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The Astronomy department organises regular research colloquia. Visiting academics, staff, postdocs and postgraduate students who have recently completed their dissertation present colloquia in the department. 

Wednesday 20 March 2019 [13:00 - 14:00] 

Speaker: Dr Maciej Bilicki (Center for Theoretical Physics, Polish Academy of Sciences, Poland)

Title: The widest-angle photometric galaxy catalogues and their applications

Abstract: Many extragalactic and cosmological applications require complete three-dimensional catalogues of galaxies covering significant fractions of the sky and reaching well beyond the local Universe. Before the advent of future very wide-angle deep surveys, such catalogues can be constructed by combining information from various existing datasets and estimating redshifts from multi-wavelength photometry. I will shortly explain the idea of the photometric redshift technique and present photo-z catalogues that I prepared with my collaborators in the recent years. I will then describe selected applications of these and other wide-angle datasets for probing the nature of the large-scale structure and its tracers, and for multi-messenger astronomy such as searches for electromagnetic counterparts of gravitational wave events.

Wednesday 13 March 2019 [13:00 - 14:00] 

Speaker: Wei Yan (Dartmouth Collega, USA)

Title: Hunting for the "Hidden Monsters" of the Universe

Abstract: Active Galactic Nuclei (AGNs) are accreting central supermassive black holes in galaxies, found throughout cosmic time. AGNs are commonly hosted by massive galaxies and the energy released by them can profoundly impact the evolution of their hosts. By probing the history of AGNs, we may have a better understanding about the way the Universe looks today; However, nearly half of the AGN population are obscured and difficult to detect. In this talk, I will present my latest work on the obscuration in AGNs with X-ray observations from NuSTAR and Chandra as well as multi-wavelength surveys in optical and infrared. I will also discuss the distribution of AGN obscuration and the connection between this obscuration with the properties of their host galaxies, to probe the co-evolution between black holes and their host environments.

Thursday 28 February 2019 [15:00 - 16:00] 

Speaker: Marie Korsaga (UCT Astronomy)

Title: Distribution of baryonic and dark matter in spiral and irregular nearby galaxies

Abstract: I will talk about the mass distribution of a sample of 121 nearby galaxies with high-quality optical velocity fields and available infrared Wide-field Infrared Survey Explorer (WISE ) 3.4 μm data. Contrary to previous studies, this sample covers all morphological types and is not biased towards late-type galaxies. These galaxies are part of the Fabry–Perot kinematical Gassendi HAlpha survey of SPirals survey of spirals and irregular nearby galaxies. Combining the kinematical data to the WISE surface brightness data probing the emission from the old stellar population, we derive mass models allowing us to compare the luminous to the dark matter (DM) halo mass distribution in the optical regions of those galaxies. DM models are constructed using the isothermal core profile and the Navarro–Frenk–White cuspy profile. We allow the mass-to-light ratios of the disc and if necessary the bulge to vary and we keep them fixed by the colour (W1- W2). We also explore the maximum disc for the pseudoisothermal model. A similar study is made on 100 galaxies using Halpha rotation curves and the optical Rc -band photometry. The mass-to light ratios are now fixed by the (B - V) colour. We compared the results obtained with those using WISE photometry. We also determine the mass models of 31 spiral and irregular nearby galaxies obtained using hybrid rotation curves combining high resolution Halpha and extended HI rotation curves, 3.4 μm photometry data and the neutral hydrogen surface density profiles to account for the atomic gas component.

Monday 25 February 2019 [13:00 - 14:00] - astro*lunch time slot

Speaker: Prof Eric Wilcots (University of Wisconsin-Madison, USA)

Title: The role of the group environment in galaxy evolution

Abstract: We are carrying out a comprehensive observational study of the gas content and galaxy transformation of a sample of ~600 galaxy groups in the local Universe. We review our understanding of the distribution of HI-rich galaxies in groups and how that depends on the dynamical evolution of the group. We present new results on how the group environment influences the transformation of galaxies via a study of stellar populations and star formation histories of galaxies. Lastly, we discuss our results tracing the distribution of the intragroup medium, what role it plays in the evolution of the gas content of resident galaxies, and how it is influenced by AGN feedback in the group environment.

Thursday 24 January 2019 [14:00 - 15:00]

Speaker: Dr Jack Radcliffe (University of Groningen, the Netherlands)

Title: Finding AGN in the faint radio sky - a high resolution perspective

Abstract: It is now widely accepted that the evolution of galaxies and the growth of the central supermassive black hole (SMBH) are intimately connected. Those SMBH which are active (hence Active Galactic Nuclei/AGN) have been shown to influence the host galaxy and such feedback mechanisms are also required by simulations. This makes understanding the abundances and influence of AGN in distant galaxies one of the hottest research topics around. While other multi-wavelength studies are invariantly affected by dust; radio, fuelled by the burgeoning capabilities of modern arrays, can provide a dust-free window into star-formation and AGN activity. However, many radio surveys are often have resolutions in excess of arcsecond which are insufficient to separate AGN and star-formation activity. To infer the existence of an AGN relies multi-wavelength diagnostics (e.g. X-rays, infra-red, radio-excess) which are often unreliable and incomplete. 

There is one key weapon missing from the AGN identification arsenal namely high resolution radio observations. Here, we can isolate high brightness temperature objects (>1E5 K) which can only be attributed to AGN in distant galaxies. In this talk, I will dispel the myth that high resolution radio surveys are constrained to small fields of view and I'll outline the developments that have made surveying degrees of the sky at milliarcsecond resolutions easy and routine (VLBI astronomers are not the black-belt radio astronomers anymore!). Using the GOODS-N field as an example, I will outline the pros and cons of AGN selection via high resolution radio observations followed by some results from the upcoming first data release from the e-MERLIN Galaxy Evolution (e-MERGE) survey. This survey provides a multi-wavelength, but crucially multi-resolution insight into the faint microJy radio source population. I'll conclude with a look to the future and the planned high resolution efforts in MeerKAT’s own deep field legacy survey, MIGHTEE. 

Tuesday 22 January 2019 [14:00 - 15:00]

Speaker: Brenda Namumba (UCT Astronomy)

Title: HI observations of Local Group dwarf  galaxies: HI distribution, kinematics, and star formation thresholds

Abstract: We investigate 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. The KAT-7 observations has revealed 23% more neutral hydrogen (HI) gas mass in the outer disk of NGC 6822 than previous studies. An overestimate of the HI extent previously reported for Sextans A has been verified.  For IC 10, the complex HI features detected with DRAO are by a factor of  2 more extended than previous interferometric HI studies. 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. The rotation curves of Sextans A and B are seen to decline in the outer parts. In the inner disk, the rotation curve of IC 10 rises steeply, then flattens until the last point where it rises again, with a maximum velocity of 30 km/s. 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. The galaxies are dark matter dominated at all radii. 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.  The critical densities for gravitational instabilities are calculated using the Toomre-Q and cloud-growth based on shear criterion. In all cases, we find that in regions of star formation, the cloud growth criterion based on shear explains better the star formation.

Venue: AST seminar room (RW James building, room 5.40) - UCT upper campus

Co-ordinator: Dr Nathan Deg