Abstract
unWISE is a wide-ranging data analysis effort aimed at repurposing the combined Wide-field Infrared Survey Explorer (WISE) plus NEOWISE imaging archive for Galactic and extragalactic astrophysics. We present updated static sky maps incorporating seven years of W1 (3.4 μm) and W2 (4.6 μm) single-exposure images, uniformly coadding nearly a quarter petabyte of input pixel data. These new unWISE atlases now constitute the deepest ever full-sky maps at wavelengths of 3–5 μm, and will be used to select millions of Dark Energy Spectroscopic Instrument quasar and luminous red galaxy targets.
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1. Introduction
NASA's Wide-field Infrared Survey Explorer (WISE; Wright et al. 2010) has transformed our view of the infrared universe. Thanks to the ongoing NEOWISE program (Mainzer et al. 2014), WISE has now mapped the entire sky more than 14 times in both its W1 (3.4 μm) and W2 (4.6 μm) channels. However, as an asteroid hunting and characterization mission, NEOWISE does not publish coadded data products.
To maximize the legacy of WISE and NEOWISE imaging for Galactic and extragalactic science, unWISE (Lang 2014) has been uniformly combining the full WISE+NEOWISE imaging data set to create the deepest possible full-sky maps (Meisner et al. 2017a, 2017b, 2018a, 2018b, 2018c, 2019, 2021) and catalogs (Schlafly et al. 2019). Here we describe our latest set of static sky ("full-depth") coadds built by incorporating all publicly available W1 and W2 single-exposure images acquired as of the 2020 March NEOWISE release.
2. Results
NEOWISE exposures spanning the 2018 December 13 to 2019 December 13 (UTC) time period were publicly released on 2020 March 26. We downloaded all ∼5 million of these single-band NEOWISE exposures to the National Energy Research Scientific Computing Center (NERSC) in preparation for unWISE processing. Combined with the previously available six years of WISE+NEOWISE imaging, our input data set consists of 18,106,932 (18,106,923) exposures in W1 (W2), totaling ∼239 terabytes in volume.
We coadded these exposures onto a set of 18,240 partially overlapping tile footprints that combine to cover the entire sky. Coaddition proceeded per Meisner et al. (2019), with no new algorithmic updates implemented. Importantly, unWISE coaddition preserves the native WISE resolution when resampling.
The resulting mean integer frame coverage over the entire sky is 245.9 (244.0) in W1 (W2), corresponding to 31.6 (31.3) minutes of integration. The minimum integer frame coverage anywhere on the sky is 94 (87) in W1 (W2); our maps have no "empty" regions. The maximum frame coverage is achieved near the north ecliptic pole, reaching 30,226 (30,183) in W1 (W2), corresponding to ∼2.7 days of total integration. We did not discard any usable data near the ecliptic poles. As a result, the peak memory usage achieved while generating our full-depth ecliptic pole coadds was 595 GB.
Our new full-depth stacks fold in an additional year of WISE imaging relative to the deepest previously published set of unWISE coadds. Figure 1 illustrates that our seven-year full-depth unWISE coadds reduce the background pixel noise by ≳2× compared to stacks that include only the first-year WISE data, such as those from AllWISE. This improvement permits correspondingly deeper source detection and forced photometry. Data Release 9 of the DESI Imaging Legacy Surveys (Dey et al. 2019) performed matched photometry on our seven-year unWISE coadds at the locations of >1.5 billion optically detected sources, thereby enabling the selection of DESI's quasar and luminous red galaxy samples. Our seven-year full-depth unWISE coadds are publicly available at https://portal.nersc.gov/project/cosmo/data/unwise/neo6/unwise-coadds/fulldepth.
Figure 1. Demonstration of reduced background pixel noise in seven-year full-depth unWISE coadds vs. unWISE stacks incorporating only the first-year of W1/W2 imaging. The left and middle columns show a 2
9 ×29 cutout within the COSMOS region, centered at (α, δ) = (149
616, 1451). The right column contains pixel value histograms for the full COSMOS tile (1497p015), showing that ≳2× narrower distributions are achieved with our seven-year full-depth coadds.
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