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

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 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 × 10¹⁰ 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.