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

TL;DR

This study uses JWST/NIRSpec to apply rest-frame near-UV absorption diagnostics (Fe II and Mg II) to star-forming galaxies at $z\gtrsim 2.5$, enabling direct comparison with lower redshift samples. Individual outflow velocities show limited correlations with galaxy properties, but composite spectra reveal that $v_{\max}$ increases with stellar mass, SFR, and dust indicators, consistent with energy-driven wind scenarios. Mg II emission, detected in about 12% of the sample, is more common in lower-mass, higher-sSFR, and less dusty systems, while Mg II absorption strength grows in higher-$M_{*}$, higher-dust bins; Na D absorption appears in more massive, dustier galaxies but with no clear net outflow. The results demonstrate that Mg II and Fe II tracers provide complementary insights into galactic winds at high redshift and highlight the value of JWST for cross-tracer, cross-epoch wind studies, setting the stage for larger, statistically robust surveys.

Abstract

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 $β$, $E(B-V)$, 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.

Figures (10)

• Figure 1: Example of AURORA spectra at 3 different redshifts that are continuum normalized: COSMOS-8363 at $z = 3.247511$ (top), GOODSN-17940 at $z = 4.11541$ (middle), and GOODSN-100067 at $z=5.187539$ (bottom). The error spectra are plotted as gray shaded regions. Blue labels indicate absorption lines that we use to analyze outflow velocities from Fe ii. Red labels indicate the Mg ii doublet. Green labels indicate emission lines. The orange label indicates the Mg i absorption line.
• Figure 2: Properties of galaxies from the AURORA survey. Left: Redshift distribution of the full survey (black histogram), of galaxies with measured Fe ii centroids (blue histogram), and of galaxies with measured Mg ii centroids (red histogram). Right: The SFR and stellar masses for the full survey (open black circles), galaxies with measured Fe ii centroids (blue circles), and of galaxies with measured Mg ii centroids (red squares). The dashed line represents the SFR main sequence from 2014Speagle at $z\sim 3$. Our Fe ii and Mg ii samples scatter well around the star-forming main sequence.
• Figure 3: Close up Mg ii profiles of absorption only (top), both absorption and emission (middle), and emission dominated (bottom). The dashed red lines indicate the rest frame wavelengths of the Mg ii$\lambda2796$ and Mg ii$\lambda2803$ lines. The gray shaded region represents the error spectrum.
• Figure 4: Outflow velocity versus various galaxy properties. Blue closed circles are velocities measured from Fe ii centroids. Red closed squares are velocities from the Mg ii doublet measured from MCMC. Large blue open circles are velocities from the Fe ii stacks. Large red open squares are velocities from the Mg ii stacks. Large red squares are $v_{\mathrm{max}}$ measured from the Mg ii stacks. We were unable to robustly determine $v_{\mathrm{max}}$ for the high sSFR composite due to its complex profile shape. Large blue circles are $v_{\mathrm{max}}$ measured from the Fe ii stacks. The dashed gray line indicates the separation between the low and high bins for each property. The individual measurements and composite spectra show no correlation between outflow velocity and galaxy property. The $v_\mathrm{max}$ measured from Mg ii suggests that the the amplitude of the outflow maximum velocity increases with $M_*$, SFR, $\beta$, $E(B-V)$, and $A_V$.
• Figure 5: Comparisons of galaxies with detected outflows ($\Delta v <0$ and the magnitude of $\Delta v$ greater than its uncertainty) from Fe ii lines (blue) or Mg ii (red) and galaxies where no outflow was detected (black) for the various galaxy properties. Average property values are printed in the upper right corner. The average $M_*$ is significantly higher for galaxies with outflows detected from either Fe ii or Mg ii, compared with galaxies in which no outflow is detected.