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We present a physical characterization of MM J100026.36+021527.9 (a.k.a. "Mambo-9"), a dusty star-forming galaxy (DSFG) at z = 5.850 ± 0.001. This is the highest-redshift unlensed DSFG (and fourth most distant overall) found to date and is the first source identified in a new 2 mm blank-field map in the COSMOS field. Though identified in prior samples of DSFGs at 850 μm to 1.2 mm with unknown redshift, the detection at 2 mm prompted further follow-up as it indicated a much higher probability that the source was likely to sit at z > 4. Deep observations from the Atacama Large Millimeter and submillimeter Array (ALMA) presented here confirm the redshift through the secure detection of 12CO(J = 6→5) and p-H2O (21,1 → 20,2). Mambo-9 is composed of a pair of galaxies separated by 6 kpc with corresponding star formation rates of 590 M⊙ yr−1 and 220 M⊙ yr−1, total molecular hydrogen gas mass of (1.7 ± 0.4) × 1011M⊙, dust mass of (1.3 ± 0.3) × 109M⊙, and stellar mass of () × 109M⊙. The total halo mass, (3.3 ± 0.8) × 1012M⊙, is predicted to exceed 1015M⊙ by z = 0. The system is undergoing a merger-driven starburst that will increase the stellar mass of the system tenfold in τdepl = 40−80 Myr, converting its large molecular gas reservoir (gas fraction of ) into stars. Mambo-9 evaded firm spectroscopic identification for a decade, following a pattern that has emerged for some of the highest-redshift DSFGs found. And yet, the systematic identification of unlensed DSFGs like Mambo-9 is key to measuring the global contribution of obscured star formation to the star formation rate density at z ≳ 4, the formation of the first massive galaxies, and the formation of interstellar dust at early times (≲1 Gyr). [ABSTRACT FROM AUTHOR]

We report the serendipitous detection of two 3 mm continuum sources found in deep ALMA Band 3 observations to study intermediate-redshift galaxies in the COSMOS field. One is near a foreground galaxy at 1.″3, but is a previously unknown dust-obscured star-forming galaxy (DSFG) at probable zCO = 3.329, illustrating the risk of misidentifying shorter wavelength counterparts. The optical-to-millimeter spectral energy distribution (SED) favors a gray λ−0.4 attenuation curve and results in significantly larger stellar mass and SFR compared to a Calzetti starburst law, suggesting caution when relating progenitors and descendants based on these quantities. The other source is missing from all previous optical/near-infrared/submillimeter/radio catalogs (“ALMA-only”), and remains undetected even in stacked ultradeep optical (>29.6 AB) and near-infrared (>27.9 AB) images. Using the ALMA position as a prior reveals faint signal-to-noise ratio ∼ 3 measurements in stacked IRAC 3.6+4.5, ultradeep SCUBA2 850 μm, and VLA 3 GHz, indicating the source is real. The SED is robustly reproduced by a massive M* = 1010.8M⊙ and Mgas = 1011M⊙, highly obscured AV ∼ 4, star-forming SFR ∼ 300 M⊙ yr−1 galaxy at redshift z = 5.5 ± 1.1. The ultrasmall 8 arcmin2 survey area implies a large yet uncertain contribution to the cosmic star formation rate density CSFRD(z = 5) ∼ 0.9 × 10−2M⊙ yr−1 Mpc−3, comparable to all ultraviolet-selected galaxies combined. These results indicate the existence of a prominent population of DSFGs at z > 4, below the typical detection limit of bright galaxies found in single-dish submillimeter surveys, but with larger space densities ∼3 × 10−5 Mpc−3, higher duty cycles of 50%–100%, contributing more to the CSFRD, and potentially dominating the high-mass galaxy stellar mass function. [ABSTRACT FROM AUTHOR]

Though half of cosmic starlight is absorbed by dust and reradiated at long wavelengths (3 μm–3 mm), constraints on the infrared through the millimeter galaxy luminosity function (or the “IRLF”) are poor in comparison to the rest-frame ultraviolet and optical galaxy luminosity functions, particularly at z ≳ 2.5. Here, we present a backward evolution model for interpreting number counts, redshift distributions, and cross-band flux density correlations in the infrared and submillimeter sky, from 70 μm–2 mm, using a model for the IRLF out to the epoch of reionization. Mock submillimeter maps are generated by injecting sources according to the prescribed IRLF and flux densities drawn from model spectral energy distributions that mirror the distribution of SEDs observed in 0 < z < 5 dusty star-forming galaxies (DSFGs). We explore two extreme hypothetical case studies: a dust-poor early universe model, where DSFGs contribute negligibly (<10%) to the integrated star formation rate density at z > 4; and an alternate dust-rich early universe model, where DSFGs dominate ∼90% of z > 4 star formation. We find that current submm/mm data sets do not clearly rule out either of these extreme models. We suggest that future surveys at 2 mm will be crucial to measuring the IRLF beyond z ∼ 4. The model framework developed in this paper serves as a unique tool for the interpretation of multiwavelength IR/submm extragalactic data sets, and will enable more refined constraints on the IRLF than can be made from direct measurements of individual galaxies’ integrated dust emission. [ABSTRACT FROM AUTHOR]

