HI 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.

arXiv comment15 pages, 19 figures, Accepted for publication in MNRAS

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