Two Exciting High-redshift Galaxy Candidates Turn Out to Be Two Exciting Ultra-cool Brown Dwarfs

Original Abstract

From the onset of observations of JWST we have discovered unexpectedly luminous galaxies at redshifts $z>10$ and as high as $z=14$. With their discovery, the question immediately followed as to where their progenitors are, since such progenitors should be within reach of existing surveys. However, the discovery of several bright candidates at $z>15$ may indicate further discrepancies between pre-JWST model predictions and current observations. Progenitors of the bright $z\sim 14$ galaxies should be visible at redshifts as high as $z\sim 20$--$30$, showing in the data as F356W and F277W dropouts. We identify two such candidates in the Bullet Cluster JWST data; however, subsequent NIRSpec follow-up data show spectra that can be well fit with ultra-cool Y dwarf templates with temperatures $T_{\rm eff} = 350^{+110}_{-80}\,\mbox{K}$ and $T_{\rm eff} = 410^{+110}_{-50}\,\mbox{K}$ and distances of $\sim 500\,\mbox{pc}$. The first is one of the lowest temperature brown dwarfs known spectroscopically. With additional NIRCam imaging taken $\sim 1$ year later, we also detect their proper motions of $(49 \pm 8)\,\mbox{mas/yr}$ and $(24 \pm 3)\,\mbox{mas/yr}$, further indicating that at least some F277W and F356W dropouts are sub-stellar cold Milky Way objects such as brown dwarfs. We find a sky density of 0.14 Y dwarfs per arcmin$^2$ and caution that the probability of detecting such objects may increase significantly in surveys at low galactic latitudes.