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Physics of Star Formation and Its Regulation

Periodic Reporting for period 3 - PhysSF (Physics of Star Formation and Its Regulation)

Reporting period: 2019-10-01 to 2021-03-31

In the past decade we learned when and where stellar mass was built up in galaxies through cosmic time, now we must
understand the physical causes in order to answer `How do galaxies form and evolve?’. This ERC project is designed
to greatly advance our understanding of the physics of the star formation (SF) process and its regulation in typical
star-forming galaxies. The ERC project consists of two complementary parts: (A) an unparalleled characterization of
the SF process in nearby galaxies through full exploitation of the revolutionary capabilities of the latest millimeter
interferometers (ALMA) and optical integral field units (MUSE). This study will constrain the key physical
parameters for the SF process on only 50pc scales - the scale of large HII regions and their predecessors, giant
molecular clouds (PHANGS -- Physics at High Angular resolution in Nearby GalaxieS). At this crucial scale, the
PHANGS ALMA-MUSE Survey will provide a characterization of the SF history, stellar/gaseous surface densities,
metallicities of stars and gas, the stellar radiation field, extinction, and stellar/gas kinematics, and thus
uncover the physical conditions that control and regulate the SF process. Part (B) will place
the results of part (A) in a cosmological context, by characterizing key galaxy quantities (e.g. gas mass fraction,
specific SF rates, gas depletion times) in fully representative galaxy samples after (z<3) and before (z>3) the peak
epoch of cosmic SF density utilising the data products from the A3COSMOS effort that is part of this
ERC project. In addition to providing the critically needed constraints on the conditions
that govern the SF process, this ERC project will provide the observational benchmark for state-of-the-art galaxy
simulations and models.
PHANGS — Physics at High Angular resolution in Nearby GalaxieS:
Two large observing programs utilising ALMA and MUSE have started to map the star-forming disks of a representative sample of nearby,
massive galaxies on the main sequence of star-forming galaxies in tracers of the molecular and ionized gas as well as the stellar populations
between October 2017 and March 2021. These datasets are complemented by a large observing program on the HST which finished in April 2021 that provides
broad-band photometry for these galaxies that allow for the identification of young stellar clusters. These datasets form the basis to study
the underlying physics of the star formation process on cloud-scales. In parallel with the reduction and analysis of these datasets new
tools for the interpretation are being developed in collaboration with members from the international PHANGS collaboration.
A publication investigating the molecular gas-star formation life-cycle demonstrates the feasibility via a pilot sample to use sightlines of
molecular gas and a star formation rate tracer to gain insights on the ubiquity of quiescent molecular gas and hints at systematic variations
across galaxies. A dedicated software to obtain the orientation of a galaxy on the sky from gaseous or stellar velocities fields has been
developed and is being applied to the molecular gas motions as seen by ALMA as well as the ionised and stellar velocity fields obtained
by MUSE. Significant effort has been spent on the development of an automatic reduction and analysis pipeline for the ALMA data which
is presented in a publication and made publicly available. The combined MUSE and ALMA data are utilised to determine the pattern speed
of galactic structures in the full sample finding that gaseous tracers are compromised relative to the use of stellar information. An
investigation of the scaling relations between the surface densities of stellar mass, molecular gas mass, and star formation rate shows that
these relations persist even at the scale of star-forming units, though with larger scatter, and an apparent dependance on galaxy environment.
The full ALMA CO data are publicly available with science-ready data products and a corresponding publication describing the survey, the
dataset and presenting a first CO atlas of nearby galaxies.

A3COSMOS — Automatic mining for the ALMA Archive in the COSMOS field:
A procedure for (semi-)automatic mining of the ALMA archive in the COSMOS field (A3COSMOS) has been developed and implemented
in collaboration with members of the German ALMA Regional Centre (ARC) node. The resulting (blind and prior-based) catalogs have
been thoroughly tested via sophisticated simulations providing robust estimates of the (sub-)mm source parameters plus
associated uncertainties, the completeness and purity. The current version of the catalog contains about 1,000 sources from more
than 1,500 images. Combined with publicly available multi-band photometric catalogs, a catalog of >650 (sub-)mm detected galaxies
including their key properties (redshift, stellar, star formation rate) has been created. The catalogs enable the comprehensive study of the
cosmic evolution of the (cold) gas content and gas depletion time of galaxies. The entire process towards assembling these catalogs has
been described in a publication; the catalogs are publicly available to the astronomical community. Scientific exploitation of these catalogs
resulted in a comprehensive analysis of the cosmic evolution of the cold gas content and cold gas depletion time showing general agreement
with results using different methodologies and revealing a bias in the sample selection which might impact the inferred relations for the
galaxy populations that are currently not probed. This publication further includes a thorough assessment of different methods to obtain
the cold gas mass from (sub-)mm continuum measurements including the potential impact of metallicity variations over cosmic time.

Tools allowing for the structural analysis of the stellar mass distribution of massive, star-forming galaxies in the early universe have been
developed and applied to a pilot sample. Showing that the stellar mass distribution is quite compact and that the overall properties of these
massive galaxies are consistent with being the progenitors of today's massive elliptical galaxies. Exploring different CO emission lines in local
and high redshift galaxies reveal a dependence of the CO excitation on the interstellar radiation field. This together with extensive modeling
of the expected CO excitation is presented in a publication.
The assembly of the first comprehensive imaging of the interstellar medium (ISM) and stars plus their clusters on cloud-scales in a
representative sample of nearby, massive main sequence galaxies is providing a legacy database for the study of the star
formation process as such as well as other physics at unprecedented detail. The to be determined relations of between these
components (ISM, stars) will enable to make significant progress in our understanding of the physics of the star formation process.

The systematic exploitation of the ALMA imaging in the COSMOS field is providing new insights into how the gas fraction and gas depletion
time of galaxy populations evolve over cosmic time. The obtained photometric catalogs in conjunction with a homogeneous sub-mm detected
galaxies catalog form a dataset of long-term value for the community. The combination of diverse observing programs have provided an insight
into the effects of selection biases when studying the gas content. Further the database holds the key to identify currently missed populations which
could potentially significantly contribute to the cosmic star formation density in the early universe.