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

الأصل · EN

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 O₃2 vs. R₂3 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 O₃2 vs. R₂3 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*<10¹0 Mₛun) 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 O₃2 vs. R₂3 diagram paves the way for using the combination of O₃2 and R₂3 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.