THE MOSDEF SURVEY: ELECTRON DENSITY AND IONIZATION PARAMETER AT z ∼ 2.3

By R. Sanders, A. Shapley, M. Kriek, Naveen A. Reddy, William Robert Freeman, A. Coil, B. Siana, B. Mobasher, I. Shivaei, S. Price and 1 more. 2015

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Using observations from the MOSFIRE Deep Evolution Field survey, we investigate the physical conditions of star-forming regions in z ∼ 2.3 galaxies, specifically the electron density and ionization state. From measurements of the [O ii]λλ3726,3729 and [S ii]λλ6716,6731 doublets, we find a median electron density of ∼250 cm−3 at z ∼ 2.3, an increase of an order of magnitude compared to measurements of galaxies at z ∼ 0. While z ∼ 2.3 galaxies are offset toward significantly higher O32 values relative to local galaxies at fixed stellar mass, we find that the high-redshift sample follows a similar distribution to the low-metallicity tail of the local distribution in the O32 versus R23 and O3N2 diagrams. Based on these results, we propose that z ∼ 2.3 star-forming galaxies have the same ionization parameter as local galaxies at fixed metallicity. In combination with simple photoionization models, the position of local and z ∼ 2.3 galaxies in excitation diagrams suggests that there is no significant change in the hardness of the ionizing spectrum at fixed metallicity from z ∼ 0 to z ∼ 2.3. We find that z ∼ 2.3 galaxies show no offset compared to low-metallicity local galaxies in emission line ratio diagrams involving only lines of hydrogen, oxygen, and sulfur, but show a systematic offset in diagrams involving [N ii]λ6584. We conclude that the offset of z ∼ 2.3 galaxies from the local star-forming sequence in the [N ii] BPT diagram is primarily driven by elevated N/O at fixed O/H compared to local galaxies. These results suggest that the local gas-phase and stellar metallicity sets the ionization state of star-forming regions at z ∼ 0 and z ∼ 2.

Subfields: Astronomy and Astrophysics, Instrumentation

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Citations in the corpus

Stellar population effects on the inferred photon density at reionization (2015, methodology): “…low mass star forming galaxies of the distant Universe (Stanway et al. 2014), although the evidence for these at intermediate redshifts (z ∼ 2− 3) and whether instead they may represent a shift in nitrogen abundance is still under discussion (e.g. see Sanders et al. 2015, and references therein).”
RECONCILING THE STELLAR AND NEBULAR SPECTRA OF HIGH-REDSHIFT GALAXIES (2016, background): “Anomalously high N/O at a given O/H has been advocated as the primary driver of the N2 BPT “offset” (Masters et al. Stellar and Nebular Spectra at z∼ 2.4 27 2014; Shapley et al. 2015; Sanders et al. 2015; Jones et al. 2015).”
Nebular Emission Line Ratios in z ≃ 2–3 Star-forming Galaxies with KBSS-MOSFIRE: Exploring the Impact of Ionization, Excitation, and Nitrogen-to-Oxygen Ratio (2016, background): “Galaxies at z ∼ 2.3 also have lower 12+log(O/H) than SDSS galaxies by a comparable amount (Steidel et al. 2014; Wuyts et al. 2014; Sanders et al. 2015), suggesting that there is a relatively constant N/O-O/H relation for all galaxies.”
De re metallica: the cosmic chemical evolution of galaxies (2018, methodology): “…Finkelstein et al., 2011a; Wuyts et al., 2012; Erb et al., 2010; Cullen et al., 2014; Zahid et al., 2014c; Wuyts et al., 2014b; Steidel et al., 2014; Sanders et al., 2015; Onodera et al., 2015; Sanders et al., 2016b; Bian et al., 2017; Sanders et al., 2018) and z ∼ 3 (Maiolino et al., 2008;…”
SDSS IV MaNGA – spatially resolved diagnostic diagrams: a proof that many galaxies are LIERs (2016, background): “Hence, the ionisation parameter can be seen as the dimensionless ratio between the number density of hydrogen-ionising photons and the number density of the total hydrogen gas (including ionised, neutral and molecular, as defined in Osterbrock & Ferland 2006, see also Sanders et al. 2015).”