arXiv subjects

Natalie Lam

Publications and source records attributed to Natalie Lam.

8 recordsLinked to original sources

arXiv2026

Unprecedented Constraints on Gas Flows at High Redshift Using Deep JWST/NIRSpec Observations from the LyC22, EXCELS, and AURORA Surveys

We investigate how low-ionization gas flows in typical star-forming galaxies at $z\sim3$ depend on galaxy intrinsic properties and viewing angle. For this analysis we use JWST/NIRSpec observations of rest-frame near-UV Fe II and Mg II absorption, and rest-frame optical Na D absorption. This study combines galaxies from the LyC22, EXCELS, and AURORA surveys and contains 176, 197, and 315 galaxies, respectively, with Fe II, Mg II, and Na D coverage. Based on both individual and composite spectra, we find no statistically significant correlations between outflow velocity and galaxy properties. However, galaxies with detected outflows tend towards higher stellar masses, SFR, and $\Sigma_{\rm SFR}$ than those without outflows, suggesting that the two samples are not drawn from the same parent population. Finally, we additionally find that Mg II emission is preferentially detected in galaxies with lower stellar mass and $A_V$, and higher sSFR, consistent with conditions that favor the escape of resonantly scattered line and ionizing continuum radiation. We present the first evidence in $z\sim3$ star-forming galaxies that properties of the absorption lines depend on galaxy inclination, with more face-on systems showing stronger absorption and higher outflow velocities, while inflowing gas is more frequently detected in more highly inclined galaxies. These trends are consistent with observations at $z\lesssim1$ and predictions from cosmological simulations in which galactic winds are launched perpendicular to the galactic disks, while accretion occurs primarily along the disk plane.

astro-ph.GA↗

arXiv2026

The JADES Mass-Metallicity and Fundamental Metallicity Relations at $z\gtrsim2$ Using New High-Redshift Metallicity Calibrations

We present measurements of the mass-metallicity relation (MZR) and fundamental metallicity relation (FMR) at $1.4<z<7.0$ using stacked JWST/NIRSpec spectra of 601 star-forming galaxies from the JWST Advanced Deep Extragalactic Survey (JADES). Using the most up-to-date strong-line metallicity calibrations based on high-redshift galaxies, we derive gas-phase metallicities from composite spectra binned by stellar mass, redshift, and star-forming main sequence (SFMS) offset. We find that the MZR slope evolves weakly from $z\sim0$ out to $z\sim5$, with $\gamma\sim0.21\pm0.03$, while the normalization decreases smoothly with redshift at a rate of $d\log(\mathrm{O/H})/dz\sim-0.1$ out to $z\sim4$. Beyond $z\gtrsim5$, the low-mass end continues to decline in metallicity while the high-mass end remains broadly consistent with lower-redshift relations, producing a steeper overall MZR. We additionally find evidence for a shallow anti-correlation between deviations from the MZR and SFMS at fixed stellar mass at $z\sim1.4-5$. This anti-correlation, albeit with weaker SFR coupling than observed locally, suggests that an FMR is already beginning to emerge by $z\sim5$. Comparisons with recent observations and cosmological simulations show broad agreement, though no single simulation simultaneously reproduces the observed slopes and normalizations across all redshifts. Our results support a picture in which bursty star formation and strong stellar feedback increasingly shape the regulation of galaxy growth at high redshift, while also highlighting the need for substantially larger spectroscopic samples to robustly constrain the evolution of galaxy scaling relations at high-redshift.

astro-ph.GA↗

arXiv2026

Balmer Decrements and Nebular-Stellar Reddening in JADES Galaxies at $2.7<z<7$

We aim to characterize nebular and stellar reddening in star-forming galaxies as a function of global galaxy properties (stellar mass, SFR, metallicity) at $2.7 < z< 7.0$. We also provide a prescription to convert SED-based $E(B-V)_{\mathrm{star}}$ to $E(B-V)_{\mathrm{gas}}$ when direct measurements of nebular reddening are unavailable. Our results are based on JWST/NIRSpec measurements of both individual spectra, with a sample of 283 galaxies, and composite spectra, including a larger sample of 327 galaxies. We estimate nebular reddening using the Balmer decrement (H$\alpha$/H$\beta$) above $10^{8.5}$ $M_{\odot}$, where the sample is representative. Stellar reddening and SFRs are derived through Prospector SED fitting, while gas-phase metallicities are estimated from strong emission-line ratios. At fixed stellar mass, Balmer decrements remain consistent within uncertainties across our redshift range, indicating that stellar mass primarily determines the overall dust column even by $z \sim 7$. We find that differential reddening ($\Delta E(B-V) \equiv E(B-V)_{\mathrm{gas}} - E(B-V)_{\mathrm{star}}$) scales linearly with mass and SFR at $z \sim 2.7 - 4.0$, but shows no mass or SFR dependence above $z \sim 4.0$. We find evidence for smaller $\Delta E(B-V)$ above $z \sim 5.0$, suggesting that nebular emission and stellar continuum probe increasingly similar dust columns towards higher redshift. Finally, we find that nebular reddening correlates strongly with metallicity out to $z \sim 5$, whereas the correlation between stellar reddening and metallicity is weaker or absent. Together, these results suggest that both dust mass and geometry play a significant role in shaping the observed reddening of high-redshift galaxies.

