arXiv subjects

Ryan A. Cooper

Publications and source records attributed to Ryan A. Cooper.

3 recordsLinked to original sources

arXiv2026

The Physical Conditions of Low-Mass Galaxies at z=3.5-7.0 from JWST Spectroscopy: The Behaviour of Spectral Line Ratios with Burstiness

We present a study of the physical conditions of galaxies at redshifts $z=3.5-7.0$, based on emission-line diagnostics of JWST NIRSpec spectra for 1889 star-forming sources spanning more than 4~dex in stellar mass. We find that the median H$\alpha$/H$\beta$ and [OIII]/H$\beta$ are constant throughout these redshifts, indicating a slow evolution of the interstellar medium conditions from the Epoch of Reionization to Cosmic Noon. The stellar-mass evolution of these line ratios follow the expected trend for chemical enrichment and dust build-up, with \ohb~being maximum for galaxies with $\rm M_{\star} \approx 10^9 \, \rm M_\odot$, and the medians of H$\alpha$/H$\beta$ and [OII]/H$\beta$ monotonically increasing from $\rm{M}_{\rm \star} \approx 10^7$ through $ \approx 10^{10}\, \rm M_\odot$. Focusing on the 1552 low stellar-mass ($<10^9 \, \rm M_\odot$) galaxies, we particularly investigate the behaviour of the main spectral-line ratios with burstiness. We find a significantly higher degree of scatter in [OIII]/H$\beta$ and [OII]/H$\beta$ for high-burstiness than for low-burstiness galaxies. Values of log([OIII]/H$\beta$)~$\gtrsim 0.8$ are almost exclusively seen amongst those with high burstiness. Meanwhile, log([OIII]/H$\beta$)~$\lesssim 0.3$ are virtually not found amongst high-burstiness galaxies, except at the lowest stellar masses $\mathrm{log(M_\star/M_\odot)}<7.5$. Our results are consistent with a scenario in which star-formation bursts produce temporary chemical enrichment that is quickly lost via stellar winds. They also suggest that metallicity measures in bursting galaxies are unstable, and that the search for the most metal-poor objects should optimally be conducted amongst low-burstiness or very low-stellar mass sources.

astro-ph.GA↗

arXiv2026

Pseudo Little Red Dot: an Active Black Hole Embedded in a Dense and Dusty, Metal-Poor Starburst Galaxy at z=5.96

We present a study of Pseudo-LRD-NOM (Pseudo little red dot with no metal lines), a highly magnified low-mass galaxy behind the lensing cluster Abell 370 at z=5.96. We classify this object as a pseudo-LRD because its red rest-frame optical colour is contaminated by a prominent Halpha line (with EW0 >~ 800 Angstroms) present in its JWST NIRSpec spectrum. Halpha is dominated by a narrow component and also has a minor broad component indicative of an active black hole with M_BH = 5.6x10^6 Msun. A narrow Hbeta emission line is also detected (with S/N = 8), producing a Balmer decrement (narrow) Halpha/Hbeta = 11. The rest-frame UV spectral slope is beta_UVspec = -1.20 +/-0.28. All these features can be ascribed to high dust attenuation. However, no [OIII]5007 or any other metal lines are detected in the spectrum, so [OIII]5007/Hbeta < 0.25, at odds with a simple dust-attenuation explanation. Accounting for all the spectral properties requires the model of a starburst with moderate colour excess E(B-V)=0.2-0.5, high gas density (n_H >~ 10^6 cm^{-3}) and low to extremely low gas/stellar metallicities (Z = 0.01-0.1 Zsun). The demagnified stellar mass is 2.25^{+1.30}_{-0.83} x10^7 Msun and the stellar-mass surface density is Sigma* = 418^{+725}_{-310} Msun/pc^2, similar to that of massive/nuclear star clusters. Pseudo-LRD-NOM provides evidence of massive black-hole growth occurring in a high-density, dusty starburst which is at the early stages of its chemical enrichment, and is likely a precursor to a real LRD.

astro-ph.GA↗

arXiv2025

High-velocity outflows in [OIII] emitters at z=2.5-9 from JWST NIRSpec medium-resolution spectroscopy

We identify galaxies hosting ionised-gas high-velocity outflows from the complete sample of medium resolution (R1000) JWST/NIRSpec MSA spectroscopy taken as part of the JWST Advanced Deep Extragalactic Survey (JADES). From a total sample of 1087 [OIII]5007 emitters we identify 40 galaxies with a blue-side broadened [OIII]5007 line at z=2.5-9. Of these, 34 are strong outflow candidates whilst 6 sources have broadening potentially driven by rotating clumps. Our outflow candidate sample is mainly composed of star-forming galaxies, including ~65% starbursts, which span the stellar mass range log10(M*/Msun)=7.5-11.0. It also includes two candidate type-2 active galactic nuclei (AGN) and a 'little red dot' (LRD). We report a median outflow velocity of 531(+146)(-159) km/s and an overall incidence rate of 3.4%. These values are significantly higher and lower respectively than recent similar works, which we accredit to the limiting resolution of the R1000 spectroscopy and a stricter outflow selection criterion. We find no correlation between the outflow velocity and the galaxy stellar mass or star-formation rate. The median ratio between outflow velocity and escape velocity is 0.77(+0.36)(-0.32), indicating that most outflows cannot escape the galaxy gravitational potentials. We do find an anti-correlation between mass loading factor and stellar mass up to M* ~(10^10)Msun, with most lowest stellar-mass M*<(10^9)Msun galaxies reaching values well above unity, as is the case for local starburst galaxies.

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