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Physics of Star Formation and Its Regulation

Periodic Reporting for period 2 - PhysSF (Physics of Star Formation and Its Regulation)

Reporting period: 2018-04-01 to 2019-09-30

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 2 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 star formation 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
in October 2017. These datasets will be complemented by a large observing program on the HST started in April 2019 that will provide
broad-band photometry for these galaxies that allows 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 MUSE data.


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 publically available to the astronomical community.

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.
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 will provide a legacy database as such for the study of the star
formation process 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 will provide new insights into how the gas fraction and gas depletion
time of galaxy populations evolve. The obtained photometric catalogs in conjunction with a homogeneous sub-mm detected galaxies catalog
forms a dataset of long-term value for the community. The combination of diverse observing programs will provide 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.