Molecular gas in the halo fuels the growth of a massive cluster galaxy at high redshift

Abstract

The largest galaxies in the Universe reside in galaxy clusters. Using sensitive observations of carbon-monoxide, we show that the Spiderweb Galaxy -a massive galaxy in a distant proto-cluster -is forming from a large reservoir of molecular gas. Most of this molecular gas lies between the proto-cluster galaxies and has low velocity dispersion, indicating that it is part of an enriched inter-galactic medium. This may constitute the reservoir of gas that fuels the widespread star formation seen in earlier ultraviolet observations of the Spiderweb Galaxy. Our results support the notion that giant galaxies in clusters formed from extended regions of recycled gas at high redshift.The formation of the largest galaxies in the Universe is thought to be a two-stage process. For the last 10 Gyr, these giant galaxies have grown mostly by cannibalizing smaller galaxies (1,2). However, computer simulations predict that in an earlier phase, lasting a few Gyr, their stars condensed directly out of large reservoirs of accreted gas (3,4).We present observational evidence for an extended gas reservoir fueling star formation in the massive Spiderweb Galaxy, MRC 1138-262, which is located in a proto-cluster at a redshift of z = 2. 161 (5-9). The Spiderweb Galaxy is not a single galaxy, but an aggregation of protocluster galaxies. They are embedded in a giant halo of atomic (neutral and ionized) hydrogen gas, which radiates Lyα emission across a region of ~200 kpc (6). The central proto-cluster galaxy has a super-massive black hole at its core, which emits jets of relativistic particles visible in radio observations (5). Observations suggest that the proto-cluster galaxies will eventually merge and evolve into a single, giant elliptical galaxy in the center of the cluster (10). We therefore refer to the Spiderweb Galaxy as the entire region encompassed by the Lyα halo, and to the gas between the proto-cluster galaxies as the inter-galactic medium (IGM).Earlier observations of line emission by carbon-monoxide revealed the presence of large amounts of molecular gas in the Spiderweb Galaxy (11). Molecular gas is the raw fuel for the formation of stars, so observations of molecular gas give us insight into the processes driving the evolution of the distant Spiderweb Galaxy. We have obtained new, sensitive observations of 12 CO (J=1→0) with the Australia Telescope Compact Array (ATCA, 90 hour exposure time) and the Karl G. Jansky Very Large Array (VLA, 8 hour exposure time) (12). The ATCA observations were optimized for detecting low-surface-brightness emission from broadly distributed CO, with a 4.8ʺ″×3.5ʺ″ resolution. The VLA observations complement the ATCA data with a higher 0.7ʺ″×0.6ʺ″ resolution, sensitive to small-scale features but not to large-scale ones. Sampling these different spatial scales allows us to obtain a complete picture of the CO distribution, from the gas in the individual proto-cluster galaxies to that across the IGM. Figure 1 shows that the CO emission in the ATCA data covers a region of ~70 kpc around the central...

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

“…There of course could still be a diffuse high volume-filling factor hot gas, but its density must be substantially lower than that in local clusters (Carilli et al 2002). More work theoretically and observationally is needed to understand our results (Emonts et al 2016) but the very extended CO disk of HAE229 is already yielding fascinating clues as to the structure of early intra-cluster media in protoclusters.…”

“…It is clear that feedback from active galactic nuclei and star-formation in cluster galaxies is necessary to provide sufficient entropy and heating to globally balance the cooling (Best et al 2006). However, the energy and momentum injection from AGN, such as from the radio galaxy, MRC 1138−262, is perhaps heating the gas, but also induces substantial cooling and dissipation (e.g., Emonts et al 2014;Voit et al 2015;Meece et al 2015;Emonts et al 2015bEmonts et al , 2016Gullberg et al 2016). Specifically, the large scale environment of MRC 1138−262 has a substantial Lyα emitting halo (Pentericci et al 1997) and a significant mass of cold molecular gas (Emonts et al 2016).…”

“…In one of the best studied protocluster fields, MRC 1138−262 at z = 2.16 (e.g., Kurk et al 2000Kurk et al , 2004aPentericci et al 2000;Hatch et al 2011;Dannerbauer et al 2014;Emonts et al 2016), using the Australia Telescope Compact Array (ATCA), Emonts et al (2013) tentatively detected the CO(1-0) emission from the Hα bright galaxy, HAE229 (Kurk et al 2004b;Doherty et al 2010). Dannerbauer et al (2014) found evidence that HAE229 -about 30 (∼250 kpc in projected distance) from the radio galaxy MRC 1138−262 -is an SMG and part of the overdensity of dusty starbursts in the field of MRC 1138−262.…”

Resolving the ISM at the Peak of Cosmic Star Formation with ALMA: The Distribution of CO and Dust Continuum in z ∼ 2.5 Submillimeter Galaxies

“…While high-resolution studies of the dust continuum emission in SMGs with ALMA have shown that this material appears to be mostly distributed in compact regions (Ikarashi et al 2015;Simpson et al 2015;Hodge et al 2016;Tadaki et al 2017a), the extended sizes revealed by the few resolved CO detections of luminous sources (Riechers et al 2010;Hodge et al 2012;Emonts et al 2016;Chen et al 2017;Dannerbauer et al 2017;Ginolfi et al 2017;Tadaki et al 2017b) challenge the general assumptions of co-spatial interwoven dust and gas generally assumed by other models. Spatially resolved observations of CO and dust continuum emission for homogeneously selected samples and modeling of their interplay, e.g., through radiative transfer approximations, may help to characterize the distributions and physics of dust and gas in the high-redshift ISM.…”

Feedback and Feeding in the Context of Galaxy Evolution withSPICA: Direct Characterisation of Molecular Outflows and Inflows

“…These inflows are usually spatially extended in comparison with the size of the nuclei, thus probing the feeding of galaxy cores (Falstad et al 2015. While simulations predict that high-z galaxies are fed by relatively pristine gas, there is growing evidence that the immediate environments of high-z sources may be metal-rich (Prochaska et al 2014;Emonts et al 2016;Neeleman et al 2017), in which case SPICA/SAFARI can play an important role in studying galaxy-scale inflows on significant cosmic timescales. In local sources, the velocities associated with these motions are low, and thus the HR mode of SPICA/SAFARI will be required to study the inflows.…”