Kinematics of the faintest gas rich galaxy in the Local Group: DDO210

Abstract

We present deep, high velocity resolution (~1.6 km/s) Giant Meterwave Radio Telescope (GMRT) HI 21cm synthesis images of the faint (M_B ~ -10.6) local group dwarf galaxy DDO210. We find that the HI distribution in the galaxy is not axi-symmetric, but shows density enhancements in the eastern and southern halves of the galaxy. The optical emission is lopsided with respect to the HI, most of the bright optical emission arises from the eastern half of the HI disk. The velocity field of the galaxy is however quite regular and shows a systematic large scale pattern, consistent with the rotational motion. The rotation curve for the galaxy shows a peak (inclination corrected) rotation velocity of only ~8 km/s, comparable to the random motions in the HI gas. After correcting for the dynamical support provided by random motions (the ``asymmetric drift'' correction), we find the corrected peak rotation velocity of ~16.0 km/s. Mass modeling of the corrected rotation curve shows that the kinematics of DDO210 can be well fit with either a modified isothermal halo (with a central density rho_0 ~ 29 X 10^{-3} M_\odot pc^{-3} for a stellar mass to light ratio of 3.4) or an NFW halo. In the case of the NFW halo however, a good fit is obtained for a wide range of parameters; the halo parameters could not be uniquely determined from the fit. Density profiles with inner slope steeper than ~1.2 however provide a poor fit to the data. Finally, the rotation curve derived using MOND also provides a reasonable fit to the observed rotation curve.