The relation between [O III]/Hβ and specific star formation in galaxies at z ∼ 2

Journal Article · Thu Sep 01 04:00:00 EDT 2016 · Astrophysical Journal Letters

DOI:https://doi.org/10.3847/2041-8205/828/1/L11· OSTI ID:22868736

Dickey, Claire Mackay; Van Dokkum, Pieter G.; Nelson, Erica J.; Leja, Joel ^[1]; Oesch, Pascal A. ^[2]; Whitaker, Katherine E. ^[3]; Momcheva, Ivelina G.; Brammer, Gabriel B. ^[4]; Franx, Marijn ^[5]

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1. Department of Astronomy, Yale University, New Haven, CT 06520 (United States)
2. Yale Center for Astronomy and Astrophysics, Department of Astronomy, Yale University, New Haven, CT 06520 (United States)
3. Department of Astronomy, University of Massachusetts, Amherst, MA 01003 (United States)
4. Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218 (United States)
5. Leiden Observatory, Leiden University, Leiden (Netherlands)

Recent surveys have identified a seemingly ubiquitous population of galaxies with elevated [O iii]/Hβ emission line ratios at z > 1, although the nature of this phenomenon continues to be debated. The [O iii]/Hβ line ratio is of interest because it is a main component of the standard diagnostic tools used to differentiate between active galactic nuclei and star-forming galaxies as well as the gas-phase metallicity indicators O {sub 23} and R {sub 23}. Here, we investigate the primary driver of increased [O iii]/Hβ ratios by median-stacking rest-frame optical spectra for a sample of star-forming galaxies in the 3D-HST survey in the redshift range z ∼ 1.4–2.2. Using N = 4220 star-forming galaxies, we stack the data in bins of mass and specific star formation rates (sSFRs), respectively. After accounting for stellar Balmer absorption, we measure [O iii]λ5007 Å/Hβ down to M ∼ 10{sup 9.2} M {sub ⊙} and sSFR ∼ 10{sup −9.6} yr{sup −1}, greater than an order of magnitude lower than previous work at similar redshifts. We find an offset of 0.59 ± 0.05 dex between the median ratios at z ∼ 2 and z ∼ 0 at fixed stellar mass, in agreement with existing studies. However, with respect to sSFR, the z ∼ 2 stacks all lie within 1σ of the median SDSS ratios, with an average offset of only −0.06 ± 0.05. We find that the excitation properties of galaxies are tightly correlated with their sSFR at both z ∼ 2 and z ∼ 0, with a relation that appears to be roughly constant over the last 10 Gyr of cosmic time.

• OSTI ID: 22868736
• Journal Information: Astrophysical Journal Letters, Journal Name: Astrophysical Journal Letters Journal Issue: 1 Vol. 828; ISSN 2041-8205
• Country of Publication: United States
• Language: English

Related Subjects

79 ASTRONOMY AND ASTROPHYSICS
EMISSION SPECTRA
EXCITATION
GALACTIC EVOLUTION
GALAXIES
GALAXY NUCLEI
MASS
METALLICITY
RED SHIFT
STARS