JWST UNCOVER: Extremely red and compact object at$z_{\mathrm{phot}}\simeq7.6$ triply imaged by Abell 2744

TL;DR

The paper presents the discovery of a highly red, ultra-compact object at $z_{phot}\simeq7.6$ that is triple-imaged by the lensing cluster Abell 2744, leveraging deep JWST UNCOVER/NIRCam data. The authors perform thorough photometry and lens modeling to show the three images are consistent with the same source, placing a stringent upper limit on the intrinsic size ($r_e\lesssim35$ pc) and revealing a red SED that points toward a UV-faint AGN–like origin, possibly a dusty quasar. They compare this scenario to alternative high-$z$ compact sources (Pop III clusters, DCBHs, etc.) and discuss the potential of future JWST/NIRSpec spectroscopy to confirm the nature. The study demonstrates JWST's capability, combined with strong lensing, to probe faint, compact high-$z$ sources and informs questions about early black hole growth and the ionizing photon budget during reionization.

Abstract

Recent JWST/NIRCam imaging taken for the ultra-deep UNCOVER program reveals a very red dropout object at $z_{\mathrm{phot}}\simeq7.6$, triply imaged by the galaxy cluster Abell 2744 ($z_{\mathrm{d}}=0.308$). All three images are very compact, i.e. unresolved, with a de-lensed size upper-limit of $r_{e}\lesssim35$ pc. The images have apparent magnitudes of $m_{\mathrm{F444W}}\sim25-26$ AB, and the magnification-corrected absolute UV magnitude of the source is $M_{\mathrm{UV},1450}=-16.81\pm0.09$. From the sum of observed fluxes and from a spectral energy distribution (SED) analysis, we obtain estimates of the bolometric luminosities of the source of $L_{\mathrm{bol}}\gtrsim10^{43} \frac{\mathrm{erg}}{\mathrm{s}}$ and $L_{\mathrm{bol}}\sim10^{44}-10^{46} \frac{\mathrm{erg}}{\mathrm{s}}$, respectively. Based on its compact, point-like appearance, its position in color-color space and the SED analysis, we tentatively conclude that this object is a UV-faint dust-obscured quasar-like object, i.e. an active galactic nucleus (AGN) at high redshift. We also discuss other alternative origins for the object's emission features, including a massive star cluster, Population III, supermassive, or dark stars, or a direct-collapse black hole. Although populations of red galaxies at similar photometric redshifts have been detected with JWST, this object is unique in that its high-redshift nature is corroborated geometrically by lensing, that it is unresolved despite being magnified -- and thus intrinsically even more compact -- and that it occupies notably distinct regions in both size-luminosity and color-color space. Planned UNCOVER JWST/NIRSpec observations, scheduled in Cycle 1, will enable a more detailed analysis of this object.

Figures (1)

• Figure 1: JWST/NIRCam composite-color image of Abell 2744 (Blue: F115W+F150W, Green: F200W+F277W, Red: F356W+F410W+F444W; see Fig. 4 in Furtak2022UNCOVER for a larger field-of-view) together with the critical curves from our SL model for sources at $z_{\mathrm{s}}=1.69$ (blue) and $z_{\mathrm{s}}=7.5$ (red). The notable three point-like red multiple images in cyan squares are the images (and zoomed-in versions) of the quasar candidate reported here at a redshift of $z_{\mathrm{phot}}\simeq7.6$. We refer to these as QSO1A, B and C. For comparison, we also show the three images (JD1A, B and candidate C image) of the Zitrin2014highz$z\simeq9.76$ object recently confirmed in Roberts-Borsani2022arXiv221015639R with JWST spectroscopy, in orange squares. All squares in the figure encompass about $2.4\hbox{$^{\prime\prime}$}\times2.4\hbox{$^{\prime\prime}$}$, and a 20$^{\prime\prime}$ bar is given in the bottom left corner. Note that despite being at a lower redshift than JD, the faint quasar candidate is significantly redder. Also, the images of JD1 show significant structure, whereas the images of our object are point-like, implying a very compact de-magnified size of $r_{e}<35$ pc, at most. The SED of the red object is shown in Figs. \ref{['fig:SED-galaxies']} and \ref{['fig:SED-AGN']}.