astro-ph.GA↗

arXiv2026

Emission-line Diagnostics at $z\gtrsim 2$: A Probe of the Ionizing Spectrum and $\alpha$ Enhancement Beyond Cosmic Noon

We analyze several key rest-optical emission-line ratios in a sample of 828 galaxies as well as composite spectra from JADES DR3 in the range $1.4 < z < 7$. These emission-line ratios include: [O III]$\lambda5008$/H$\beta$, [N II]$\lambda 6585$/H$\alpha$, [S II]$\lambda\lambda 6718,6733$/H$\alpha$, [O I]$\lambda 6302$/H$\alpha$, O32, R23, Ne3O2, and RO2Ne3. We find evidence for a harder ionizing spectrum at $z\sim 3.5$ compared to $z\sim 2$ at fixed gas-phase metallicity, resulting in a pronounced shift in the star-forming galaxy locus on the [N II]/H$\alpha$ BPT diagram and the O32 vs. R23 diagram. At $z\gtrsim 3.5$, star-forming galaxies occupy a common locus, indicating that ISM ionizing conditions at fixed gas-phase metallicity do not evolve strongly at these early cosmic times. There is a connection between ISM ionizing conditions and the chemical abundance patterns (i.e., $\alpha$/Fe) in massive stars providing the ionizing radiation field. Therefore, the lack of evolution in ISM ionizing conditions at $z\gtrsim 3.5$, followed by evolution towards a softer ionizing spectrum at fixed nebular metallicity as cosmic time proceeds to $z\sim 2$ and lower redshift mirrors the chemical abundance patterns in Milky Way stars as a function of iron abundance. Our results highlight the diagnostic power of emission-line diagrams in the era of JWST to further our understanding of the ISM conditions into the Epoch of Reionization.

astro-ph.GA↗

arXiv2025

The AURORA Survey: Constraining Chemical Enrichment Pathways at Cosmic Noon with Argon Abundances

We present argon abundances from a sample of 46 star-forming galaxies at $z=2-3.5$ from the Assembly of Ultradeep Rest-Optical Observations Revealing Astrophysics (AURORA) program. Although argon is an $\alpha-$element produced by Core Collapse Supernovae (CCSNe), the latest supernova yield models suggest additional argon production and enrichment by Type Ia supernovae (SNe Ia), unlike other $\alpha-$elements such as oxygen. To study the relationship between argon and oxygen abundances, we construct two median-stacked composite spectra for separate $z=2.0-2.6$ and $z=2.8-3.5$ redshift bins, presenting more representative measurements than previous samples that require individual detection of faint lines. Abundance ratios were determined using an empirical calibration based on the strength of the [ArIII]$\lambda7137$ emission line relative to the [OIII]$\lambda5008$ emission line. With this calibration, we estimate argon abundances (Ar/O) of $0.42^{+0.12}_{-0.10}\mbox{(Ar/O)}_{\odot}$ for the $\langle z \rangle = 2.26$ bin and $0.42^{+0.12}_{-0.11}\mbox{(Ar/O)}_{\odot}$ for the $\langle z \rangle = 3.15$ bin, suggesting minimal SNe Ia and dominant CCSNe enrichment in this sample. Comparison of our abundance measurements of $z\sim 2-3$ AURORA galaxies with chemical evolution modeling of Milky Way stars shows consistency with the Milky Way Bulge component, suggesting a rapid star-formation timescale. However, even larger samples of actively star-forming galaxies with available argon abundances, as well as comparisons between argon abundance and other critical galaxy properties (e.g., sSFR) and models (e.g., one tuned specifically to this redshift range) are needed to draw stronger conclusions on the role of argon in galactic chemical enrichment at Cosmic Noon.

astro-ph.GA↗

arXiv2025

The Star-forming Main Sequence and Bursty Star-formation Histories at $z>1.4$ in JADES and AURORA

