Stellar properties determine the ionizing continua of massive stars

Ionizing photons from stars power nearly all of the observations of star-forming galaxies, but little is actually known about star's ionizing continua because it cannot be observed. Rather, we must use indirect observations (nebular emission lines or FUV observations) to infer the number and shape of the ionizing continua. FUV observations of massive stars are the most direct link to the ionizing continua because they describe the properites of the stars emitting ionizing photons. This is emphasized by theoretical models of the stellar continuum, which show that the ratio of the ionizing flux at 900Å to the non-ionizing flux at 1500Å strongly depends on both the age and metallicity of the stars (see the plot to the right). Consequently, if the stellar age and metallicity are constrained, the number of ionizing photons produced by stars can be predicted (this is crucial to determine the source of cosmic reionization). 

Massive star populations are either a single burst or a mixture of ages

The stellar continuum fits to the observed data illustrate that the light from massive stars either comes from a single burst of star formation or a mix of various ages. This is most easily seen by looking at how the ratio of the ionizing flux (at 900Å) to the non-ionizing flux (at 1500Å) scales with the fitted age of the stellar population (to the right). Half of our galaxies lie along the curves found for a theoretical stellar population (blue and gold curves), while the other half lie in a tight relationship consistent with a random mix of ages (gray line). 

​This "mixed age" population is similar in spirit to the conical "continuous star formation" model, but uses spectral features to determine the shape and strength of the ionizing photons. In fact, the continuous star formation model is only correct for a very small subset of models. FUV models of stellar populations can accurately determine the number of ionizing photons produced by stars.

​The figure to the right shows that even if a stellar population "appears" to be 10 Myr, it can still produce similar amounts of ionizing photons as a 3 Myr population. These observationally constrained fits have profound impacts for how we calculate star formation rates and determine the number of ionizing photons produced by stellar populations. 

Constraining Stellar Populations with FUV spectroscopy

 The far ultraviolet (1200-2000Å) hosts a variety of stellar lines that probe the age and metallicity of a stellar population. Specifically, these features arise in stellar winds (orange features in the left picture) and in stellar photospheres (green features). Stellar wind features are especially useful because their shape is determined independently by both the age and metallicity of the stars, while they are also broad and strong enough to efficiently diagnose the properties of massive stars, even at modest signal-to-noise ratios. 

Stars and gas have similar metallicities

By fitting theoretical models to the observed FUV continuum, I derived stellar metallicity estimates for a sample of low redshift and z = 2 galaxies. The ISM metallicities, from Oxygen emission lines, are very similar to the inferred stellar metallicities (see plot to the left). If observers want to determine the ionizing properties of stars, they can use ISM metallicities, which have been measured for decades in star-forming galaxies. 

The tight correlation means that the metallicities of galaxies can be inferred from the FUV stellar continuum. This is especially powerful at high-redshifts where optical emission lines that are typically used to determine the metallicity are redshifted out of the optical. This method promises to describe the metal enrichment history of the highest redshift galaxies.