COLDz: A High Space Density of Massive Dusty Starburst Galaxies ∼1 Billion Years after the Big Bang

Abstract

We report the detection of CO(J=2→1) emission from three massive dusty starburst galaxies at z>5 through molecular line scans in the NSF's Karl G. Jansky Very Large Array (VLA) CO Luminosity Density at High Redshift (COLDz) survey. Redshifts for two of the sources, HDF 850.1 (z=5.183) and AzTEC-3 (z=5.298), were previously known. We revise a previous redshift estimate for the third source GN10 (z=5.303), which we have independently confirmed through detections of CO J=1→0, 5→4, 6→5, and [C II] 158 μm emission with the VLA and the NOrthern Extended Milllimeter Array. We find that two currently independently confirmed CO sources in COLDz are "optically dark", and that three of them are dust-obscured galaxies at z>5. Given our survey area of ∼60 arcmin 2 , our results appear to imply a ∼6-55 times higher space density of such distant dusty systems within the first billion years after the Big Bang than previously thought. At least two of these z>5 galaxies show star formation rate surface densities consistent with so-called "maximum" starbursts, but we find significant differences in CO excitation between them. This result may suggest that different fractions of the massive gas reservoirs are located in the dense, star-forming nuclear regions-consistent with the more extended sizes of the [C II] emission compared to the dust continuum and higher [C II]-to-far-infrared luminosity ratios in those galaxies with lower gas excitation. We thus find substantial variations in the conditions for star formation between z>5 dusty starbursts, which typically have dust temperatures that are ∼57%±25% warmer than starbursts at z=2-3 due to their enhanced star formation activity.

The ALPINE-ALMA [CII] survey

“…By considering the volumes corresponding to each source in our survey, we derive a space density of HST+near-IR dark galaxies in ALPINE of ∼(1.2 ± 0.7) × 10 −4 and (5.0 ± 3.8) × 10 −5 Mpc −3 at z 3 and 5, respectively. The value found in the highest redshift interval is higher (though consistent within the uncertainties) than the source density of dark galaxies estimated by Williams et al (2019) at z 4.1-5.7 (2.9 × 10 −5 Mpc −3 ) and by Riechers et al (2020) at z > 5 (i.e. (1.0 ± 0.7) × 10 −5 Mpc −3 ).…”

“…We note that in the common luminosity bins, the COLDz derivation is in very good agreement with our estimate, with the CO LFs extending the high bright end to even higher luminosities. Indeed, the recent finding that in the COLDz survey there may be an overdensity at z ∼ 5 capturing three bright dusty starbursts (Riechers et al 2020) could partially explain such a high CO LF at 4.5 < z < 6 (see e.g. our LF obtained by including the two [C II] emitters likely associated with the ALPINE targets shown in Fig.…”

“…16) represent the obscured and unobscured SFRD from this work and from Bouwens et al (2015), respectively. densities, such as (0.042 ± 0.028) arcmin −2 , are reported by Riechers et al (2020) for optically dark CO emitters at z > 5 detected down to an equivalent 870 µm flux density of ∼5 mJy.…”

“…The HST-dark galaxies also tend to be serendipitously found in CO line scan surveys (see e.g. Riechers et al 2020, who found two of them at z > 5), possibly with space densities higher than expected even at the bright end of the CO luminosity functions (LFs). These results indicate the existence of a prominent population of dusty star-forming galaxies at z > 4, which is fainter than the confusion limit of the single-dish sub-mm surveys that discovered the SMGs, but with much larger space densities, providing a significant contribution to the SFRD at high-z, even higher than that of the UV-bright galaxies at the same redshifts (e.g.…”

ALMA Deep Field in SSA22

“…For instance, Simpson et al (2014) found ten SMGs which were detected at 3.6 µm, but not detected at shorter wavelengths. While obtaining a spectroscopic redshift of these types of near-infrared-dark SMGs is still not easy, A&A 640, L8 (2020) a handful of these galaxies are now placed at z ∼ 4−5 via molecular and atomic line detections at (sub)millimeter wavelengths and the number is increasing (e.g., Swinbank et al 2012;Riechers et al 2017;Oteo et al 2018;Casey et al 2019;Jin et al 2019;Jiménez-Andrade et al 2020;Riechers et al 2020).…”