Periodic Reporting for period 4 - EMPIRE (Galaxy Evolution in the ALMA Era - The Baryon Cycle and Star Formation in Nearby Galaxies)
Periodo di rendicontazione: 2021-10-01 al 2022-12-31
Our grant is exploiting recent technological developments regarding radio astronomical instrumentation, which allows us to examine emission from many different spectral lines from various molecules in interstellar space beyond the Milky Way. This is a major step forward compared to examining emission from CO (carbon monoxide), the most abundant molecule in the universe (aside from molecular hydrogen), which was the main focus for the past decades. The diversity of spectral lines from different molecules, such as HCN, HCO+ or HNC, but also various previously unaccessible CO lines, allows us in turn to constrain the conditions in the interstellar medium: this includes physical properties like masses, pressures, or energy budgets as well as chemical abundances. A particular focus are constraints on the volume density of the molecular gas, which is a key parameter regulating star formation as it determines how fast a gas cloud can collapse under its own gravity. While it cannot be measured directly, it has to be inferred from models using such new observations as the ones obtained as part of this ERC grant.
We have furthermore investigated under which conditions different spectral lines in the CO molecule are excited across other galaxies. Carbon monoxide is the workhorse tool to trace molecular hydrogen in galaxies across the universe. In currently ongoing work we calibrate systematically and comprehensively such CO line excitation across and among a sample of nearby spiral galaxies paying particular focus to systematic effects. We find surprisingly little variation of the ratio of the first two base transitions so that assuming a constant "ratio" seems to be a simple yet plausible approach, applicable to a wide range of astronomical observations.
Pushing towards extremely high resolution observations, we managed to link "spectroscopic" observations of different molecules to the individual properties of molecular clouds (sizes, masses, energy balance) to study: how do the properties of individual molecular clouds link to and regulate the actual physical and chemical conditions in the star forming gas (densities, temperatures)? How are these conditions related to the formation of stars? Our first results indeed find clear links for the first time between cloud properties and gas physics.
Key context comes from comparing extragalactic studies to studies of individual star forming regions in our own galaxy. Hence, bot approaches are highly complimentary though usually pursued by distinct communities. Specifically, we participate in a large observing campaign examining multiple molecular clouds in the Milky Way. This program is ongoing and we have completed several studies of massive star forming regions in our galaxy. They show in detail under what conditions (and from what regions) the molecular lines we study in other galaxies actually emit: the gas turns out to be dense, as expected, but relatively warm. This is quite surprising, as lines like HCN or HCO+ were expected to trace largely dense and quite cold molecular gas immediately about to form stars.
We have also extended our work to studies of the ionized gas in star forming regions from optical observations with the VLT and the Hubble Space Telescope. This is an important complement to obtain a holistic picture of the interstellar medium and how its properties regulate star formation.
As one of very few groups, we also capitalized on directly liking observations in other galaxies to those in our Galaxy and hence bridging these two communities. We achieved high enough spatial resolution to make the link to large scale observations in the Milky Way and have -towards the end of the grant- begun making direct one-to-one comparisons between star forming regions/molecular clouds in our galaxy and in other galaxies. This comprehensive approach of linking different size scales and environments provides key context for the Milky Way community as well as a wide range of extragalactic observations.