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

Abstract

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.