$JWST$ Insight Into a Lensed $HST$-dark Galaxy and its Quiescent Companion at $z=2.58$

Abstract

Using the novel $JWST$/NIRCam observations in the Abell 2744 field, we present a first spatially resolved overview of an $HST$-dark galaxy, spectroscopically confirmed at $z=2.58$ with magnification $\hat{I}¼\approx 1.9$. While being largely invisible at $\sim$1 $\hat{I}¼$m with NIRCam, except for sparse clumpy sub-structures, the object is well-detected and resolved in the long-wavelength bands with a spiral shape clearly visible in F277W. By combining ancillary ALMA and $Herschel$ data, we infer that this object is an edge-on dusty spiral with an intrinsic stellar mass log$(M_{\ast }/M_{\odot })\sim 11.3$ and a dust-obscured SFR$\sim 300M_{\odot }$~yr${}^{-1}$. A massive quiescent galaxy (log$(M_{\ast }/M_{\odot })\sim 10.8$) with tidal features lies 2\farcs{0} away ($r$$\sim$9 kpc), at a consistent redshift as inferred by $JWST$ photometry, indicating a potential major merger. The dusty spiral lies on the main-sequence of star formation, and shows high dust attenuation in the optical ($3<A_{\mathrm{V}}<4.5$). In the far-infrared, its integrated dust SED is optically thick up to $\hat{I}{»}_0\sim 500$ $\hat{I}¼$m, further supporting the extremely dusty nature. Spatially resolved analysis of the $HST$-dark galaxy reveals a largely uniform $A_{\mathrm{V}}\sim 4$ area spanning $\sim$57 kpc${}^2$, which spatially matches to the ALMA 1 mm continuum emission. Accounting for the surface brightness dimming and the depths of current $JWST$ surveys, unlensed analogs of the $HST$-dark galaxy at $z>4$ would be only detectable in F356W and F444W in UNCOVER-like survey, and become totally $JWST$-dark at $z\sim 6$. This suggests that detecting highly attenuated galaxies in the Epoch of Reionization might be a challenging task for $JWST$.

15 pages, 5 figures, 1 table. Accepted to ApJL