Professor Andrew Bunker

Abstract:

Abstract The UV continuum slope of galaxies, β, is a powerful diagnostic of the metallicity and ages of stars, nebular gas properties, dust content, and the escape of Lyman continuum photons. In this study, we present β measurements for 395 spectroscopically confirmed galaxies at 5 < z < 14.3 selected primarily from JADES, using high quality JWST NIRSpec/PRISM spectra. We find a median β = −2.15, finding a mild increase in blueness of β with increasing redshift and fainter UV magnitudes. Interestingly, we find evidence for reddening of the average β at z > 9.5, deviating from the trend observed at z < 9.5. Using stacked spectra in bins of redshift and β, we derive trends between β and dust attenuation, metallicity, ionization parameter, and stellar age indicators, finding a lack of dust attenuation to be the dominant driver of bluer β values. We further report five galaxies with β ≤ −2.9, which show a range of spectroscopic properties and signs of significant LyC photon leakage. Finally, we show that the redder β values at z > 9.5 may require rapid build-up of dust reservoirs in the very early Universe or a significant contribution from the nebular continuum emission to the observed UV spectra, with the nebular continuum fraction depending on the gas temperatures and densities. We show that in the absence of dust, nebular emission at ne > 10, 000 cm−3 can reproduce the range of red β that we see in our sample. Higher gas densities can also redden the nebular continuum emission, potentially explaining the observed β values.

Abstract:

Abstract We present JWST/NIRSpec dense-shutter spectroscopy (DSS). This novel observing strategy with the NIRSpec/MSA deliberately permits a high number of controlled spectral overlaps to reach extreme multiplex while retaining the low background of slit spectroscopy. In a single configuration over the JADES Origins Field, we opened shutters on all faint (mF444W < 30 mag) zphot > 3 candidates, prioritising emission-line science and rejecting only bright continuum sources. Using 33.6 and 35.8 ks on-source in G235M and G395M, we observed a single mask with ∼850 sources, obtaining spectroscopic redshifts for ∼540 galaxies over 2.5 ≲ z ≲ 8.9. The per-configuration target density in DSS mode is 4–5× higher than standard no- and low-overlap MSA strategies (<200 sources), with no loss in redshift precision or accuracy. Line-flux sensitivities are 30 percent lower at fixed exposure time, matching the expected increase in background noise, but the gain in survey speed is 5× in our setup, more than justifying the penalty. The measured line sensitivity exceeds NIRCam/WFSS by at least ∼5 × (~25 × in exposure time) at λ ∼ 4 μm, demonstrating that DSS is a compelling method to gain deep, wide-band spectra for large samples. Crucially, NIRSpec/MSA could deliver even higher target allocation densities than those used here. We derive H α-based SFRs, gas-phase metallicities (including a large sample suitable for strong-line calibrations), and identify rare mini-quenched galaxies and broad-line AGN. DSS is immediately applicable wherever deep imaging enables robust pre-selection and astrometry, providing an efficient method to obtain large samples of faint emission-line galaxies, a compelling middle ground between the completeness of slitless surveys and the sensitivity and bandwidth of NIRSpec/MSA.

Abstract:

Transient astronomy of the early, high-redshift (z > 3) Universe is an unexplored regime that offers the possibility of probing the first stars and the epoch of reionization. During Cycles 1 and 2 of the James Webb Space Telescope (JWST), the JWST Advanced Deep Extragalactic Survey program enabled one of the first searches for transients in deep images (∼30 AB mag) over a relatively wide area (25 arcmin2). One transient, AT 2023adsv, was discovered with an F200W magnitude of 28.04 AB mag, and subsequent JWST observations revealed that the transient is a likely supernova (SN) in a host with zspec = 3.613 ± 0.001 and an inferred metallicity at the position of the SN of Z* = 0.3 ± 0.1 Z⊙. At this redshift, the first detections in F115W and F150W show that AT 2023adsv had bright rest-frame UV flux at the time of discovery. The multiband light curve of AT 2023adsv is best matched by a template of a Type IIP SN (SN IIP) with a peak absolute magnitude of MB ≈ −18.3 AB mag. We find a good match to a 20 M⊙ red supergiant progenitor star with an explosion energy of 2 × 1051 erg, likely higher than normally observed in the local Universe, but consistent with SNe IIP drawn from local, lower-metallicity environments. AT 2023adsv is the most distant photometrically classified SN IIP yet discovered with a spectroscopic redshift measurement, and may represent a global shift in SN IIP properties as a function of redshift.

Abstract:

The discovery of high-redshift galaxies exhibiting a steep spectral UV downturn potentially indicative of two-photon continuum emission marks a turning point in our search for signatures of star formation following a top-heavy IMF in the early Universe. We develop a photometric search method for identifying further nebular-dominated galaxy candidates, whose nebular continuum dominates over the starlight, due to the high ionising photon production efficiencies ξion associated with massive star formation. We utilise the extensive medium-band imaging from JADES, which enables the identification of Balmer jumps across a wide range of redshifts (1.5 < z < 8.5), through the deficit in rest-frame optical continuum level. As Balmer jumps are a general recombination feature of young starbursts (≲ 3 Myr), we further demand a high observed log (ξion, obs/(Hz erg−1)) > 25.60 to power the strong nebular continuum, together with a relatively non-blue UV slope (mF115W − mF200W > −0.4 at z = 6) indicating a lack of stellar continuum emission. Our nebular-dominated candidates, constituting ∼11 per cent of galaxies at z ∼ 6 (decreasing to ∼2 per cent at z ∼ 2, not completeness-corrected) are faint in the rest-frame optical (median Mopt = −17.95) with extreme line emission (median EWHα, rest = 1567 Å, EW[O III] + Hβ, rest = 2292 Å). However, hot H ii region temperatures, collisionally-enhanced two-photon continuum emission, and strong UV lines are expected to accompany top-heavy star formation. Thus nebular-dominated galaxies do not necessarily exhibit the biggest Balmer jumps, nor the largest ξion, obs or reddest UV slopes. Hence continuum spectroscopy is ultimately required to establish the presence of a two-photon downturn in our candidates, thus advancing our understanding of primordial star formation and AGN.