Professor Andrew Bunker

Abstract:

Recent JWST/NIRSpec observations revealed high-z star-forming galaxies depart from the fundamental metallicity relation (FMR), yet the z = 0 FMR has not been well-characterized in the low-mass regime ( log(M⋆/M⊙)≲9 ) for an appropriate comparison of low- and high-z systems. We attempt to rectify this limitation through a meta-analysis, providing a local, observational comparison for future high-z FMR studies. We analyzed common FMR fitting methods for ∼4000 [O III] λ4363 emitters, with the majority being below log(M⋆/M⊙)=9 at z ∼ 0 where the canonical FMR has not been well explored. We find no evidence of the canonical FMR in this low-mass regime through any method, suggesting that slowly evolving, quasi-steady state gas reservoirs are not yet established in these systems. We find a weak positive correlation between metallicity and star formation, and that these systems are gas-rich with substantial diversity in effective yields (yeff) spanning ∼1.5 dex. We demonstrate that increasing yeff correlates with decreasing FMR offsets, which, combined with the nonequilibrium gas models of Dalcanton et al., suggests that star formation bursts rapidly return and eject metals from the ISM before subsequent gas balancing. Pristine infall cannot lead to the yeff diversity we measure, and thus is not the primary process behind FMR deviations. With this characterization, we demonstrate z ≳ 2 [O III] λ4363 systems are indeed more metal-poor than z ∼ 0 counterparts ( Δ12+log(O/H)=0.3dex ) at fixed M⋆.

Abstract:

The James Webb Space Telescope ( ) opened a new observational window on the primordial Universe. Here we present new JWST NIRSpec integral field spectroscopy (IFS) observations of the z=7.54 quasar ULAS J1342+0928 obtained as part of the Galaxy Assembly with NIRSpec IFS (GA-NIFS) GTO programme. The new data-set obtained with both the prism (R , with a mean (median) value of 480 (390) ̊m M_⊙ , JWST and the high-resolution grating (R emission on ∼7 kpc scales, well above the typical galaxy size at this redshift, likely associated with a past outflow event. Additionally, the high-resolution observations show a more energetic ionised outflow on nuclear scales (łesssim 0.8 kpc). The total ionised mass outflow rate ranges between ∼100 and 2200 ̊m M_⊙ , yr^ allow for a complete description of the quasar emission from the rest-frame UV to optical bands. The low-resolution data reveal the presence of O iii -1 -1 , where the significant spread is mostly due to the lack of tight constraints on the electron density and outflow geometry. This range broadly encompasses the star formation rate range (85--545 ̊m M_⊙ , yr^ -1 ), yet this only represents a lower limit to the total mass outflow rate. By employing an accretion disc modelling, for the first time on data, we manage to robustly estimate the black hole mass and the bolometric luminosity, ̊m łog(M_ JWST BH /(M_⊙))=9.1± 0.2 and ̊m łog(L_ bol /(erg , s^ -1 ))=46.8± 0.1, respectively. We derive an Eddington ratio of ̊m λ_ Edd ∼ 0.4, challenging the paradigm of widespread super-Eddington accretion in quasars at the epoch of re-ionisation. Lastly, we measure the most distant broad line ratios ( / and / ) so far. Interpreting these ratios as proxies to the broad line region metallicity, we find evidence for an early metal enrichment in quasars within 700 Myrs from the Big Bang. Fe ii UV Mg ii Fe ii opt H β

Abstract:

Little red dots (LRDs) are compact, red sources discovered by JWST at high redshift (z ≳ 4), marked by distinctive “V-shaped” spectral energy distributions (SEDs) and often interpreted as rapidly accreting active galactic nuclei (AGNs). Their true nature remains unclear though, and their evolutionary connection to their lower-redshift counterparts is still poorly constrained. Thus, we present WISEA J123635.56+621424.2 (here dubbed the Saguaro), a z = 2.0145 galaxy in GOODS-North, as a possible analog of high-redshift LRDs and a potential missing link in their evolutionary path toward lower-redshift systems. It features a compact LRD-like nucleus surrounded by a face-on spiral host. Its connections to LRDs include the following: (1) its nuclear spectrum shows a clear “V-shaped” SED, and (2) when redshifted to z = 7, surface brightness dimming makes the host undetectable, thus mimicking an LRD. This suggests that high-redshift LRDs may be embedded in extended hosts. To test this, we stack rest-frame UV images of 99 photometrically selected LRDs, revealing faint, diffuse emission. Stacking in redshift bins reveals mild radial growth, consistent with the expected galaxy size evolution. A simple analytic model confirms that surface brightness dimming alone can explain their compact appearance. Lastly, we show that the Saguaro is not unique by describing similar objects from the literature at z ≲ 3.5. Taken together, our results support a scenario in which LRDs may not be a distinct population, but could be the visible nuclei of galaxies undergoing a short-lived, (perhaps) AGN-dominated evolutionary phase, with their compact, red appearance driven largely by observational biases.