The Highly Flattened Dark Matter Halo of NGC 4244

In a previous paper (Olling 1995, \aj, 110, 591; astro-ph/9505002) a method was developed to determine the shapes of dark matter halos of spiral galaxies from an accurate determination of the rotation curve, the flaring of the gas layer and the velocity dispersion in the HI. Here this method is applied to the almost edge-on Scd galaxy NGC 4244 for which the necessary parameters are determined in the accompanying paper (AJ, Aug. 1996; astro-ph/9605110). The observed flaring of the HI beyond the optical disk puts significant constraints on the shape of the dark matter halo, which are almost independent of the stellar mass-to-light ratio. NGC 4244's dark matter halo is found to be highly flattened with a shortest-to-longest axis ratio of 0.2 (-0.1)(+0.3). If the dark matter is disk-like, the data presented in this paper imply that the vertical velocity dispersion of the dark matter must be 10% - 30% larger than the measured tangential dispersion in the HI. Alternatively, the measured flaring curve is consistent with a round halo if the gaseous velocity dispersion ellipsoid is anisotropic. In that case the vertical dispersion of the gas is 50 - 70% of the measured tangential velocity dispersion.Comment: 16 pages LaTeX, uses aaspptwo style (357kByte). Includes 3 figures. Complete paper, is also available at http://www.astro.soton.ac.uk/~olling/PrePrints/Paper_03/ or via anonymous ftp at ftp.astro.soton.ac.uk, cd pub/olling/Paper_03 To be published in the Aug. 1996 issue of the Astronomical Journa

NGC 4244: A Low Mass Galaxy with a Falling Rotation Curve and a Flaring Gas Layer

I present sensitive high resolution HI VLA B+C+D array observations of the edge-on Scd galaxy NGC 4244. The gas layer of NGC 4244 is rather symmetric in all respects, i.e. the surface density distribution, flaring and warping, so that the rotation curve (RC) is determined accurately, despite the fact that i~90^o. The RC rises slowly in the inner 6 kpc, is ~ constant at 100 km/s out to 10kpc, and decreases in Keplerian fashion by 15% at the last measured point at 14 kpc. The RC constrains the stellar mass-to-light ratio to lie between 50 and 100% of the ``maximum-disk'' value. A new technique is presented to determine simultaneously the inclination and the thickness of the gas layer from HI observations which uses the apparent widths at many channels and works for i >= 60^o. The inclination of the un- warped disk is about 84.5 degrees: beyond D25/2 the inclination changes gra- dually till 82.5^o, while at large radii the disk seems to warp back to the plane defined by the inner disk. The gaseous velocity dispersion is roughly constant within the optical disk (8.5+/-1 km/s) and increases slightly beyond. The thickness of the gas layer increases gradually from 0.4 to 1.5 kpc. In an accompanying paper (AJ, Aug. 1996; astro-ph/9605111). I use the measurements presented in this paper to infer that the dark matter halo of NGC 4244 is highly flattened.Comment: 30 pages LaTeX, uses aaspptwo style (439kByte). Includes 3 figures. Complete document including 16 figures (5.4 MByte) is available at http://www.astro.soton.ac.uk/~olling/PrePrints/Paper_02/ or via anonymous ftp at ftp.astro.soton.ac.uk, cd pub/olling/Paper_02 To be published in the Aug. 1996 issue of the Astronomical Journa

On the usage of Flaring Gas Layers to determine the Shape of Dark Matter Halos

I present a new method of deriving the shape of the dark matter (DM) halos of spiral galaxies. The method relies on the comparison of model predictions with high spectral and spatial resolution HI observations of the gas layer. The potential arising from the {\em total} mass distribution of the galaxy is used in the calculation of the vertical distribution of the gas. I developed a new algorithm to calculate the force field of an arbitrary, azimuthally symmetric, density distribution. This algorithm is used to calculate the forces due to the radially truncated stellar disk as well as of the flaring gas layer. I use a simple two-parameter family of disk-halo models which have essentially the same observed equatorial rotation curve but different vertical forces. This mass model is composed of a stellar disk with constant M/L, and a DM-halo with a given axial ratio. I approximate the radial force due to the gaseous disk, and iteratively determine the vertical force due to the global distribution of the gas. The thickness of the gaseous disk is sensitive to both the flattening of the DM-halo and the self-gravity of the gas, but not to the particular choice of disk-halo decomposition. I show that the determination of the thickness of the gas layer is not restricted to edge-on galaxies, but can be measured for moderately inclined systems as well.Comment: 40 pages including 14 Postscript figures (2MByet) also available via anonymous ftp at ftp://parsifal.phys.columbia.edu/pub/olling/ as files Halo-Shape.ps(.Z) (2360 kb, uncompressed), Halo-Shape-txt.ps.Z (the text, 362 kb, uncompressed) Halo-Shape-F##.ps.Z (the figures, 1173 kb, uncompressed

Luminous and Dark Matter in the Milky Way

(Abridged) Axisymmetric models of the Milky Way exhibit strong interrelations between the Galactic constants (R_0 and T_0), the stellar columndensity (S_*) and the shape of the dark matter (DM) halo. Here we present analytical relations that can be used to investigate the effects of the uncertain gaseous velocity dispersion on the HI flaring constraints. The contribution of cosmic rays and magnetic fields to the pressure gradients is small. A significantly flattened dark matter halo is only possible if R_0 <~ 6.8 kpc. If R_0 is larger than ~7 kpc, or T_0 >~ 170 km/s, we can rule out two DM candidates that require a highly flattened DM halo: 1) decaying massive neutrinos; and 2) a disk of cold molecular hydrogen. It is only possible to construct self-consistent models of the Galaxy based on the IAU-recommended values for the Galactic constants in the unlikely case that the the stellar columndensity is smaller than ~18 M_sun/pc^2. If we assume that the halo is oblate and S_* = 35 +/- 5 M_sun/pc^2, R_0 <~ 8 kpc and T_0 <~ 200 km/s. Combining the best kinematical and star-count estimates of S_*, we conclude that: 25 <~ S_* <~ 45 M_sun/pc^2. Kuijken & Gilmore's (1991) determination of the columndensity of matter with |z|<=1.1 kpc is robust and valid over a wide range of Galactic constants. Our mass models show that the DM density in the Galactic centre is uncertain by a factor 1000. In the Solar neighbourhood we find: rho_DM ~0.42 GeV/c^2/cm^3 or (11 +/- 5) mM_sun/pc^3 -- roughly 15% of rho_tot.Comment: Accepted for publication in MNRA