Professor Andrew Bunker

Abstract:

Abstract Understanding how galaxies assemble their mass during the first billion years of cosmic time is a central goal of extragalactic astrophysics, yet joint constraints on their sizes and kinematics remain scarce. We present one of the first statistical studies of the Hα size-mass relation at high redshift with a sample of 213 galaxies at spectroscopic redshifts of z ≈ 4 − 6 from the FRESCO and CONGRESS NIRCam grism surveys. We measure the Hα morphology and kinematics of our sample using the novel forward modelling Bayesian inference tool geko, and complement them with stellar continuum sizes in the rest-frame FUV, NUV, and optical, obtained from modelling of imaging data from the JADES survey with Pysersic. At z ≈ 5, we find that the average Hα sizes are larger than the stellar continuum (FUV, NUV and optical), with $r_{\rm e, \mathrm{H}\alpha } = 1.17 \pm 0.05$ kpc and re, cont ≈ 0.9 kpc for galaxies with $\log (M_{\star } \rm [{\rm M}_{\odot } ]) = 9.5$. However, we find no significant differences between the stellar continuum sizes at different wavelengths, suggesting that galaxies are not yet steadily growing inside-out at these epochs. Instead, we find that the ratio $r_{\rm e, \mathrm{H}\alpha }/r_{\rm e, NUV}$ increases with the distance above the star-forming main sequence ($\Delta \rm MS$), consistent with an expansion of Hα sizes during episodes of enhanced star formation caused by an increase in ionising photons. As galaxies move above the star-forming main sequence, we find an increase of their rotational support v/σ0, which could be tracing accreting gas illuminated by the Hα emission. Finally, we find that about half of the elongated systems (b/a < 0.5) are not rotationally supported, indicating a potential flattened/prolate galaxy population at high redshift.

Abstract:

Abstract JWST has revealed a population of ‘Little Red Dots’ (LRDs): compact, red objects at redshifts z = 2–9 with ‘v’-shaped spectral energy distributions, broad permitted lines, and, often, hydrogen Balmer absorption. We use NIRSpec/IFS data from the BlackTHUNDER survey to study the H α line in the LRD Abell2744-QSO1 at z = 7.04, which is a confirmed AGN due to time-variable equivalent width (EW) in its broad emission lines. The H α spectral profile is non-Gaussian, requiring at least two Gaussian components. We also detect a narrow-line Gaussian component, and strong H α absorption (EW relative to the continuum $\sim 22_{+12}^{-7}\mathring{\rm A}$), confirming a connection between the strong Balmer break and line absorption. The absorber is at rest with respect to broad H α, suggesting that the gas cannot be interpreted as an inflow or outflow, forming instead a long-lived structure. Its velocity dispersion is $\sigma _abs = 110^{+20}_{-10}$ km s−1, consistent with the value inferred from the analysis of the Balmer break. Based on H α, we infer a black hole mass of log (M•/M⊙) = 7.2, smaller but close to the previous estimates based on H β. The Eddington ratio is 0.09. Combining the high signal-to-noise ratio of the narrow H α line with the spectral resolution R = 3, 700 of the G395H grating, we infer a narrow-line intrinsic dispersion $\sigma _\mathrm{n}=22_{-6}^{+5}$ km s−1, which places a stringent constraint on the black-hole-to-dynamical-mass ratio of this system to be M•/Mdyn = 0.15–1.2, confirming the overmassive nature of the black hole and potentially leaving little room for a host galaxy.