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

Abstract

We present JWST/Near Infrared Spectrograph (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 of ≲104 M⊙ and an absolute luminosity of MUV > −11.2 (mUV > 35.6), confirmed at redshift 6.639 ± 0.004. The system is strongly amplified (μ ≳ 100) by straddling a critical line of the Hubble Frontier Field galaxy cluster MACS J0416. Although the stellar continuum is currently not detected in the Hubble and JWST/Near Infrared Camera (NIRCam) and Near Infrared Imager and Slitless Spectrograph (NIRISS) imaging, arclet-like shapes of Lyman and Balmer lines, Lyα, Hγ, Hβ and Hα are detected with NIRSpec IFS with signal-to-noise ratios (S/N) of approximately 5 − 13 and large equivalent widths (> 300 − 2000 Å), along with a remarkably weak [O III]λλ4959, 5007 at S/N ≃ 4. LAP1 shows a large ionizing photon production efficiency, log(ξion[erg Hz−1]) > 26. From the metallicity indexes R23 = ([O III] + [O II])/Hβ ≲ 0.74 and R3 = ([O III]/Hβ) = 0.55 ± 0.14, we derive an oxygen abundance of 12 + log(O/H)≲6.3. Intriguingly, the Hα emission is also measured in mirrored subcomponents where no [O III] 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). The formal stellar mass limit of the subcomponents would correspond to ∼103 M⊙ or MUV fainter than −10. Alternatively, this metal-free, pure line-emitting region could be the first case of a fluorescing H I 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.

MIDIS: The Relation between Strong (Hβ + [O iii]) Emission, Star Formation, and Burstiness around the Epoch of Reionization

“…These kinds of effects only matter at very low SFR (∼0.1 M e yr −1 ) at low redshifts, but the bursty nature characterizing many high-z galaxies may still produce a stochastic sampling of the star-forming units within a galaxy (Vikaeus et al 2020;Pallottini & Ferrara 2023). This, in turn, can have important implications for the estimation of metallicities in low-SFR galaxies at high redshifts (e.g., Vanzella et al 2023).…”

Diverse Oxygen Abundance in Early Galaxies Unveiled by Auroral Line Analysis with JWST

“…Through the robust determination of oxygen abundance, our data reveal the distribution of early galaxies in the massmetallicity plane. Despite probing a wide range of both redshift and stellar mass, our galaxies turn out to be moderately enriched in metallicity, Recently, Vanzella et al (2023) reported an upper limit of ( ) log O H +12 < 6.3 on a low-mass star complex (10 4 M e ), through calibrated strong line methods. The system is a part of lensed arcs strongly amplified by the foreground galaxy cluster.…”