Galaxy formation and evolution

Abstract:

Reliable indirect diagnostics of LyC photon escape from galaxies are required to understand which sources were the dominant contributors to reionization. While multiple LyC escape fraction (f_esc) indicators have been proposed to trace favourable conditions for LyC leakage from the interstellar medium of low-redshift ‘analogue’ galaxies, it remains unclear whether these are applicable at high redshifts where LyC emission cannot be directly observed. Using a library of 14 120 mock spectra of star-forming galaxies with redshifts 4.64 ≤ z ≤ 10 from the SPHINX²⁰ cosmological radiation hydrodynamics simulation, we develop a framework for the physics that leads to high f_esc. We investigate LyC leakage from our galaxies based on the criteria that successful LyC escape diagnostics must (i) track a high-specific star formation rate, (ii) be sensitive to stellar population age in the range 3.5–10 Myr representing the times when supernova first explode to when LyC production significantly drops, and (iii) include a proxy for neutral gas content and gas density in the interstellar medium. O₃₂, Σ_SFR, M_UV, and H β equivalent width select for one or fewer of our criteria, rendering them either necessary but insufficient or generally poor diagnostics. In contrast, UV slope (β), and E(B − V) match two or more of our criteria, rendering them good f_esc diagnostics (albeit with significant scatter). Using our library, we build a quantitative model for predicting f_esc based on direct observables. When applied to bright z > 6 Ly α emitters observed with JWST, we find that the majority of them have 𝑓esc≲10 per cent⁠.

Abstract:

We present the first measurement of HI mass of star-forming galaxies in different large scale structure environments from a blind survey at z ∼ 0.37. In particular, we carry out a spectral line stacking analysis considering 2875 spectra of colour-selected star-forming galaxies undetected in HI at 0.23 < z < 0.49 in the COSMOS field, extracted from the MIGHTEE-HI Early Science datacubes, acquired with the MeerKAT radio telescope. We stack galaxies belonging to different subsamples depending on three different definitions of large scale structure environment: local galaxy overdensity, position inside the host dark matter halo (central, satellite, or isolated), and cosmic web type (field, filament, or knot). We first stack the full star-forming galaxy sample and find a robust HI detection yielding an average galaxy HI mass of MHI = (8.12 ± 0.75) × 109 M⊙ at ∼11.8σ. Next, we investigate the different subsamples finding a negligible difference in MHI as a function of the galaxy overdensity. We report an HI excess compared to the full sample in satellite galaxies (MHI = (11.31 ± 1.22) × 109, at ∼10.2σ) and in filaments (MHI = (11.62 ± 0.90) × 109. Conversely, we report non-detections for the central and knot galaxies subsamples, which appear to be HI-deficient. We find the same qualitative results also when stacking in units of HI fraction (fHI). We conclude that the HI amount in star-forming galaxies at the studied redshifts correlates with the large scale structure environment.