Victoria Toptun

Galaxy groups are one of the most common environments in the Universe, but at the same time, due to their low mass, they are very sensitive to non-gravitational processes. In our team at ESO, we use observations and simulations together to study very faint groups at the low end of the mass range, which are very hard to reach with individual observations. Over the last four years, from surface-brightness stacking of eFEDS data we concluded that groups have a much more reduced gas content than we would expect, as well as lower gas concentrations. With Magneticum simulations, we not only took a more detailed look at the selection effects of observational samples, but also traced the impact of the assembly history on the gas content.

My part in this project is to use spectral stacking on eRASS1 data for optically selected samples to reach the spectra of low-mass groups. Modeling these X-ray spectra gives the temperature of the hot gas, which is in turn one of the direct proxies of halo mass. With this in hand, I first investigated how the mass-temperature relation evolves toward the low-mass end. In parallel, I applied my stacking procedure to mock X-ray data from the Magneticum simulation, to make sure it reproduces the original properties of the hot gas. Then I traced how the brightest central galaxy in a group is connected to its halo, which is closely tied to the interplay between stellar feedback in the single-galaxy regime and AGN feedback in the cluster regime. Now I'm actively working on a closer look at the centers of galaxy groups: stay tuned for new results by the end of 2026!

EMPOWER is a VLT/KMOS integral-field survey. For now I've contributed as a rotating member of its steering committee and as a visitor-mode observer, but I plan to be involved in scientific projects in the future.

For the 4MOST ChANGES survey I worked on selecting the parent sample, which led into this recent paper.

For RCSEDv2 I assembled and calibrated the photometry and built the database, which gave me experience with big data in astronomy, databases and the virtual observatory. The photometric-calibration questions it raised also led to my master's paper.

The topic of my master's thesis. The mechanisms of growth of supermassive black holes are still unclear: they may grow from massive seeds (10⁵-10⁶ M_☉) or from low-mass ones (~100 M_☉), and the second route should leave behind a population of intermediate-mass black holes (10²-10⁵ M_☉).

Fitting spectra from the SDSS survey, we selected a sample of IMBH candidates and confirmed their AGN nature with X-rays. My main job was the X-ray analysis: taking optically selected candidates and confirming the active nucleus from its X-ray emission, then measuring its luminosity and accretion rate. This built a larger sample of bona-fide IMBHs and low-mass supermassive black holes (<10⁶ M_☉), using our own XMM-Newton, Chandra and Swift observations together with public archives. I also helped with the photometric decomposition of the host galaxies. Part of this work led to a paper on one peculiar source with a very low accretion rate, to which I contributed as a co-author and was responsible for all of the X-ray analysis.