Constraints on the halo density profile using HI flaring in the outer Galaxy

Abstract

Abstract. The observed flaring of HI disk in the outer region of galaxies has been used in the past to determine the shape of the dark matter halo. Previous studies based on this concept suggest a slightly oblate halo (axis ratio ∼0.8) for our Galaxy. We reinvestigate this problem by calculating the HI scaleheight in the outer Galaxy to a larger radial distance, and by studying its dependence on the shape and the density profile of the halo. We find that a simple isothermal infinite halo of any shape-oblate or prolate, is not able to account for the observed flaring. Instead we show that a spherical halo with density falling faster than isothermal halo in the outer region provides a better fit to the observed HI flaring as well as the observed rotation curve of our Galaxy. These halos have about 95% of their mass within a few hundreds of kpc for R • = 8.5 kpc and Θ • = 220 km s −1 , the central density and core radius can be constrained to the range ρ • = 0.035−0.06 M pc −3 and R c = 8−10 kpc. Our claim for such "finite-sized" spherical halos is supported by recent literature on numerical simulation studies of halo formation as well as analyses of SDSS data.

Dark matter in the Milky Way

“…First we discuss non-singular spheroids with flattening q (see e.g. Olling & Merrifield 2001;Narayan et al 2005),…”

“…Oblate halos have q < 1, prolate q > 1. ρ 0 is the density at the center, R c defines the core radius, and p is the power index (Narayan et al 2005). We consider the case p = 1 first.…”

“…They conclude that significantly flattened halos are possible only in the case that the distance of the Sun from the Galactic center is smaller than 6.8 kpc. Narayan et al (2005) south, q=1 DM spheroid, q =64 DM spheroid, q =8 DM spheroid, q =2 DM spheroid, q =1 DM spheroid, q =1/2 DM spheroid, q =1/4 DM spheroid, q =1/8…”

“…Applying a similar analysis to the Milky Way, however, has led to conflicting results (Olling & Merrifield 1998, 2000. Narayan et al (2005) show that neither an oblate nor a prolate isothermal halo can explain the Milky Way flaring. They advocate a steeper fall-off for the halo density.…”

Does the Stellar Distribution Flare? A Comparison of Stellar Scale Heights With Lab H I Data

“…Olling (1995) demonstrated that the shape of the flaring H i layer is affected by the flattening of the dark matter component. Olling & Merrifield (2000, 2001, and later Narayan & Jog (2002), Kalberla (2003), Narayan et al (2005), and Kalberla et al (2007) applied this basic idea to model the Milky Way mass distribution using fullsky H i data. According to these investigations, it became evident that stellar flaring is also expected to be observed (Jog 2007).…”