Promotionskolloquium
Shilpa Ranchod	SCHEDULED

MPIfR

Magnetic fields are ubiquitous in the Universe and play a key role in galaxy evolution, influencing star formation, cosmic-ray transport, and feedback. Radio polarisation observations provide a powerful probe of cosmic magnetic fields through Faraday rotation. Large samples of polarised radio galaxies enable rotation measure (RM) grids that trace both extragalactic sources and the magnetised interstellar medium of the Milky Way. As modern interferometers push to µJy sensitivities and dramatically increase RM grid densities, interpreting these measurements requires a better understanding of both foreground Faraday effects and the faint polarised source population. This thesis addresses two key aspects for improving RM grid interpretation: (i) the observational biases and physical origins of Faraday complexity at low Galactic latitudes, and (ii) the nature of the extragalactic polarised source population. Using broadband spectro-polarimetric ATCA observations of polarised extragalactic sources, I show that the observed increase in Faraday complexity toward low Galactic latitudes in the SPASS/ATCA catalogue is largely driven by contamination from diffuse Galactic polarised emission. Our relatively higher-resolution observations allow this contamination to be filtered out, revealing that 42% of sources still show Faraday complexity. We find this complexity to be consistent with turbulent injection scales in the Galactic spiral arms and toward the Galactic centre. In addition, I present the linear polarisation component of the VLA-COSMOS 3 GHz survey, yielding the deepest polarised source counts (2.6 µJy/beam) at this frequency to date. Through these broadband observations, we identify the dominant depolarisation mechanisms in this sample and model the frequency dependence of polarised source counts. Furthermore, these results show that even the µJy polarised sky is dominated by active galactic nuclei, with no detection of star-forming galaxies, implying that significantly deeper observations will be required to probe this population, even at higher frequencies.

	Special Colloquium
Dr. Sasha Plavin	SCHEDULED

Black Hole Initiative, Harvard University

Blazars exhibit dramatic optical variability, but the precise location of these flares — accretion flow, jet base, or extended jet — remains debated. I present a novel approach using Gaia astrometry together with VLBI to localize the flaring regions directly. Among blazars with strong optical emission from extended jets, I demonstrate a universal pattern: optical flares occur very close to the central engine. These measurements use the latest Gaia DR3 and place a typical flare within ~0.5 mas (a few pc) of the VLBI core position, consistent with an origin in the innermost jet or accretion region. Such a direct localization can constrain electron acceleration and seed photon fields, and provide a model-independent anchor for gamma-ray and higher-energy emission zones. The upcoming Gaia DR4 will bring per-epoch astrometry, sharpening these constraints further.

	Special Colloquium
Dr. Stefano Berta	SCHEDULED

IRAM Grenoble

The New IRAM KID Array 2 (NIKA2) is a dual-band continuum camera operating at the IRAM 30m telescope on the Sierra Nevada, in Spain. It is equipped with three kilo-pixels arrays of Lumped Element Kinetic Inductance Detectors (LEKID) cooled at 150mK. NIKA2 observes simultaneously at 1.2 and 2.0 mm, and has polarimetry capabilities at 1.2 mm. I will introduce the NIKA2 instrument, its state of the art KIDs technology, its current performance and its bright future. I will then present the most recent science results: NIKA2 studies the magnetic fields of massive star forming regions and supernova remnants, the interplay between star formation and interstellar medium components in nearby galaxies, the structure of the intracluster medium in massive clusters of galaxies, and the evolution of the dust cosmic density all the way back to an epoch when the Universe was less than 1 Gyr old. Join me in this journey across the millimeter sky!