Professor Andrew Bunker

Abstract:

The James Webb Space Telescope (JWST) discovered 79 transients out to z ∼ 4.8 through the JADES Transient Survey (JTS), but the JTS did not find any z > 5 transients. We present the first photometric evidence of a z > 5 transient/variable source with JWST. The source, AT 2023adya, resides in a zspec = 5.274 galaxy in GOODS-N, which dimmed from mF356W = 26.05 ± 0.02 mag to 26.24 ± 0.02 mag in the rest-frame optical over approximately 2 rest-frame months, producing a clear residual signal in the difference image (mF356W = 28.01 ± 0.17 mag; SNvar = 6.09) at the galaxy center. Shorter-wavelength bands (F090W/F115W) show no rest-frame UV brightness change. Based on its rest-frame V-band absolute magnitude (MV = −18.48 mag), AT 2023adya could be any core-collapse supernova (SN) subtype or an SN Ia. However, due to low SN Ia rates at high redshift, the SN Ia scenario is unlikely. Alternatively, AT 2023adya may be a variable active galactic nucleus (AGN). The NIRCam/Grism spectrum shows no broad Hα emission line (FWHM = 130 ± 26 km s−1), but we cannot exclude the existence of a faint broad line and therefore cannot exclude the AGN scenario. AT 2023adya is unlikely to be a tidal disruption event (TDE) because the TDE models matching the observed brightness changes have low event rates. Although it is not possible to determine AT 2023adya’s nature based on the two-epoch single-band photometry alone, this discovery pushes the transient/variable science frontier past z = 5 and toward the Epoch of Reionization.

Abstract:

The James Webb Space Telescope is revolutionising our ability to understand the host galaxies and local environments of high-z quasars. Here we obtain a comprehensive understanding of the host galaxy of the z=7.08 quasar by combining NIRSpec integral field spectroscopy with NIRCam photometry of the host continuum emission. Our emission-line maps reveal that this quasar host is undergoing a merger with a bright companion galaxy. The quasar host and the companion have similar dynamical masses of ∼10^ M_⊙, suggesting that this is a major galaxy interaction. Through detailed quasar subtraction and SED fitting using the NIRCam data, we obtained an estimate of the host stellar mass of M_*=(3.0^ with M_ for the companion galaxy. Using the Balmer line, we estimated a virial black hole mass of M_ BH M_⊙. Thus has an extreme black hole--stellar mass ratio of M_ BH /M_*=0.63^ which is ∼3 dex larger than expected by the local scaling relations between black hole and stellar mass. is powered by an overmassive black hole with the highest reported black hole--stellar mass ratio in a quasar host that is currently undergoing a major merger. These new insights highlight the power of JWST for measuring and understanding these extreme first quasars.

Abstract:

We present deep JWST/NIRCam and MIRI imaging of Ion1, a previously confirmed Lyman continuum (LyC)-emitting galaxy at zspec = 3.794. Together with existing Hubble Space Telescope imaging, these new observations from the JWST Advanced Deep Extragalactic Survey program enable a joint analysis of Ion1’s LyC, rest-frame UV, stellar, and dust emission with unprecedented detail. We report the first detection of dust emission at rest-frame ∼3 μm in a high-redshift LyC-emitting galaxy using MIRI/F1500W. Our analysis suggests a porous distribution of dust in Ion1, with regions exhibiting evidence of dust deficit coinciding both with LyC-emitting regions and with the peak of Hα emission. Furthermore, multiband NIRCam imaging reveals a strong far-UV-to-optical color gradient, where LyC-emitting regions appear significantly bluer than the rest of Ion1. Spatially resolved spectral energy distribution fitting confirms that this color gradient is primarily driven by spatially varying dust attenuation. Together, these findings suggest that Ion1’s LyC emission originates from a compact star-forming complex near its stellar-light centroid, where stellar feedback carves out low-H i-column-density channels, facilitating LyC escape. However, only a fraction of these LyC photons—specifically those along sightlines with minimal H i obscuration—ultimately escape and reach observers. This work underscores the critical role of dust and neutral gas geometry in shaping LyC escape in galaxies at high redshifts. Anisotropic LyC escape may be a common feature in the early Universe, which must be properly incorporated to constrain the epoch of reionization.