The ALMA Spectroscopic Survey in the Hubble Ultra Deep Field: Continuum Number Counts, Resolved 1.2 mm Extragalactic Background, and Properties of the Faintest Dusty Star-forming Galaxies
Smail, I.; Magnelli, B.; Daddi, E.; Elbaz, D.; Ivison, R. J.; Inami, H.; Infante, L.; Bauer, F. E.; Ibar, E.; Weiss, A.; Walter, F.; Riechers, D. A.; Swinbank, A. M.; van der Werf, P.; Karim, A.; Bertoldi, F.; Aravena, M.; Decarli, R.; Assef, R. J.; Bouwens, R.; Popping, G.; Cox, P.; Wagg, J.; Carilli, C. L.; Anguita, T.; Bacon, R.; Hodge, J.; Sheth, K.; Da Cunha, E.; Bell, E.; Ota, K.; Gónzalez-López, J.; Cortes, P.; Le Le Fèvre, O.
Chile, Germany, United States, Australia, United Kingdom, France, Netherlands
Abstract
We present an analysis of a deep (1σ = 13 μJy) cosmological 1.2 mm continuum map based on ASPECS, the ALMA Spectroscopic Survey in the Hubble Ultra Deep Field. In the 1 arcmin2 covered by ASPECS we detect nine sources at \gt 3.5σ significance at 1.2 mm. Our ALMA-selected sample has a median redshift of z=1.6+/- 0.4, with only one galaxy detected at z > 2 within the survey area. This value is significantly lower than that found in millimeter samples selected at a higher flux density cutoff and similar frequencies. Most galaxies have specific star formation rates (SFRs) similar to that of main-sequence galaxies at the same epoch, and we find median values of stellar mass and SFRs of 4.0× {10}10 {M}⊙ and ∼ 40 {M}⊙ yr-1, respectively. Using the dust emission as a tracer for the interstellar medium (ISM) mass, we derive depletion times that are typically longer than 300 Myr, and we find molecular gas fractions ranging from ∼0.1 to 1.0. As noted by previous studies, these values are lower than those using CO-based ISM estimates by a factor of ∼2. The 1 mm number counts (corrected for fidelity and completeness) are in agreement with previous studies that were typically restricted to brighter sources. With our individual detections only, we recover 55% ± 4% of the extragalactic background light (EBL) at 1.2 mm measured by the Planck satellite, and we recover 80% ± 7% of this EBL if we include the bright end of the number counts and additional detections from stacking. The stacked contribution is dominated by galaxies at z∼ 1{--}2, with stellar masses of (1-3) × 1010 M {}⊙ . For the first time, we are able to characterize the population of galaxies that dominate the EBL at 1.2 mm.