Characterization of Two 2mm-detected Optically-Obscured Dusty Star-Forming Galaxies

Abstract

The 2 mm Mapping Obscuration to Reionization with ALMA (MORA) Survey was designed to detect high redshift (z 4), massive, dusty star-forming galaxies (DSFGs). Here we present two, likely high redshift sources, identified in the survey whose physical characteristics are consistent with a class of optical/near-infrared (OIR) invisible DSFGs found elsewhere in the literature. We first perform a rigorous analysis of all available photometric data to fit spectral energy distributions and estimate redshifts before deriving physical properties based on our findings. Our results suggest the two galaxies, called MORA-5 and MORA-9, represent two extremes of the "OIR-dark" class of DSFGs. MORA-5 (z phot = 4.3 +1.5 −1.3 ) is a significantly more active starburst with a star-formation rate of 830 +340 −190 M yr −1 compared to MORA-9 (z phot = 4.3 +1.3 −1.0 ) whose star-formation rate is a modest 200 +250 −60 M yr −1 . Based on the stellar masses (M ≈ 10 10−11 M ), space density (n ∼ (5 ± 2) × 10 −6 Mpc −3 , which incorporates two other spectroscopically confirmed OIR-dark DSFGs in the MORA sample at z = 4.6 and z = 5.9), and gas depletion timescales (< 1 Gyr) of these sources, we find evidence supporting the theory that OIR-dark DSFGs are the progenitors of recently discovered 3 < z < 4 massive quiescent galaxies.

An ALMA Spectroscopic Survey of the Brightest Submillimeter Galaxies in the SCUBA-2-COSMOS field (AS2COSPEC): Survey Description and First Results

“…However, assuming a typical lifetime of 100 Myr for our sample SMGs (Liao et al in preparation) the duty cycle corrected number densities are 3.3 × 10 −6 cMpc −3 , 1.9 × 10 −6 cMpc −3 , and 1.0 × 10 −6 cMpc −3 for the three redshift bins, respectively. Our number density estimate at z > 4 is similar to that presented by searches based on Herschel (Ivison et al 2016) and ALMA 2 mm survey (Manning et al 2021), although sample selections are different thus direct comparisons are not straightforward.…”

Mapping Obscuration to Reionization with ALMA (MORA): 2 mm Efficiently Selects the Highest-redshift Obscured Galaxies

“…Three of the 12 sources are spectroscopically confirmed at z = 2.472 (MORA-10, known as 450.09, with the synchrotron component), z = 4.625 (MORA-3, known as AzTEC-2), and z = 5.850 (MORA-4, known as MAMBO-9); of the remaining nine sources, eight have some form of OIR-based photometric redshift (Laigle et al 2016;Marsan et al 2017;Gómez-Guijarro et al 2018;Weaver et al 2021). The last source (MORA-5) lacks an OIR counterpart and any redshift constraint; MORA-5 along with MORA-9, whose OIR phot-z is highly uncertain, have hybrid photometric redshift fits provided in our accompanying manuscript (Manning et al 2021).…”

“…These sources are MORA-3 (known as AzTEC-2, spectroscopically confirmed at z = 4.63), MORA-4 (known as MAMBO-9, spectroscopically confirmed at z = 5.85), MORA-5, and MORA-9. As these represent the potential highestredshift subset of the 2 mm selected sample, a more thorough analysis of them is given in the accompanying paper by Manning et al (2021). It appears that MORA-3 (AzTEC-2) is the only galaxy in the sample that is gravitationally lensed…”

“…Z21 uses number counts from the MORA Survey, alongside 1.2 mm number counts from ASPECS and 3 mm number counts, to derive constraints on the high-z IRLF, even in the absence of direct redshift measurements of individual galaxies. Thus it is important to emphasize that the predicted redshift The two unconfirmed OIR-dark sources with the most uncertain redshift constraints are MORA-5 and MORA-9, described in more detail in our accompanying manuscript, Manning et al (2021). The redshifts for these two galaxies are derived jointly using OIR and millimeter constraints, here called hybrid.…”

“…The light purple shaded regions in the M ISM -z plane also shows the theoretical maximum M ISM mass limit as a function of redshift based on the Harrison & Hotchkiss (2013) intrinsically massive; thus the survey volume itself sets an upper limit to its mass. The last two exceptions to the standard MAGPHYS stellar mass fitting approach are sources MORA-5 and MORA-9, which are the two OIR-dark sources explored in more detail in Manning et al (2021), as they lack good redshift constraints. Their stellar masses are fit using the MAGPHYS+photoz code (Battisti et al 2019), which is a further update to MAGPHYS allowing for redshift uncertainty; this was deemed necessary in the case of MORA-5 and MORA-9 in particular, given the significant uncertainties on both sources' photometric redshifts.…”