The BPT diagram and mass-metallicity relation at z~2.3:

Insights from KBSS-MOSFIRE

Steidel et al. (2014) - Strong nebular line ratios in the spectra of z=2-3 star-forming galaxies: First results from KBSS-MOSFIRE - arXiv:1405.5473

• Obtained rest-frame optical spectroscopy of 251 emission-line galaxies between 2.0 < z < 2.6 from Keck.

• 8.6 < log(M*/Mʘ) < 11.4 2 < SFR [Mʘ/yr] < 200 7.9 ≤ Zg(O3N2) ≤ 8.6

• The star-forming sequence on the BPT diagram at z~2.3 is shifted upwards with respect to that at z=0.

• This is due to a) higher ionisation parameter, b) harder ionising radiation field (i.e. higher Teff), and c) higher N/O.

• The mass-metallicity relation (MZR) at z~2.3 is lower than that at z=0 by ~0.32 dex, at all stellar masses.

• The dependence of Zg on SFR is much weaker than at z=0, suggesting no FMR extension to these redshifts.Fig. 1 Fig. 2

1

The BPT diagram at z~2.3:

Fig. 3

• High-z galaxies (symbols) lie above the z=0 relation from SDSS (grey points). See also Brinchmann+08, Kewley+13b.

• The scatter around the best fit (orange line) is similar to that at z=0 (~0.1 dex).

• This shift away from the local relation implies that locally calibrated strong-line diagnostics will give inconsistent values of Zg at higher redshift.

• This is because galaxies no longer lie on the expected 1D relations (red curves)…

BPT diagram at z=0:

• Can be used to distinguish star-forming galaxies (along main ridge, e.g. red lines) from AGN hosts (high [OIII]/Hβ and high [NII]/Hα). See e.g. Kewley+01, Kauffmann+03c.

• Also tells us about metallicity distribution, as Zg for SF galaxies increases to the bottom-right of the plot.

2

Increasing Zg

The BPT diagram at z~2.3:

• …for example, the N2 diagnostic provides a higher Zg than the O3N2 diagnostic at z~2.3, even when calibrated to the same low-z sample of Te galaxies (Pettini & Pagel 04).

• Therefore, conversions between different diagnostics calibrated at z=0 (e.g. Kewley & Ellison 08) are not applicable at higher z (see also Cullen+14). This is a problem for studies of MZR and FMR evolution (e.g. Maiolino+08; Mannucci+10).

Fig. 4

3

• Using photoionisation models, Steidel+14 found that both higher ionisation parameter, Γ, and higher Teff are required to reproduce observations at z~2.3 (i.e. match Figs. 3 and 4).

• ne=1000 cm-3, -2.9 < log(Γ) < -1.8, and Teff~50000 K are required.

• However, note the small dependence of BPT position on Zg at fixed Γ in Fig. 5… Are strong-line diagnostics mainly tracing Γ and/or Teff at high-z?

Fig. 5

Z/Zʘ = 0.2 Z/Zʘ = 1.0

O/H dependence on N/O:

• There is evidence that O/H is nearly independent of N/O at high z, unlike the positive correlation assumed at low z.

• When assuming no N/O dependence in the photoionisation models, the sensitivity of the N2 diagnostic to Zg is weakened (Fig. 7).

• The stronger N/O-dependence of the N2 diagnostic causes the over-estimate of Zg at high z relative to the O3N2 diagnostic.

• Recalibrations of the low-z diagnostics specifically for high z give good correspondence with Te-based metallicities.

4

Fig. 6

Fig. 7

The MZR at z~2.3:

5

• Assuming that the (locally calibrated) O3N2 diagnostic is better (as it has a weaker N/O dependence and closer correspondence to the few Te metallicities available), the MZR at z~2.3 is plotted (Fig. 8).

• An offset of around -0.32 dex in Zg from the z=0 MZR is found, similar to the average offset found by Erb+06a using N2.

• However, no clear mass-dependence in the Zg offset is seen (see also Moustakas+11).

• This is in contradiction to the ‘chemical downsizing’ claimed by other works using other diagnostics, e.g. locally-calibrated N2 and R23 (e.g. Maiolino+08, Zahid+13b).

• Compared to Erb+06a metallicities: a) Lower Zg at high M* due to weaker N/O dependence. b) Higher Zg at low M* due to better correspondence with Te metallicities (i.e. higher SNR).

Fig. 8

The MZR at z~2.3:

• The scatter in the z~2.3 MZR is remarkably small (σsc~0.10 dex), similar to that at z=0. Note that the diagnostic used (even when calibrated to be optimal at high-z) has a larger scatter (σO3N2~0.14 dex).

• This suggests that there should be an even tighter correlation between M* and line intensity (therefore, Γ) than between M* and Zg…

• Also, there is no clear dependence of Zg on SFR at fixed mass in the MOSFIRE sample (Fig. 9). This suggests the FMR doesn’t hold at these high redshifts/SFRs…

Zahid+14Moustakas+11

6Is chemical downsizing really occurring in galaxies?...

Fig. 10 Fig. 11

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Fig. 12

Fig. 9

Conclusions:

7

• Very young stellar populations, or very top-heavy IMF are not required to reproduce shifted BPT diagram at high redshift. Instead, binarity and fast rotation of low-metallicity, massive stars provide the conditions needed (i.e. high ionisation parameter and high Teff, and increased primary N production).

• Eight AGN hosts (determined by their far-UV, mid-IR, and X-ray properties) do not lie within the star-forming sequence of the BPT diagram at z~2.3. Therefore, this diagram can still be used to distinguish AGN optically.

• Strong-line diagnostics are likely tracing ionisation parameter more than Zg at high redshift. However, the O3N2 diagnostic (re-calibrated at z=0) appears to be the most accurate available currently, due to weaker N/O dependence and closer correspondence to Te-method metallicities.

• Metallicites from strong-line diagnostics differ from each other in different ways at low and high redshift. Therefore, conversions between diagnostics calibrated at z=0 won’t work at high redshift.

• No mass-dependent MZR evolution from z~2.3 to z=0 found. Previous high-z metallicity estimates using N2 and R23 are likely to be less accurate than the O3N2 diagnostic used here (which, in turn, is worse than an ‘ideal’ high-z-calibrated diagnostic, or Te metallicities).

• Uncomfortably small scatter in the MZR at all redshifts (compared to the error in the metallicity diagnostics used) suggests that a) a low range of Teff should be present at each epoch, and b) there should be a more fundamental, monotonic relation between M* and Γ.

• The fundamental metallicity relation (FMR) does not match the distribution of the z~2.3 sample in M*-SFR-Zg space. Nor does this projection onto this space reduce the scatter compared to the MZR (see also Cullen+14).

• Te-based metallicites, using either weak optical lines or rest-UV intercombination lines (e.g. Erb+10), and high-z calibrations of strong-line ratios from them, are the best ways forward for studying Zg at high redshift.