Vorträge, Seminare, Ereignisse

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The spatial distribution and clustering of stellar feedback across galaxy discs is a key driver of galactic outflows and therefore galaxy evolution. Using a suite of high-resolution isolated galaxy simulations spanning the star-forming main sequence, I will show that clustered supernova explosions are accounted for by spatially- and temporally-coherent star formation occurring in the most massive molecular clouds. These massive molecular clouds are sustained by the constant accretion of new dense gas from the surrounding environment, in competition with the constant ejection of gas by the momentum associated with the expanding ionised regions around young massive stars. By parametrising the gas accretion rate in terms of properties of the large-scale galactic environment, I will discuss how the detailed spatial distribution of star formation and stellar feedback could be modelled as a sub-grid process in future cosmological simulations.

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The study of exoplanets, planets that orbit stars other than the Sun, is at the forefront of the public imagination in space and an exciting field of research. The question "how do stars and planetary systems form and evolve?" is one of the biggest in Astronomy, and is at the root of one of the most important questions in science today: "How did we get here?". To resolve these questions we need to observe, interpret, and understand the nature of planets beyond our Solar System. In this talk I will go through the methods, observations, and the physics behind some of the interpretations we are making about the nature of these strange new worlds. As an observer I will show some of the work we have been doing measuring the transmission spectra of giant exoplanet atmospheres looking for the tale-tale absorption of water vapor and the presence of exotic clouds. I will discuss some of the challenges associated with the measurement of atmospheric abundances, a brief look at the role clouds can play in understanding dynamics and chemistry, and what future measurements can help further constrain our understanding of planetary atmospheres. 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://zoom.us/j/94202622849?pwd=dGlPQXBiUytzY1M2UE5oUDRhbzNOZz09 Dr Wakeford will be based at the Max-Planck Institut fuer Astronomie during her visit to Heidelberg, and will be available for meetings by arrangement with her host, Laura Kreidberg (kreidberg@mpia.de).

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In this talk I will describe two complementary and novel approaches to understanding the assembly of galaxies beyond the MW in a cosmological context using observational tracers. In the first approach, we infer the origin of globular clusters from observables towards the goal of reconstructing the assembly histories of galaxies in upcoming wide-field surveys. Using the E-MOSAICS simulations to follow the formation and co-evolution of ca. 1000 galaxies and their star clusters, we explored the use of supervised machine learning to classify observed GCs into accreted and in-situ populations. Assessing the performance using a subset of the simulations and the known origin of the MW clusters, we obtain an accuracy of ca. 90% for 2/3 of the sample and successfully identify accreted debris buried deep within the Galaxy. In the second approach we study hundreds of high quality galaxy rotation curves to understand the impact of the large-scale environment on the structure of their host dark matter (DM) haloes. Galaxies in high density environments show a systematic shift in their DM density profile at large radii that is consistent with a relatively early assembly of their host haloes. The effect is manifest in the well known radial acceleration relation (RAR) as a slight downturn at the lowest accelerations for galaxies in dense environments. This environmental dependence can be understood in the context of assembly bias within the Lambda-CDM cosmological paradigm, implying that the RAR can provide useful constraints on the fundamental relation between dark and luminous matter.

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Hubble's constant, H0, quantifies the expansion rate of the Universe today and is of fundamental importance for cosmology. For example, H0 is related to the age of the Universe, its observable size, and its critical density, among others. Yet, recent observations have established a 5 sigma discrepancy between H0 measured in the local Universe and the value predicted by flat Lambda CDM cosmology based on observations of the early Universe. To clarify this looming cosmological crisis, the H1PStars project seeks to measure H0 to an accuracy of 1% using stellar standard candles that calibrate the absolute magnitudes of type-Ia supernovae. Starting with a brief overview of the current Hubble tension, I will discuss recent improvements in the calibration of and upcoming opportunities for the nearest rungs of the distance ladder. In turn, I will discuss ongoing work to improve the absolute calibration of Cepheid luminosities, to mitigate biases affecting Cepheid distances, and to obtain new insights into Cepheid stars from a stellar variability perspective that will help to improve our astrophysical basis for using Cepheids as highly accurate distance tracers. I will close by discussing these improvements in the context of the 1% measurement of Hubble?s constant required to understand the ongoing cosmological crisis. 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://zoom.us/j/94202622849?pwd=dGlPQXBiUytzY1M2UE5oUDRhbzNOZz09 Dr. Anderson will be based at the ARI institute for his visit to Heidelberg and will be available for meetings by arrangement with his host, Dominika Wylezalek (dominika.wylezalek@uni-heidelberg.de).

