Main Colloquium
Prof. Dr. Selma de Mink	CANCELED

MPA

+++postponed+++

	Main Colloquium
Dr. Jacques Roland	SCHEDULED

Observatoire de Paris

Variations of the flux density of the nucleus of an extragalactic radio source can be due either to the variations of the synchrotron spectra of the black holes contained in the nucleus, or to the ejection of VLBI components. As a consequence of the variations of the synchrotron spectra of the black holes, there will be periods during which the low frequency and the high frequency observations will be aligned on the same BH, say BH1, and periods during which the low frequency and the high frequency observations will be aligned on different BHs, say BH1 and BH2. We obtained centroid position time series at 15 GHz from MOJAVE model fitting giving epoch positions of the VLBI components and for each epoch, we calculated the 15~GHz flux barycenter positions using a beam $B = 2$ mas which corresponds to the $8$~GHz beam. The comparison of the $8$~GHz and $15$~GHz time series shows that: a) the time series are generally correlated, but they can be anti-correlated during some periods and b) the absolute position of the ICRF3 survey corresponds to the mean position of the $15$~GHz flux density barycenter within $\leq 0.100$ mas$. The comparison of the $8$ and $15$~GHz has been done for 2201+315 which structure of the nucleus is known and for 0851+202 and 0923+392 which structures of the nucleus are not known.

	Lunch Colloquium
Janhavi Baghel	SCHEDULED

National Center for Radio Astrophysics, Pune, India

Blazars are active galactic nuclei (AGN) with jets oriented at small angles to our lines of sight that result in their extreme properties due to Doppler boosting effects. They are delineated based on their spectral properties into quasars and BL Lac objects. The underlying reasons for this blazar divide are not ambiguously clear. In this talk, I will present polarization sensitive data from the VLA and GMRT on the Palomar Green (PG) blazar sample. These data reveal differences in the kiloparsec (kpc) scale jets between the blazar subclasses. The PG quasars show a great diversity in their morphology not always consistent with their purported parent sample of FRII radio galaxies. While many BL Lacs exhibit core-halo morphology consistent with FRI radio galaxies, some possess FRII-like hotspots. Overall, polarimetric observations reveal differences in the inferred magnetic fields in the kpc-scale jets of blazars that may be the result of intrinsic differences in accretion disk structures along with extrinsic differences in their surrounding environments.

	Promotionskolloquium
Moritz Haslbauer	SCHEDULED

Helmholtz-Institut für Strahlen- und Kernphysik, Universität Bonn

Investigating the current standard model of cosmology (Lambda Cold Dark Matter - LCDM - framework) by performing tests on different astrophysical scales could indicate if the missing mass problem implies indeed the existence of cold dark matter (CDM) or rather emerges because of a breakdown of Newton's law of gravity. In my talk, I will argue that the performed tests during my doctoral studies disprove the LCDM framework over all probed scales ranging from galactic to large-scale structures. The tests established a consistent picture of the Universe in which dynamical friction on galactic scales is next to absent and structure formation is more enhanced than predicted by the LCDM framework disfavoring the existence of CDM on galactic scales and pointing to a long-range correction of Newtonian gravity as provided by Milgromian Dynamics (MOND). Recent observations of high-redshift galaxy candidates by the James Webb Space Telescope illustrate that testing cosmological models requires also assumptions about the underlying stellar initial mass function (IMF) which seems to depend on the global star formation rate and metallicity. As the interpretation of photometric measurements depends on the properties of the stellar IMF, I will discuss that galaxy evolution and cosmological models have to be considered in a more holistic picture by bridging stellar population of galaxies with cosmology. [Referees: Prof. Dr. Pavel Kroupa, Prof. Dr. Karl Menten, PD. Dr. Akaki Rusetsky, Prof. Dr. Estela Suarez]

	Main Colloquium
Prof. Heinz Andernach	SCHEDULED

Th"uringer Landessternwarte, Tautenburg, Germany, on leave from University of Guanajuato, Mexico

Giant Radio Galaxies (GRGs) with a projected linear extent over 1 Mpc were first found in 1974, and over 5,000 are now known, plus another 5,000 larger than 0.7 Mpc, the currently adopted threshold for GRGs. I present results based on my compilation of GRGs (a) from literature, and my own visual inspection of (b) large-scale radio surveys in general (e.g. NVSS, SUMSS, RACS, LoTSS DR2, etc.), and of specific regions like (c) the LoTSS Deep Fields (LDF), as well as (d) the equatorial SWAG-X/eFEDS/LOFAR field. Although very time-consuming, systematic visual inspection clearly provides a higher ratio of GRGs <1 Mpc to those >1 Mpc than literature samples, which tend to disfavor smaller GRGs. I also show examples of published mis-identifications based on automated algorithms, lacking the necessary human control of the results. In order to shed light on the open question why GRGs can reach their large sizes, I look at (a) their location as function of environment density in the LDF and SWAG-X fields, and (b) I select the very largest 115 GRGs (from 3 to 6.6 Mpc), to study how they were discovered (mostly by LOFAR), their distribution in redshift and radio luminosity, and their bending angle, and compare these with GRGs of more modest sizes.