Dr Jacopo Chevallard

Abstract:

The James Webb Space Telescope provides an unprecedented opportunity for unbiased surveys of Hα-emitting galaxies at z > 4 with the NIRCam's wide-field slitless spectroscopy (WFSS). In this work, we present a census of Lyα escape fraction (f esc,Lyα ) of 165 star-forming galaxies at z = 4.9–6.3, utilizing their Hα emission directly measured from FRESCO NIRCam/WFSS data. We search for Lyα emission of each Hα-emitting galaxy in the Very Large Telescope/MUSE data. The overall f esc,Lyα measured by stacking is 0.090 ± 0.006. We find that f esc,Lyα displays a strong dependence on the observed UV slope (β obs) and E(B − V), such that the bluest galaxies (β obs ∼ −2.5) have the largest escape fractions (f esc,Lyα ≈ 0.6), indicative of the crucial role of dust and gas in modulating the escape of Lyα photons. f esc,Lyα is less well related to other parameters, including the UV luminosity and stellar mass, and the variation in f esc,Lyα with them can be explained by their underlying coupling with E(B − V) or β obs. Our results suggest a tentative decline in f esc,Lyα at z ≳ 5, implying increasing intergalactic medium attenuation toward higher redshift. Furthermore, the dependence of f esc,Lyα on β obs is proportional to that of the ionizing photon escape fraction (f esc,LyC), indicating that the escape of Lyα and ionizing photon may be regulated by similar physical processes. With f esc,Lyα as a proxy to f esc,LyC, we infer that UV-faint (M UV > −16) galaxies contribute >70% of the total ionizing emissivity at z = 5–6. If these relations hold during the epoch of reionization, UV-faint galaxies can contribute the majority of UV photon budget to reionize the Universe.

Abstract:

We explore the kinematic gas properties of six 5.5 < z < 7.4 galaxies in the JWST Advanced Deep Extragalactic Survey (JADES), using high-resolution JWST/NIRSpec multi-object spectroscopy of the rest-frame optical emission lines [O III ] and H α . The objects are small and of low stellar mass (∼1 kpc; M * ∼ 10 7 − 9 M ⊙ ), less massive than any galaxy studied kinematically at z > 1 thus far. The cold gas masses implied by the observed star formation rates are about ten times higher than the stellar masses. We find that their ionised gas is spatially resolved by JWST, with evidence for broadened lines and spatial velocity gradients. Using a simple thin-disc model, we fit these data with a novel forward-modelling software that accounts for the complex geometry, point spread function, and pixellation of the NIRSpec instrument. We find the sample to include both rotation- and dispersion-dominated structures, as we detect velocity gradients of v ( r e )∼100 − 150 km s −1 , and we find velocity dispersions of σ 0 ∼ 30 − 70 km s −1 that are comparable to those at cosmic noon. The dynamical masses implied by these models ( M dyn ∼ 10 9 − 10 M ⊙ ) are higher than the stellar masses by up to a factor 40, and they are higher than the total baryonic mass (gas + stars) by a factor of ∼3. Qualitatively, this result is robust even if the observed velocity gradients reflect ongoing mergers rather than rotating discs. Unless the observed emission line kinematics is dominated by outflows, this implies that the centres of these galaxies are dominated by dark matter or that star formation is three times less efficient, leading to higher inferred gas masses.

Abstract:

We present the properties of 17 faint Lyman-α emitting galaxies (LAEs) at z > 5.8 from the JWST Advanced Deep Extragalactic Survey (JADES) spectroscopic data in the Hubble Ultra Deep Field/GOODS-S. These LAEs span a redshift range z ≈ 5.8 − 8.0 and a UV magnitude range MUV ≈ −17 to −20.6, with the Lyα equivalent width (EW) in the range ≈25 − 350 Å. The detection of other rest-optical emission lines in the spectra of these LAEs enables the determination of accurate systemic redshifts and Lyα velocity offsets, as well as the physical and chemical composition of their stars and interstellar media. These faint LAEs are consistent with metal-poor systems with high ionization parameters, similar to the general galaxy population at z > 6. We measured an average ionizing photon production efficiency, log(ξion/erg−1 Hz) ≈ 25.57 across our LAEs, which does not evolve strongly with redshift. We report an anti-correlation between the Lyα escape fraction and the velocity offset from systemic redshift, consistent with model expectations. We further find that the strength and velocity offset of Lyα are neither correlated with galaxy spectroscopic properties nor with ξion. We find a decrease in Lyα escape fractions with redshift, indicative of decreasing sizes of ionized bubbles around LAEs at high redshifts. We used a range of galaxy properties to predict Lyman continuum escape fractions for our LAEs, finding that the ionizing photon output into the intergalactic medium from our LAEs remains roughly constant across the observed Lyα EW, showing a mild increase at fainter UV magnitudes and at higher redshifts. We derived correlations between the ionizing photon output from LAEs and their UV magnitudes, Lyα strengths and redshifts, which can be used to constrain the ionizing photon contribution of LAEs at z > 6 towards cosmic reionization