Dr. Sthabile Kolwa
The University of Johannesburg
skolwa [at] uj.ac.za
Hello! I am Sthabile, a lecturer at the Physics Department of the University of Johannesburg.
I am also an early-career astronomer whose main research interests center around two overarching topics (i) active galactic nuclei (AGN) and (ii) source populations selected by deep extragalactic radio surveys. Currently, I am carrying out much of this research as a member of the MeerKAT Large Survey Project working group MIGHTEE, superMIGHTEE (MIGHTEE + commensal uGMRT observations), and LOFAR collaborations. I am also involved in efforts to develop anomaly detection software pipelines for the Vera C. Rubin Observatory / LSST.
Before I slide into the details of my research, I reckon you should consider sending in an abstract for a contributed talk or poster for the GA 2024: XXXII IAU General Assembly which will take place from the 6 - 15 August 2024 in the supremely gorgeous city of Cape Town, South Africa.
Dr. Sarah V. White and I will be chairing the IAU General Assembly 2024 Symposium 394 titled All-inclusive AGN.
![...](assets/img/IAU_AGN-symposium-2024.png)
Active Galactic Nuclei (AGN)
- Observational constraints on AGN feedback The tightly constrained black-hole and bulge mass (KH13) and black-hole mass velocity dispersion (G09) relations in astrophysics demonstrate the inextricable link between the evolution of supermassive black-holes and their host galaxies. In studying the intricate ways in which active galactic nuclei (AGN), through jets and radiative winds, impact star-formation, we are capable of developing a deeper understanding of how black-holes co-evolve with their galaxies.
- The impact of large-scale (Mpc) structure on AGN activity While it is commonly accepted that AGN have a significant impact on the host galaxy's stars, dust and gas, the role of environment on AGN triggering is rather contentious. Results are biased by the wavelength-selectin of AGN and the distance-scale on which environment density is measured. My latest interest is in studying the influence of cosmological Mpc-scale structures on occurrence and recurrence of AGN.
- The evolution of radio-AGN and star-forming galaxies In deep extragalactic radio continuum surveys, we can select wide samples radio-active galactic nuclei and star-forming galaxy populations. Combining these sensitive radio detections with infrared, optical and X-ray detections, we can trace emission from the stars, dust, gas and central black-hole of galaxy. With such multiwavelength datasets, we place observational constraints on the relations between physical components within the host galaxies in hopes of understanding how the galaxies have evolved.
- Quantifying systematic bias and selection-effects Radio continuum detections are incredibly useful in providing dust-unbiased observations of galaxies. However, problems such as incompleteness and blending do arise when we cross-identify radio-detected sources with multi-wavelength (optical, infrared and X-ray) catalogues. Incompleteness and blending in multi-wavelength cross-identification can lead to biases in observed scaling relations, luminosity and mass distributions of radio-selected sources. One focal point of my research is the development of new and innovative ways to quantify, predict and correct the impact of instrument-related and observational biases.
In an effort to understand how kinetic or mechanical feedback operates within galaxies hosting radio-loud active galactic nuclei, my collaborators and I examined optical integral field unit (IFU) datacubes from the Very Large Telescope (VLT) UT4 instrument, MUSE, as well as mm/sub-mm data from ALMA for a handful of radio galaxies.
Of this sample, we selected the radio galaxy MRC 0943-242 for single-source study on the extended emission nebular region. This work was published in Kolwa et al. 2019b. where we provide evidence for jet-gas interactions in HeII 1640Å emission lines (shown below) based on the VLT/MUSE observations. Additionally, we trace the cool, ionised gas component (T~10,000 K) within the extended halo of a high-power radio-AGN host galaxy at z=2.9.
![...](assets/img/HeII_nucleus_offset_NB_img.jpg)
The VLT/MUSE observations also provide evidence of a large-scale (<60 kpc wide) expanding shell of metal-enriched and ionised gas which results the observed absorption lines superimposed on the Lyman-α, CIV and NV (shown below) emission profiles detected across the projected scale of the extended Lyman-α nebulae surrounding this galaxy.
![...](assets/img/CIV_MUSE_fit.png)
![...](assets/img/NV_MUSE_fit.png)
![...](assets/img/NV_CIV_cloudy.jpg)
In the past, I studied the kpc-scale environments of radio-AGN. In Kolwa et al. 2019a, we obtained a sample of VLA-detected radio sources in the SDSS Stripe 82 field and found significant and consistent over-densities within galaxy group scale environments of nearby radio-AGN z < 0.8. In agreement with the majority of findings on radio-AGN, we found that the kpc-scale environments of radio-AGN host galaxies are over-dense relative to non-AGN (or inactive galaxies) and radio-power is not strongly correlated with environment over kpc3 volumes, in the nearby Universe.
![...](assets/img/radio_AGN_environments.jpeg)
Radio Source Populations
In Kolwa et al. (2023), we obtain constraints and upper limits on molecular gas mass from neutral carbon line emission in seven high-redshift radio galaxies (HzRGs). These detections allowed us to determine their gas fractions and star-forming efficiencies (seen below).
![](assets/img/SFE_fgas_lit.png)
In current work, we study spectral curvature of the faint microJansky radio source population selected by MeerKAT/MIGHTEE L-band and uGMRT (superMIGHTEE) surveys which span observing frequencies of approximately 100 - 1800 MHz. We do so with the purpose of determining the physical mechanisms underlying radio emission in these sources (mainly radio-AGN and star-forming galaxies). Owed to the fact that our radio source sample is incomplete, relations such as the radio colour-colour plot (seen below) are systematically biased towards specific spectral shapes. In this case, predicting the unbiased distribution via simulations of radio source populations is crucial.
![...](assets/img/radio-colour-colour.png)