Jorge Sanchez Almeida, Ignacio Trujillo, Sebastian F. Sanchez, Mireia Montes
Published 2026-02-11, To appear as a Research Note of the American Astronomical Society (RNAAS). All authors contribute equally
The linear structure reported by van Dokkum et al. (2023) has been proposed as either a massive stellar wake produced by a runaway supermassive black hole (SMBH) or a bulgeless edge-on galaxy. New JWST/NIRSpec IFU observations target the tip of the structure, where a SMBH would produce a bow shock, whereas a normal galaxy would host an HII region. Using standard BPT diagrams ([OIII]5007/Hb vs [NII]6583/Ha and [OIII]5007/Hb vs [OII]6716,6731/Ha), we find that the line ratios at the tip fall on the locus of low-metallicity low-extinction HII regions. This region does not overlap with loci typical of shocks in merging galaxies. Thus, these results are consistent with the interpretation that the linear structure is a star-forming galaxy, with the bright knot representing one of its HII regions.
Clara L. Pollock, Kasper E. Heintz, Joris Witstok, Rashmi Gottumukkala, Gabriel Brammer, Sownak Bose, Alex J. Cameron, Pratika Dayal, Pieter van Dokkum, Johan Fynbo, Viola Gelli, Matthew J. Hayes, Akio K. Inoue, Claudia del P. Lagos, Peter Laursen, Romain A. Meyer, Rohan Naidu, Pascal Oesch, Lucie E. Rowland, Nial R. Tanvir, Sandro Tacchella, Chamilla Terp, Francesco Valentino, Fabian Walter, John Weaver, Callum Witten
Published 2026-02-12, Submitted to A&A
One of the major conundrums in contemporary extragalactic astrophysics is the apparent overabundance of a remarkable population of UV-bright galaxies at redshifts $z\gtrsim 9$. We analyse galaxies spectroscopically observed by JWST/NIRSpec Prism and confirmed to lie at $z>9$, with sufficient signal-to-noise to carefully model their rest-frame UV to optical continua and line emission. In particular, we model the damped Lyman-$α$ (Ly$α$) absorption (DLA) features of each galaxy to place observational constraints on the gas assembly of neutral atomic hydrogen (HI) onto the galaxy halos at the onset of cosmic reionisation. Based on the derived HI column densities and star-formation rate (SFR) surface densities, we show that all galaxies are highly efficient at forming stars on rapid $\sim 10-100\,$Myr depletion timescales, greatly in excess compared to the canonical local universe Kennicutt-Schmidt relation and predictions from state-of-the-art galaxy formation simulations. The dense HI gas appears to also drive the offset from the fundamental-metallicity relation of these galaxies though its dust-to-gas ratio is seemingly consistent with values derived for local galaxies except for the lowest metallicity sight-lines. Our results provide the first robust observational constraints on the impact of pristine HI gas on early galaxy assembly, and imply that a combination of highly efficient star formation and low dust obscuration can likely explain the UV-brightness of galaxies at cosmic dawn.