分类: 天文学 >> 天文学 提交时间: 2023-02-19
摘要: Recent JWST/NIRcam imaging taken for the ultra-deep UNCOVER program reveal a very red, triply imaged, compact dropout object at $z_{\mathrm{phot}}\simeq7.66$ which is prominently lensed by the galaxy cluster Abell 2744 ($z_{\mathrm{d}}=0.308$). All three images are very compact, i.e. unresolved, with an inferred de-lensed size upper limit of $r_{e}\lesssim35$ pc. The observed F444W magnitude of the three images is $m\sim25-26$ AB and the source's absolute UV magnitude is $M_{\mathrm{UV},1450}=-16.81\pm0.09$, after correcting for magnification. From the sum of observed fluxes and from a spectral energy distribution (SED) fit we obtain lower limits on the bolometric luminosities of $L_{\mathrm{bol}}\gtrsim10^{43} \frac{\mathrm{erg}}{\mathrm{s}}$ and $L_{\mathrm{bol}}\gtrsim10^{44} \frac{\mathrm{erg}}{\mathrm{s}}$ for the source, respectively. Based on its compact, point-like appearance, its positions in color-color and $M_{\mathrm{UV}}$-size diagrams and the SED analysis, we tentatively conclude that this object could be a faint quasar-like object, i.e., an extreme emission line object whose nebular emission is boosted by an active galactic nucleus (AGN). We discuss various alternative origins for the object's emission features, including a massive star cluster, a cluster of 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. The planned UNCOVER JWST/NIRSpec observations (scheduled in Cycle 1) will enable a more detailed analysis of this object.
分类: 天文学 >> 天文学 提交时间: 2023-02-19
摘要: We report the discovery of a triply-imaged active galactic nucleus (AGN), lensed by the galaxy cluster MACS J0035.4-2015 ($z_{\mathrm{d}}=0.352$). The object is detected in Hubble Space Telescope (HST) imaging taken for the RELICS program. It appears to have a quasi-stellar nucleus consistent with a point-source, with a de-magnified radius of $r_e\lesssim100$ pc. The object is spectroscopically confirmed to be an AGN at $z_{\mathrm{spec}}=2.063\pm0.005$ showing broad rest-frame UV emission lines, and is detected in both X-ray observations with \textit{Chandra} and in ALCS ALMA band 6 (1.2 mm) imaging. It has a relatively faint rest-frame UV luminosity for a quasar-like object, $M_{\mathrm{UV},1450}=-19.7\pm0.2$. The object adds to just a few quasars or other X-ray sources known to be multiply lensed by a galaxy cluster. Some faint, diffuse emission from the host galaxy is also seen around the nucleus, and nearby there is another fainter object sharing the same multiple-imaging symmetry and geometric redshift, which may be an interacting galaxy or a star-forming knot in the host. We present an accompanying lens model, calculate the magnifications and time delays, and infer physical properties for the source. We find the rest-frame UV continuum and emission lines to be dominated by the AGN, and the optical emission to be dominated by the relatively young ($\sim100$ Myr) host galaxy of modest stellar mass $M_{\star}\simeq10^{9.2} \mathrm{M}_{\odot}$. We also observe variations in the AGN's emission, which may suggest that the AGN used to be more active. This object adds a low-redshift counterpart to several relatively faint AGN recently uncovered at high redshifts with HST and JWST.