Constraining the Metallicities, Ages, Star Formation Histories, and Ionizing Continua of Extragalactic Massive Star Populations^∗

Abstract

We infer the properties of massive star populations using the far-ultraviolet stellar continua of 61 star-forming galaxies: 42 at low redshift observed with the Hubble Space Telescope and 19 at z∼2 from the MEGaSaURA sample. We fit each stellar continuum with a linear combination of up to 50 single-age and single-metallicity STARBURST99 models. From these fits, we derive light-weighted ages and metallicities, which agree with stellar wind and photospheric spectral features, and infer the spectral shapes and strengths of the ionizing continua. Inferred light-weighted stellar metallicities span 0.05-1.5 Z e and are similar to the measured nebular metallicities. We quantify the ionizing continua using the ratio of the ionizing flux at 900 Å to the non-ionizing flux at 1500 Å and demonstrate the evolution of this ratio with stellar age and metallicity using theoretical single-burst models. These single-burst models only match the inferred ionizing continua of half of the sample, while the other half are described by a mixture of stellar ages. Mixed-age populations produce stronger and harder ionizing spectra than continuous star formation histories, but, contrary to previous studies that assume constant star formation, have similar stellar and nebular metallicities. Stellar population age and metallicity affect the far-UV continua in different and distinguishable ways; assuming a constant star formation history diminishes the diagnostic power. Finally, we provide simple prescriptions to determine the ionizing photon production efficiency (ξ ion ) from the stellar population properties. The ξ ion inferred from the observed star-forming galaxies has a range of log(ξ ion )=24.4-25.7 Hz erg −1 that depends on the stellar population age, metallicity, star formation history, and contributions from binary star evolution. These stellar population properties must be observationally determined to accurately determine the number of ionizing photons generated by massive stars.

Gravitational lensing reveals ionizing ultraviolet photons escaping from a distant galaxy

“…To test how strong the assumptions built in to the stellar models affect the derived escape fractions, we constructed an alternative model based on the BPASS library (44). The best-fit BPASS model (53) has age, metallicity and non-ionizing UV spectral shape consistent with the STARBURST99 model, but predicts a higher intrinsic LyC flux (see figure S6). We infer a lower relative and absolute escape fraction than from STARBURST99, with f BPASS esc,rel = 76 +17 −8 %, and f BPASS esc,abs = 24 +5 −3 %.…”

Lyman continuum escape fraction in Ly α emitters at z ≃ 3.1

“…L 1500 /L 820 is the intrinsic luminosity density ratio between 1500Å and 820Å. Here, we adopt two L 1500 /L 820 values, L 1500 /L 820 = 3 and L 1500 /L 820 = 7, to cover a wide range of possible star formation history (e.g., Siana et al 2007;Chisholm et al 2019). exp(τ IGM,820 ) ≡ 1/T is the mean IGM opacity at 820Å at z = 3.1 (e.g., Madau 1995), and T is the mean IGM transmission.…”

Massive Star Formation in the Ultraviolet Observed with the Hubble Space Telescope

“…The integrated spectra of young massive star clusters and star forming galaxies hold the key to revealing the content of massive stars. A comprehensive analysis of the far-UV spectra of 61 star-forming galaxies has been performed by [41]. A major tool for determining the relevant properties is population synthesis, which is the topic of the next section.…”