TY - JOUR
T1 - The FMOS-COSMOS Survey of Star-forming Galaxies at z ≈ 1.6. IV. Excitation State and Chemical Enrichment of the Interstellar Medium
AU - Kashino, D.
AU - Silverman, John D.
AU - Sanders, D.
AU - Maier, Christian
N1 - Publisher Copyright:
© 2017. The American Astronomical Society. All rights reserved.
PY - 2017/1/20
Y1 - 2017/1/20
N2 - We investigate the physical conditions of ionized gas in high-z star-forming galaxies using diagnostic diagrams based on the rest-frame optical emission lines. The sample consists of 701 galaxies with an Hα detection at 1.4≲ z≲ 1.7, from the Fiber Multi-Object Spectrograph (FMOS)-COSMOS survey, that represent the normal star-forming population over the stellar mass range 109.6≲ M* /M⊙ ≲ 1011.6, with those at M* > 1011 M⊙ being well sampled. We confirm an offset of the average location of star-forming galaxies in the Baldwin-Phillips-Terlevich (BPT) diagram ([O III]/Hβ versus [N II]/Hα ), primarily toward higher [O III]/Hβ , compared with local galaxies. Based on the [S II] ratio, we measure an electron density (ne=220-130+170 cm-3), which is higher than that of local galaxies. Based on comparisons to theoretical models, we argue that changes in emission-line ratios, including the offset in the BPT diagram, are caused by a higher ionization parameter both at fixed stellar mass and at fixed metallicity, with additional contributions from a higher gas density and possibly a hardening of the ionizing radiation field. Ionization due to active galactic nuclei is ruled out as assessed with Chandra. As a consequence, we revisit the mass-metallicity relation using [N II]/Hα and a new calibration including [N II]/[S II] as recently introduced by Dopita et al. Consistent with our previous results, the most massive galaxies (M* ≳ 1011 M⊙ ) are fully enriched, while those at lower masses have metallicities lower than local galaxies. Finally, we demonstrate that the stellar masses, metallicities, and star formation rates of the FMOS sample are well fit with a physically motivated model for the chemical evolution of star-forming galaxies.
AB - We investigate the physical conditions of ionized gas in high-z star-forming galaxies using diagnostic diagrams based on the rest-frame optical emission lines. The sample consists of 701 galaxies with an Hα detection at 1.4≲ z≲ 1.7, from the Fiber Multi-Object Spectrograph (FMOS)-COSMOS survey, that represent the normal star-forming population over the stellar mass range 109.6≲ M* /M⊙ ≲ 1011.6, with those at M* > 1011 M⊙ being well sampled. We confirm an offset of the average location of star-forming galaxies in the Baldwin-Phillips-Terlevich (BPT) diagram ([O III]/Hβ versus [N II]/Hα ), primarily toward higher [O III]/Hβ , compared with local galaxies. Based on the [S II] ratio, we measure an electron density (ne=220-130+170 cm-3), which is higher than that of local galaxies. Based on comparisons to theoretical models, we argue that changes in emission-line ratios, including the offset in the BPT diagram, are caused by a higher ionization parameter both at fixed stellar mass and at fixed metallicity, with additional contributions from a higher gas density and possibly a hardening of the ionizing radiation field. Ionization due to active galactic nuclei is ruled out as assessed with Chandra. As a consequence, we revisit the mass-metallicity relation using [N II]/Hα and a new calibration including [N II]/[S II] as recently introduced by Dopita et al. Consistent with our previous results, the most massive galaxies (M* ≳ 1011 M⊙ ) are fully enriched, while those at lower masses have metallicities lower than local galaxies. Finally, we demonstrate that the stellar masses, metallicities, and star formation rates of the FMOS sample are well fit with a physically motivated model for the chemical evolution of star-forming galaxies.
KW - ACTIVE GALACTIC NUCLEI
KW - CHANDRA-DEEP-FIELDS
KW - DIGITAL SKY SURVEY
KW - EMISSION-LINE GALAXIES
KW - GREATER-THAN 2
KW - H-II REGIONS
KW - HIGH-REDSHIFT GALAXIES
KW - MASS-METALLICITY RELATION
KW - NEAR-INFRARED SPECTROSCOPY
KW - POINT-SOURCE CATALOG
KW - galaxies: ISM
KW - galaxies: abundances
KW - galaxies: evolution
KW - galaxies: high-redshift
KW - galaxies: star formation
UR - http://www.scopus.com/inward/record.url?scp=85011294832&partnerID=8YFLogxK
U2 - 10.3847/1538-4357/835/1/88
DO - 10.3847/1538-4357/835/1/88
M3 - Article
SN - 0004-637X
VL - 835
JO - The Astrophysical Journal
JF - The Astrophysical Journal
IS - 1
M1 - 88
ER -