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

TL;DR

The study addresses whether the $45$ kpc linear structure at $z\sim1$ is a runaway SMBH wake or a bulgeless edge-on galaxy. It employs JWST/NIRSpec IFU observations of the structure's tip and standard emission-line diagnostics via two BPT diagrams, with the Balmer line Hb inferred from Ha or previous data, yielding $\log([OIII]5007/Hb) \approx 0.66$–$0.86$, $\log([NII]6583/Ha) \approx -1.7$, and $\log([SII]6716,6731/Ha) \approx -1.1$, placing the region in the low-metallicity HII regime. The results show no overlap with shock or merger-driven excitation, arguing against the runaway SMBH scenario and supporting a bulgeless edge-on galaxy with the bright knot as an HII region. This demonstrates the effectiveness of JWST emission-line spectroscopy for distinguishing exotic SMBH interpretations from ordinary star-forming galaxies at high redshift.

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.

Figures (1)

• Figure 1: BPT diagrams showing the location of the bright knot at the tip of the linear structure, proposed to be either a bulgeless edge-on galaxy 2023AA...673L...9S or a runaway SMBH 2023ApJ...946L..50V. Left panel: [OIII]5007/${\rm H}\beta$ versus [NII]6583/${\rm H}\alpha$. Right panel: [OIII]5007/${\rm H}\beta$ versus [SII]6716,6731/${\rm H}\alpha$. The colored dots are included to illustrate the line ratios observed in real galaxies. Each point corresponds to the central 3 region sampled by the SDSS fiber 2000AJ....120.1579Y for approximately half a million local galaxies of all types, as selected by 2025AA...704A.145S. The lines indicate the classical dividing curves defined by 2001ApJ...556..121K and 2003MNRAS.346.1055K, below which star-forming regions are found. The dots are color-coded by the equivalent width of ${\rm H}\alpha$ (shown in the side vertical bar), which, in the case of star-forming regions, serves as a proxy for the relative importance of the current starburst compared to past episodes. The symbols mark the tip of the structure for three different estimates of ${\rm H}\beta$ (approach 1 in the text is labeled as LRIS@Keck I, whereas approach 2 corresponds to the orange and blue stars, with the third estimate overlapping the LRIS@Keck I value). In both BPT diagrams, the tip of the linear structure lies directly on the ridge of low-metallicity HII regions, actively forming stars at present 2016ApJ...819..110S. This part of the BPT diagrams does not overlap with where shocks in merging galaxies are typically found 2011ApJ...734...87R2014MNRAS.444.3894H. Thus, the observed line ratios are fully consistent with the edge-on galaxy scenario.