ALMACAL. XI. Over-densities as signposts to proto-clusters? A cautionary tale

Abstract

It may be unsurprising that the most common approach to finding proto-clusters is to search for over-densities of galaxies. Upgrades to submillimetre (submm) interferometers and the advent of the James Webb Space Telescope will soon offer the opportunity to find more distant candidate proto-clusters in deep sky surveys without any spectroscopic confirmation. In this letter, we report the serendipitous discovery of an extremely dense region centred on the blazar, J0217-0820, at z=0.6 in the ALMACAL sky survey. Its density is eight times higher than that predicted by blind submm surveys. Among the seven submm-bright galaxies, three are as bright as conventional single-dish submm galaxies, with S_870um > 3mJy. The over-density is thus comparable to the densest known and confirmed proto-cluster cores. However, their spectra betray a wide range of redshifts. We investigate the likelihood of line-of-sight projection effects using light cones from cosmological simulations, finding that the deeper we search, the higher the chance that we will suffer from such projection effects. The extreme over-density around J0217-0820 demonstrates the strong cosmic variance we may encounter in the deep submm surveys. Thus, we should also question the fidelity of galaxy proto-cluster candidates selected via over-densities of galaxies, where the negative K correction eases the detection of dusty galaxies along an extraordinarily extended line of sight.

arXiv comment7 pages, 5 figures, update with the accepted version

References in corpus (65)

