The GOODS-South Field
The GOODS-South field is one of the best explored cosmological fields at present. This region of 70 arcmin2 covers only one-tenth of the Moon's surface, which is about the surface of the sky covered by the lead with a pencil held at arm's length. It has already been observed by many space and terrestrial telescopes, covering a wide range of wavelengths, from X-rays to radio waves, including images obtained by the Hubble, Herschel, Chandra and Spitzer space telescopes. The idea of the ALMA survey was to explore the deepest part of this field without introducing any bias other than the depth of ALMA observations.
To associate the counterparts of the different wavelengths as well as possible, we have done intensive work on the astrometry of the different instruments involved. In addition to an already known systemic offset in this field between HST and ALMA, we have discovered a local offset depending on the location in the field that can reach for 0.15" on the edge of GOODS-South.
Optically dark galaxies
We showed that ALMA surveys are efficient to reveal new galaxies so strongly dust-obscured that they escape the deepest observations (H>28) of the Hubble Space Telescope but are unveiled by their thermal dust emission. The discovery of these optically dark galaxies, which represent ~15-20% of the ALMA detections, suggests that the number of massive star-forming galaxies in the distant Universe may be larger than previously expected. Preliminary evidence of their spatial distribution suggests that they exhibit stronger clustering than field galaxies and could even serve as beacons to identify the most distant galaxy clusters at their formation epoch.
The modeling of the spectral energy distributions (SEDs) of the ALMA detected galaxiies, using the SED fitting code CIGALE has made it possible to derive the key physical properties of the galaxies detected by ALMA. I derived star formation rates (SFRs), the masses of gas, dust, and stars, dust temperatures, the time required for galaxies to consume their gas, the relationship between infrared luminosity and radio luminosity, and the excess of the infrared component in the spectra of galaxies.
Additionally, the precise characterization of this large ALMA survey has led to the number counts, i.e., the estimation of the number of galaxies in the sky per unit area and flux density. The derived differential and cumulative number counts of our detections have allowed us to partly alleviate the degeneracy observed above 1 mJy.beam-1 in previous (sub)millimeter studies.
The analysis of the derived properties, in addition to a comparative study of UV and infrared sizes (measured with GALFIT), reveals that contrary to some theories, the progenitors of IR compact galaxies are not UV compact galaxies. Indeed, the very compact IR sizes (similar to the optical sizes of passive galaxies with comparable masses and redshifts) compared to their optical sizes (in the range of expected sizes) show that these galaxies are undergoing a morphological transition and rapidly assembling their bulge.
Furthermore, this study showed that massive galaxies detected by ALMA could consume their gas reservoirs in a secular way and quench without evoking external or violent phenomena (slow-dowfall).