Evolution of cold gas in active galaxies.

Master of Chemistry and Physics. University of KwaZulu-Natal, Durban 2015.Abstract available in PDF file

HI survey of local group dwarf galaxies

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

H i kinematics, mass distribution and star formation threshold in NGC 6822, using the SKA pathfinder KAT-7

We present high sensitivity H I observations of NGC 6822, obtained with the Karoo Array Telescope (KAT-7). We study the kinematics, the mass distribution and the star formation thresholds. The KAT-7 short baselines and low system temperature make it sensitive to large-scale, low surface brightness emission. The observations detected ∼ 23 per cent more flux than previous Australian Telescope Compact Array observations. We fit a tilted ring model to the H I velocity field to derive the rotation curve (RC). The KAT-7 observations allow the measurement of the rotation curve of NGC 6822 out to 5.8 kpc, ∼1 kpc further than existing measurements. NGC 6822 is seen to be dark matter dominated at all radii. The observationally motivated pseudo-isothermal dark matter (DM) halo model reproduces well the observed RC while the Navarro Frank-White DM model gives a poor fit to the data. We find the best-fitting mass-to-light ratio (M/L) of 0.12 ± 0.01 which is consistent with the literature. The modified Newtonian dynamics gives a poor fit to our data. We derive the star formation threshold in NGC 6822 using the H I and H α data. The critical gas densities were calculated for gravitational instabilities using the Toomre-Q criterion and the cloud-growth criterion. We found that in regions of star formation, the cloud-growth criterion explains star formation better than the Toomre-Q criterion. This shows that the local shear rate could be a key player in cloud formation for irregular galaxies such as NGC 6822

Mightee-h i: Possible interactions with the galaxy NGC 895

The transformation and evolution of a galaxy is strongly influenced by interactions with its environment. Neutral hydrogen (H i) is an excellent way to trace these interactions. Here, we present H i observations of the spiral galaxy NGC 895, which was previously thought to be isolated. High-sensitivity H i observations from the MeerKAT large survey project MIGHTEE reveal possible interaction features, such as extended spiral arms and the two newly discovered H i companions, that drive us to change the narrative that it is an isolated galaxy. We combine these observations with deep optical images from the Hyper Suprime Camera to show an absence of tidal debris between NGC 895 and its companions

an overview of the MHONGOOSE survey: Observing nearby galaxies with MeerKAT

© Copyright owned by the author(s). MHONGOOSE is a deep survey of the neutral hydrogen distribution in a representative sample of 30 nearby disk and dwarf galaxies with H I masses from ∼ 106 to ∼ 1011 M, and luminosities from MR ∼ 12 to MR ∼ −22. The sample is selected to uniformly cover the available range in log(MHI). Our extremely deep observations, down to H I column density limits of well below 1018 cm−2 — or a few hundred times fainter than the typical H I disks in galaxies — will directly detect the effects of cold accretion from the intergalactic medium and the links with the cosmic web. These observations will be the first ever to probe the very low-column density neutral gas in galaxies at these high resolutions. Combination with data at other wavelengths, most of it already available, will enable accurate modeling of the properties and evolution of the mass components in these galaxies and link these with the effects of environment, dark matter distribution, and other fundamental properties such as halo mass and angular momentum. MHONGOOSE can already start addressing some of the SKA-1 science goals and will provide a comprehensive inventory of the processes driving the transformation and evolution of galaxies in the nearby universe at high resolution and over 5 orders of magnitude in column density. It will be a Nearby Galaxies Legacy Survey that will be unsurpassed until the advent of the SKA, and can serve as a highly visible, lasting statement of MeerKAT’s capabilities

MeerKAT-16 H I observation of the dIrr galaxy WLM

We present observations and models of the kinematics and the distribution of the neutral hydrogen (HI) in the isolated dwarf irregular galaxy, Wolf-Lundmark-Melotte (WLM). We observed WLM with the Green Bank Telescope (GBT) and as part of the MeerKAT Early Science Programme, where 16 dishes were available. The HI disc of WLM extends out to a major axis diameter of 30 arcmin (8.5 kpc), and a minor axis diameter of 20 arcmin (5.6 kpc) as measured by the GBT. We use the MeerKAT data to model WLM using the TiRiFiC software suite, allowing us to fit different tilted-ring models and select the one that best matches the observation. Our final best-fitting model is a flat disc with a vertical thickness, a constant inclination and dispersion, and a radially-varying surface brightness with harmonic distortions. To simulate bar-like motions, we include second-order harmonic distortions in velocity in the tangential and the vertical directions. We present a model with only circular motions included and a model with non-circular motions. The latter describes the data better. Overall, the models reproduce the global distribution and the kinematics of the gas, except for some faint emission at the 2-sigma level. We model the mass distribution of WLM with a pseudo-isothermal (ISO) and a Navarro-Frenk-White (NFW) dark matter halo models. The NFW and the ISO models fit the derived rotation curves within the formal errors, but with the ISO model giving better reduced chi-square values. The mass distribution in WLM is dominated by dark matter at all radii.Comment: Accepted for publication in MNRAS, 25 pages, 21 figures, 5 table