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Although dust in elliptical galaxies is ubiquitous, its origin is not well understood, given the absence of young stellar populations with classical dust producers like post-AGB stars or supernovae. Many authors still promote an “external” origin through galaxy mergers. However, at least in one case, we have shown that the dust has its origin in nuclear dusty outflows (NGC 1316 or Fornax A). This talk presents two more key galaxies: M87, the central galaxy of a Virgo subcluster, and NGC 4696, the central galaxy of the Centaurus galaxy cluster. The discussion is based on HST images and on wide-field and narrow-field MUSE data cubes. In M87, dust filaments emerge from the nucleus and seemingly from jet knots. More dust than found in the literature is present, including a dust filament starting from the tip of the jet. The jet itself is fine-structured down to the HST resolution limit, so its relativistic character is in doubt. NGC 4696 shows similar dust properties. Better visible is here an optical continuum radiation from the dust which directly indicates cooling of a hot ISM in magnetic fields. All evidence support a scenario, where dust forms in situ as the final cooling product of a multi-phase magnetised ISM.

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Newly formed stars will ionize and enrich the surrounding gas and thereby contribute to regulating the star formation cycle. However, fragmentation of the cloud means that stellar feedback is not perfectly coupled to the gas. The PHANGS survey provides a large sample of galaxies that is perfectly suited to study the different phases of the ISM, which is necessary to put constraints on this process. With PHANGS-MUSE we detect over 24,000 HII regions across 19 galaxies and with PHANGS-HST we identify a catalogue of over 15,000 compact stellar associations. Both catalogues are matched and we obtain a sample of 4,177 clearly matched HII regions and ionizing clusters. By comparing the observed Halpha flux to the number of ionizing photons predicted by stellar population synthesis models, we can quantify the leaking radiation. This allows us to measure the escape fraction for one of the largest samples of HII regions in the literature and study correlations with other properties of the cloud.

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Magnetars are highly-magnetized (ca. 1016 G) neutron stars which may randomly emit bursts of high energy radiation, in the form of hard X-rays or much more rarely giant flares of gamma-rays during their active phase. Recent observations of GRB 200415A, a short and very bright pulse of gamma-rays, have been claimed to be an extragalactic magnetar giant flare (MGF) whose host galaxy is the Sculptur Galaxy (NGC 253). However, as the redshift of the transient object was not able to be measured, it is possible that the measured location of the transient on the celestial sphere and the location of the local galaxy merely coincide. Thus, its real progenitor could have been arbitrarily far away, leaving the standard model of short gamma-ray bursts (SGRBs), the merger of two compact objects, as an explanation for the observations. In this talk, I will present an estimate of the false-alarm rate of SGRBs being incorrectly identified as MGFs using population synthesis to simulate data collected by the Gamma-ray Burst Monitor onboard the Fermi Gamma-ray Space Telescope.

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Stellar feedback plays an important role in the regulation of the structure, kinematics and chemical abundance of the interstellar medium. Multiple stellar winds and supernovae explosions inject energy and momentum into the interstellar medium. These processes are probable drivers of the turbulence of the ISM. They can also create large holes and superbubbles with sizes varying from several pc to several kpc. Modern integral-field spectrographs allow us to study the resolved properties of the ionized gas, and the high-resolution images from space base telescopes provide information about young stars in the nearby galaxies. Thus, using the combined data set, we can connect directly the ionized gas to the young stellar population. In my talk, I will focus on how the mechanical feedback impacts the small-scale morphology and kinematics of the ISM of nearby galaxies as observed in the PHANGS-MUSE and PHANGS-HST data. Also, I will consider how the feedback from young stars affects the dust content in the nearby galaxies based on the new PHANGS-JWST observations.

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Indigenous Elders of the world are expert observers of the stars. They teach that everything on the land is reflected in the sky, and everything in the sky is reflected on the land. These living systems of knowledge challenge conventional ideas about the nature of science and the longevity of oral tradition. This talk will explore the scientific layers of Australian Indigenous star knowledge with a focus on observations of variable stars, cataclysmic stars, stellar scintillation, the motions of planets, and transient phenomena, showing how this can guide modern scientific research.

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