The MOSDEF-LRIS Survey: The connection between massive stars and ionized gas in individual galaxies at z ∼ 2

Abstract

We present constraints on the massive star and ionized gas properties for a sample of 62 star-forming galaxies at z ∼ 2.3. Using BPASS stellar population models, we fit the rest-UV spectra of galaxies in our sample to estimate age and stellar metallicity which, in turn, determine the ionizing spectrum. In addition to the median properties of well-defined subsets of our sample, we derive the ages and stellar metallicities for 30 high-SNR individual galaxies – the largest sample of individual galaxies at high redshift with such measurements. Most galaxies in this high-SNR subsample have stellar metallicities of 0.001 < Z* < 0.004. We then use Cloudy+BPASS photoionization models to match observed rest-optical line ratios and infer nebular properties. Our high-SNR subsample is characterized by a median ionization parameter and oxygen abundance, respectively, of log (U)med = −2.98 ± 0.25 and 12 + log (O/H)med = 8.48 ± 0.11. Accordingly, we find that all galaxies in our sample show evidence for α-enhancement. In addition, based on inferred log (U) and 12 + log (O/H) values, we find that the local relationship between ionization parameter and metallicity applies at z ∼ 2. Finally, we find that the high-redshift galaxies most offset from the local excitation sequence in the BPT diagram are the most α-enhanced. This trend suggests that α-enhancement resulting in a harder ionizing spectrum at fixed oxygen abundance is a significant driver of the high-redshift galaxy offset on the BPT diagram relative to local systems. The ubiquity of α-enhancement among z ∼ 2.3 star-forming galaxies indicates important differences between high-redshift and local galaxies that must be accounted for in order to derive physical properties at high redshift.

The NIRVANDELS Survey: a robust detection of α-enhancement in star-forming galaxies at z ≃ 3.4

“…(O/Fe) to > 5 × (O/Fe) . A direct comparison to the Topping et al (2020b) results is shown in Fig. 9 where it can be seen that the basic result (i.e., g > ★ ) is fully consistent with the result presented here.…”

“…However, when interpreting the redshift evolution of emission lines, the known evolution towards more extreme H region conditions at high redshift must also be considered (e.g., Steidel et al 2014;Shapley et al 2015). Most recent results, as well as the results presented in this paper, suggest that the primary cause of this evolution is the harder ionizing spectra emitted by oxygen-enhanced, low-metallicity (i.e., iron-poor), stars at high redshift (e.g., Steidel et al 2016;Strom et al 2017;Topping et al 2020b, see Section 4.3 for further discussion). As a consequence, not all of the observed redshift evolution in line ratios can be attributed purely to changes in g (e.g., Cullen et al 2016).…”

“…Moreover, below a certain threshold S/N estimates of ★ can become biased. For example, Topping et al (2020b) find that unbiased estimates of ★ require S/N ≥ 5.6 per resolution element (five of the six galaxies with individual ★ estimates satisfy this slightly higher threshold). Stellar metallicity estimates resulting from this procedure are listed in Tables 2 and 3, and examples of fits to the individual and stacked FUV spectra are shown in Figs.…”

“…In this sense, O/H in ionized gas should also be representative of O/H in massive stars. Therefore, to a reasonable approximation, combining optically-derived estimates of g with FUV-based estimates of ★ traces the O/Fe abundance ratio of the massive stellar populations in star-forming galaxies (Steidel et al 2016;Topping et al 2020b). More generally, because O is an − process element, the observed O/Fe enhancement traces the enhancement of elements, and is also referred to as − enhancement.…”

“…Large filled square data points represent the low-★ and high-★ stacks. The grey circular data points show the data for star-forming galaxies at 2.3 presented in Topping et al (2020b). The various lines show constant g / ★ ratios as indicated by the labels, with the solid line representing the one-to-one relation.…”