A novel approach to correcting $T_e$-based mass-metallicity relations
Two modes of LyC escape from bursty star formation: implications for [C II] deficits and the sources of reionization
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 fesc>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 M1500Å 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 fesc>20 per cent dominate the ionizing photon budget at z ≳ 7.5 but the contribution from galaxies with fesc<5 per cent becomes significant at the tail-end of reionization.
SDSS-IV MaStar: [α/Fe] for the MaNGA Stellar Library from synthetic model spectra
Abstract:
We calculate the α-enhancement ratio [α/Fe] for the Mapping Nearby Galaxies at APO (MaNGA) Stellar Library (MaStar) while also fitting for the fundamental atmospheric parameters effective temperature, surface gravity, and metallicity – Teff, log g, [Fe/H]. This approach builds upon a previous catalogue of stellar parameters, whereby only the fundamental atmospheric parameters are fit with solar-scaled models. Here, we use the same Markov Chain Monte Carlo method with the additional free parameter [α/Fe]. Using the full spectral fitting code pPXF, we are able to fit multiple lines sensitive to [α/Fe] for a more robust measurement. Quality flags based on the convergence of the sampler, errors in [α/Fe] and a cut in the χ2 of the model fit are used to clean the final catalogue, returning 17 214 spectra and values in the range of −0.25 < [α/Fe] < 0.48. Comparing our calculated [α/Fe] with literature values reveals a degeneracy in cool stars with log g ≥ ∼4; this comparison is then used to create an alternative and calibrated parameter set. We also plot the final catalogue in an [Fe/H] versus [α/Fe] diagram and recover the expected result of increasing [α/Fe] with decreasing [Fe/H] for Milky Way disc-halo stars. We apply our method to a subsample of spectra of uniform resolution and higher signal to noise that finds that our results are independent of this higher signal to noise. In the context of stellar population models, we are able to cover a parameter space for the creation of intermediate to old age models at solar-scaled [α/Fe], high [Fe/H] and enhanced [α/Fe], low [Fe/H].