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

H I 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 H 1 extent of the galaxies. We derive H I extends of 30 and 20 arcmin and total H I fluxes of 181 +/- 2.0 and 105 +/- 1.4 Jy km s(-1) for Sextans A and B, respectively. This result shows clearly the overestimate of the H 1 extent and total flux of 54 arcmin 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 arcsec (similar to 3.5 kpc) and 650 arcsec (similar to 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 an 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 an M/L of 0.2 represents best the data. Using the MOdified Newtonian Dynamics, 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.