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

Abstract

We present the characteristics of 2 mm selected sources from the largest Atacama Large Millimeter/submillimeter Array (ALMA) blank-field contiguous survey conducted to date, the Mapping Obscuration to Reionization with ALMA (MORA) survey covering 184 arcmin2 at 2 mm. Twelve of 13 detections above 5σ are attributed to emission from galaxies, 11 of which are dominated by cold dust emission. These sources have a median redshift of 〈 z 2 mm 〉 = 3.6 − 0.3 + 0.4 primarily based on optical/near-infrared photometric redshifts with some spectroscopic redshifts, with 77% ± 11% of sources at z > 3 and 38% ± 12% of sources at z > 4. This implies that 2 mm selection is an efficient method for identifying the highest-redshift dusty star-forming galaxies (DSFGs). Lower-redshift DSFGs (z < 3) are far more numerous than those at z > 3 yet are likely to drop out at 2 mm. MORA shows that DSFGs with star formation rates in excess of 300 M ⊙ yr−1 and a relative rarity of ∼10−5 Mpc−3 contribute ∼30% to the integrated star formation rate density at 3 < z < 6. The volume density of 2 mm selected DSFGs is consistent with predictions from some cosmological simulations and is similar to the volume density of their hypothesized descendants: massive, quiescent galaxies at z > 2. Analysis of MORA sources’ spectral energy distributions hint at steeper empirically measured dust emissivity indices than reported in typical literature studies, with 〈 β 〉 = 2.2 − 0.4 + 0.5 . The MORA survey represents an important step in taking census of obscured star formation in the universe’s first few billion years, but larger area 2 mm surveys are needed to more fully characterize this rare population and push to the detection of the universe’s first dusty galaxies.

Searching Far and Long. I. Pilot ALMA 2 mm Follow-up of Bright Dusty Galaxies as a Redshift Filter

“…Nevertheless, our data can be used to compare galaxyintegrated Rayleigh-Jeans slopes to other high-z samples and commonly adopted literature values of β for high-z data sets of similar quality. Here, our finding of 〈β〉 = 2.4 ± 0.3 suggests that the distribution of integrated β indices skews high, in line with other recent works, including a well-studied galaxy in SSA22 with β = 2.3 (Kato et al 2018) as well as other samples with dust continuum data at λ obs  2 mm (Jin et al 2019;Casey et al 2021). The aggregate best-fit SED results for our sample (see Table 2) show relatively steep β values ranging from 1.8 < β < 3.0, all steeper than the standard value often adopted in the literature, typically β = 1.8 (e.g., Scoville et al 2016), justified by measurements of the β from the Milky Way's ISM (e.g., Paradis et al 2009;Planck Collaboration et al 2011).…”

“…While the redshift distribution of galaxies selected at 2 mm is expected to be relatively high with 〈z〉 ≈ 3.6 (Casey et al 2021), the selection of these targets at 850 μm leads to a lower median redshift, consistent with what has been previously found for 850 μm-selected DSFGs (e.g., Dudzevičiūtė et al 2020;da Cunha et al 2021). Though we expect the follow up 2 mm observations to filter out lower-redshift sources (Casey et al 2021;Zavala et al 2021;Manning et al 2022), our source selection here is based on 850 μm, and thus for this study the redshift distribution is reflective of the 850 μm DSFG redshift distribution. Previous millimeter studies demonstrate that deeper surveys tend to select for lower-redshift DSFGs; in other words brighter DSFGs tend to sit at higher redshifts (e.g., Béthermin et al 2015).…”

“…Predictions from the SHARK model (Lagos et al 2020) are also in line with this observation. The MORA survey-the largest ALMA 2 mm blank-field contiguous survey (184 arcmin 2 )-demonstrates this, finding an average redshift z 3.6 0.3 0.4 á ñ = -+ , with 77% of sources at z > 3 and 30% at z > 4, effectively filtering out lower-redshift DSFGs (Casey et al 2021). Other millimeter wavelengths have been leveraged to select for higher-redshift sources; for example, (Zavala et al 2018a,) conduct a blind search at 3 mm and detect 16 sources at >5σ, and Williams et al (2019) serendipitously discover a z ∼ 5-6 source at similar wavelengths.…”

“…For example, Koprowski et al (2017), Rowan-Robinson et al (2016), Dudzevičiūtė et al (2020), and Gruppioni et al (2020) all present very different conclusions on the number density of IR-luminous galaxies at z > 3, due to either limited sample sizes at high redshift or fundamental differences in survey strategy. Additionally, using a blankfield 2 mm Atacama Large Millimeter/submillimeter Array (ALMA) survey (Mapping Obscuration to Reionization with ALMA, or MORA), Zavala et al (2021) and Casey et al (2021) found that DSFGs contribute ∼35% at z = 5 and only 20%-25% at z = 6-7, suggesting dust-obscured star formation plays a minor role in total star formation at early epochs. On the other hand, Gruppioni et al (2020) find a flatter IR luminosity function (IRLF) slope from 4.5 < z < 6, with a significant contribution to the CSFRD from obscured sources at high redshift, though again, using a very different survey strategy.…”

No Redshift Evolution of Dust Temperatures from $0 < z < 2$

“…The median emissivity, β = 1.96 is fully consistent with several recent publications that find similarly steep values for the emissivity spectral index in various high-redshift galaxies with high quality far-IR constraints (e.g. Kato et al 2018;Casey et al 2021). We choose to adopt these priors because the minority of galaxies which produce unphysical fits when run with flat priors have more physical fits using normal priors.…”