The clustering of DESI-like luminous red galaxies using photometric redshifts

Abstract

We present measurements of the redshift-dependent clustering of a DESI-like luminous red galaxy (LRG) sample selected from the Legacy Survey imaging dataset, and use the halo occupation distribution (HOD) framework to fit the clustering signal. The photometric LRG sample in this study contains 2.7 million objects over the redshift range of 0.4 < z < 0.9 over 5655 sq. degrees. We have developed new photometric redshift (photo-z) estimates using the Legacy Survey DECam and WISE photometry, with σNMAD = 0.02 precision for LRGs. We compute the projected correlation function using new methods that maximize signal-to-noise while incorporating redshift uncertainties. We present a novel algorithm for dividing irregular survey geometries into equal-area patches for jackknife resampling. For a 5-parameter HOD model fit using the MultiDark halo catalog, we find that there is little evolution in HOD parameters except at the highest-redshifts. The inferred large-scale structure bias is largely consistent with constant clustering amplitude over time. In an appendix, we explore limitations of MCMC fitting using stochastic likelihood estimates resulting from applying HOD methods to N-body catalogs, and present a new technique for finding best-fit parameters in this situation. Accompanying this paper we have released the PRLS (Photometric Redshifts for the Legacy Surveys) catalog of photo-z’s obtained by applying the methods used in this work to the full Legacy Survey Data Release 8 dataset. This catalog provides accurate photometric redshifts for objects with z < 21 over more than 16,000 square degrees of sky.

Discovering New Strong Gravitational Lenses in the DESI Legacy Imaging Surveys

“…Many of our lens candidates have spectroscopic or photometric redshifts z  0.8, greater than the typical redshifts of 0.3-0.8 for the current known lensing sample (e.g., Brownstein et al 2012;Wong et al 2018). In fact, the highest spectroscopic redshift from SDSS DR16 is 0.8924 (DESI-241.7346+42.1102) and the highest photometric redshift (Zhou et al 2021) is 1.021 ± 0.061 (DESI-072.0873-19.4173). In addition, the angular scales of our systems are typically between 1 5 and 5″ (see Figure 10), significantly larger than the typical value for previously known galaxy lensing systems (1 5).…”

“…We have checked our candidate list against the spectroscopic database from the Sloan Digital Sky Survey (SDSS) DR16 19 and found that for approximately half of them the putative lensing galaxy has a spectroscopic redshift. For the rest, we have found photometric redshifts from Zhou et al (2021).…”

The Evolutionary Map of the Universe pilot survey

“…At that redshift, the largest angular size across the lobes is 530 kpc, which is not unusual for double-lobed radio galaxies. An alternative identification (marked B in Figure 23) of the host is an isolated unresolved 200 μJy radio source which appears to be coincident with the galaxy DES J204835.43-491137.5, at z = 0.937 (Zhou et al 2021). If the 'Smoking Gun' were actually at this redshift, then the source's large inferred radio size (2.25 Mpc) would make it a member of the rare class of 'Giant Radio Galaxies' (see Section 6.4) and it would also be very luminous (∼4 × 10 23 W/Hz).…”

Real-time discovery of AT2020xnd: a fast, luminous ultraviolet transient with minimal radioactive ejecta

“…ZTF20acigmel passed the filter on 2020-10-12 under criterion 2 above: it is a new source coincident with a faint extended Legacy Survey source (type "REX") with a high photometric redshift ( = 1.33 +0.76 −0.40 ;Zhou et al 2021). It was not immediately identified as a transient of interest during scanning: the event was relatively faint ( ∼ 19.7 mag, ∼ 20.1 mag), and the most recent upper limit was three days prior and relatively shallow ( > 20.2 mag).…”