Winter Term 2021/22

All measurements of cosmic star formation and stellar mass must assume the stellar initial mass function (IMF) to extrapolate from massive stars to total. Traditional determinations of the IMF in galaxies are made at ultraviolet, optical and near-infrared wavelengths, which cannot be probed in dusty gas-rich galaxies. The C-13/O-18 abundance ratio in the ISM – which are produced mainly by low-and-intermediate-mass stars and massive stars, respectively -- has been demonstrated as a sensitive index of the IMF in gas-rich galaxies. Besides the C-13/O-18 ratio, which is associated with different elements, O-17 and O-18 are likely even more sensitive to the IMF. Here we will present new ALMA observations and archival data of 17O and 18O in nearby galaxies. A systematic trend of 17O/18O ratios from starbursts to dwarf galaxies implies that a variable IMF is inevitable. At the end, we will present recent benchmarking on the CNO isotope gradient of the Milky Way, extending to the low-metallicity outer region.

New observations with ALMA have provided a census of the density of molecular gas in the cosmic volume defined by the Hubble Ultra-Deep Field. This molecular gas density shows an order of magnitude decrease as a function of redshift from z~2 to z=0. It follows, to first order, the dependence of the cosmic star formation rate density. This is markably different from the atomic gas phase that shows a rather flat redshift dependence. At low redshift, observations of the interstellar medium of nearby galaxies (in particular the HERACLES survey of molecular gas and the THINGS survey of atomic hydrogen) have demonstrated that the atomic gas is significantly more extended than the molecular gas (the latter being tightly correlated with star formation activity). A similar picture is also emerging in observations of high-redshift galaxies. Assuming a simple galaxy model based on these findings, and using other measurements from the literature, the ALMA Hubble Ultra-Deep Field data are used to put observational constraints on the gas (net) accretion flows in galaxies. These gas flows are needed to explain the build-up of the stellar mass in galaxies, and are further compared to cosmological galaxy formation simulations.

Most stars in our Galaxy are born in clusters. How molecular clouds collapse and fragment to give rise to a cluster of stars with a range of stellar masses remains an open question. The physical conditions in clouds harboring protoclusters limit the Jeans mass to about 1 Msun. This creates a puzzle for massive star formation since dense cores much greater than 1 Msun tend to further fragment into lower mass cores. Recent sensitive and high angular resolution observations of cluster forming clouds begin to unravel the complex process of fragmentation. My talk will review the recent progress in fragmentation studies. I will present observations of protocluster forming clumps and discuss the role of turbulence and magnetic fields in the formation of protostellar cores.

Chemistry determines many important aspects of the interstellar medium (ISM): for example, the abundance of observable species, heating and cooling of the gas, and the ionization fraction. Understanding chemistry is important for both understanding the physical processes that control star formation, and interpreting observations of molecular clouds in the local universe and beyond. In this talk, I will summarise our efforts in adding chemistry to numerical simulations of the ISM. I will give examples of applying chemistry in different contexts: the environmental dependence of the X_CO conversion factor, self-consistent chemistry and thermodynamics, formation of molecules in HI clouds, and more.

Over the next decade, new and upcoming facilities will secure observations of the restframe UV/optical spectral energy distributions (SEDs) for millions of galaxies across the entire span of cosmic time. Measurements of star formation rates (SFRs), masses, and other physical parameters from those SEDs will enable comparisons with models of galaxy evolution with unprecedented accuracy. Dust in galaxies provides an important limitation to the reliability of those measurements. (Sub)mm facilities have limited areal coverage and sensitivity and will not be able to observe those millions of galaxies. This implies that IR-based dust corrections will be difficult to extend to large galaxy samples, and dust removal from the SEDs will require the use of attenuation curves. I will review the current state of our understanding of attenuation curves at low and high redshift, highlight both their strengths and weaknesses, and discuss what steps may be able to move the field forward. During this presentation I will also highlight some recent results from the Large Millimeter Telescope and the insights they provide on star formation processes.

The Cryogenic Storage Ring (CSR) at the Max Planck Institute for Nuclear Physics in Heidelberg is the largest electrostatic storage ring project in the world. The CSR combines electrostatic ion optics with extreme vacuum and cryogenic temperatures. The storage ring has a circumference of 35 m, and all deflectors are housed in experimental vacuum chambers that can be cooled down to 5K. It has been shown that within a few minutes of storage inside the CSR infrared-active molecular ions (e.g., CH+, HeH+ and OH-) cool to their lowest rotational quantum states by spontaneous emission of radiation. Equipped with an ion-neutral collision setup and a low-energy electron cooler, the CSR offers unique possibilities for astrochemical experiments under true interstellar conditions. I will present an overview of the capabilities of the CSR, along with first experimental results on collision experiments between cold molecular ions and neutral atoms, free electrons, and photons, yielding quantum state-selective rate coefficients for astrophysically important processes.

A large-scale magnetic field permeates our Galaxy and is involved in a variety of astrophysical processes such as star formation and cosmic ray propagation. Dust polarization has been proven one of the most powerful observables for studying the magnetic field properties in the interstellar medium (ISM). However, it does not provide a direct measurement of its strength. Indirect methods have been developed, based on the assumption of equipartition between turbulent kinetic energy and fluctuating magnetic energy. However, these are rather inaccurate. In the era of large polarimetric surveys that will multiply the number of stars with measured polarimetric properties by a factor of 1000, it is important to find ways to mine high-accuracy information about magnetic field strengths from polarimetric data. In this talk, I will discuss how this is possible, by better accounting for the properties of interstellar medium magnetized turbulence.

All ingredients to make stars like our Sun and planets like our Earth are present in dense cold interstellar clouds. In these "stellar-system precursors" an active chemistry is already at work, as demonstrated by the presence of a rich variety of organic molecules in the gas phase and icy mantles encapsulating the sub-micrometer dust grains, the building blocks of planets. Here, I’ll present a journey from the earliest phases of star formation to protoplanetary disks, with links to our Solar System, highlighting the crucial role of astrochemistry as powerful diagnostic tool of the various steps present in the journey.