Paper

An extremely metal poor star complex in the reionization era: Approaching Population III stars with JWST

Ravindranath, S, Zanella, A, Caputi, K I, Gronke, M, Caminha, G B, Bradac, M, Acebron, A, Ricotti, M, Gilli, R, Castellano, M, Willott, C, Pascale, R, Vanzella, E, Mestric, U, Annibali, F, Grillo, C, Sani, E, Strait, V, Martis, N, Annunziatella, M, Comastri, A, Calura, F, Dickinson, M, Mignoli, M, Mercurio, A, Rihtarsic, G, Rosati, P, Bolamperti, A, Bergamini, P, Adamo, A, Gruppioni, C, Loiacono, F, Meneghetti, M, Messa, M

2023

Overview

We present JWST/NIRSpec integral field spectroscopy (IFS) of a lensed Population III candidate stellar complex (dubbed Lensed And Pristine 1, LAP1), with a lensing-corrected stellar mass ~<10^4 Msun, absolute luminosity M_UV > -11.2 (m_UV > 35.6), confirmed at redshift 6.639 +/- 0.004. The system is strongly amplified (\\mu >~ 100) by straddling a critical line of the Hubble Frontier Field galaxy cluster MACS J0416. Despite the stellar continuum is currently not detected in the Hubble and JWST/NIRCam and NIRISS imaging, arclet-like shapes of Lyman and Balmer lines, Lya, Hg, Hb and Ha are detected with NIRSpec IFS with signal-to-noise ratios SNR=5-13 and large equivalent widths (>300-2000A), along with a remarkably weak [OIII]4959-5007 at SNR ~ 4. LAP1 shows a large ionizing photon production efficiency, log(\\xi_{ion}[erg~Hz^{-1}])>26. From the metallicity indexes R23 = ([OIII]4959-5007 + [OII]3727) / Hb ~< 0.74 and R3 = ([OIII]5007 / Hb) = 0.55 +/- 0.14, we derive an oxygen abundance 12+log(O/H) ~< 6.3. Intriguingly, the Ha emission is also measured in mirrored sub-components where no [OIII] is detected, providing even more stringent upper limits on the metallicity if in-situ star formation is ongoing in this region (12+log(O/H) < 6, or Z < 0.002 Zsun). The formal stellar mass limit of the sub-components would correspond to ~10^{3} Msun or M_UV fainter than -10. Alternatively, such a metal-free pure line emitting region could be the first case of a fluorescing HI gas region, induced by transverse escaping ionizing radiation from a nearby star-complex. The presence of large equivalent-width hydrogen lines and the deficiency of metal lines in such a small region, make LAP1 the most metal poor star-forming region currently known in the reionization era and a promising site that may host isolated, pristine stars.

• Publisher: Cornell University Library, arXiv.org
• Subject: Equivalence / Integral field spectroscopy / Ionization / Ionizing radiation / Luminosity / Metallicity / Population III stars / Red shift / Signal to noise ratio / Star & galaxy formation / Star formation / Stellar mass