Empirical Calibration of Na I D and Other Absorption Lines as Tracers of High-Redshift Neutral Outflows

Lorenzo Moretti, Sirio Belli, Gwen C. Rudie, Andrew B. Newman, Minjung Park, Amir H. Khoram, Nima Chartab, Darko Donevski

First listed 2025-07-09 | Last updated 2026-03-09

Abstract

Recent JWST observations of massive galaxies at z > 2 have detected blueshifted absorption in Na I D and other resonant absorption lines, indicative of strong gas outflows in the neutral phase. However, the measured mass outflow rates are highly uncertain because JWST observations can only probe the column density of trace elements such as sodium, while most of the gas is in the form of hydrogen. The conversion between the column density of sodium and that of hydrogen is based on observations of gas clouds within the Milky Way, and has not been directly tested for massive galaxies at high redshift. In order to test this conversion, we study a unique system consisting of a massive quiescent galaxy (J1439B) at z = 2.4189 located at a projected distance of 38 physical kpc from the bright background quasar QSO J1439. The neutral outflow from the galaxy is observed as a sub-damped Lyman-alpha absorber in the spectrum of the background quasar, which enables a direct measurement of the hydrogen column density from Lyman transitions. We obtain new near-infrared spectroscopy with Magellan/FIRE and detect Na I D and other resonant absorption lines from Mg II, Mg I, and Fe II. We are thus able to derive new, empirical calibrations between the column density of trace elements and the hydrogen column density, that can be used to estimate the mass and the rate of neutral gas outflows in other massive quiescent galaxies at high redshift. The calibration we derive for Na I is only 30% lower than the local relation that is typically assumed at high redshift, confirming that the neutral outflows observed with JWST at z > 2 are able to remove a large amount of gas and are thus likely to play a key role in galaxy quenching. However, using the local calibration for Mg II yields an order-of-magnitude discrepancy compared to the empirical calibration, possibly because of variations in the dust depletion.

Short digest

A cleverly aligned system lets the authors directly calibrate neutral outflow tracers: a massive quiescent galaxy J1439B at z=2.4189 sits 38 kpc from the sightline to QSO J1439+1117, where its outflow appears as a sub-DLA enabling an H I column density measurement. New Magellan/FIRE spectra detect Na I D, Mg II, Mg I, and Fe II in the quasar spectrum and tie their column densities to N(H I), delivering empirical conversions usable for JWST-selected neutral outflows. The Na I calibration is just ~30% below the Milky Way relation commonly assumed at high z, supporting large neutral mass outflow estimates relevant to quenching. In contrast, applying the local Mg II calibration would err by an order of magnitude, likely due to dust depletion variations.

Key figures to inspect

• Figure 3: Inspect the FIRE quasar spectrum at z=2.585 to see the identified Na I D, Mg II, Mg I, and Fe II absorption from the sub-DLA at z=2.41837; check continuum placement and S/N that underpin the equivalent-width and column-density estimates.
• Figure 4: Velocity-space stacks relative to J1439B’s systemic show the −47 and −164 km/s components; use these to connect metal-line kinematics to the H I component structure from UVES (O I, Fe II, Al II) and to evaluate blending in the EW measurements.
• Figure 2: The SED fit for J1439B establishes its massive, low-SFR, quiescent nature; verify the placement below the star-forming main sequence that motivates outflow-driven quenching relevance.
• Figure 1: Geometry cartoon clarifies the 38 kpc impact parameter and near-zero Δv (~47 km/s) between galaxy and main absorber—key evidence linking the sub-DLA to J1439B’s outflow rather than an intervening system.