30.01.2025

M. Kraus & R. Taylor

Joint Journal Club

Joint Journal Club in Sporilov. Two papers will be presented: - From GPS/ASU, Rhys Taylor will present the paper "Dark-matter-free Dwarf Galaxy Formation at the Tips of the Tentacles of Jellyfish Galaxies" - https://ui.adsabs.harvard.edu/abs/2024ApJ...969...24L/abstract[TBD] - From STE/ASU, M. Kraus will present the paper "The ESO UVES/FEROS Large Programs of TESS OB pulsators: II. The physical origin of macroturbulence" - https://ui.adsabs.harvard.edu/abs/2024A%26A...692A.245S/abstract Zoom link is distributed separately.

03.03.2025

Thibault Barnouin

The long awaited come back of UV polarimetry of Active Galactic Nuclei

The last UV detectors capable of measuring the polarized signals from Active Galactic Nuclei (AGN) were the Faint Object Spectrograph (FOS) and Faint Object Camera (FOC) on board the Hubble Space Telescope (HST), decommissioned in 1997 and 2001, respectively, and the Wisconsin Ultraviolet Photo-Polarimeter Experiment (WUPPE), which flew on two NASA Space Shuttle missions in December 1990 and March 1995. The FOC’s imaging resolution of 22x22 milliarcseconds squared at wavelengths as short as 115 nm is still unmatched to this day. Used together with polarizing filters, it allowed the study of 25 AGN, providing great insight into the structure of the scattering medium of type 2 AGN and the precise location of their obscured nucleus. The FOS's far- to near-UV high spectral resolution polarimeter — achieved using a prism and rotating waveplate — enabled the study of 50 AGN. During its only two flights, WUPPE obtained spectropolarimetry for only 5 AGN. With the exception of one radio-quiet AGN (NGC 1068), WUPPE's AGN observations had very poor spectral resolution in polarimetry. Although limited in number, these observations provided critical insights into the physics of these highly asymmetrical and spatially unresolved objects. During this presentation, I will go through the AGN samples in the polarimetric archives of HST/FOC, HST/FOS, and WUPPE, highlighting their groundbreaking contributions and the unanswered questions that remain since their decommissioning. Finally, I will discuss how a POLLUX-like instrument — to be proposed by CNES as a fourth instrument for NASA’s Habitable Worlds Observatory — could not only build upon this legacy but also surpass it.

02.04.2025

Ondřej Pejcha

Star Dance: From Wide Pairs to Collisions

Our Sun is a single star, which is somewhat of an exception in the Universe as most stars are found in binary and multiple systems. For a wide range of initial conditions at stellar birth, the two stars in a binary will interact by exchanging mass, exploding, or merging to a single object. Computer simulations of binary systems are indispensable in revealing the fundamental physical processes governing the binary's structure and evolution, yet they are complicated by a lack of any useful symmetry. In this talk I will review results of radiation / magneto-hydrodynamical simulations of a common envelope evolution phase studied with the help of ERC StG and describe plans for a new set of simulations with discontinuous Galerkin method to be done with the support of ERC CoG.

08.04.2025

Harrison Souchereau

Inner Tail Gas Asymmetries and Fallback in the Jellyfish Galaxy NGC 4858

Gaseous evolution due to ram pressure acceleration of material that has been pushed out of the disk, but not fully stripped, can be a complex process. We explore this ”inner tail” evolution using high-resolution (∼1” = 460pc) ALMA CO(2-1) observations of the Coma cluster jellyfish galaxy NGC 4858, obtained from the ALMA-JELLY large program. We compare this spectacular observational data to a suite of state of the art ENZO ”wind tunnel” galaxy simulations, where we vary the disk-wind angle between runs. In our observations, we find numerous structural and kinematic features indicative of the effects from strong, inclined ram pressure, including an asymmetric inner gas tail. We also find kinematic signatures of fallback: gas clumps that had been previously pushed out of the disk but are now falling inwards. We propose a simple, torque-focused mechanism that would create asymmetric inner-tail morphologies regardless of the presence of pre-existing spiral structure. These asymmetries are supported by the results of our simulation suite which also confirm where fallback is to be expected based on the galaxy’s rotation and the ram pressure wind direction. We find that while fallback occurs in all simulations more inclined than 45 degrees, the fallback rates are largest for angles closer to edge-on. We also find that star formation is elevated for more inclined winds, and can lead to the generation of a ”leading edge plume”: shell-like structures of stars upstream from the galaxy disk.

