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The mosdef survey: Excitation properties of z ∼ 2.3 star-forming galaxies

  • Author(s): Shapley, AE
  • Reddy, NA
  • Kriek, M
  • Freeman, WR
  • Sanders, RL
  • Siana, B
  • Coil, AL
  • Mobasher, B
  • Shivaei, I
  • Price, SH
  • Groot, LD
  • et al.

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We present results on the excitation properties of z ∼ 2.3 galaxies using early observations from the MOSFIRE Deep Evolution Field (MOSDEF) Survey. With its coverage of the full suite of strong rest-frame optical emission lines, MOSDEF provides an unprecedented view of the rest-frame optical spectra of a representative sample of distant star-forming galaxies. We investigate the locations of z ∼ 2.3 MOSDEF galaxies in multiple emission-line diagnostic diagrams. These include the [O iii]λ5007/Hβ vs. [N ii]/Hα and [O iii]λ5007/Hβ vs. [S ii]λλ6717, 6731/Hα "BPT" diagrams, as well as the O vs. R excitation diagram. We recover the well-known offset in the star-forming sequence of high-redshift galaxies in the [O iii]λ5007/Hβ vs. [N ii]/Hα BPT diagram relative to Sloan Digital Sky Survey star-forming galaxies. However, the shift for our rest-frame optically selected sample is less significant than for rest-frame-UV selected and emission-line selected galaxies at z ∼ 2. Furthermore, we find that the offset is mass-dependent, only appearing within the low-mass half of the z ∼ 2.3 MOSDEF sample, where galaxies are shifted toward higher [N ii]/Hα at fixed [O iii]/Hβ. Within the [O iii]λ5007/Hβ vs. [S ii]/Hα and O vs. R diagrams, we find that z ∼ 2.3 galaxies are distributed like local ones, and therefore attribute the shift in the [O iii]λ5007/Hβ vs. [N ii]/Hα BPT diagram to elevated N/O abundance ratios among lower-mass () high-redshift galaxies. The variation in N/O ratios calls into question the use at high redshift of oxygen abundance indicators based on nitrogen lines, but the apparent invariance with redshift of the excitation sequence in the O vs. R diagram paves the way for using the combination of O and R as an unbiased metallicity indicator over a wide range in redshift. This indicator will allow for an accurate characterization of the shape and normalization of the mass-metallicity relationship over more than 10 Gyr. 32 23 32 23 32 23 32 23

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