April 28, 2025
Constraining the physical and chemical evolution of molecular clouds is essential to our understanding of star formation.These investigations often necessitate knowledge of some local representative number density of the gas along the line of sight.However, constraining the number density is a difficult endeavor. Robust constraints on the number density often require line observations of specific molecules along with radiation transfer modeling, which provides densities traced by that specific molecule.Column density maps of molecular clouds are more readily available, with many high-fidelity maps calculated from dust emission and extinction, in particular from surveys conduction with the Herschel Space Observatory.We introduce a new probabilistic model which is based on the assumption that the total hydrogen nuclei column density along a line of sight can be decomposed into a turbulent component and a gravitationally dominated component.Therefore, for each pixel in a column density map, the line of sight was decomposed into characteristic diffuse (dubbed “turbulent”) and dense (dubbed “gravitational”) gas number densities from column density maps.The method thus exploits a physical model of turbulence to decouple the random turbulent column from gas in dense bound structures empirically using the observed column density maps.We find the model produces reasonable turbulent and gravitational densities in the Taurus L1495/B213 and Polaris Flare clouds.The model can also be used to infer an effective attenuating column density into the cloud, which is useful for astrochemical models of the clouds.We conclude by demonstrating an application of this method by predicting the emission of the [C II] 1900 GHz, [C I] 492 GHz, and CO (J = 1–0) 115 GHz lines across the Taurus L1495/B213 region at the native resolution of the column density map utilizing a grid of photodissociation-region models.
Laure Bouscasse
IRAM Grenoble
“Emergence and inheritance of chemical complexity during star formation.”
During star formation the dense gas undergoes chemical evolution, where a large number of complex organic molecules (COMs) as well as unsaturated carbon chain molecules emerge providing pre-requisites for the emergence of life and for chemistry in planet-forming disks. Star formation processes are found to be at the initial stage of this increasing chemical complexity, therefore, it is crucial to answer the open questions: how the diverse and complex molecular complexity emerges, how does the environment impact the chemical complexity, how does the physical parameters of the central protostar influence the complexity of COMs? In this seminar, I will present our ongoing effort to study the evolution of chemistry and the influence of physical parameters as unveiled by various telescopes, such as APEX, ALMA, IRAM-30m and NOEMA. I will show that the chemical complexity is already present during the earliest stages of high-mass star formation with saturated COMs in the coldest stages. I will also present a new evolutionary stage prior to the emergence of hot cores, where molecular abundances of COMs are lower than that of hot cores and resemble more that of hot corinos. In a second step, I will show our complementary studies towards unsaturated complex molecules challenging our current view on the dichotomy between WCCC and hot corino chemical laboratories. All these studies will take us to the need for multi scale, statistical and broad band studies to reconcile the large scales and small scales, and the observations towards comets and star forming regions.
April 14, 2025
Constraining the physical and chemical evolution of molecular clouds is essential to our understanding of star formation.These investigations often necessitate knowledge of some local representative number density of the gas along the line of sight.However, constraining the number density is a difficult endeavor. Robust constraints on the number density often require line observations of specific molecules along with radiation transfer modeling, which provides densities traced by that specific molecule.Column density maps of molecular clouds are more readily available, with many high-fidelity maps calculated from dust emission and extinction, in particular from surveys conduction with the Herschel Space Observatory.We introduce a new probabilistic model which is based on the assumption that the total hydrogen nuclei column density along a line of sight can be decomposed into a turbulent component and a gravitationally dominated component.Therefore, for each pixel in a column density map, the line of sight was decomposed into characteristic diffuse (dubbed “turbulent”) and dense (dubbed “gravitational”) gas number densities from column density maps.The method thus exploits a physical model of turbulence to decouple the random turbulent column from gas in dense bound structures empirically using the observed column density maps.We find the model produces reasonable turbulent and gravitational densities in the Taurus L1495/B213 and Polaris Flare clouds.The model can also be used to infer an effective attenuating column density into the cloud, which is useful for astrochemical models of the clouds.We conclude by demonstrating an application of this method by predicting the emission of the [C II] 1900 GHz, [C I] 492 GHz, and CO (J = 1–0) 115 GHz lines across the Taurus L1495/B213 region at the native resolution of the column density map utilizing a grid of photodissociation-region models.
