No abstract

In this paper, we model dwarf galaxies as a two‐component system of gravitationally coupled stars and atomic hydrogen gas in the external force field of a pseudo‐isothermal dark matter halo, and numerically obtain the radial distribution of H i vertical scale heights. This is done for a group of four dwarf galaxies (DDO 154, Ho II, IC 2574 and NGC 2366) for which most necessary input parameters are available from observations. The formulation of the equations takes into account the rising rotation curves generally observed in dwarf galaxies. The inclusion of self‐gravity of the gas into the model at par with that of the stars results in scale heights that are smaller than what was obtained by previous authors. This is important as the gas scale height is often used for deriving other physical quantities. The inclusion of gas self‐gravity is particularly relevant in the case of dwarf galaxies where the gas cannot be considered a minor perturbation to the mass distribution of the stars. We find that three out of four galaxies studied show a flaring of their H i discs with increasing radius, by a factor of a few within several disc scale lengths. The fourth galaxy has a thick H i disc throughout. This flaring arises as a result of the gas velocity dispersion remaining constant or decreasing only slightly while the disc mass distribution declines exponentially as a function of radius.

In this paper, we use the outer-galactic HI scale height data as well as the observed rotation curve as constraints to determine the halo density distribution of the Andromeda galaxy (M31). We model the galaxy as a gravitationallycoupled system of stars and gas, responding to the external force-field of a known Hernquist bulge and the dark matter halo, the density profile of the latter being characterized by four free parameters. The parameter space of the halo is optimized so as to match the observed HI thickness distribution as well as the rotation curve on an equal footing, unlike the previous studies of M31 which were based on rotation curves alone. We show that an oblate halo, with an isothermal density profile, provides the best fit to the observed data. This gives a central density of 0.011 M ⊙ pc −3 , a core radius of 21 kpc, and an axis ratio of 0.4. The main result from this work is the flattened dark matter halo for M31, which is required to match the outer galactic HI scale height data. Interestingly, such flattened halos lie at the most oblate end of the distribution of halo shapes found in recent cosmological simulations.

A galactic disk in a spiral galaxy is generally believed to be embedded in an extended dark matter halo, which dominates its dynamics in the outer parts. However, the shape of the halo is not clearly understood. Here we show that the dark matter halo in the Milky Way Galaxy is prolate in shape. Further, it is increasingly more prolate at larger radii, with the vertical-to-planar axis ratio monotonically increasing to 2.0 at 24 kpc. This is obtained by modeling the observed steeply flaring atomic hydrogen gas layer in the outer Galactic disk, where the gas is supported by pressure against the net gravitational field of the disk and the halo. The resulting prolate-shaped halo can explain several long-standing puzzles in galactic dynamics, for example, it permits long-lived warps thus explaining their ubiquitous nature.

Superthin galaxies are low surface brightness disc galaxies, characterised by optical discs with strikingly high values of planar-to-vertical axes ratios (> 10), the physical origin and evolution of which continue to be a puzzle. We present mass models for three superthin galaxies: IC5249, UGC7321 and IC2233. We use high resolution rotation curves and gas surface density distributions obtained from HI 21 cm radio-synthesis observations, in combination with their two-dimensional structural surface brightness decompositions at Spitzer 3.6 µm band, all of which were available in the literature. We find that while models with the pseudo-isothermal (PIS) and the Navarro-Frenk-White (NFW) dark matter density profiles fit the observed rotation curves of IC5249 and UGC7321 equally well, those with the NFW profile does not comply with the slowly-rising rotation curve of IC2233. Interestingly, for all of our sample galaxies, the best-fitting mass models with a PIS dark matter density profile indicate a compact dark matter halo i.e., R c /R D < 2 where R c is the core radius of the PIS dark matter halo, and R D is the radial scale-length of the exponential stellar disc. The compact dark matter halo may be fundamentally responsible for the superthin nature of the stellar disc, and therefore our results may have important implications for the formation and evolution models of superthin galaxies in the universe.