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A Tight Relation between N/O Ratio and Galaxy Stellar Mass Can Explain the Evolution of Strong Emission Line Ratios with Redshift

Masters, Daniel and Faisst, Andreas and Capak, Peter (2016) A Tight Relation between N/O Ratio and Galaxy Stellar Mass Can Explain the Evolution of Strong Emission Line Ratios with Redshift. Astrophysical Journal, 828 (1). Art. No. 18. ISSN 0004-637X. doi:10.3847/0004-637X/828/1/18.

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The offset of high-redshift star-forming galaxies in the [O III]/Hβ versus [N II]/Hα (O3N2) diagram in comparison with the local star-forming galaxy sequence is now well established. The physical origin of the shift is the subject of some debate and has important implications for metallicity measurements based on strong lines at all redshifts. To investigate the origin of the O3N2 offset, we use a sample of ~100,000 star-forming galaxies from the Sloan Digital Sky Survey DR12 to understand how measurable galaxy physical properties (Σ_(SFR), ionization parameter, nitrogen-to-oxygen (N/O) ratio, and stellar mass) drive galaxy position in two key diagnostic diagrams: O3N2 and [O III]/Hβ versus [S II]/Hα (O3S2). At fixed [O III]/Hβ, galaxies close to the high-redshift locus in O3N2 have higher Σ_(SFR), stellar mass, and N/O ratio. We conclude that higher N/O ratios at fixed [O III]/Hβ are the proximate cause of the O3N2 shift. We also find a tight correspondence in the distributions of stellar mass and N/O in the diagnostic diagrams. This relation, spanning a range of galaxy evolutionary states, suggests that the N/O–M* relation is more fundamental than the relation between N/O and O/H. We argue that a more fundamental N/O–M* relation is well-motivated physically. Because the mass–metallicity relation evolves more rapidly with redshift than N/O–M*, the N/O ratios of high-redshift galaxies are elevated in comparison with local galaxies with the same gas-phase O/H. The O3N2 shift and elevated N/O ratios observed in high-redshift galaxies, therefore, come about as a natural consequence of the N/O–M* relation combined with the evolution of the mass–metallicity relation.

Item Type:Article
Related URLs:
URLURL TypeDescription Paper
Masters, Daniel0000-0001-5382-6138
Faisst, Andreas0000-0002-9382-9832
Capak, Peter0000-0003-3578-6843
Additional Information:© 2016 The American Astronomical Society. Received 2016 May 12; revised 2016 June 29; accepted 2016 July 5; published 2016 August 24. We thank the anonymous referee for a thoughtful and constructive report that improved this paper. D.M. is grateful to Alaina Henry, Samir Salim, Charles Steinhardt, John Silverman, Lin Yan, Jin Koda, and Louis Abramson for helpful and enjoyable conversations. D.M. acknowledges the organizers and participants of the "First Carnegie Symposium in Honor of Leonard Searle", held at the Carnegie Observatories in July 2015, for many enlightening talks and conversations. A.F. acknowledges support from the Swiss National Science Foundation.
Group:COSMOS, Infrared Processing and Analysis Center (IPAC)
Funding AgencyGrant Number
Swiss National Science Foundation (SNSF)UNSPECIFIED
Subject Keywords:galaxies: evolution; galaxies: high-redshift; galaxies: ISM; galaxies: abundances; HII regions; ISM: abundances
Issue or Number:1
Record Number:CaltechAUTHORS:20160826-132511755
Persistent URL:
Official Citation:Daniel Masters et al 2016 ApJ 828 18
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:69978
Deposited By: Tony Diaz
Deposited On:26 Aug 2016 21:45
Last Modified:11 Nov 2021 04:22

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