JWST spectra are consistent with the edge-on star-forming galaxy scenario for the "runaway supermassive black hole"

TL;DR

New JWST observations suggest the linear structure is a star-forming galaxy with HII regions, rather than a shock from a runaway supermassive black hole, based on emission line diagnostics.

Contribution

The study provides the first spectroscopic evidence supporting the galaxy scenario over the SMBH wake hypothesis for the linear structure.

Findings

Line ratios match low-metallicity HII regions

Structure does not show shock signatures typical of merging galaxies

Results favor a star-forming galaxy interpretation

Abstract

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.