The little galaxy that could: kinematics of Camelopardalis B

Abstract

We present deep, high velocity resolution (^~1.6 km s⁻¹) Giant Meterwave Radio Telescope HI 21 cm synthesis images, as well as optical broad band images, for the faint (M_B~-10.9) dwarf irregular galaxy Camelopardalis B. We find that the HI in the galaxy has a regular velocity field, consistent with rotational motion. Further, the implied kinematical major axis is well aligned with the major axis of both the HI flux distribution as well as that of the optical emission. Camelopardalis B is the faintest known galaxy with such relatively well behaved kinematics. From the HI velocity field we derive a rotation curve for the galaxy. The rotation curve can be measured out to galacto-centric distances >4 times the optical scale length. The peak (inclination corrected) rotation velocity v_o is only ~7 km σ⁻¹-the high velocity resolution of our observations was hence critical to measuring the rotation curve. Further, the peak rotational velocity is comparable to the random velocity s of the gas, i.e. v_o/σ~1. This makes it crucial to correct the observed rotation velocities for random motions before trying to use the kinematics to construct mass models for the galaxy. After applying this correction we find a corrected peak rotation velocity of ~20 km s⁻¹. On fitting mass models to the corrected rotation curve we find a good fit for a constant density halo with a density of ρ₀~12 M_⊙ pc⁻³. This density is well determined, i.e. it has a very weak dependence on the assumed mass to light ratio of the stellar disk. We also find that the corrected rotation curve cannot be fit with an NFW halo regardless of the assumed mass to light ratio. Finally we compile from the literature a sample of galaxies (ranging from normal spirals to faint dwarfs) with rotation curves obtained from HI synthesis observations. The complete sample covers a luminosity range of ~12 magnitudes. From this sample we find: (i) that Camelopardalis B lies on the Tully-Fisher relation defined by these galaxies, provided we use the corrected rotation velocity, and (ii) a weak trend for increasing halo central density with decreasing galaxy size. Such a trend is expected in hierarchical models of halo formation.