Vorträge, Seminare, Ereignisse

Abstract
Cosmic rays (CRs) are high energy particles with with an energy density comparable to the other energy densities in the interstellar medium (ISM) and very distinct cooling and transport processes. The low cooling efficiency of CRs allows them to travel long distances from their location of acceleration. As a result, they have an impact on star formation at small scales and high densities. Furthermore, the pressure exerted by CRs fills the galaxy, enabling the launching of galactic-scale outflows, which have a significant influence on galactic evolution. In this talk, I will focus on the dynamical impact of CRs, demonstrating how they shape the ISM and contribute to the launching of galactic outflows. There are significant uncertainties in our understanding of CR astrophysics, particularly regarding the transport speed and the efficiency with which they lose energy. I will highlight the efforts to address these uncertainties by incorporating CRs into hydrodynamical simulations. These simulations aim to accurately model their evolution from non-relativistic to relativistic regimes, while also considering the full magneto-hydrodynamical evolution. This improves the accuracy of the CR-driven dynamics and allows us to generate more accurate synthetic observables such as gamma rays and synchrotron radiation, which provide valuable insights into the transport and cooling properties of CRs. Those unable to attend the colloquium in person are invited to participate online through Zoom (Meeting ID: 942 0262 2849, passcode 792771) using the link: https://eu02web.zoom-x.de/j/94202622849?pwd=dGlPQXBiUytzY1M2UE5oUDRhbzNOZz09

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To a great extent, our knowledge of Astronomy hinges upon our understanding of how massive stars (M > 8 Msun) behave and evolve across different eras of the Universe. Despite having short lives and being far outnumbered by their low-lass analogs, high-mass stars deeply impact their surroundings due to their strong winds (high mass-loss rates and wind speed), ionizing fluxes, and usually violent death as supernovae. Moreover, they are progenitors of neutron stars and black holes, which produce gravitational waves. Massive stars spend most of their main-sequence lifetime as O- and B-type objects. However, most of them are expected to evolve towards cooler temperatures and high luminosity, becoming blue supergiants/hypergiants. The full evolutionary picture, however, remains unclear as many factors play important roles in their evolution, such as internal mixing, rotation, and mass-loss rates. For instance, the status and origins of B supergiants (BSGs) and B hypergiants (BHG) are still under heavy debate. Additionally, important aspects of the nature of their atmospheres/winds are still not well understood. Even less is the connection between their stellar and wind properties -- for instance, the behavior of their mass-loss rates with stellar temperatures. To address this problem and deepen our understanding of the atmospheric/wind properties of BSG/BHGs, we analyze their spectra using state-of-the-art comoving-frame stellar atmosphere codes. In the first study we present in the colloquium, we use CMFGEN (Hillier et al. 1998) to produce the largest multi-wavelength spectral analysis of BSGs in the Small Magellanic Clouds in the context of the ULLYSES/XShootU collaboration -- dedicated to studying hot stars in low metallicity environments. The properties of the late BSGs are compatible with H-shell burning objects whereas the early BSG have a more unclear status. Concerning the wind, we find a sharp decrease of the wind terminal velocity at B1 spectral type, but no corresponding increase in mass-loss rates towards cooler temperatures is present. This aligns better with recent theoretical "mass loss recipes" which challenge the current scenario, that predicts an increase in mass-loss rates at low temperatures due to the recombination of Fe IV to Fe III at inner layers. In the second study, we use PoWR^HD (Sander et al. 2017, 2018) to produce the first hydrodynamically consistent model of BHGs. Through that, we investigate the conditions and mechanisms behind the mass loss and driving of the wind. We find Fe III is the ion responsible for the wind acceleration in these stars (even at lower metalicities), with very little contribution of other metals to the velocity field, which reveals a very shallow acceleration. Additionally, we find evidence for a clumped atmosphere already from sub-photospheric layers. These models also allow us to investigate the impact of different properties (e.g. clumping, turbulence, mass) on the wind properties. Our current findings reveal that higher clumping in the inner layers increases the wind density, producing spectra more similar to those of luminous blue variables.

