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arXiv · 1409.7071

The MOSDEF Survey: Excitation Properties of $z\sim 2.3$ Star-forming Galaxies

Alice E. Shapley·Naveen A. Reddy·Mariska Kriek·William R. Freeman·Ryan L. Sanders·Brian Siana·Alison L. Coil·Bahram Mobasher·Irene Shivaei·Sedona H. Price·Laura de Groot
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Abstract

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 [OIII]/Hb vs. [NII]/Ha and [OIII]/Hb vs. [SII]/Ha "BPT" diagrams, as well as the O_32 vs. R_23 excitation diagram. We recover the well-known offset in the star-forming sequence of high-redshift galaxies in the [OIII]/Hb vs. [NII]/Ha BPT diagram relative to SDSS 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 towards higher [NII]/Ha at fixed [OIII]/Hb. Within the [OIII]/Hb vs. [SII]/Ha and O_32 vs. R_23 diagrams, we find that z~2.3 galaxies are distributed like local ones, and therefore attribute the shift in the [OIII]/Hb vs. [NII]/Ha BPT diagram to elevated N/O abundance ratios among lower-mass (M_*<10^10 M_sun) 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_32 vs. R_23 diagram paves the way for using the combination of O_32 and R_23 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.

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Alice E. Shapley, Naveen A. Reddy, Mariska Kriek, William R. Freeman, Ryan L. Sanders, Brian Siana, Alison L. Coil, Bahram Mobasher, Irene Shivaei, Sedona H. Price, Laura de Groot. 2015-01-13. The MOSDEF Survey: Excitation Properties of $z\sim 2.3$ Star-forming Galaxies. https://doi.org/10.1088/0004-637x%2F801%2F2%2F88

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