Galaxy Evolution in the ALMA Era - The Baryon Cycle and Star Formation in Nearby Galaxies

Project Information

EMPIRE

Grant agreement ID: 726384

DOI

10.3030/726384

Project closed

EC signature date 16 January 2017

Start date 1 July 2017

End date 31 December 2022

Funded under

• EXCELLENT SCIENCE - European Research Council (ERC)

Total cost

€ 1 659 451,00

EU contribution

€ 1 659 451,00

Coordinated by

RHEINISCHE FRIEDRICH-WILHELMS-UNIVERSITAT BONN
Germany

Periodic Reporting for period 4 - EMPIRE (Galaxy Evolution in the ALMA Era - The Baryon Cycle and Star Formation in Nearby Galaxies)

Reporting period: 2021-10-01 to 2022-12-31

Summary of the context and overall objectives of the project

The conversion of gas into stars is one of the key processes taking place in the evolution of galaxies over billions of years of cosmic evolution. How do we end up with the stars in galaxies we see today? How will the formation of stars continue to evolve? How does this shape the properties of galaxies? are key questions in this context. Stars form in collapsing clouds of gas and dust and, at the end of their life span, eject matter, radiation and energy back into the interstellar medium. This matter cycle is a continuous process, changing the structure, composition and properties of galaxies. Understanding how matter is cycled through its different phases in and across galaxies is of paramount interest to astronomy, but also sheds light on some of the fundamental questions for mankind, like the production of "heavy elements" in stars (carbon or oxygen). This provides a close link between the formation of stars and planetary systems. Understanding the many aspects of the cosmic matter cycle, how galaxies form stars, how these stars change the conditions in the interstellar matter, the physical and chemical conditions, and how this process impacts galaxies as a whole are the central science topics of this grant.

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.

Work performed from the beginning of the project to the end of the period covered by the report and main results achieved so far

A significant amount of work went into finishing data processing and producing final data products of the comprehensive EMPIRE survey (empiresurvey.webstarts.com) taking two years to observe, and from many follow-up observing campaigns from state of the art facilities, like ALMA, IRAM facilities, APEX, SOFIA or the VLA. Multiple spectral lines from various different molecules are covered in our observations, such as hydrogen cyanide or prussic acid HCN, HCO+ or carbon monoxide, CO, which all emit efficiently under very different conditions in the interstellar medium. Expanding on previous work, we have developed new, sophisticated modeling techniques related to the radiative transfer of such spectral line emission. Of primary interest are actual gas densities and as a secondary goals also temperature, as these are immediately related to the ability of the molecular gas to collapse and form stars. The key result from this comprehensive project is that we can indeed link an increase in molecular gas density to the star formation activity across galaxies. This tool box of models has been made publicly available by us via an openly accessible web interface (www.densegastoolbox.com).

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.

Progress beyond the state of the art and expected potential impact (including the socio-economic impact and the wider societal implications of the project so far)

We have surveyed an entire sample of nearby galaxies in a suite of molecular spectral lines only rarely observed across entire galaxies in the past. We have expanded the way such observations are used to learn about the physical conditions (volume densities, temperatures) in the gas and on existing tools to perform the (more realistic) radiative transfer computations. This work was also the basis to obtain new observations at some of the most competitive observatories to extend such observations to ultra-high resolution to study how gas density varies on the scales of individual star forming regions and how the properties of molecular clouds link to gas physics and hence how star formation is regulated. Another focus was to achieve detailed insights into the molecular gas conditions traced by emission from the CO molecule by observing many spectral lines from different CO isotpologues. This is of paramount interest to estimate (molecular) gas masses for galaxies throughout the universe.
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.

Spectral line observations from different molecules covering the entire nearby disk galaxy NGC 4321