Abstract
Stars represent only a small fraction of the atoms in today’s Universe. Most of the normal matter resides, instead, in a dilute plasma located around and between galaxies throughout the cosmic web. Supermassive black holes (SMBH) in halo-central galaxies are known to interact with their hot gaseous atmospheres over scales ranging from sub- to hundreds-of-kiloparsec. This is believed to be a main mechanism that couples black hole and galaxy co-evolution, and fundamentally shapes galaxy formation by regulating the balance between heating versus radiative cooling and star formation. To understand the necessarily dynamical processes associated with SMBH feedback, it is critical to probe the detailed velocity structure of the diffuse X-ray emitting media surrounding brightest cluster and brightest group galaxies. High-resolution X-ray spectroscopy is poised to revolutionise this field by providing routine, robust measurements of the widths and shifts of emission lines with remarkable precisions down to few km/s. In this talk, I will review and compare the existing estimates of the velocities of the hot baryons in the cores of clusters of galaxies, and give a first glimpse into new results obtained with the Resolve spectrometer, onboard the recently launched XRISM mission. Those unable to attend the colloquium in person are invited to participate online through Zoom (Meeting ID: 942 0262 2849, passcode 792771) using the link: https://eu02web.zoom-x.de/j/94202622849?pwd=dGlPQXBiUytzY1M2UE5oUDRhbzNOZz09 During her visit to Heidelberg, Dr Simionescu will be available for meetings by arrangement with her host, Annalisa Pillepich(pillepich@mpia.de).

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Gustav Kirchhoff (1824 - 1887) discovered spectral analysis together with Robert Bunsen at Heidelberg University in the mid-19th century, paving the way for modern astrophysics. In 1859, Bunsen – a chemist at Heidelberg – attempted to identify the chemical composition of various salts by examining their different flame colors. Physicist Kirchhoff suggested to spectrally split up this light by using a prism. Between 1859 and 1861, Kirchhoff and Bunsen discovered that the spectrum of each element contains characteristic lines, a specific "fingerprint". In particular, Kirchhoff and Bunsen were able to use this to explain the Fraunhofer lines in the solar spectrum: They indicate the chemical composition of the sun! This new branch of research - spectroscopy - made it possible to not just determine the chemical composition of distant stars, but also their temperatures, pressure conditions, and even their (radial) velocities: "Astrophysics" was born - the investigation of stars using physical methods. The lecture will show how Kirchhoff's discovery revolutionized "the study of the heavens". The highly successful path of "astrophysics" since then will be briefly outlined, finally it will be discussed whether we are "astronomers" or "astrophysicists".

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Studying the AGN feedback effect on the cold molecular gas of their host galaxies is key to understanding its impact on the local star formation. I will present a study of the CO(2-1) emission line distribution and kinematics in a sample of four local Seyfert galaxies with luminosities L_AGN ~ 10^44 erg/s. They were observed with ALMA, using a spatial resolution of ~100 – 400 pc, and covering up to ~10 kpc radii. Comparing the CO(2-1) observations with imaging data of [O III]lambda5007 emission lines from HST, we find that the ionized gas is generally observed in regions deficient in molecular gas, which we interpret to be caused by the AGN radiation partially destroying it. Although the kinematics of the cold molecular gas is dominated by rotation, all Seyfert galaxies present regions with double peaks in CO(2-1), which trace clouds with more complex motions. In particular, for NGC 3281 and NGC 6860, the cold molecular gas outflows were detected at the edges of their bipolar [O III] emission, surrounding it. I will also discuss my ongoing project to analyze the complex kinematics of the ionized gas in high-redshift radio galaxies (z ~ 3) obtained with JWST.

Abstract
The need for understanding the winds of Wolf-Rayet (WR) stars cannot be understated: the light of these stars, their mass-loss rates, ionization capabilities and ultimately their further evolution is all greatly affected by the behaviour of their wind. Despite WR-star winds being notoriously difficult to model, advancements on this matter have been made. One approach is using non-LTE, co-moving frame computations with the Potsdam Wolf-Rayet (PoWR) code where now hydrodynamic consistency throughout the wind domain is enforced. While already applied multiple times for the regime of hot, hydrogen-free WR stars, we now present their first wide-range application in the regime of nitrogen-rich late-type WN stars that still contain hydrogen in their spectra (WNLh type). A newly generated temperature sequence of these WNLh-star models reveals a sudden change in the wind regimes: Below 30 kK, the mass-loss rates increase significantly, while the terminal wind velocity drops strongly, accompanied with large changes in the emergent model spectra. This discontinuous behaviour greatly resembles the well-known bi-stability jump in B-supergiants. Examining the models, we discover that our obtained regime change does not correspond to the switch from Fe IV to Fe III as expected, but is linked to the higher ionization switch of Fe V to Fe IV, therefore also coinciding with higher stellar temperatures. Hence, this bi-stable behaviour occurs both due to a different cause and in a different temperature regime as the "classical" case for B-supergiants, making it a different phenomenon altogether; a new bi-stability jump for Wolf-Rayet stars.

