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.

7 pages, 5 figures, update with the accepted version

References in corpus (22)

• Simba: Cosmological Simulations with Black Hole Growth and Feedback
• Formation of Massive Galaxies at High Redshift: Cold Streams, Clumpy Disks and Compact Spheroids
• Distant Radio Galaxies and their Environments
• A massive proto-cluster of galaxies at a redshift of z {\approx} 5.3
• The Massive Hosts of Radio Galaxies Across Cosmic Time
• 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
• The Nascent Red Sequence at z~2
• Characterising and Identifying Galaxy Protoclusters
• A lensed protocluster candidate at $z=7.66$ identified in JWST observations of the galaxy cluster SMACS0723-7327
• Constraining galaxy overdensities around three z~6.5 quasars with ALMA and MUSE
• ALMACAL II: Extreme star-formation-rate densities in a pair of dusty starbursts at $z=3.442$ revealed by ALMA 20-milliarcsec resolution imaging
• Submillimetre galaxies in two massive protoclusters at z = 2.24: witnessing the enrichment of extreme starbursts in the outskirts of HAE density peaks
• Massive galaxy formation caught in action at z~5 with JWST
• ALMACAL IX: multi-band ALMA survey for dusty star-forming galaxies and the resolved fractions of the cosmic infrared background
• SCUBA2 High Redshift Bright Quasar Survey: Far-infrared Properties and Weak-line Features
• A Multiwavelength Study of ELAN Environments (AMUSE${}^2$): Ubiquitous dusty star-forming galaxies associated with enormous Ly$\alpha$ nebulae on megaparsec scales
• Imaging the environment of a z = 6.3 submillimeter galaxy with SCUBA-2
• ALMA unveils wider environment of distant red protocluster core
• Candidate high-redshift protoclusters and lensed galaxies in the Planck list of high-z sources overlapping with Herschel-SPIRE imaging

Cited by in corpus (5)

• NOEMA formIng Cluster survEy (NICE): Characterizing eight massive galaxy groups at $1.5<z<4$ in the COSMOS field
• Noema formIng Cluster survEy (NICE): Discovery of a starbursting galaxy group with a radio-luminous core at z=3.95
• 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
• Licking the plate: dusty star-forming galaxies buried in the ALMA calibration data