ASPECS - The ALMA Spectroscopic Survey in the Hubble Ultra Deep Field

I will report on some of the results emerging from the ALMA large program ASPECS (www.aspecs.info). ASPECS obtained deep imaging in the 1mm and 3mm bands of the Hubble Ultra-Deep Field (H-UDF) through frequency scans. The observations provide a full census of dust and molecular gas in the H-UDF, down to masses that are typical of main-sequence galaxies at redshifts 1-4. The resulting data products enable a great range of studies, from the characterisation of individual galaxies, capitalizing on the unique multi-wavelength dataset of the H-UDF, to CO excitation studies to constrain the gas properties of the distant galaxies. A 3D stacking analysis using precise redshifts from major VLT/MUSE initiatives on the field helped in recovering additional emission of galaxy samples that are too faint to be detected individually. Stacking in both the continuum and line (capitalizing on 100s of spectroscopic redshifts from major VLT/MUSE initiatives on the H-UDF) pushed the detection limits further. The nature of the observations (full spectral scans) provides a census of dust and molecular gas in the cosmic volume defined by the H-UDF. The resulting cosmic molecular gas density as a function of redshift shows an order of magnitude decrease from z=2 to z=0. This is markably different from independent measurements of the atomic gas phase that shows a rather flat redshift dependence. These measurements can be used to put new constraints on the gas accretion process that is needed to explain the build-up of stellar mass in galaxies through cosmic history.