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

Abstract

Using deep JWST/NIRSpec spectra from the Blue Jay survey, we perform the first systematic investigation of neutral gas content in massive galaxies at Cosmic Noon based on the Ca II H, K absorption lines. We analyze a sample of 9 galaxies at 1.8 < z < 2.8 with stellar masses > 10.6, for which we detect 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 fit 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 measure 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 measure 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, 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 discussing possible reasons for this behavior, we derive 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. This calibration offers a new way to estimate the outflow mass and the mass outflow rate for the neutral phase from current and future JWST observations of massive galaxies at Cosmic Noon and beyond

Figures (13)

• Figure 1: Distribution of stellar mass and sSFR for 141 galaxies from the Blue Jay survey. Gray crosses mark systems with no detection of excess Na I D absorption. Blue squares represent the sample of galaxies with excess Na I D absorption due to neutral gas studied by Davies_2024. The galaxies analyzed in the present work are represented by red circles, and they have both Na I and Ca II absorption excess clearly detected.
• Figure 2: NIRSpec spectrum of galaxy COSMOS 9395 at $z \sim 2.1$. The observed spectrum is shown in blue, and the shaded area is the 1$\sigma$ uncertainty, while the dark pink curve is the best-fit stellar continuum provided by Prospector. Some of the most important emission and absorption lines visible in this wavelength range, including Ca II K, Ca II H and Na I D, are highlighted.
• Figure 3: Stack of Ca II triplet lines for the sample galaxies. Left panel: Stack of the observed spectra (blue) and stack of the Prospector stellar models (dark pink) for the nine galaxies in the sample, around the Ca II triplet. The stack is normalized so that the level of the continuum is approximately 1. The Ca II triplet is precisely reproduced by the stellar models. Right panel: Stack of the residuals, i.e., observed spectra divided by their best-fit stellar models. The dashed vertical lines mark the wavelengths of the absorption lines that are part of the Ca II triplet. Shaded areas represent the 1$\sigma$ uncertainty.
• Figure 4: Corner plot showing the posterior distributions of the Ca II parameters inferred by emcee, for galaxy COSMOS 18071. For the posterior distribution function of each parameter, the blue line is the median and the two dashed black lines are the 16$^{th}$ and the 84$^{th}$ percentiles.
• Figure 5: Observed NIRSpec spectra divided by their stellar continuum (blue), with the best-fit model (orange) for galaxies 11142 and 16874. The blue shadow is the flux uncertainty. For each galaxy the left panel shows the Ca II K, H absorption and H$\epsilon$ emission, while the right panel shows the Na I D absorption and He I emission. The dashed vertical lines mark the systemic wavelength of the absorption lines.