H i observations of Sextans A and B with the SKA pathfinder KAT-7

Abstract

HI observations of the Local Group dwarf irregular galaxies Sextans A and B, obtained with the Karoo Array Telescope (KAT-7) are presented. The KAT-7 wide field of view and excellent surface brightness sensitivity allows us to verify the true HI extent of the galaxies. We derive HI extends of 30$^{\prime}$ and 20$^{\prime}$ and total HI fluxes of 181 $\pm$ 2.0 Jy.km.s$^{-1}$ and 105 $\pm$ 1.4 Jy.km.s$^{-1}$ for Sextans A and B respectively. This result shows clearly the overestimate of the HI extent and total flux of 54$^{\prime}$ and 264 Jy.km.s$^{-1}$ reported for Sextans A using the Effelsberg observations. Tilted ring models allow us to derive the rotation curves (RCs) of Sextans A and B out to 550$^{\prime \prime}$ ($\sim$ 3.5 kpc) and 650$^{\prime \prime}$ ($\sim$ 4 kpc) respectively. The RCs of the two galaxies are seen to decline in the outer parts. The dark matter distribution in Sextans A is better described by the pseudo-isothermal halo model when a M/L ratio of 0.2 is used. For Sextans B, the mass model fits are not as good but again an isothermal sphere with a M/L of 0.2 represents best the data. Using the MOdified Newtonian Dynamics (MOND), better fits are obtained when the constant a$_{0}$ is allowed to vary. The critical densities for gravitational instabilities are calculated using the Toomre-$Q$ and cloud-growth based on shear criterion. We find that in regions of star formation, the cloud growth criterion based on shear explains better the star formation in both Sextans A and B.