Deep, pencil-beam surveys from ALMA at 1.1–1.3 mm have uncovered an apparent absence of high-redshift dusty galaxies, with existing redshift distributions peaking around z ∼ 1.5–2.5. This has led to a perceived dearth of dusty systems at z ≳ 4 and the conclusion, according to some models, that the early universe was relatively dust-poor. In this paper, we extend the backward-evolution galaxy model described by Casey et al. to the ALMA regime (in depth and area) and determine that the measured number counts and redshift distributions from ALMA deep field surveys are fully consistent with constraints of the infrared luminosity function (IRLF) at z < 2.5 determined by single-dish submillimeter and millimeter surveys conducted on much larger angular scales (∼1–10 deg2). We find that measured 1.1–1.3 mm number counts are most constraining for the measurement of the faint-end slope of the IRLF at z ≲ 2.5 instead of the prevalence of dusty galaxies at z ≳ 4. Recent studies have suggested that UV-selected galaxies at z > 4 may be particularly dust-poor, but we find that their millimeter-wave emission cannot rule out consistency with the Calzetti dust attenuation law, even by assuming relatively typical, cold-dust (Tdust ≈ 30 K) spectral energy distributions. Our models suggest that the design of ALMA deep fields requires substantial revision to constrain the prevalence of z > 4 early universe obscured starbursts. The most promising avenue for detection and characterization of such early dusty galaxies will come from future ALMA 2 mm blank-field surveys covering a few hundred arcmin2 and the combination of existing and future dual-purpose 3 mm data sets. [ABSTRACT FROM AUTHOR]

A decade of study has established that the molecular gas properties of star-forming galaxies follow coherent scaling relations out to z ∼ 3, suggesting remarkable regularity of the interplay between molecular gas, star formation, and stellar growth. Passive galaxies, however, are expected to be gas-poor and therefore faint, and thus little is known about molecular gas in passive galaxies beyond the local universe. Here we present deep Atacama Large Millimeter/submillimeter Array observations of CO(2–1) emission in eight massive (Mstar ∼ 1011M⊙) galaxies at z ∼ 0.7 selected to lie a factor of 3–10 below the star-forming sequence at this redshift, drawn from the Large Early Galaxy Astrophysics Census survey. We significantly detect half the sample, finding molecular gas fractions ≲0.1. We show that the molecular and stellar rotational axes are broadly consistent, arguing that the molecular gas was not accreted after the galaxies became quiescent. We find that scaling relations extrapolated from the star-forming population overpredict both the gas fraction and gas depletion time for passive objects, suggesting the existence of either a break or large increase in scatter in these relations at low specific star formation rate. Finally, we show that the gas fractions of the passive galaxies we have observed at intermediate redshifts are naturally consistent with evolution into local, massive early-type galaxies by continued low-level star formation, with no need for further gas accretion or dynamical stabilization of the gas reservoirs in the intervening 6 billion years. [ABSTRACT FROM AUTHOR]

Drawing from the LEGA-C data set, we present the spectroscopic view of the stellar population across a large volume- and mass-selected sample of galaxies at large look-back time. We measure the 4000 Å break (Dn4000) and Balmer absorption line strengths (probed by Hδ) from 1019 high-quality spectra of z = 0.6–1.0 galaxies with M* = 2 × 1010M⊙ to 3 × 1011M⊙. Our analysis serves as a first illustration of the power of high-resolution, high signal-to-noise ratio continuum spectroscopy at intermediate redshifts as a qualitatively new tool to constrain galaxy formation models. The observed Dn4000–EW(Hδ) distribution of our sample overlaps with the distribution traced by present-day galaxies, but z ∼ 0.8 galaxies populate that locus in a fundamentally different manner. While old galaxies dominate the present-day population at all stellar masses >2 × 1010M⊙, we see a bimodal Dn4000–EW(Hδ) distribution at z ∼ 0.8, implying a bimodal light-weighted age distribution. The light-weighted age depends strongly on stellar mass, with the most massive galaxies >1 × 1011M⊙ being almost all older than 2 Gyr. At the same time, we estimate that galaxies in this high-mass range are only ∼3 Gyr younger than their z ∼ 0.1 counterparts, at odds with purely passive evolution given a difference in look-back time of >5 Gyr; younger galaxies must grow to >1011M⊙ in the meantime, or small amounts of young stars must keep the light-weighted ages young. Star-forming galaxies at z ∼ 0.8 have stronger Hδ absorption than present-day galaxies with the same Dn4000, implying larger short-term variations in star formation activity. [ABSTRACT FROM AUTHOR]