A clear detection of proper motion confirms that the claimed z ≃ 32 galaxy candidate, “Capotauro”, is a Y-type brown dwarf

Abstract

The compact red source “Capotauro”, discovered in deep JWST imaging in 2025, has been suggested as a possible galaxy candidate at redshift z ≃ 32 on account of its extreme NIRCam colours. However, given the lack of evidence that this source is spatially resolved, a very cool brown dwarf, at a distance consistent with the scale-height of the Milky Way stellar disc (≃ 500 pc), provides an alternative explanation of the observed photometry. Here we exploit new medium-band NIRCam imaging obtained ≃ 3.5 years after the discovery data to test this hypothesis. Using filter-dependent point-spread-function fitting, a robust local relative astrometric frame defined by 68 compact reference sources, and a joint four-image injection–recovery analysis, we show that Capotauro has moved by 132 ± 20 mas over the 3.5-yr interval span of the observations, ruling out the possibility that it is an extragalactic source (galaxy, AGN, or supernova) at >6σ. The observed apparent proper motion is $(\mu _\xi ,\mu _\eta )=(+9.4^{+6.4}_{-4.8},-35.6^{+5.1}_{-5.9})$ mas yr−1, or $37.6^{+5.5}_{-5.6}$ mas yr−1 in total. Through comparison of its photometric spectral energy distribution with empirical templates, we find that Capotauro is best described as a brown dwarf of spectral type Y1.0 ± 0.5 (${T_{\rm eff}} \approx 350$ K) at a distance of 730 ± 110 pc (although the existing data and spectral templates are insufficient to rule out an even colder, later-type brown dwarf). Capotauro is thus one of the most distant Y dwarfs found to date, as expected given its discovery in a deep JWST extragalactic survey field. This result demonstrates the value of multi-epoch imaging for identifying substellar contaminants among the most extreme photometric-redshift candidates.