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

I present sensitive high resolution VLA B, C, and D array observations of the almost edge-on Scd galaxy NGC 4244 in the 21-cm spectral line of neutral atomic hydrogen. The gas layer of NGC 4244 is rather symmetric in all respects, i.e. the surface density distribution, flaring and warping. This symmetry allows for a reliable determination of the rotation curve, despite the fact that the galaxy is close to edge-on. The rotation curve rises slowly in the inner 6 kpc, is roughly constant at 100 km s out to 10 kpc, and decreases in Keplerian fashion by 15% at the last measured point at 14 kpc. The rotation curve 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 high resolution H I observations. This procedure uses the apparent widths at many azimuths (many channels) and can be used at inclinations as low as 60 . Kinematic information is used to separate flaring from warping. The inclination of the unwarped disk is about 84.5, while the small warp coincides with a decreasing inclination (to 82.5 ± 1 ). The data indicates that at large radii the disk warps back to the plane defined by the inner disk. The measured gaseous velocity dispersion is roughly constant within the optical disk (8.5 ± 1 km s) and increases slightly beyond. On both sides of the galaxy the thickness of the gas layer increases gradually from ∼400 pc at 5 kpc to ∼1.5 kpc at the last measured point (at 13 kpc). The strong gradients in the inferred thickness which bracket the spiral arms probably result from streaming motions associated with the arms and are not intrinsic to the galaxy. In an accompanying paper (AJ, Aug. 1996) I use the measurements presented in this paper to infer that the dark matter halo of NGC 4244 is highly flattened.