Brandt Gaches
University of Duisburg-Essen
“Rapid density estimation in giant molecular clouds”
March 18, 2025
Chemical complexity in the molecular interstellar medium (ISM) is driven by fast ion-molecule reactions. This network of chemical reactions requires a source of ionization, and as molecular gas is generally well-shielded from ionizing UV photons, cosmic rays provide the dominant source of ionization in such environments. The impact of cosmic rays on atomic and molecular hydrogen is parameterized as the cosmic-ray ionization rate (CRIR; number of ionizations per atom/molecule per unit time), which serves as an important input variable in astrochemical modeling. Our understanding of cosmic rays in both diffuse and dense gas has vastly improved over the past decade as more detailed chemical models have been developed, and as more sensitive observations of molecules that respond to the CRIR have been made. The recent creation of 3D dust maps using Gaia differential extinction measurements allows, for the first time, ionization rates inferred from observations of molecular absorption lines to be assigned to a physical location in the nearby Galaxy. By combining this information we are beginning to build the first map of the CRIR in the solar neighborhood. I will discuss our ongoing work on this project, and how we can use such a map to better understand cosmic-ray acceleration and propagation.
Nick Indriolo
Space Telescope Science Institute (STScI) Baltimore, US
“Mapping the Cosmic-Ray Ionization Rate in the Solar Neighborhood“
February 25, 2025
Active galactic nuclei (AGN) make the most significant contribution to the overall energy balance in the Universe in all electromagnetic bands not dominated by the cosmic microwave background. A good understanding of physical processes and phenomena driving this contribution is paramount for addressing the key challenges in astrophysics and cosmology, including accretion onto black holes, electromagnetic fields, and shock waves in relativistic plasma. These factors can be best studied with comprehensive programs combining dedicated multi-band and multi-messenger measurements with ultra-high angular resolution imaging in the radio regime enabled by the technique of very long baseline interferometry (VLBI).
Yuri Kovalev
MPIfR, Bonn
“Multi-messenger Lighthouses of the Universe: The Many Extremes of Active Galactic Nuclei“
In this talk, I will present recent results from several studies that probe key aspects of AGN physics. Observations with space VLBI have revealed extraordinarily high brightness temperatures in blazars, pushing the known limits of energy dissipation in relativistic plasma. Growing observational evidence for neutrino production in blazar-type AGN is shedding new light on proton acceleration mechanisms, whether near supermassive black holes or within shocks embedded in relativistic jets. The recent detection of an extreme 220 PeV neutrino by the European KM3NeT telescope presents further challenges and opportunities for understanding these processes.
Looking ahead to the emerging era of multi-messenger astrophysics, I will conclude by introducing the MuSES ERC project, which aims to unravel the mechanisms of particle acceleration and neutrino production in AGN.
February 24, 2025
Dr. Paola Dominguez Fernandez (CFA Harvard, US) who is visiting our group.
Paola Dominguez Fernandez
CFA Harvard, US
“AGN jets in merging galaxy clusters“
AGN bubbles in cool-core galaxy clusters are believed to facilitate the transport of cosmic ray electrons (CRe) throughout the cluster. Recent radio observations are revealing complex morphologies of cluster diffuse emission, potentially linked to interactions between AGN bursts and the cluster environment. We perform three-dimensional magneto-hydrodynamical simulations of binary cluster mergers and inject a bi-directional jet at the center of the main cluster. Kinetic, thermal, magnetic and CR energy are included in the jet and we use the two-fluid formalism to model the CR component. We explore a wide range of cluster merger and jet parameters. We discuss the formation of various wide-angle-tail (WAT) and X-shaped sources in the early evolution of the jet and merger. During the last phase of the evolution, we find that the CR material efficiently permeates the central region of the cluster reaching radii of ∼ 1–2 Mpc within ∼ 5–6 Gyr, depending on the merger mass ratio. We find that solenoidal turbulence dominates during the binary merger and explore the possibility for the CR jet material to be re-accelerated by super-Alfv`enic turbulence and contribute to cluster scale radio emission. We find high volume fractions, ≳ 70%, at which the turbulent acceleration time is shorter than the electron cooling time. Finally, we study the merger shock interaction with the CRe material and show that it is unlikely that this material significantly contributes to the radio relic emission associated with the shocks. We suggest that multiple jet outbursts and/or off-center radio galaxies would increase the likelihood of detecting these merger shocks in the radio due to shock re-acceleration.
December 16, 2024
Understanding the evolution of galaxies from the Big Bang to the present day is a difficult task, both observationally and theoretically. In this talk, I describe how, despite these challenges, we have been able to make significant progress in this field of research over the last 20 years, thanks in particular to the accurate measurement of the star formation rate, gas mass, and morphology of thousands of high-redshift galaxies using infrared/submillimetre observations from Spitzer, Herschel, ALMA, and JWST/MIRI. These measurements have led to the emergence of a new paradigm on the universality of star formation: the majority of stars in the Universe have formed in long, continuous phases, driven by the secular transformation into stars of gas accreted by galaxies from the cosmic web. In this talk, I will begin by summarizing our current knowledge of the cosmic density of star formation and molecular gas, and discuss how the simultaneous variation of these quantities with redshift (rising from early cosmic time, peaking at z~2, and decaying sharply to the present day) suggests a common mode of star formation, controlled by the availability of molecular gas in galaxies. I will then introduce the main sequence of star-forming galaxies and discuss how the study of galaxy properties in this context has led to the development of this new paradigm of a universal, secular mode of star formation across cosmic time, which is well described by so-called gas regulator models. I will conclude by describing some prospects in this area of research, based in particular on new observatories such as Euclid, SKA, CCAT-prime, and possibly AtLAST.
