Galaxy formation and evolution

Abstract:

In the hierarchical view of star formation, giant molecular clouds (GMCs) undergo fragmentation to form small-scale structures made up of stars and star clusters. Here we study the connection between young star clusters and cold gas across a range of extragalactic environments by combining the high resolution (1″) PHANGS-ALMA catalogue of GMCs with the star cluster catalogues from PHANGS-HST. The star clusters are spatially matched with the GMCs across a sample of 11 nearby star-forming galaxies with a range of galactic environments (centres, bars, spiral arms, etc.). We find that after 4 - 6 Myr the star clusters are no longer associated with any gas clouds. Additionally, we measure the autocorrelation of the star clusters and GMCs as well as their cross-correlation to quantify the fractal nature of hierarchical star formation. Young (≤10 Myr) star clusters are more strongly autocorrelated on kpc and smaller spatial scales than the >, 10 Myr stellar populations, indicating that the hierarchical structure dissolves over time.

Abstract:

We use the SPHINX²⁰ cosmological radiation hydrodynamics simulation to study how Lyman continuum (LyC) photons escape from galaxies and the observational signatures of this escape. We define two classes of LyC leaker: Bursty Leakers and Remnant Leakers, based on their star formation rates (SFRs) that are averaged over 10 Myr (SFR₁₀) or 100 Myr (SFR₁₀₀). Both have f_esc>20 per cent and experienced an extreme burst of star formation, but Bursty Leakers have SFR₁₀ > SFR₁₀₀, while Remnant Leakers have SFR₁₀ < SFR₁₀₀. The maximum SFRs in these bursts were typically ∼100 times greater than the SFR of the galaxy prior to the burst, a rare 2σ outlier among the general high-redshift galaxy population. Bursty Leakers are qualitatively similar to ionization-bounded nebulae with holes, exhibiting high ionization parameters and typical H II region gas densities. Remnant Leakers show properties of density-bounded nebulae, having normal ionization parameters but much lower H II region densities. Both types of leaker exhibit [C II]_158μm deficits on the [C II]–SFR₁₀₀ relation, while only Bursty Leakers show deficits when ₁₀ is used. We predict that [C II] luminosity and SFR indicators such as Hα and M_1500Å can be combined to identify both types of LyC leaker and the mode by which photons are escaping. These predictions can be tested with [C II] observations of known z = 3–4 LyC leakers. Finally, we show that leakers with f_esc>20 per cent dominate the ionizing photon budget at z ≳ 7.5 but the contribution from galaxies with f_esc<5 per cent becomes significant at the tail-end of reionization.