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Mapping the Cosmic Gas Supply with ALMA

Periodic Reporting for period 2 - Cosmic_Gas (Mapping the Cosmic Gas Supply with ALMA)

Reporting period: 2019-05-01 to 2020-10-31

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, characterizing this fuel supply in galaxies as a function of cosmic epoch is still in its infancy. The ALMA facility has redefined our ability to map out the cosmic cold gas supply. This ERC program is based on extensive observational ALMA programs led by the PI: ASPECS is 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 provides 3D molecular scans in two ALMA bands of the Hubble Ultra Deep Field -- the iconic cosmological deep field. A second focus is the detailed characterization 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 project capitalizes on the unparalleled capabilities of ALMA and other facilities (e.g. VLT/MUSE, and in the future JWST) to map out the cosmic gas supply through cosmic history, and will provide crucial insights to define observational strategies for JWST. The final goal of this program is to define the global state-of-the-art in cosmological galaxy evolution through high-redshift ISM studies. Its results will put stringent constraints on numerical modelling and similations of both the earliest structures in the Universe as well as galaxy evolution through cosmic time.
In the first half of the period of this ERC program a large number of key science goals have already been met. 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 already been published from this project, and the final set of papers (including the ‘summary’ paper on the cosmic baryon cycle by the PI) will be published later this year. For project B, we have 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. 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. This ultimately resulted in kpc-resolution imaging of the interstellar medium in dozens of quasar host galaxies that allow to spatially resolve the morphology and kinematics of galaxies and (in many cases) their companion galaxies. These studied culminated in the unprecedented 400pc imaging of a quasar host galaxy that is seen when the universe is less than 1Gyr old. In parallel, we have been focusing on multi-line imaging of these galaxies, and have demonstrated that the interstellar medium of the most distant host galaxy known (at redshift 7.5 when the Universe was a mere ~750 Myr old) was already highly enriched with heavy elements which poses significant constraints on early galaxy evolution models.
In the next steps the program will finalise the full work done on the ALMA Large program, with an anticipated data and press release later this year. We will also push the highest-resolution imaging possible for the most distant quasar population with ALMA, possibly through another large ALMA initiative. Significant technical progress was made in developing new analysis tools to interpret data from the ALMA interferometer. These include new methods to assess the noise properties and highly accurate fluxes in interferometric imaging. Milestone papers that have pushed the state of the art is the 400pc imaging of a quasar host galaxies when the Universe was less than 1 Gyr old, and a in-depth characterisation of the interstellar medium of the most distant quasar known to date, demonstrating that it is already highly enriched when the Universe was a mere ~750 Myr old. A number of key papers from the ASPECS survey are very close to acceptance, including the deepest millimetre continuum map obtained by ALMA to date. In a parallel paper, we are discussing the full baryon cycle in the Universe using the latest ASPECS results, which will put new constraints on cosmic gas accretion onto galaxies. Eventually, JWST will provide observing capabilities beyond what is possible currently to characterise some of the earliest cosmic structures known. A breakthrough result would be the first detection of the stellar components of the first quasar host galaxies using JWST.