Professor Andrew Bunker

Abstract:

We systematically search for galaxy overdensities using 20 independent fields with a minimum of six filters (F090W, F115W, F150W, F277W, F356W, and F444W) from BEACON, the JWST Cycle 2 NIRCam pure-parallel imaging survey. We apply an adaptive kernel-density estimation method that incorporates the full photometric redshift probability distribution function of each galaxy to map galaxy overdensities, and identify 207 significant (>4σ) overdensities at 1.5 < z < 5. We measure the quenched galaxy fraction, the average specific star formation rate (sSFR), the total dark matter mass, and the local galaxy density of each system. By investigating the correlation among these observables, we find that galaxy quenching proceeds in two paths. (i) Overdensities within more massive halos exhibit higher quenched fractions and lower averaged sSFRs. This trend weakens at z ≳ 2, consistent with cold gas streams penetrating shock-heated massive halos and sustaining star formation activity at early times. (ii) We also find a dependence of the same parameters on local densities at z < 2, where the quenched fraction increases and the sSFR decreases toward higher densities. The environmental trend in sSFR weakens at z ∼ 2–3 and shows tentative evidence for a reversal at z > 3, potentially due to a larger cold gas supply in earlier times. Our study reveals a complex interplay between individual galaxies and large-scale environmental properties, marking the onset of environmental effects on galaxy quenching in massive halos at cosmic noon.

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:

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.