Benjamin Magnelli
CEA Paris-Saclay, France
“An infrared/submillimeter view on the universality of star-formation across cosmic time”
December 9, 2024
SPHEREx, an upcoming NASA satellite mission, is set to launch in early 2025. With its all-sky near-infrared spectral imaging survey covering 0.75–5 μm, SPHEREx will address key questions in cosmology, galaxy formation, and the origins of biogenic signatures, using its low-resolution spectroscopic catalog and near-infrared spectral images. In this talk, I will provide an overview of the SPHEREx mission, its main science targets in cosmology and galaxy evolution, and discuss additional opportunities with the SPHEREx dataset.
Yun-Ting Cheng
Research Scientist in astrophysics and cosmology at Jet Propulsion Laboratory/California Institute of Technology
“Studying Cosmology and Galaxy Evolution with SPHEREx”
November 28, 2024
Observations of spectral lines are performed to improve our understanding of the interstellar medium (ISM). However, assessing quantitatively the potential of a line to constrain the physical conditions of the ISM is complex, even more so for combinations of lines. Observation campaigns thus usually observe as many lines as possible for as long as possible.
Lucas Einig
IRAM
“Quantifying the informativity of emission lines to infer physical conditions in giant molecular clouds”
In this paper, we propose an approach that quantifies how well observing one or more lines can constrain a physical parameter. To do so, we resort to information theory and in particular mutual information. In fact, this statistic quantifies the average reduction in uncertainty on a given physical parameter resulting from the observation of a given set of lines. We illustrate our approach on synthetic observations — produced from an fast emulator of the Meudon PDR code and a simulator of IRAM 30m observations — to constrain the cloud visual extinction Av and the intensity of the incident UV field G0. We study how mutual information evolves with integration time, which could allow observers to optimize their proposals to achieve the highest constraining power with the minimum time budget. We also propose visualizations, called “information maps”, of how mutual information evolves as a function of the physical regime (Av, G0) for a given line or set of lines. We show that combining lines provides information in regimes where none of the individual lines is informative, thanks to line complementarities. Finally, we compare mutual information values for multiple lines and sets of lines to determine which one best constrains a physical parameter. This could allow observers to select the most suitable lines to observe for a given physical environment. Due to line complementarities, the most informative set of lines does not necessarily combine the most informative individual lines.”
November 18, 2024
The observed eRosita bubbles — massive, bi-lobed structures extending above and below the Milky Way’s disk — have shed new light on the complex dynamics in the galaxy’s center and raised questions about their origin and energy source. These bubbles, reminiscent of the previously observed Fermi bubbles, provide a unique laboratory to understand energetic processes in the galactic center. In this talk, I will present results from state-of-the art 3D magnetohydrodynamic (MHD) simulations of Milky Way-like galaxies, carefully constructed to include a realistic gravitational potential of the central galactic region. These simulations reveal that supernova (SN) driven explosions can indeed produce large-scale outflows comparable in size, dynamics, energetics, and X-ray emission characteristics to the observed eRosita bubbles. However, they fall short of reproducing the gamma-ray emission observed in similar structures, highlighting shortcomings of the included physical processes in current
Philipp Girichidis
Institute for theoretical Astrophysics (ITA), University of Heidelberg
“eRosita bubbles in Milky Way galaxies”
simulations. I will illustrate the physical mechanisms by which these bubbles are generated and discuss future paths to explore a more detailed comparison to observations via gamma rays.
September 17, 2024
Stars, particularly massive ones, are ubiquitous visible light sources in the universe and drive galactic chemical evolution by dramatically altering their surrounding environments through radiation, stellar winds, and supernova explosions. Therefore, understanding the properties of stars is crucial, and numerical modeling has been employed to grasp their complex characteristics. In this talk, the speaker, the lead developer of the Japanese stellar evolution code HOSHI, will present recent research conducted using the HOSHI code. The main topics include the incorporation of large-scale stellar magnetic fields in a general stellar evolution and the disk accretion evolution in the protostellar phase, with brief discussions on the gravitational collapse of massive stars.
Koh Takahashi
Division of Science, National Observatory of Japan
“Research on Stellar Evolution Using the HOSHI Code”