The implications of the surprising existence of a large, massive CO disk in a distant protocluster

Abstract

Itis not yet known if the properties of molecular gas in distant protocluster galaxies are significantly affected by their environment as galaxies are in local clusters. Through a deep, 64 hours of effective on-source integration with the Australian Telescope Compact Array (ATCA), we discovered a massive, M mol = 2.0 ± 0.2 × 10 11 M , extended, ∼40 kpc, CO(1-0)-emitting disk in the protocluster surrounding the radio galaxy, MRC 1138−262. The galaxy, at z CO = 2.1478, is a clumpy, massive disk galaxy, M * ∼ 5 × 10 11 M , which lies 250 kpc in projection from MRC 1138−262 and is a known Hα emitter, HAE229. HAE229 has a molecular gas fraction of ∼30%. The CO emission has a kinematic gradient along its major axis, centered on the highest surface brightness rest-frame optical emission, consistent with HAE229 being a rotating disk. Surprisingly, a significant fraction of the CO emission lies outside of the UV/optical emission. In spite of this, HAE229 follows the same relation between star-formation rate and molecular gas mass as normal field galaxies. HAE229 is the first CO(1-0) detection of an ordinary, star-forming galaxy in a protocluster. We compare a sample of cluster members at z > 0.4 that are detected in low-order CO transitions with a similar sample of sources drawn from the field. We confirm findings that the CO-luminosity and FWHM are correlated in starbursts and show that this relation is valid for normal high-z galaxies as well as those in overdensities. We do not find a clear dichotomy in the integrated Schmidt-Kennicutt relation for protocluster and field galaxies. Our results suggest that environment does not impact the "star-formation efficiency" or the molecular gas content of high-redshift galaxies. Not finding any environmental dependence in these characteristics, especially for such an extended CO disk, suggests that environmentally-specific processes such as ram pressure stripping are not operating efficiently in (proto)clusters. We discuss why this might be so.

NOEMA redshift measurements of bright Herschel galaxies

“…Figure 13 displays the relation between the apparent CO luminosities, L ′ CO(1−0) , and the width (∆V) of the CO emission lines for the sources presented in this work and a compilation of high-z lensed and unlensed SMGs, as well as local ULIRGs from the literature (see figure caption for details and references). This relationship has already been presented and discussed in previous studies (e.g., Harris et al 2012;Bothwell et al 2013;Carilli & Walter 2013;Aravena et al 2016;Yang et al 2017;Dannerbauer et al 2017;Isbell et al 2018). Figure 13 includes CO measurements of ∼160 galaxies in total; it should be noted that none of the gravitationally lensed sources in this plot was corrected for lensing magnification.…”

Radio Spectra and Sizes of Atacama Large Millimeter/submillimeter Array-identified Submillimeter Galaxies: Evidence of Age-related Spectral Curvature and Cosmic-Ray Diffusion?

“…At 1.4 GHz (rest-frame ∼ 5 GHz), a subset of our SMGs are spatially resolved on ∼ 1.3 ′′ (r 1.4 GHz ∼ 10 kpc) scales, in good agreement both with previously measured 1.4 GHz SMG sizes in the Lockman Hole field obtained with high-resolution (∼ 0.2 ′′ ) imaging from the Multi-Element Radio-Linked Interferometer Network (MER-LIN; Biggs & Ivison 2008), and with the stellar disk sizes of SMGs in the ECDFS field measured via near-IR imaging with HST (Aguirre et al 2013;Chen et al 2015). Moreover, observations of cold gas tracers in SMGs show the ISM of SMGs to be extended on scales of 10 kpc Thomson et al 2012;Emonts et al 2016;Dannerbauer et al 2017;Gullberg et al 2018).…”

On the Gas Content, Star Formation Efficiency, and Environmental Quenching of Massive Galaxies in Protoclusters at z ≈ 2.0–2.5

“…For example, detected CO(3−2) line emission in seven star-forming galaxies associated with the protocluster 4C23.56 at z = 2.49 and derived a median SFE consistent with the reference sample (although different results have been reported in the same structure, as discussed below; Tadaki et al 2019). Dannerbauer et al (2017) detected an extended CO(1 − 0) emitting disk in a proto-cluster member galaxy at z ≈ 2.15, whose gas properties and SFR follow the same relation as normal field galaxies. Similarly, Darvish et al (2018) used ALMA dust continuum observations to investigate the role of environmental density on the molecular gas content in a large sample of ∼ 400 massive (M ⋆ 10 10 M ⊙ ) galaxies within z ≈ 0.5 − 3.5, where the density was estimated by the projected surface density of galaxies over different redshift slices, and concluded that the SFE is independent of galaxy overdensity.…”

“…The work by Bouwens et al (2016) is, however, based on observations a factor of ∼ 3x deeper. Dannerbauer et al 2017), and starvation (when gas accretion is stopped).…”