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Abstract:

Cosmic ray (CR) feedback has been proposed as a powerful mechanism for driving warm gas outflows in galaxies. We use cosmological magnetohydrodynamic simulations to investigate the impact of CR feedback on neutral hydrogen (H_I) in a 10¹¹²Ñ⊙ dark matter halo at 2<z<4. To this end, we post-process the simulations with ionizing radiative transfer and perform Monte Carlo Lyman-α (Lyα) transfer calculations. CR feedback reduces H_I column densities around young stars, thereby allowing more Lyα photons to escape and consequently offering a better match to the Lyα luminosities of observed Lyα emitters. Although galaxies with CR-driven outflows have more extended H_I in the circumgalactic medium, two Lyα line properties sensitive to optical depth and gas kinematics - the location of the red peak in velocity space (v_red) and relative strength of the blue-to-red peaks (B/R) - cannot distinguish between the CR-driven and non-CR simulations. This is because Lyα photons propagate preferentially along low HI density channels created by the ionizing radiation, thereby limiting the scattering with volume-filling H_I. In contrast, the observed low flux ratios between the valley and peak and the surface brightness profiles are better reproduced in the model with CR-driven outflows because the Lyα photons interact more before escaping, rather than being destroyed by dust as is the case in the non-CR simulation. We discuss the potential cause of the paucity of sightlines in simulations that exhibit prominent red peaks and large v_red, which may require the presence of more volume-filling H_I.

Abstract:

JWST spectroscopy has built large emission line samples at z ≳ 4, but it has yet to confidently reveal many galaxies with the hard radiation fields commonly associated with active galactic nucleus photoionization. While this may indicate a weaker UV ionizing spectrum in many z > 4 active galactic nuclei or obscuration from dense neutral gas and dust, the complete picture remains unclear owing to the small number of deep rest-UV spectra. Here, we characterize the strength of high-ionization lines in 53 new galaxies observed with NIRSpec R = 2700 grating spectroscopy. We present new detections of narrow N v λ1240 in two galaxies. One is a previously confirmed z = 6.98 little red dot (LRD) with broad Hβ, and the other is a z = 8.72 galaxy with a narrow-line spectrum. Neither source exhibits C iv or He ii emission, indicating large N v/C iv and N v/He ii ratios that may reflect a combination of nitrogen-enhancement and resonant scattering effects. We investigate the incidence of narrow high-ionization lines in a large database of 851 NIRSpec grating spectra, and we separately quantify the fraction of LRDs with narrow high-ionization UV emission lines. Our results likely suggest that hard radiation fields are indeed present in a small subset of LRDs ( 12 . 5 − 10.4 + 23.7 % ) and UV-selected galaxies ( 2 . 2 − 1.0 + 1.7 % ) at z > 4. The identification of narrow high-ionization lines in the population of LRDs with strong Balmer absorption suggests that the dense neutral hydrogen gas may not uniformly cover the nucleus. The strong N v (coupled with weak C iv and He ii) suggests that efforts to identify high-ionization lines should extend down in wavelength to the N v doublet.