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| Main Authors: | , , , , , , , , |
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| Format: | Preprint |
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2025
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| Subjects: | |
| Online Access: | https://arxiv.org/abs/2506.06831 |
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| _version_ | 1866915332999872512 |
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| author | Mondal, Chayan Saha, Kanak Borgohain, Anshuman Smith, Brent M. Windhorst, Rogier A. Reddy, Naveen Chen, Chian-Chou Umetsu, Keiichi Jansen, Rolf A. |
| author_facet | Mondal, Chayan Saha, Kanak Borgohain, Anshuman Smith, Brent M. Windhorst, Rogier A. Reddy, Naveen Chen, Chian-Chou Umetsu, Keiichi Jansen, Rolf A. |
| contents | He II $λ$1640 emission in galaxies indicates the presence of sources that produce extreme ionizing photons. Here, we report the discovery of a He II $λ$1640 emitting galaxy, GNHeII J1236+6215, at $z=$ 2.9803 in the GOODS-north field. We use photometry in 17 wavebands from near-UV to infrared to characterize the galaxy SED and combine Keck LRIS and JWST NIRSpec spectra to identify 15 emission lines including He II $λ$1640. We infer that the He$^+$ ionization in the galaxy could be driven by small pockets of young Population III stars or low-metallicity Very Massive Stars (VMSs) rather than AGN or metal-rich Wolf-Rayet stars. The galaxy has a highly ionized ISM ([OIII]5007/[OII]3727 = 7.28$\pm$0.11, [SIII]/[SII] = 1.97$\pm$0.48 and detected Ly$α$, H$α$, H$β$, H$γ$ lines), little reddening by dust (E(B$-$V) = 0.04$\pm$0.12), low metallicity (12 + log(O/H) = 7.85$\pm$0.22), and high star formation rate (SFR$_{\rm SED}$ = 12.2$\pm$2.0 M$_{\odot}$ yr$^{-1}$). In addition to these ISM conditions, we also notice a significant [SII] deficiency ([SII]6718,6732/H$α$ = 0.08$\pm$0.02, $Δ$[SII] = $-$0.12) which may indicate the presence of density-bounded optically thin H~II regions that combined with the low dust extinction favor leaking of ionizing Lyman continuum (LyC) photons. Our best-fit SED model also infers a high nebular ionization (log U = $-2.0$) and a low stellar mass M = 7.8$\pm3.1\times$10$^8$M$_{\odot}$. This discovery not only adds one important object to the known sample of high-redshift He~II emitters but also highlights a potential connection between He$^+$ ionization and favorable ISM conditions for the leakage of ionizing photons from galaxies. |
| format | Preprint |
| id |
arxiv_https___arxiv_org_abs_2506_06831 |
| institution | arXiv |
| publishDate | 2025 |
| record_format | arxiv |
| spellingShingle | GNHeII J1236+6215: A He II $λ$1640 emitting and potentially LyC leaking galaxy at $z$ = 2.9803 unveiled through JWST & Keck observations Mondal, Chayan Saha, Kanak Borgohain, Anshuman Smith, Brent M. Windhorst, Rogier A. Reddy, Naveen Chen, Chian-Chou Umetsu, Keiichi Jansen, Rolf A. Astrophysics of Galaxies He II $λ$1640 emission in galaxies indicates the presence of sources that produce extreme ionizing photons. Here, we report the discovery of a He II $λ$1640 emitting galaxy, GNHeII J1236+6215, at $z=$ 2.9803 in the GOODS-north field. We use photometry in 17 wavebands from near-UV to infrared to characterize the galaxy SED and combine Keck LRIS and JWST NIRSpec spectra to identify 15 emission lines including He II $λ$1640. We infer that the He$^+$ ionization in the galaxy could be driven by small pockets of young Population III stars or low-metallicity Very Massive Stars (VMSs) rather than AGN or metal-rich Wolf-Rayet stars. The galaxy has a highly ionized ISM ([OIII]5007/[OII]3727 = 7.28$\pm$0.11, [SIII]/[SII] = 1.97$\pm$0.48 and detected Ly$α$, H$α$, H$β$, H$γ$ lines), little reddening by dust (E(B$-$V) = 0.04$\pm$0.12), low metallicity (12 + log(O/H) = 7.85$\pm$0.22), and high star formation rate (SFR$_{\rm SED}$ = 12.2$\pm$2.0 M$_{\odot}$ yr$^{-1}$). In addition to these ISM conditions, we also notice a significant [SII] deficiency ([SII]6718,6732/H$α$ = 0.08$\pm$0.02, $Δ$[SII] = $-$0.12) which may indicate the presence of density-bounded optically thin H~II regions that combined with the low dust extinction favor leaking of ionizing Lyman continuum (LyC) photons. Our best-fit SED model also infers a high nebular ionization (log U = $-2.0$) and a low stellar mass M = 7.8$\pm3.1\times$10$^8$M$_{\odot}$. This discovery not only adds one important object to the known sample of high-redshift He~II emitters but also highlights a potential connection between He$^+$ ionization and favorable ISM conditions for the leakage of ionizing photons from galaxies. |
| title | GNHeII J1236+6215: A He II $λ$1640 emitting and potentially LyC leaking galaxy at $z$ = 2.9803 unveiled through JWST & Keck observations |
| topic | Astrophysics of Galaxies |
| url | https://arxiv.org/abs/2506.06831 |