24.04.2025

L. Ferrero and P. Jachym

Joint Journal Club

Joint Journal Club Two papers will be presented: - From STE/ASU, Leticia Ferrero will present the paper: "A kinematical study of the launching region of the blueshifted HH 46/47 outflow with SINFONI K-band observations", Birney et al. 2024 https://ui.adsabs.harvard.edu/abs/2024A%26A...692A.143B/abstract - From GPS/ASU, Pavel Jachym will present [TBD] Zoom link is distributed separately.

07.05.2025

Daniela Pérez

Survival of wormholes in a cosmological bounce

The standard cosmological model, called the Λ-CDM model, is a 6-parameter model that includes Einstein’s gravity with the cosmological constant, baryons, neutrinos, photons, cold dark matter, and a hot initial state. Though it works very well to explain the available astronomical data, it is, nonetheless, not free of problems. One major issue is that the ΛCDM model is singular, a feature that is inherent in the incompleteness of the background theory, in this case general relativity. Another approach to avoid the singularity is through a cosmological bounce from a previous contracting phase. During the contraction, the density and temperature of the cosmic fluid increases in such a way that all structure is expected to be erased. However, it has been shown that black holes can survive the bounce. Another type of object that might survive a bounce is a wormhole. Although these are conjectural objects, the potential astrophysical effects caused by them have been studied extensively. In this talk, I will show that there is one solution of Einstein's field equations that represents a dynamical wormhole that survives a cosmological bounce. I will also discuss some possible astrophysical manifestations of surviving wormholes in a bouncing universe.

20.05.2025

Mark Morris

Origin and Manifestations of The Galactic Wind

Galactic winds can be seen in a variety of nearby spiral galaxies, most clearly if those galaxies are edge-on to our line of sight. Such winds usually emanate primarily from the galaxies’ central regions. But the recognition of a central wind blowing outward from our Milky Way Galaxy into the Galactic halo has become evident only in the last few decades, perhaps because we are “too close” to the action to easily see the full picture. In this talk, I will describe the various known manifestations of the Galactic wind, starting with the earliest observations of the hot, coronal wind using absorption lines against distant quasars. Other manifestations of the wind include radio continuum and radio line emission, gamma-ray emission from the gigantic Fermi Bubbles, and an X-ray emitting plasma, extending from a central “chimney” on scales of hundreds of parsecs all the way out to the eRosita X-ray bubbles on a scale exceeding 10 kpc. But what is the origin of the Galactic wind? I will present evidence that there are at least two sources, the dominant ones being the collectivity of supernovae occurring in the Galaxy’s Central Molecular Zone, and separately in the 3 to 5 kpc ring of star formation in the inner regions of the Galactic disk. Of course, episodic outflows from the Galactic black hole are probably also a non-negligible contributor during periods of enhanced accretion. Finally, I will suggest what the next steps might be for the exploration of the Galactic wind.

23.05.2025

Michal Bílek

Fornax dwarf spheroidal an its globular clusters in MOND

The presence of globular clusters (GCs) in Fornax dwarf is important for testing different dark matter and modified gravity theories. The problem is that in most of them, dynamical friction acting on the GCs is so strong, that the GCs would settle in the center of Fornax faster than is the observed stellar age of GCs. I will present our high-resolution N-body simulations aiming to answer whether the GCs of Fornax are consistent with the MOND modified gravity. Following older MOND studies of the Local Group, we assumed that Fornax is a tidal dwarf galaxy formed in a Milky Way - Andromeda flyby 7.5 Gyr ago. We newly propose that the GCs of Fornax started their lives as massive star clusters in the young Milky Way. Our results are the following: 1) The scenario can explain why the Milky Way has a disk of satellites, whose member Fornax is. 2) The simulations show that the probability that Fornax has five surviving GCs, as observed, is 20%. 3) They also explain the observe spatial distribution of the GCs around Fornax. 4) For the latest proper motion, MOND gravity implies an orbit of Fornax whose perigalacticons perfectly agree with the observed peaks of star formation in Fornax and with the independently determined time of the Milky Way - Andromeda flyby. 5) Without any tuning, the simulations reproduce the observed photometric "halo" component of Fornax. 6) Fornax is predicted to have tidal tails which are at the current observational limits. 7) By an additional simulation, we show that if Fornax started its life as a rotating disky tidal dwarf galaxy, the varying tidal forces from the Milky Way were able to transform it into a non-rotating spheroid.

