The GLASS-JWST Early Release Science Program. V. H$α$ luminosity functions at $z\sim1.3$ and $z\sim2.0$

TL;DR

This study uses JWST/NIRISS WFSS data from the GLASS-JWST ERS program, combined with gravitational lensing in ABELL-2744, to measure the H$ luminosity functions at $z \sim 1.3$ and $z \sim 2.0$ and to reach faint intrinsic luminosities of $L_{\rm H\alpha} \sim 10^{40.5}$ and $10^{40.9}$ erg s$^{-1}$. A forward- modeling framework with FSPS templates is developed to estimate completeness and effective survey volume, resulting in a sample of 99 H$ emitters with $S/N > 5$ and two redshift bins: $z=1.05-1.5$ and $z=1.8-2.2$, where the H$ flux is blended with [N II]. The authors constrain the faint-end slopes to $\alpha = -1.50^{+0.14}_{-0.08}$ at $z \sim 1.3$ and $\alpha = -1.60^{+0.17}_{-0.09}$ at $z \sim 2.0$, and derive cosmic star formation rate densities of $0.097^{+0.015}_{-0.016}$ and $0.129^{+0.025}_{-0.030}$ $M_\odot$ yr$^{-1}$ Mpc$^{-3}$, respectively, with negligible bright AGN contribution. The methodology enables robust H$ LF measurements for JWST slitless data and is readily applicable to future wide-field surveys (e.g., Euclid, Roman, CSST).

Abstract

We present H$α$ luminosity function (LF) measurements at redshifts $z\sim1.3$ and $z\sim2.0$ using JWST NIRISS grism data from the GLASS-JWST survey. Based on emission lines spectroscopically identified in the F115W, F150W and F200W filters, we select 99 H$α$ emitters. Through detailed effective volume and completeness analysis for each source, we construct the H$α$ LF in two redshift bins. Thanks to the sensitivity of NIRISS WFSS and gravitational lensing magnification, our sample reaches intrinsic H$α$ luminosities $\sim$10 times deeper than previous grism surveys, down to $L_{\rm Hα}\sim10^{40.5}~\rm erg~s^{-1}$ at $z\sim1.3$ and $L_{\rm Hα}\sim10^{40.9}~\rm erg~s^{-1}$ at $z\sim2.0$ with completeness larger than 0.8, corresponding to star formation rates of 0.4 and 1.0 $M_{\odot}~\rm yr^{-1}$, respectively. We robustly constrain the faint-end slope of the H$α$ luminosity function to be $-1.50^{+0.14}_{-0.08}$ at $z\sim1.3$ and $-1.60^{+0.17}_{-0.09}$ at $z\sim2.0$ after considering the cosmic variance of $\sim 20\%$, consistent with previous estimations. The emission-line samples presented here will enable further detailed studies of galaxy properties including metallicities. We find a negligible contribution from bright active galactic nuclei in our sample. We estimate integrated cosmic star formation rate densities of $0.097^{+0.015}_{-0.016}~M_{\odot}~\rm yr^{-1}~Mpc^{-3}$ at $z\sim1.3$ and $0.129^{+0.025}_{-0.030}~M_{\odot}~\rm yr^{-1}~Mpc^{-3}$ at $z\sim2.0$. The methodology presented here can be readily applicable to other JWST slitless spectroscopic datasets and future wide-field slitless surveys, including those from Euclid, Roman, and the Chinese Space Station Telescope.

Figures (2)

• Figure 1: ID-2300 as an example to illustrate the 2D emission line maps, 2D spectra, and 1D spectra of the H$\alpha$ emitters in our sample. Top panels, from left to right: a color-composite cutout (RGB image at 30 mas pixel scale; R: F200W, G: F150W, B: F115W), and surface-brightness maps of the emission lines ([O II], H$\beta$, [O III], H$\alpha$) derived from the NIRISS slitless spectroscopy. Middle panels: continuum-subtracted 2D grism spectra obtained in the three filters (F115W, F150W, F200W). Bottom panels: The optimally extracted 1D observed flux $F_{\lambda}$ (in units of [$10^{-19}~\rm erg~s^{-1}~cm^{-2}$, its $1\sigma$ uncertainty, and the forward-modeled spectra, shown as the black stepped curves, grey shaded region, and red dash line, respectively. The physical scale, redshift, and $\chi^2$ value of the fit are annotated in the upper-left panel.
• Figure 2: H$\alpha$ properties and spatial distribution of our sample. Left panel: Cutout images (right) and H$\alpha$ spectra (left) for two representative galaxies in our sample, ID-4116 and ID-1795. The 1D and 2D spectra are displayed in the upper and lower sub-panels, respectively. The measured H$\alpha$ flux, physical scale, and magnification factor are annotated. Right panel: Spatial distribution of H$\alpha$ emitters at $z\sim1.3$ (blue squares) and $z\sim2.0$ (red circles) in the ABELL-2744 field. The background is the NIRISS RGB image (R: F200W, G: F150W, B: F115W).