Establishing a new technique for discovering large-scale structure using the ORELSE survey

Abstract

The Observations of Redshift Evolution in Large Scale Environments (ORELSE) survey is an ongoing imaging and spectroscopic campaign initially designed to study the effects of environment on galaxy evolution in high-redshift (z ∼ 1) large-scale structures. We use its rich data in combination with a powerful new technique, Voronoi tessellation Monte-Carlo (VMC) mapping, to search for serendipitous galaxy overdensities at 0.55 < z < 1.37 within 15 ORELSE fields, a combined spectroscopic footprint of ∼1.4 square degrees. Through extensive tests with both observational data and our own mock galaxy catalogs, we optimize the method's many free parameters to maximize its efficacy for general overdensity searches. Our overdensity search yielded 402 new overdensity candidates with precisely measured redshifts and an unprecedented sensitivity down to low total overdensity masses (M tot > ∼ 5 × 10 13 M ). Using the mock catalogs, we estimated the purity and completeness of our overdensity catalog as a function of redshift, total mass, and spectroscopic redshift fraction, finding impressive levels of both 0.92/0.83 and 0.60/0.49 for purity/completeness at z = 0.8 and z = 1.2, respectively, for all overdensity masses at spectroscopic fractions of ∼20%. With VMC mapping, we are able to measure precise systemic redshifts, provide an estimate of the total gravitating mass, and maintain high levels of purity and completeness at z ∼ 1 even with only moderate levels of spectroscopy. Other methods (e.g., red-sequence overdensities and hot medium reliant detections) begin to fail at similar redshifts, which attests to VMC mapping's potential to be a powerful tool for current and future wide-field galaxy evolution surveys at z ∼ 1 and beyond. 1 The particular type of i filter curve will differ from field to field, e.g., I + (equivalent to SDSS i ) or Cousins I , and some fields have multiple i-bands available. This is also true for the r-and z-bands used. For the sake of simplicity in this paper, we will refer to all variants of these bands by their the redshift of the targeted large-scale structure was greater than 0.95), a magnitude range that encompasses nearly all high-quality ORELSE objects. Every field's detection limit is fainter than 24.5, so our magnitude cut homogenizes the completeness statistics for all fields. Optical/Near-infrared Imaging and Photometry

An optical observational cluster mass function at z ∼ 1 with the ORELSE survey

“…Our previous work in Hung et al (2020) searched for serendipitous cluster candidates in the ORELSE (Lubin et al 2009) survey, a large multiwavelength photometric and spectroscopic campaign targeted at several known large-scale structures over redshifts of 0.6 < z < 1.3. It was designed to look for surrounding largescale structure in each field, but it also probed the full dynamic range of environments at all redshifts by targeting galaxies along the line of sight (Gal et al 2008;Lubin et al 2009).…”

“…ORELSE's extensive data set provides thousands of high-quality photometric and spectroscopic redshifts ideal for a cluster search. We identified galactic overdensities using a powerful new technique, VMC mapping, described in detail in Lemaux et al (2018) and applied to look specifically for structure in ORELSE in Hung et al (2020). A Voronoi tessellation is a density field estimator that splits a 2D plane by assigning a polygonal cell to every object in the plane whose area is the region closer to its host object than any other object.…”

“…We repeat this 100 times, and the final VMC map of the slice is then computed by median combining the densities from all realizations. For full details on the VMC methodology within the context of ORELSE, see Hung et al (2020). Overdensities are first found in the redshift slices (see Section 3.1), and then linked together across neighbouring slices (see Section 3.2).…”

“…Our work in this paper aims to similarly derive a cluster mass function 1 from an optical/NIR perspective for the first time outside of the relatively local universe. In Hung et al (2020), we found galaxy clusters using a powerful new technique known as Voronoi tessellation Monte Carlo (VMC) mapping and apply it to optical and NIR photometric and spectroscopic data over the redshift range 0.55 < z < 1.37. Unlike other cluster search algorithms, VMC mapping makes no assumptions about cluster geometry or morphology.…”

“…In Hung et al (2020), we demonstrated VMC mapping's sensitivity to detecting unprecedentedly low-mass structures, quantifying purity and completeness estimates down to total masses of 10 13.5 M . Our search recovered 51 previously known structures and found 402 new overdensity candidates, with estimated masses between 10.2 <log (M/M ) < 14.8 2 and a spectroscopic redshift fraction of at least 5 per cent.…”