Probing neutral outflows in z  ∼  2 galaxies using JWST observations of Ca II H and K absorption lines

¹ Dipartimento di Fisica e Astronomia (DIFA), Università di Bologna Bologna, Italy
² INAF, Osservatorio di Astrofisica e Scienza dello Spazio Via Piero Gobetti 93/3 I-40129 Bologna, Italy
³ Centre for Astrophysics and Supercomputing, Swinburne University of Technology Hawthorn Victoria, Australia
⁴ ARC Centre of Excellence for All Sky Astrophysics in 3 Dimensions (ASTRO 3D), Australia
⁵ Center for Astrophysics – Harvard & Smithsonian Cambridge MA, USA
⁶ Department of Astronomy & Astrophysics, The Pennsylvania State University University Park PA, USA
⁷ Institute for Gravitation and the Cosmos, The Pennsylvania State University University Park PA, USA
⁸ Institute for Computational & Data Sciences, The Pennsylvania State University University Park PA, USA
⁹ Kavli Institute for Cosmology, University of Cambridge Cambridge, UK
¹⁰ Cavendish Laboratory, University of Cambridge Cambridge, UK
¹¹ Research School of Astronomy and Astrophysics, Australian National University Canberra ACT, Australia
¹² Leibniz Institute for Astrophysics Potsdam, Germany

^★ Corresponding author: This email address is being protected from spambots. You need JavaScript enabled to view it.

Received: 6 June 2025
Accepted: 17 November 2025

Abstract

Using deep JWST/NIRSpec spectra from the Blue Jay survey, we performed the first systematic investigation of neutral gas content in massive galaxies at Cosmic Noon based on the Ca II H, K absorption lines. We analyzed a sample of nine galaxies at 1.8 < z < 2.8 with log M_*/M_⊙ > 10.6, for which we detected neutral gas absorption both in Ca II and in Na I. After removing the stellar continuum using the best-fit model obtained with Prospector, we fitted the excess absorption due to neutral gas in the Ca II H, K doublet and in the Na I D doublet, together with nearby emission lines produced by ionized gas. We measured covering fractions between 0.2 and 0.9 from the Ca II H and K lines, which are spectrally well resolved in the NIRSpec R ∼ 1000 observations, unlike the absorption lines in the Na I D doublet. We measured the velocity shift, velocity dispersion, and column density separately for Ca II and Na I. About half of the galaxies present blueshifted Ca II, indicative of an outflow of neutral gas and consistent with previous results based on Na I. The velocity shift and the column density measured from Ca II are correlated with those measured from Na I, implying that these absorption lines trace gas in similar physical conditions. However, the column densities are not in a 1:1 relation, meaning that the relative amount of Ca II and Na I atoms along the line of sight varies with the gas column density. After investigating possible reasons for this behavior, we derived an empirical relation between the column density of Ca II and the column density of Na I and, in a more indirect way, of neutral hydrogen H I. With this calibration, we measured the mass outflow rates in Blue Jay galaxies using the Ca II lines, finding values in the range ∼2.7 − 56 M_⊙/yr, broadly consistent (within a factor of roughly five) with previous studies of the Na I lines. The employment of Ca II lines offers a new way to infer properties of neutral gas from current and future JWST observations of massive galaxies at Cosmic Noon and beyond.