dc.description.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. |
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