• Unveiling Dust-enshrouded Star Formation in the Early Universe: a Sub-mm Survey of the Hubble Deep Field
• UniverseMachine: The Correlation between Galaxy Growth and Dark Matter Halo Assembly from z=0-10
• A Deep Sub-millimeter Survey of Lensing Clusters: A New Window on Galaxy Formation and Evolution
• How Filaments are Woven into the Cosmic Web
• Simba: Cosmological Simulations with Black Hole Growth and Feedback
• Formation of Massive Galaxies at High Redshift: Cold Streams, Clumpy Disks and Compact Spheroids
• Dusty star forming galaxies at high redshift
• Lyman Alpha Imaging of a Proto-Cluster Region at <z>=3.09
• A Large Structure of Galaxies At Redshift z~3 and its Cosmological Implications
• A unified multi-wavelength model of galaxy formation
• Distant Radio Galaxies and their Environments
• Ancient Light from Young Cosmic Cities: Physical and Observational Signatures of Galaxy Proto-Clusters
• The Canada-UK Deep Sub-Millimeter Survey II: First identifications, redshifts and implications for galaxy evolution
• The realm of the galaxy protoclusters
• A Subaru Search for Ly-alpha Blobs in and around the Proto-cluster Region at Redshift z=3.1
• A massive proto-cluster of galaxies at a redshift of z {\approx} 5.3
• The ALPINE-ALMA [CII] Survey: data processing, catalogs, and statistical source properties
• The Massive Hosts of Radio Galaxies Across Cosmic Time
• Galaxy Clusters around radio-loud AGN at 1.3 < z < 3.2 as seen by Spitzer
• The Discovery of Primeval Large-Scale Structures with Forming Clusters at Redshift 6
• A massive core for a cluster of galaxies at a redshift of 4.3
• High-redshift star formation in the ALMA era
• An extreme proto-cluster of luminous dusty starbursts in the early Universe
• The impact of clustering and angular resolution on far-infrared and millimeter continuum observations
• An excess of dusty starbursts related to the Spiderweb galaxy
• Gas filaments of the cosmic web located around active galaxies in a proto-cluster
• Early results from GLASS-JWST. XIV: A spectroscopically confirmed protocluster 650 million years after the Big Bang
• The space density of luminous dusty star-forming galaxies at $z>4$: SCUBA-2 and LABOCA imaging of ultrared galaxies from $Herschel$-ATLAS
• H-alpha emitters in z~2 proto-clusters: evidence for faster evolution in dense environments
• ALMACAL I: First dual-band number counts from a deep and wide ALMA submm survey, free from cosmic variance
• A star-bursting proto-cluster in making associated to a radio galaxy at z=2.53 discovered by H_alpha imaging
• An ALMA survey of the SCUBA-2 Cosmology Legacy Survey UKIDSS/UDS field: number counts of submillimeter galaxies
• From the far-ultraviolet to the far-infrared -- galaxy emission at $0\le z\le 10$ in the Shark semi-analytic model
• A search for clusters at high redshift - IV. Spectroscopy of H-alpha emitters in a proto-cluster at z=2.16
• An ALMA survey of the brightest sub-millimetre sources in the SCUBA-2 COSMOS field
• A Chandra study of X-ray sources in the field of the z=2.16 radio galaxy MRC 1138-262
• Spatially unassociated galaxies contribute significantly to the blended submillimetre galaxy population: predictions for follow-up observations of ALMA sources
• The Nascent Red Sequence at z~2
• Reproducing sub-millimetre galaxy number counts with cosmological hydrodynamic simulations
• Discovery of an Excess of Halpha Emitters around 4C 23.56 at z=2.48
• Physical properties and evolution of (Sub-)millimeter selected galaxies in the galaxy formation simulation Shark
• Characterising and Identifying Galaxy Protoclusters
• Resolving the optical emission lines of Lya blob 'B1' at z=2.38: another hidden quasar
• A lensed protocluster candidate at $z=7.66$ identified in JWST observations of the galaxy cluster SMACS0723-7327
• Submillimetre galaxies in cosmological hydrodynamical simulations -- an opportunity for constraining feedback models
• ALMACAL VI: Molecular gas mass density across cosmic time via a blind search for intervening molecular absorbers
• ALMA Deep Field in SSA22: Survey Design and Source Catalog of a 20 arcmin^2 Survey at 1.1mm
• The ALMA Spectroscopic Survey in the HUDF: A model to explain observed 1.1 and 0.85 millimeter dust continuum number counts
• AzTEC 1.1 mm observations of high-z protocluster environments: SMG overdensities and misalignment between AGN jets and SMG distribution
• ALMACAL II: Extreme star-formation-rate densities in a pair of dusty starbursts at $z=3.442$ revealed by ALMA 20-milliarcsec resolution imaging
• Constraining galaxy overdensities around three z~6.5 quasars with ALMA and MUSE
• Ultra-Red Galaxies Signpost Candidate Proto-Clusters at High Redshift
• Overdensities of Submillimetre-Bright Sources around Candidate Protocluster Cores Selected from the South Pole Telescope Survey
• Submillimetre galaxies in two massive protoclusters at z = 2.24: witnessing the enrichment of extreme starbursts in the outskirts of HAE density peaks
• SCUBA-2 follow-up of Herschel-SPIRE observed Planck overdensities
• ALMACAL IX: multi-band ALMA survey for dusty star-forming galaxies and the resolved fractions of the cosmic infrared background
• Massive galaxy formation caught in action at z~5 with JWST
• The Herschel view of the environment of the radio galaxy 4C+41.17 at z = 3.8
• A Multiwavelength Study of ELAN Environments (AMUSE${}^2$): Ubiquitous dusty star-forming galaxies associated with enormous Ly$\alpha$ nebulae on megaparsec scales
• SCUBA2 High Redshift Bright Quasar Survey: Far-infrared Properties and Weak-line Features
• Imaging the environment of a z = 6.3 submillimeter galaxy with SCUBA-2
• Is a top-heavy initial mass function needed to reproduce the submillimeter galaxy number counts?
• ALMA unveils wider environment of distant red protocluster core
• A millimeter-wave redshift search for the unlensed HyLIRG, HS1700.850.1
• Candidate high-redshift protoclusters and lensed galaxies in the Planck list of high-z sources overlapping with Herschel-SPIRE imaging

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• NOEMA formIng Cluster survEy (NICE): Characterizing eight massive galaxy groups at $1.5<z<4$ in the COSMOS field
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• ASW${}^2$DF: Census of the obscured star formation in a galaxy cluster in formation at $z=2.2$
• An ALMA Band 7 survey of SDSS/Herschel quasars in Stripe 82: I. The properties of the 870 micron counterparts
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