High Resolution HI Observations of the Galaxy NGC404: a dwarf S0 with abundant Interstellar gas

Abstract

(Abridged) We present HI maps of the S0 galaxy NGC404, at a spatial resolution of 15.2" x 14.4" and a velocity resolution of 2.6km/s. The HI has been traced out to a radius R~8R_25 or 48 disc scale-lengths, making it one of the largest HI extents reported (800" or 12.6kpc at the assumed distance of 3.3Mpc). Approximately 75% of the HI resides in a doughnut which is seen close to face-on and the optical galaxy fits snugly within the hole of the doughnut. The remaining 25% of the neutral gas is found in an annulus concentric with the doughnut and with a somewhat larger ellipticity. A total HI mass of 1.52 x 10^8 Msol is found, which implies an M(HI)/L_B = 0.22 in solar units. We argue that most if not all of this gas is of external origin, most likely due to the merger of a dwarf irregular galaxy with M_B ~ -15.5mag. The velocity field shows a steeply declining observed rotation curve, compatible with Keplerian decline. However, because the galaxy is close to face-on there is a degeneracy in the determination of the intrinsic rotation curve and inclination. We therefore analysed two extreme cases, producing tilted-ring model fits forcing either a Keplerian or a Flat rotation curve through the observations; both approaches result in equally plausible fits. Assuming a distance of 3.3Mpc, a total mass of 3 x 10^10 Msol is found on the basis of the Keplerian rotation curve. This implies a global M/L_B ratio of ~44 in solar units which is high and likely a reflection of the low blue luminosity of the galaxy (~15 times lower than the average S0 luminosity). Values for a Flat rotation curve are a factor of 4 higher.