Periodic Reporting for period 5 - Cosmic_Gas (Mapping the Cosmic Gas Supply with ALMA)
Berichtszeitraum: 2023-11-01 bis 2024-10-31
The molecular gas phase is the material in galaxies out of which stars form. As such, it is this quantity that controls the star formation rate of a galaxy, thereby the overall stellar mass build-up, and ultimately galaxy evolution through cosmic time. In contrast to the decades-long studies of the stellar mass and star formation, characterising this fuel supply (ie molecular gas) in galaxies as a function of cosmic epoch has been in its infancy at the beginning of the project. The ALMA facility however redefined our ability to map out the cosmic cold gas supply, which triggered the ideas underlying this ERC program, through extensive observational ALMA programs led by the PI. In particular ASPECS was the first-ever approved ALMA large (150h) program of a cosmological deep field, aimed at providing a comprehensive view of the baryon cycle from gas to stars over cosmic time. ASPECS provided 3D molecular scans in two ALMA bands of the Hubble Ultra Deep Field (HUDF) -- the iconic cosmological deep field. Through this ERC, the molecular gas content of galaxies could thus be traced through cosmic history by targeting the iconic HUDF with ALMA. As vast multi-wavelength data for this field was available by previous international efforts, the molecular gas measurements could be immediately put in context with e.g. the stellar properties and star formation activity of the galaxies in the field. With these measurements, the project could thus constrain the cosmic baryon cycle, and quantify the gas flows within galaxies for the first time, providing a major breakthrough in the field. A second focus has been the detailed characterisation of the gas content in the host galaxies of the most distant quasars, when the Universe was less than one billion year old (<1/14th of today's age). These studies assess the role of cold gas in the build-up of the first massive cosmic structures in the Universe. This was achieved through a two-step process: first, a survey of dozens of quasar hosts with ALMA: this survey demonstrated the ubiquitousness of gas and dust already in the first Gyr of the universe, and showed that many of the targets lend themselves for high spatial resolution imaging. Indeed, follow-up observations on spatial scales of only a few hundred parsecs (typically only achieved in the local universe) constrained the kinematics of the quasar hosts, as well as their morphologies. One of the big surprises of these studies has been that the quasar host galaxies do not look like merging systems ("train wrecks"), but in many cases like simple rotating disks. This was unexpected, as these galaxies harbour accreting supermassive black holes with masses beyond one billion solar masses. However, their host galaxies appear to be unaffected by these central engines. In summary, the project capitalised on the unparalleled capabilities of ALMA and other facilities to map out the cosmic gas supply through cosmic history, from the first Gyr (cosmic dawn) to the peak of cosmic star formation (cosmic noon) to today's universe.
This ERC program met all anticipated key science goals. The focus has been on two related aspects: A) Detailed studies of the cosmic evolution of the molecular gas content through the PI-led ALMA Large Program ASPECS, and B) In-depth studies of the interstellar medium in the most distant quasar host galaxies with ALMA. For project A, significant efforts went into calibrating, reducing, and analysing the data from the first ALMA deep field: ASPECS exploits the unprecedented sensitivity of ALMA to advance our understanding of the molecular gas and dust content in distant galaxies. Targeting the iconic Hubble Ultradeep Field (UDF) with available 30-band photometry, we implemented a novel observing strategy to study the molecular gas properties in distant galaxies. Instead of targeting molecular gas emission lines in individual galaxies pre-selected through multi-wavelength observations, this approach employed a ‘spectral scanning’ method to obtain an unbiased measurement of the molecular gas content in a well-defined cosmological volume. At the same time, these spectral scans delivered the deepest-ever dust continuum maps of the universe. Taken together, these methods have allowed for a full characterisation of molecular gas and dust in the cosmological volume probed by the UDF, down to galaxy masses that encompass the bulk of the luminosity and masses out to redshifts ~4, i.e. when the Universe was only 1/7th of today’s age. A number of key publications have been published from this project, including the ‘summary’ paper on the cosmic baryon cycle by the PI. This work has been presented at many conferences, and a dedicated webpage (www.aspecs.info) was set up that shares the data with the community, and includes key information and easily digestible infographics and movies for the public). For project B, this project has pushed ALMA to obtain key measurements of the interstellar medium of some of the most extreme objects in the early universe. These effort started with a highly successful survey of dust and gas in ~25 of these objects, which resulted in an almost 100% detection rate, implying that the universe was already highly enriched with heavy elements which poses significant constraints on early galaxy evolution models. These objects turned out to be so bright with ALMA that follow-up spatially resolved imaging is possible, which the group has subsequently pushed in a number of publications that show stunning images of the earliest accessible quasar hosts. By the end of this project, this has resulted in stunning few-hundred parsec imaging of quasar host galaxies when the universe was less than 1 Gyr old. In total, the ERC project efforts resulted in 40 peer-reviewed publications in leading international journals.
In many regards, this ERC has more than fulfilled its original promise and goals, to an extent that the available instrumentation has been pushed to the limit both in terms of molecular deep fields, and extremely high-resolution imaging of quasar host galaxies. Indeed, of order 400 hours of high-quality ALMA time (secured by the PI's team), including one ALMA large program, were analyzed and published through the group's efforts, which implies that major, future progress is only possible with a 10x times increase in observing time (to reach 10 times larger samples, or sqrt(10)~3 times deeper images). However, 4000h ALMA programs (equivalent of one full year of observing time) are currently without reach, and provide a main motivation to push the development of the next generation of facilities. Indeed, the science cases addressed in this ERC have greatly guided the discussions of such future facilities, in particular the next generation Very Large Array (ngVLA) and the next major ALMA upgrade (ALMA2030/2040).