to form weaker bars. In the case of the FIRE-2 galaxies these properties combine to produce ellipsoidal bars with strengths A2/A0 ∼ 0.1–0.2, bar lengths smaller than the corotation (mean bar radius ∼ 1.53 kpc), a wide range of bar pattern speeds (36–97 km s−1 kpc−1), and bars that live for a wide range of dynamical times (2–160 bar rotations). Our last project is motivated from the dark matter halo shape measurements for dwarf galaxies in Das et al. (2023). The authors find oblate and spherical dark matter halo shapes of dwarf galaxies in their sample. We investigate the possibility of prolate shapes of dark matter halos of dwarf galaxies using isolated N-Body simulations of galaxies having a baryonic disk and a dark matter halo and also with dark matter-only simulations. We model the dark matter halos of dwarf galaxies with flat-cored pseudoisothermal density profiles based on realistic galaxy and dark matter halo properties from empirical relations derived from observations to study the stability of prolate dark matter halo shapes. We compare our results with simulated models of similar mass cuspy Hernquist density profiles of dark matter halos. We find that prolate halos with flat-cored density profiles are much less stable than those in cuspy dark matter halos with similar parameters. Our results suggest that prolate halos cannot survive in low mass galaxies with stellar masses 107 to 109M⊙. The studies presented in the Thesis and in previous literature suggests a connection between the characteristics of baryonic disks, stellar bars, and the underlying dark matter halos of galaxies. Utilizing the connection between stellar structures within galaxies and the properties of dark matter halos offers an opportunity to investigate the dark matter halo properties of observed galaxies.