HI survey of local group dwarf galaxies

Doctoral Thesis


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This thesis investigates the HI properties of 3 dwarf irregular galaxies and one blue compact dwarf galaxy of the Local Group. The data set of each dwarf irregular galaxy was obtained with the Karoo Array Telescope (KAT-7) while the blue compact dwarf was observed with the Dominion Radio Astrophysical Observatory (DRAO) synthesis telescope. While the main purpose of KAT-7 was to test technical solutions for MeerKAT, its capabilities, and that of DRAO, have enabled us to produce interesting Square Kilometer Array (SKA) pathfinder science. The compact configuration of the two telescopes, coupled with low system temperatures (Ts ys) provide a unique HI dataset on nearby dwarf galaxies; sensitivity to extended HI emission while providing intermediate resolution to derive the large scale kinematics. The following properties are investigated in details: i) the HI distribution, ii) kinematics and mass distribution, iii) star formation thresholds, and iv) possible mechanisms for the chaotic HI structure and kinematics in a blue compact dwarf galaxy. The main results of our study are: from the HI distribution, the KAT-7 results have revealed even more neutral gas mass in the outer disk of NGC 6822. We detect 23% more HI mass in NGC 6822 than what was detected with the Australian Telescope Compact Array (ATCA). The KAT-7 results have been able to show an overestimate of the HI extent previously reported for Sextans A using Effelsberg observations. For IC 10, the complex HI features detected with DRAO are by a factor of ∼ 2 more extended than previous interferometric HI studies. Rotation velocities are derived for each galaxy. For NGC 6822, the rotation curve probes the gravitational potential out to 5.8 kpc, ∼ 1 kpc further than existing measurements. The rotation curves of Sextans A and B decline in the outer regions and extends out to 3.5 kpc and 4 kpc respectively. The central region of IC 10 has a regularly rotating disk, and the rotation velocities rise slowly in the inner region < 0.35 kpc. Beyond this radius the velocities are almost constant until the last point where it rises again reaching a maximum velocity of ∼ 30 km s−1 . Mass models are used to describe the distribution of dark matter. The dark matter models used are the Navarro-Frenk and White (NFW) and pseudo-isothermal (ISO) sphere. The dark matter distribution in dwarf irregular galaxies is better described by the ISO model when a mass to light ratio of 0.2 is used. This M/L value was derived for the Spitzer 3.6 µm band surface brightness profiles. The galaxies are dark matter dominated at all radii, and the stellar potential is insignificant to account for the total observed kinematics. In the case of IC 10, the kinematics of the inner disk can be described without the need of a dark matter halo. This result does not exclude the possible presence of dark matter on larger scales. Critical densities for gravitational instabilities are calculated using a one-dimensional Toomre-Q and cloud-growth based on shear criterion for the 3 dwarf irregular galaxies. In all cases, we find that in regions of star formation, the cloud growth criterion based on shear explains better the star formation. This suggests that the local shear rate could be a key player in cloud formation for irregular galaxies. Simulations are carried out to investigate if an interaction between IC 10 and M 31 can reproduce the observed HI morphology and kinematics of IC 10. The simulations are carried out using the GALACTICS code. From these simulations, it is unlikely that the HI features we see in IC 10 are caused by an interaction with M 31. The features seen in the simulations are both larger and at lower column density than what can be reached by current observations. The HI extensions with different kinematics seen south, east and west of the main core of IC 10 are more likely the result of accretion.