Hunting Wandering 3<z<8 Black Holes: via Spatial Offsets in Ionization Ratio and Continuum Emission

Urvi Thakurdesai, Anthony J. Taylor, Steven L. Finkelstein, Gene C. K. Leung, Oscar A. Chavez Ortiz, Jonathan R. Trump, Bren E. Backhaus, Nikko J. Cleri, Francesco D'Eugenio, Fabio Pacucci, Anton M. Koekemoer, Pablo Arrabal Haro, Micaela Bagley, Mark Dickinson, Jeyhan Kartaltepe, Casey Papovich, Nor Pirzkal

First listed 2026-07-01 | Last updated 2026-06-29

Abstract

The early growth and assembly of supermassive black holes (SMBHs) remain key topics of interest in galaxy evolution. One of the scenarios predicted by theoretical models is that frequent minor mergers and asymmetric gas inflows may cause SMBHs to temporarily reside off-center within their host galaxies in the early universe. To observationally test this scenario, we investigate whether spatially offset ionization signatures-which may be indicative of active galactic nuclei (AGN)-can be identified. Using JWST NIRSpec PRISM spectroscopy from the Cosmic Evolution Early Release Science (CEERS) survey, we analyze the 2D spectra of 90 high-redshift galaxies (3 < z < 8), including two known broad-line AGN. By measuring key emission lines such as Hα, H\b{eta}, [OIII]λ5007, [NeIII]λ3868, and [OII]λλ3727, 3729 we derive spatial flux ratio profiles, and focus on [OIII]/Hβ as a tracer of high-ionization mechanisms that may indicate AGN activity. We identify 26 galaxies (~30% of the sample) with significant localized peaks in [OIII]/Hβ. Out of these 26 galaxies, 12 sources (~46%) exhibit significant spatial offsets between the peak [OIII]/Hβ ratio and the stellar continuum center. Six of these sources show the highest amount (> 1.5) pixel spatial offsets. This spatial offset between ionization structure and stellar centers offers a promising avenue to probe early SMBH evolution and its connection to galaxy formation.

Short digest

Using CEERS JWST/NIRSpec PRISM 2D spectroscopy, this paper searches 90 slit-centered galaxies at 3 < z < 8, including two known broad-line AGN, for localized high-ionization regions by comparing spatial [OIII]/Hβ profiles with the stellar continuum center. The authors identify 26 systems with significant localized [OIII]/Hβ peaks, and 12 of those show statistically significant offsets between the ionization peak and the continuum peak, with six reaching offsets larger than 1.5 pixels. They further test whether the offsets track line-ratio strength, finding no significant relation with the maximum [OIII]/Hβ value but a positive trend with the contrast between the off-center peak and the value at the continuum center. The main result is that spatially offset ionization in JWST slit spectra may be a practical way to flag wandering or off-nuclear black-hole growth in the early universe across samples much larger than targeted IFU programs.

Key figures to inspect

• Figure 1. Use this figure to show how the visually inspected CEERS parent set is reduced to the final 90-galaxy sample through the intra-slit positional cut. That selection is central to the paper because the search for wandering black holes depends on trustworthy cross-dispersion spatial profiles that are not dominated by slit-edge losses or nodded-background contamination.
• Figure 5. This is the clearest figure for the paper’s core measurement because it shows three concrete spatial-profile outcomes: a coincident ionization and continuum peak, a genuinely offset [OIII]/Hβ peak, and a uniform-ionization case. The combination of fitted spatial profiles, posterior peak locations, and NIRCam context images makes the offset definition intuitive and demonstrates exactly what kind of evidence promotes a source into the candidate off-nuclear category.
• Figure 6. Include this figure because it distills an important non-result: the size of the spatial offset does not significantly correlate with the maximum [OIII]/Hβ value. That helps readers avoid equating stronger ionization ratios with stronger wandering-black-hole evidence and sharpens the interpretation of what the offset metric is actually capturing.
• Figure 7. This figure carries one of the paper’s main quantitative synthesis results by showing that larger offsets are associated with larger spatial variation in [OIII]/Hβ across the central rows. It is especially worth highlighting because the caption explicitly notes the likely selection bias toward high-contrast systems, so the figure both supports and appropriately qualifies one of the strongest trends in the analysis.
• Figure 8. This OHNO diagnostic plot is the best figure for connecting the spatial-offset measurement to physical interpretation. It shows where the offset sources fall in line-ratio space relative to the AGN versus star-formation boundary, while also preserving the paper’s key caveat that the calibration of this diagnostic at high redshift remains uncertain because of harder ionizing conditions.