A clear detection of proper motion confirms that the claimed $\mathbf{z}\simeq 32$ 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 $z\simeq 32$ 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 ($\simeq 500$ pc), provides an alternative explanation of the observed photometry. Here we exploit new medium-band NIRCam imaging obtained $\simeq 3.5$ 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 $132\pm 20$ 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 $>6\sigma$. The observed apparent proper motion is $({\mu }_{\xi },{\mu }_{\eta })=(+9.4_{-4.8}^{+6.4},-35.6_{-5.9}^{+5.1})$ mas yr${}^{-1}$, or $37.6_{-5.6}^{+5.5}$ 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 Y$1.0\pm 0.5$ ($T_{\mathrm{eff}}\approx 350$ K) at a distance of $730\pm 110$ 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.

14 pages, 5 figures, 4 tables, submitted to MNRAS