2020ApJ...897...91G

We present the results from the 1.2 mm continuum image obtained as part of the Atacama Large Millimeter/submillimeter Array Spectroscopic Survey in the Hubble Ultra Deep Field. The 1.2 mm continuum image has a size of 2.9 (4.2) arcmin² within a primary beam response of 50% (10%) and an rms value of 9.3µJybeam^–1. We detect 35 sources at high significance (Fidelity >=0.5); 32 have well-characterized near-infrared Hubble Space Telescope counterparts. We estimate the 1.2 mm number counts to flux levels of < 30µJy in two different ways: we first use the detected sources to constrain the number counts and find a significant flattening of the counts below S_ν ∼ 0.1 mJy. In a second approach, we constrain the number counts using a probability of deflection statistics (P(D)) analysis. For this latter approach, we describe new methods to accurately measure the noise in interferometric imaging (employing jackknifing in the cube and in the visibility plane). This independent measurement confirms the flattening of the number counts. Our analysis of the differential number counts shows that we are detecting ∼93% (∼100% if we include the lower fidelity detections) of the total continuum dust emission associated with galaxies in the Hubble Ultra Deep Field. The ancillary data allow us to study the dependence of the 1.2 mm number counts on redshift (z = 0-4), galaxy dust mass (M_dust=10⁷–10⁹M_☉), stellar mass (M_*=10⁹–10¹²M_☉), and star formation rate (SFR=1-1000M_☉yr^–1). In an accompanying paper we show that the number counts are crucial to constrain galaxy evolution models and the understanding of star-forming galaxies at high redshift.