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.
| Original language | English |
|---|---|
| Pages (from-to) | 1652-1665 |
| Number of pages | 14 |
| Journal | Monthly Notices of the Royal Astronomical Society |
| Volume | 499 |
| Issue number | 2 |
| DOIs | |
| State | Published - Dec 1 2020 |
Bibliographical note
Publisher Copyright:© 2020 The Author(s).
Funding
We thank the anonymous referee for their constructive comments. We acknowledge support from National Science Foundation (NSF) Astronomy and Astrophysics Research Grants (AAG) AST1312780, 1312547, 1312764, and 1313171, grant AR13907 from the Space Telescope Science Institute, and grant NNX16AF54G from the NASA Astrophysics Data Analysis Program (ADAP) program. We also acknowledge a NASA contract supporting the 'Wide Field InfraRed Survey Telsescope (WFIRST) Extragalactic Potential Observations (EXPO) Science Investigation Team' (15-WFIRST15- 0004), administered by Goddard Space Flight Center (GSFC). This work made use of v2.2.1 of the Binary Population and Spectral Synthesis (BPASS) models as described in Eldridge et al. 2017 and Stanway & Eldridge (2018). We wish to extend special thanks to those of Hawaiian ancestry on whose sacred mountain we are privileged to be guests. Without their generous hospitality, most of the observations presented herein would not have been possible. This work is based on data obtained at the W.M. Keck Observatory, which is operated as a scientific partnership among the California Institute of Technology, the University of California, and the National Aeronautics and Space Administration, and was made possible by the generous financial support of the W.M. Keck Foundation.
| Funders | Funder number |
|---|---|
| National Science Foundation (NSF) | AST1312780, AR13907 |
| Directorate for Mathematical and Physical Sciences | 1312764, 1313171, 1312547, 1312780 |
| National Aeronautics and Space Administration | 15-WFIRST15- 0004 |
| W. M. Keck Foundation | |
| University of California, Los Angeles | |
| Goddard Space Flight Center | |
| Space Telescope Science Institute | NNX16AF54G |
Keywords
- galaxies: ISM
- galaxies: evolution
- galaxies: high-redshift
ASJC Scopus subject areas
- Astronomy and Astrophysics
- Space and Planetary Science