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

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.

Figures (5)

• Figure 1: Artistic illustration of the system, consisting of the galaxy J1439B and its outflow, detected as a sub-DLA in the spectrum of the background quasar J1439+1117.
• Figure 2: SED fit to the broadband photometry of galaxy J1439B. The blue line represents the model spectrum, red squares represent the best-fit model photometry, and black circles with error bars indicate the observed photometry.
• Figure 3: Magellan/FIRE spectrum of QSO J1439+1117 (in blue; smoothed with a 4-pixel Gaussian kernel) and spectral uncertainty (in orange). Red dashed lines mark the main emission lines from the QSO at z = 2.585, while green dashed lines mark absorption lines from the sub-DLA at z = 2.41837. Spectral windows with poor atmospheric transmission are marked in gray.
• Figure 4: Absorption features in the QSO J1439+1117 spectrum in velocity space, with zero corresponding to the systemic velocity of galaxy J1439B. Normalized flux is shown on the $y$-axis of each panel. The top panels show Fe II, Mg II, Mg I and Na I transitions in the FIRE spectrum. The shaded areas represent the spectral regions used in the calculation of the EW for the component at -47 km/s (blue shaded areas), or for the sum of the components at -47 and -164 km/s when they are blended (red shaded areas). The bottom panels show O I, Fe II, and Al II in the higher-resolution UVES data. Vertical dashed lines mark the absorbers used to fit the H I distribution in Srianand2008 and Noterdaeme2008.
• Figure 5: Column density calibration for different species. The local calibration based on Milky Way gas clouds is shown in red, with the error bar showing the range due to variations in dust depletion. Our empirical calibration derived from J1439B is shown in blue. The two calibrations yield consistent results for Na and Fe, but are inconsistent for Mg, suggesting stronger dust depletion of Mg in high-redshift outflows compared to the Milky Way.