The unWISE Catalog: Two Billion Infrared Sources from Five Years of WISE Imaging (1901.03337v1)

Abstract: We present the unWISE Catalog, containing the positions and fluxes of roughly two billion objects observed by the Wide-field Infrared Survey Explorer (WISE) over the full sky. The unWISE Catalog has two advantages over the existing WISE catalog (AllWISE): first, it is based on significantly deeper imaging, and second, it features improved modeling of crowded regions. The deeper imaging used in the unWISE Catalog comes from the coaddition of all publicly available 3$-$5 micron WISE imaging, including that from the ongoing NEOWISE-Reactivation mission, thereby increasing the total exposure time by a factor of 5 relative to AllWISE. At these depths, even at high Galactic latitudes many sources are blended with their neighbors; accordingly, the unWISE analysis simultaneously fits thousands of sources to obtain accurate photometry. Our new catalog detects sources at 5-sigma roughly 0.7 magnitudes fainter than the AllWISE catalog and more accurately models millions of faint sources in the Galactic plane, enabling a wealth of Galactic and extragalactic science. In particular, relative to AllWISE, unWISE doubles the number of galaxies detected between redshifts 0 and 1 and triples the number between redshifts 1 and 2, cataloging more than half a billion galaxies over the whole sky.

Overview of the unWISE Catalog of Infrared Sources

The paper, "The unWISE Catalog: Two Billion Infrared Sources from Five Years of WISE Imaging" by Schlafly et al., presents the comprehensive unWISE catalog, a significant advancement in infrared astronomical data resources. This catalog results from combining five years of data from the Wide-field Infrared Survey Explorer (WISE), aiming to enhance the detection capabilities over previous iterations such as AllWISE.

Key Contributions and Results

1. Enhanced Imaging Depth and Accuracy: The unWISE Catalog is derived from deeper imaging data, utilizing co-addition of all publicly available WISE imaging, including data from the ongoing NEOWISE-Reactivation mission. This approach achieves a depth approximately 0.7 magnitudes fainter than AllWISE.
2. Improved Crowded-Field Modeling: A core achievement of this work is the improved handling of crowded regions through the crowdsource analysis pipeline. This method allows simultaneous fitting of thousands of sources, which is particularly beneficial for intricate, densely populated areas like the Galactic plane.
3. Massive Source Detection: The unWISE Catalog detects around two billion objects, offering approximately double the number of detected galaxies in the redshift range of 0 to 1, and tripling the detections between redshifts 1 and 2 compared to AllWISE.
4. Practical Tools and Data Accessibility: The catalog includes model images, sky and depth maps, and PSF images, aiding researchers in independent analyses. These tools are complemented by a user-friendly public data release available online.

Implications for Galactic and Extragalactic Astronomy

The enhanced depth and source detection capabilities translate to significant implications for both Galactic and extragalactic research. Within the Galaxy, the unWISE data can unveil brown dwarfs in the solar neighborhood and further explore the structure of the Galactic bulge. For extragalactic studies, the catalog provides robust data for studying mid-infrared properties of galaxies and enables the identification of quasars at substantial redshifts.

Astrometric and Photometric Precision

The comparison between unWISE and previously established datasets shows minor systematic differences, on the order of milliarcseconds in astrometry, a testament to the catalog's precision. Photometric comparisons also highlight uniformity across sky regions with sub-percent level discrepancies.

Future Directions and Limitations

The paper acknowledges several areas for potential enhancement:

• Non-linearity and Saturation Effects: Addressing trends in the photometry with magnitude, especially around the saturation threshold, could refine source measurements further.
• Sky Subtraction and Extended Sources: Improvements in sky modeling and more sophisticated handling of extended sources could refine catalogs particularly in nebulous regions.
• Integration of Source Motions: Future iterations might incorporate motion models leveraging the wealth of temporal data available, particularly from NEOWISE.
• Astrometric Enhancements: Further calibrations, possibly referencing modern astrometric datasets like Gaia, could augment accuracy levels.

In conclusion, the unWISE Catalog represents a major step forward for infrared survey astronomy, offering vast, reliable, and deeper coverage, thereby providing the community with an invaluable tool for diverse astrophysical applications. The comprehensive approach and significant improvements over previous efforts promise to substantially contribute to ongoing and future astronomical research endeavors.