Reaching for the stars -- JWST/NIRSpec spectroscopy of a lensed star candidate at $z=4.76$
Authors: Lukas J. Furtak (Ben-Gurion University of the Negev), Ashish K. Meena (Ben-Gurion University of the Negev), Erik Zackrisson (Uppsala University), Adi Zitrin (Ben-Gurion University of the Negev), Gabriel B. Brammer (Cosmic Dawn Center), Dan Coe (Space Telescope Science Institute), José M. Diego (Instituto de Física de Cantabria), Jan J. Eldridge (University of Auckland), Yolanda Jiménez-Teja (Instituto de Astrofísica de AndalucÍa), Vasily Kokorev (Kapteyn Astronomical Institute), Massimo Ricotti (University of Maryland), Brian Welch (NASA Goddard Space Flight Center), Rogier A. Windhorst (Arizona State University), Abdurro'uf (Space Telescope Science Institute), Felipe Andrade-Santos (Berklee College of Music), Rachana Bhatawdekar (European Space Astronomy Centre), Larry D. Bradley (Space Telescope Science Institute), Tom Broadhurst (16 and 17 an 18), Wenlei Chen (University of Minnesota), Christopher J. Conselice (University of Manchester), Pratika Dayal (Kapteyn Astronomical Institute), Brenda L. Frye (Steward Observatory), Seiji Fujimoto (The University of Texas at Austin), Tiger Y. -Y. Hsiao (The Johns Hopkins University), Patrick L. Kelly (University of Minnesota), Guillaume Mahler (Durham University), Nir Mandelker (The Hebrew University), Colin Norman (Space Telescope Science Institute), Masamune Oguri (Chiba University), Norbert Pirzkal (Space Telescope Science Institute), Marc Postman (Space Telescope Science Institute), Swara Ravindranath (Space Telescope Science Institute), Eros Vanzella (Osservatorio di Astrofisica e Scienza dello Spazio di Bologna), Stephen M. Wilkins (University of Sussex)
Abstract: We present JWST/NIRSpec observations of a highly magnified star candidate at a photometric redshift of $z_{\mathrm{phot}}\simeq4.8$, 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 $z_{\mathrm{spec}}=4.758\pm0.004$, 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 $T_{\mathrm{eff,B}}\simeq15000$ K with either a redder F-type companion ($T_{\mathrm{eff,F}}\simeq6250$K) or significant dust attenuation ($A_V\simeq0.82$) 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, $r\lesssim0.5$ 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 $\sim1.2$ Gyr old showcases the ability of JWST to study early stars through extreme lensing.
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