We analyze JWST spectroscopic and HST+JWST photometric observations of 659 star-forming galaxies at $1.4<z<9$ from DR3 of the JADES survey and the AURORA Cycle 1 program. We measure the star-forming main sequence (SFMS) for galaxies above $10^{8.5}\rm\ M_\odot$ where the sample is largely representative, estimating star-formation rates (SFRs) using the H$\alpha$ line flux and rest-frame far UV (1600\AA) continuum measurements, each independently corrected for dust attenuation. We find that the intrinsic, measurement-error-subtracted scatter in the SFMS ($\sigma_{\rm int}$) increases with decreasing stellar mass for the H$\alpha$-based SFMS, and we find no mass dependence of $\sigma_{\rm int}$ in the UV-based SFMS. Additionally, we find that $\sigma_{\rm int}$ decreases with increasing redshift, from $0.36^{+0.02}_{-0.02}$ dex to $0.22^{+0.08}_{-0.07}$ dex (H$\alpha$ SFMS), and from $0.28^{+0.02}_{-0.02}$ dex to $0.20^{+0.08}_{-0.07}$ dex (UV SFMS) between $z\sim2$ and $z\sim 6.5$. We also measure the redshift evolution of the specific SFR and find that, assuming $\rm sSFR\propto (1+z)^\gamma$, $\gamma=1.89^{+0.16}_{-0.15}$ for the H$\alpha$-based SFMS, and $\gamma=1.36^{+0.13}_{-0.13}$ for the UV-based SFMS. Analyzing the observed H$\alpha$/UV luminosity ratios and star-formation histories from the {\sc prospector} fitting code, we find that 41--60\% of the sample is inconsistent with having a constant star-formation history. Finally, we find tentative evidence for shorter SFR burst timescales with increasing redshift based on the distribution of $\rm L_{H\alpha}/\nu L_{\nu,1600}$ vs. $\Delta\rm \log(L_{H\alpha})$. Taken together, these results are consistent with recent theoretical predictions (e.g., the {\sc thesan-zoom} simulations) of bursty star formation in the early Universe and provide valuable constraints for theoretical models of galaxy evolution.

astro-ph.GA↗

arXiv2025

MOSDEF-3D: Keck/OSIRIS Maps of the Ionized ISM in $z \sim 2$ Galaxies

We present spatially-resolved rest-frame optical emission line maps of four galaxies at $z \sim 2$ observed with Keck/OSIRIS to study the physical conditions of the ISM at Cosmic Noon. Our analysis of strong emission line ratios in these galaxies reveals an offset from the local star-forming locus on the BPT diagram, but agrees with other star-forming galaxies at similar redshifts. Despite the offset towards higher [O III]$\lambda5008$/H$\beta$ and [N II]$\lambda6585$/H$\alpha$, these strong-line ratios remain consistent with or below the maximum starburst threshold even in the inner $\sim 1$ kpc region of the galaxies, providing no compelling evidence for central AGN activity. The galaxies also exhibit flat radial gas-phase metallicity gradients, consistent with previous studies of $z \sim 2$ galaxies and suggesting efficient radial mixing possibly driven by strong outflows from intense star formation. Overall, our results reveal the highly star-forming nature of these galaxies, with the potential to launch outflows that flatten metallicity gradients through significant radial gas mixing. Future observations with JWST/NIRSpec are crucial to detect fainter emission lines at higher spatial resolution to further constrain the physical processes and ionization mechanisms that shape the ISM during Cosmic Noon.

astro-ph.GA↗

arXiv2025

The AURORA Survey: Tracing Galactic Outflows at $z\gtrsim2.5$ with JWST/NIRSpec NUV Absorption Lines

We probe galactic-scale outflows in star-forming galaxies at $z\gtrsim2.5$ drawn from the \textit{JWST}/NIRSpec AURORA program. For the first time, we directly compare outflow properties from the early universe to the present day using near-UV absorption lines. We measure ISM kinematics from Fe\,{\sc ii} and Mg\,{\sc ii} absorption features in 41 and 43 galaxies, respectively, and examine how these kinematics correlate with galaxy properties. We find that galaxies with outflows tend to have higher stellar masses, and that maximum outflow velocities increase with stellar mass, SFR, UV slope $\beta$, $E(B-V)$, and $A_V$. We also find that Mg\,{\sc ii} emission is more common in galaxies with lower masses, higher sSFRs, and less dust. These trends are consistent with those in star-forming galaxies at $z<2$ when using the same outflow tracers, suggesting that the feedback from star formation has played a persistent role in shaping galaxy evolution over cosmic time. We also directly compare near-UV and far-UV features in the same NIRSpec spectrum for a $z=5.19$ galaxy, finding consistent ISM kinematics and demonstrating that different tracers yield comparable measurements. We also detect Na\,D absorption in 10 galaxies, which have higher stellar mass, SFR, and dust attenuation compared to galaxies without Na\,D absorption, which is consistent with $z\sim0$ studies. The broad continuum coverage and sensitivity of NIRSpec will enable future studies with larger samples, allowing for robust tests of these trends across a wider dynamic range of galaxy properties.

astro-ph.GA↗