Exploring the Polarization Perspective on Kiloparsec scale AGN jets

Lunch Colloquium
Janhavi Baghel
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.

From the stellar IMF to large-scale structure formation: How the LCDM model is incompatible with observations over all probed astrophysical scales

Moritz Haslbauer
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]

Indication of non-thermal radio emission from the Orion nebula: Cosmic ray acceleration from stellar wind?

Special Colloquium
Rashid Mohammed
Indian Institute of Science, Bangalore, India

The physical conditions of the interstellar medium (ISM) profoundly influence the star formation activity of galaxies. However, the exact role of the physical properties of ISM is yet to be understood in regulating star formation. Hence, it is crucial to constrain these properties through observations in the star-forming regions. The famous Orion HII region is one such region hosting a variety of objects, e.g., young star clusters, predominantly atomic photodissociation region (PDR), and layered ionized gases. Moreover, its proximity (414 ± 7 pc) makes it an ideal target for studying vital properties of the ISM in star-forming regions and physical processes happening in them. HII regions dominantly emit radiation through thermal mechanisms. However, a handful of HII regions are reported to emit nonthermal emissions. Recent studies have made efforts to explain such atypical nonthermal emissions through physical models. However, such observational results are fairly new and yet to be understood clearly. Using the enhanced capabilities of the upgraded Giant Metrewave Radio Telescope (uGMRT), we have studied the ISM in the Orion region. In this talk, I will present the results of the wide-band observations of the Orion nebula at unique low-frequency bands of uGMRT. From deep continuum images of band-3 (300 MHz – 500 MHz) and band-4 (635 MHz – 735MHz) of uGMRT, we have produced a reliable spectral index map. We are reporting indications of nonthermal emission from some parts of the region. To establish the reliability of the spectral index map, we are also validating the reliability of the spectral map produced through simulated data of uGMRT. I will also discuss how the stellar winds from the massive stars residing in the massive star clusters of the Orion nebula may give rise to such physical scenarios to explain this new finding.

Giant Radio Galaxies at 50: With Ten Thousand known we are not a bit wiser ?

Main Colloquium
Prof. Heinz Andernach
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.