Expanding the Y Dwarf Census with Spitzer Follow-up of the Coldest CatWISE Solar Neighborhood Discoveries

We present Spitzer 3.6 and 4.5 μm follow-up of 170 candidate extremely cool brown dwarfs newly discovered via the combination of Wide-field Infrared Survey Explorer (WISE) and NEOWISE imaging at 3–5 μm. CatWISE, a joint analysis of archival WISE and NEOWISE data, has improved upon the motion measurements of AllWISE by leveraging a >10× time baseline enhancement, from 0.5 yr (AllWISE) to 6.5 yr (CatWISE). As a result, CatWISE motion selection has yielded a large sample of previously unrecognized brown dwarf candidates, many of which have archival detections exclusively in the WISE 4.6 μm (W2) channel, suggesting that they could be both exceptionally cold and nearby. Where these objects go undetected in WISE W1 (3.4 μm), Spitzer can provide critically informative detections at 3.6 μm. Of our motion-confirmed discoveries, 17 have a best-fit Spitzer [3.6]–[4.5] color most consistent with spectral type Y. It is likely that CWISEP J144606.62–231717.8 (μ ≈ 1."3 yr⁻¹) is the reddest, and therefore potentially coldest, member of our sample with a very uncertain [3.6]–[4.5] color of 3.71 ± 0.44 mag. We also highlight our highest proper-motion discovery, WISEA J153429.75–104303.3, with μ ≈ 2."7 yr⁻¹. Given that the prior list of confirmed and presumed Y dwarfs consists of just 27 objects, the Spitzer follow-up presented in this work has substantially expanded the sample of identified Y dwarfs. Our new discoveries thus represent significant progress toward understanding the bottom of the substellar mass function, investigating the diversity of the Y dwarf population, and selecting optimal brown dwarf targets for James Webb Space Telescope spectroscopy.