04.06.2025

Chand Swadesh

Accretion Geometry of Black Hole X-ray Binaries in the Hard State

Black hole X-ray binaries (BHXRBs) serve as ideal laboratories for studying the complex interplay between accretion and ejection processes in strong gravitational fields. Most of the low-mass BHXRBs are transient in nature and exhibit sporadic outbursts. During a typical outburst, these systems transition through various spectral states, from hard to soft, each associated with distinct spectral and timing properties. Despite decades of studies, the exact physical picture of these systems in the hard spectral state remains an open question, particularly regarding the geometry of the corona and extent of the inner accretion disk. While some studies argue for a disk that extends close to the innermost stable circular orbit (ISCO) in the hard state, others propose a significantly truncated disk scenario. In this talk, I will present findings from some of our recent broadband spectral and timing studies of BHXRBs in their hard state, which provide insights into the nature and evolution of the inner accretion flow.

09.06.2025

Benedikt Bergmann

Timepix detectors in Space: From radiation monitoring in low earth orbit to astroparticle physics

Hybrid pixel detectors (HPD) of Timepix [1,2] technology have become increasingly interesting for space applications. While up to date, common space radiation monitors rely on silicon diodes, achieving particle (mainly electron and proton) separation by pulse-height analysis, detector stacking, shielding or electron removal by a magnetic field, the key advantage of HPDs is that, in addition to the energy deposition measurement, particle signatures in the sensor are seen as tracks with a rich set of features. These track characteristics can be exploited for identification of particle type, energy, and its trajectory. Determining these pieces of information on a single layer bypasses the need for sensor stacking or complex shielding geometries, so that HPD based space radiation devices provide science-class data with a large field of view at an order of magnitude lower weight and approximately half of the power consumption compared with commonly used space radiation monitors. The first Timepix (256 x 256 pixels, 55 µm pitch) used in open space is SATRAM (Space Application of Timepix Radiation Monitor), attached to the Proba-V satellite launched to low earth orbit (LEO, 820 km, sun-synchronous) in 2013. During this time, it has been providing data for mapping out the fluxes of electrons and protons trapped in the Van-Allen radiation belts [3], e.g. by in-orbit maps of the ionizing dose rate dominated by electrons in polar horn regions, as well as protons in the South Atlantic Anomaly. Noiseless detection of individual particles allows to measure even rare signatures of highly ionizing events from galactic cosmic rays. I will discuss different data analysis methodology, relying on extraction and characterization of track features, novel machine learning approaches (e.g., [5]) and using statistical interpolation. Based on the success of SATRAM, advanced and miniaturized space radiation monitors based on Timepix3 [2] and Timepix2 [4] technology have been developed for the European Space Agency (ESA). These will be flown on the GOMX-5 mission (launch in 2023) and used within the European Space Radiation Array. Large area Timepix3 detectors (512 x 512 pixels, 55 µm pitch) were developed for the demonstrator of the penetrating particle analyzer [5] (mini.PAN), a compact magnetic spectrometer (MS) designed to measure the properties of cosmic rays in the 100 MeV/n – 20 GeV/n energy range in deep space with unprecedented accuracy, thus providing novel results to investigate the mechanisms of origin, acceleration and propagation of galactic cosmic rays and of solar energetic particles, and producing unique information for solar system exploration missions. Precise per-pixel time measurement together with a high spatial segmentation allows its use a single layer Compton camera, thus making it an interesting tool also for directional gamma-ray detection and polarization measurement.

09.06.2025

Farida Sial

social seminar by Afghan journalist

The speaker has worked as a journalist with Radio Azadi in Prague (part of Radio Free Europe/ Free Liberty) for nearly three years. Before this role, she spent several years in Afghanistan, where she worked with various media outlets. Her journalistic focus has consistently centered on critical issues such as women’s rights, the challenges and restrictions faced by women, unemployment, migration, human rights, and the ongoing humanitarian crises in Afghanistan. At Radio Azadi, her reporting has shed light on underreported topics within Afghan media, including human rights violations, arbitrary arrests, restrictions on girls’ education, limitations faced by Afghan youth, and other urgent social matters. The speaker holds a Bachelor’s degree in journalism and a Master’s degree in international relations. As a representative of Radio Azadi, she actively participates in international conferences that focus on issues affecting Afghan women. She remains deeply committed to amplifying the voices of Afghan women and all individuals who are deprived of their fundamental human rights.