Reaching for the stars - JWST/NIRSpec spectroscopy of a lensed star candidate at z = 4.76
Ravindranath, Swara; Wilkins, Stephen M.; Mahler, Guillaume; Broadhurst, Tom; Frye, Brenda L.; Conselice, Christopher J.; Fujimoto, Seiji; Kelly, Patrick L.; Oguri, Masamune; Bhatawdekar, Rachana; Brammer, Gabriel B.; Zitrin, Adi; Coe, Dan; Andrade-Santos, Felipe; Windhorst, Rogier A.; Postman, Marc; Jiménez-Teja, Yolanda; Pirzkal, Norbert; Norman, Colin; Bradley, Larry D.; Zackrisson, Erik; Kokorev, Vasily; Dayal, Pratika; Furtak, Lukas J.; Vanzella, Eros; Welch, Brian; Abdurro'uf; Mandelker, Nir; Eldridge, Jan J.; Ricotti, Massimo; Meena, Ashish K.; Diego, José M.; Chen, Wenlei; Hsiao, Tiger Y. -Y.
Israel, Sweden, Denmark, United States, Spain, New Zealand, Brazil, Netherlands, United Kingdom, Japan, Italy, Malta
Abstract
We present JWST/NIRSpec observations of a highly magnified star candidate at a photometric redshift of zphot ≃ 4.8, previously detected in JWST/NIRCam imaging of the strong lensing (SL) cluster MACS J0647+7015 (z = 0.591). The spectroscopic observation allows us to precisely measure the redshift of the host arc at zspec = 4.758 ± 0.004, and the star's spectrum displays clear Lyman- and Balmer-breaks commensurate with this redshift. A fit to the spectrum suggests a B-type super-giant star of surface temperature $T_{\mathrm{eff,B}}\simeq 15\, 000$ K with either a redder F-type companion ($T_{\mathrm{eff,F}}\simeq 6\, 250$ K) or significant dust attenuation (AV ≃ 0.82) along the line of sight. We also investigate the possibility that this object is a magnified young globular cluster rather than a single star. We show that the spectrum is in principle consistent with a star cluster, which could also accommodate the lack of flux variability between the two epochs. However, the lack of a counter image and the strong upper limit on the size of the object from lensing symmetry, r ≲ 0.5 pc, could indicate that this scenario is somewhat less likely - albeit not completely ruled out by the current data. The presented spectrum seen at a time when the Universe was only ~1.2 Gyr old showcases the ability of JWST to study early stars through extreme lensing.