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We aim to trace the evolution of eight edge-on star-forming disc galaxies through the analysis of stellar population properties of their (thin and thick) discs. We use Multi-Unit Spectroscopic Explorer (MUSE) observations and full-spectrum fitting to produce spatially resolved maps of ages, metallicities and [Mg/Fe] abundances and extract the star formation histories of stellar discs. Our maps show thick discs that are on average older, more metal-poor and more ?-enhanced than thin discs. However, age differences between thin and thick discs are small (around 2 Gyr) and the thick discs are younger than previously observed in more massive and more quiescent galaxies. Both thin and thick discs show mostly sub-solar metallicities, and the vertical metallicity gradient is milder than previously observed in similar studies. [Mg/Fe] differences between thick and thin discs are not sharp. The star formation histories of thick discs are extended down to recent times, although most of the mass in young stars was formed in the thin discs. Our findings show thick discs that are different from old thick discs previously observed in more massive galaxies or more quiescent galaxies. We propose that thick discs in these galaxies did not form quickly at high redshift, but slowly in an extended time. The thin discs were formed also slowly, but with a larger mass fraction at very recent times.

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Fast inward migration of planetary cores embedded in gaseous disks is a common problem in the current planet formation paradigm. Even though dust is ubiquitous in protoplanetary disks, its dynamical role in the migration history of planetary embryos has not been considered until recently. In this talk, I will show that a planetesimal embedded in a dusty disk leads to an asymmetric dust-density distribution that can exert a net torque under conditions relevant to planetary embryos up to several Earth masses. Building on the results or a large suite of numerical simulations for measuring this dust torque under a wide range of conditions, I will present the first study showing that dust torques can have a significant impact on the migration and formation history of planetary embryos. Those unable to attend the colloquium in person are invited to participate online through Zoom (Meeting ID: 942 0262 2849, passcode 792771) using the link: https://eu02web.zoom-x.de/j/94202622849?pwd=dGlPQXBiUytzY1M2UE5oUDRhbzNOZz09 During his visit to Heidelberg, Prof. Pessah will be available for meetings by arrangement with his host, Maria Bergemann (bergemann@mpia.de).

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Even small-aperture (amateur) telescopes are indespensible for astronomical research. This way, deep exposures of galaxies in the local universe reveal a complexity of substructures. Here I will show selected highlights from a dedicated campaign, the HERON survey, that allowed us to investigate the formation channels of galaxies with some peculiar morphologies such as boxy halos.

Abstract
The interstellar medium plays a central role in the galaxy evolution process; it is the reservoir that fuels galaxy growth via star formation, the repository of material formed by these stars, and a sensitive tracer of internal and external processes that affect entire galaxies (e.g. accretion and feedback). In this overview talk, I will discuss how observations of the interstellar medium are shedding light on the vast range of physics and scales at play in the star formation and galaxy evolution processes, using results from recent observing campaigns with (sub)mm/radio facilities (IRAM, ALMA, JCMT, APEX) as well as large optical spectroscopic surveys (DESI). By connecting these observations with theory and simulations, a picture emerges where galaxy evolution is driven by gas availability on galactic- and molecular cloud-scales and the efficiency of the star formation process out of this gas, depending on local conditions in the interstellar medium. These results highlight the multi-scale nature of star formation and galaxy evolution, and help draw a path forward to understand mass assembly in the Universe. Those unable to attend the colloquium in person are invited to participate online through Zoom (Meeting ID: 942 0262 2849, passcode 792771) using the link: https://eu02web.zoom-x.de/j/94202622849?pwd=dGlPQXBiUytzY1M2UE5oUDRhbzNOZz09 During her visit to Heidelberg, Prof. Saintonge will be available for meetings by arrangement with her host, Dominika Wylezalek (dominika.wylezalek@uni-heidelberg.de).

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Globular clusters are not the simple stellar populations we used to think they were. The vast majority of them consists of two main populations, dubbed the 1P (pristine) and 2P (polluted) populations, with distinct light-element chemical abundances. How multiple stellar populations unfold remains a riddle. A decade of observations has shown unambiguously that the fraction of 1P stars in clusters, F_1P, is a decreasing function of their present-day mass. That is, the multiple-stellar-population phenomenon is exacerbated in massive clusters. The present-day distribution of Galactic globular clusters in the (mass, F_1P) space must therefore hold clues regarding the formation of their multiple stellar populations. In this talk, I will decipher this distribution, detailing the processes and parameters shaping it.

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