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 (1$\sigma$=13 $\mu$Jy) cosmological 1.2-mm
continuum map based on ASPECS, the ALMA Spectroscopic Survey in the Hubble
Ultra Deep Field. In the 1 arcmin$^2$ covered by ASPECS we detect nine sources
at $>3.5\sigma$ significance at 1.2-mm. Our ALMA--selected sample has a median
redshift of $z=1.6\pm0.4$, 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 cut-off and similar frequencies. Most
galaxies have specific star formation rates similar to that of main sequence
galaxies at the same epoch, and we find median values of stellar mass and star
formation rates of $4.0\times10^{10}\ M_\odot$ and $\sim40~M_\odot$ yr$^{-1}$,
respectively. Using the dust emission as a tracer for the ISM mass, we derive
depletion times that are typically longer than 300 Myr, and we find molecular
gas fractions ranging from $\sim$0.1 to 1.0. As noted by previous studies,
these values are lower than using CO--based ISM estimates by a factor $\sim$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 $55\pm4\%$ of the
extragalactic background light (EBL) at 1.2 mm measured by the Planck
satellite, and we recover $80\pm7\%$ 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 $z\sim1-2$, with stellar masses of
(1-3)$\times$10$^{10}$ M$_\odot$. For the first time, we are able to
characterize the population of galaxies that dominate the EBL at 1.2 mm.
