The Y-dwarf population with HST : unlocking the secrets of our coolest neighbours – III. Near-infrared photometry

ABSTRACT Y dwarfs represent the coldest class of brown dwarfs, with effective temperatures below 500 K, and provide unique analogues to cold giant exoplanets. We present a large compilation of uniform nearinfrared photometry from the Hubble Space Telescope for 21 Y-dwarfs across multiple WFC3/IR filters, including the F105W, F125W, and F160W bands. We employed refined Point Spread Function fitting and calibration procedures to reach photometric uncertainties at the 0.02–0.05 mag level for most targets. Combined with precise parallax measurements, our data reveal well-defined Y-dwarf sequences in near-infrared colour–magnitude diagrams, observed with a markedly improved tightness. Known photometric trends emerge with minimal scatter, including the continuous redward progression in F125W–F160W with decreasing temperature, and the blueward trend in F105W–F125W with possible hints of a reversal around 350 K. Comparisons to the ATMO, Sonora Elf Owl, and Lacy & Burrows atmospheric models highlight systematic discrepancies, in particular F105W–F125W and F105W–F160W colours predicted to be too red. Low-metallicity grids provide the best fits to the global Y-dwarf population, although closer inspection across wavelengths shows that these preferences likely reflect compensating effects in missing or incomplete physics rather than true population level abundances. While some atmospheric diversity is expected among Y dwarfs, their tight observational sequences and systematic offsets from model predictions reveal that key physical and chemical processes remain inadequately captured in current grids. Our results underscore the importance of high precision, internally consistent data sets in robustly tracing the Y-dwarf cooling sequence and providing the empirical constraints needed to advance theoretical models at the lowest temperatures.