General Seminar

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General Seminar takes place every Monday at 11.15 in Piwnice, radioastronomy seminar room.
Remote participation via the BigBlueButton (BBB).

25 November 2024

“Topological term in the Einstein equation and consequences for cosmology”

dr Quentin Vigneron (Institute of Astronomy, Nicolaus Copernicus University)

Abstract:
I will present a modification of General Relativity in which a term depending on the topology of the Universe is added in the Einstein equation. The main motivation is purely theoretical and linked to the existence of a Newtonian limit. A major consequence of this modification for cosmology is that the expansion of the Universe no longer depends on the presence of spatial curvature, i.e. \Omega = 1 \forall \Omega_K. This is a radical (parameter-free) departure from the Standard Model, which I will confront with observations of the Cosmic Microwave Background. Finally, we will see that the advantages of this modification go beyond the Newtonian limit, as it allows to have a well-defined inflationary model regardless of the curvature of the Universe.

2 December 2024

“Halo Asymmetry in the Modeling of Galaxy Clustering”

dr Anna Durkalec (National Centre for Nuclear Research)

Abstract:
Studies of galaxy clustering show that the relationship between the luminous structure and the underlying dark matter distribution is not straightforward. It depends on various, often interrelated, elements, such as the properties of the galaxies or the properties of the environment in which they reside.

Halo Occupation Distribution (HOD) modelling of the galaxy correlation function, usually assumes that the dark matter halo is spherically symmetric. However, both modern N-body simulations and observational data suggest that this assumption is too simplistic – halos have different shapes and are almost never spherical. To account for this, we propose a new, modified HOD model that takes into account halo asymmetries and improves estimates of dark matter halo masses. Using simulations, we show that this model accurately retrieves the halo asymmetry along with other halo parameters. It can therefore be successfully applied in new work.

Using our model, we find that the shape of the dark matter halos depends on the halo masses and is therefore correlated with the stellar mass of the galaxies. Furthermore, based on the observational results, we find 3 – 5% differences between the halo masses estimated using the HOD model, which assumes spherical symmetry, and our model.

9 December 2024

“Cluster winds and how they depend on the stellar population parameters”

mgr Hanno Stinshoff (Institute of Astronomy, Nicolaus Copernicus University)

Abstract:
Young massive clusters (YMCs) are affected in their evolution by the feedback of their stars, which makes massive stars with high mass loss an important contributor. Under certain conditions the collective winds of the stars (resulting in the cluster wind) form dense, warm clumps, that can provide a starting place for a second generation of stars, that inherits abundances of the first generation. This is then observed later in globular clusters (GCs) as abundance anomalies. This scenario hinges on many factors like the amount of mass available for the second generation (the so-called mass budget) and the cluster wind power. To investigate this, I varied the cluster wind velocity prescriptions for synthetic populations to see the consequences. This way the conditions for winds that result in the formation of a second generation can be outlined. For this I am showing the results of a semi-analytic code solving spherically symmetric hydrodynamic equations describing the star cluster wind and second stellar generations (“WINDCALC”, Wünsch et al., 2017). I applied it to the “BoOST” stellar evolution model grids from Szécsi et al. (2022) to ensure a broad range of stellar parameters as basis for the population.