Disentangling the physical parameters of gaseous nebulae and galaxies

Daichi Kashino; Akio K. Inoue

doi:10.1093/mnras/stz881

paper

Disentangling the physical parameters of gaseous nebulae and galaxies

arXiv:1812.06939 · doi:10.1093/mnras/stz881

Abstract

We present an analysis to disentangle the connection between physical quantities that characterize the conditions of ionized HII regions -- metallicity ( $Z$ ), ionization parameter ( $U$ ), and electron density ( $n_{e}$ ) -- and the global stellar mass ( $M_{*}$ ) and specific star formation rate ( $sSFR = SFR / M_{*}$ ) of the host galaxies. We construct composite spectra of galaxies at $0.027 \leq z \leq 0.25$ from Sloan Digital Sky Survey, separating the sample into bins of $M_{*}$ and sSFR, and estimate the nebular conditions from the emission line flux ratios. Specially, metallicity is estimated from the direct method based on the faint auroral lines [OIII] $λ$ 4363 and [OII] $λλ$ 7320,7330. The metallicity estimates cover a wide range from $12 + lo g O/H \sim 7.6 - 8.9$ . It is found that these three nebular parameters all are tightly correlated with the location in the $M_{*}$ --sSFR plane. With simple physically-motivated ansätze, we derive scaling relations between these physical quantities by performing multi regression analysis. In particular, we find that $U$ is primarily controlled by sSFR, as $U \propto sSFR^{0.43}$ , but also depends significantly on both $Z$ and $n_{e}$ . The derived partial dependence of $U \propto Z^{- 0.36}$ is weaker than the apparent correlation ( $U \propto Z^{- 1.52}$ ). The remaining negative dependence of $U$ on $n_{e}$ is found to be $U \propto n_{e}^{- 0.29}$ . The scaling relations we derived are in agreement with predictions from theoretical models and observations of each aspect of the link between these quantities. Our results provide a useful set of equations to predict the nebular conditions and emission-line fluxes of galaxies in semi-analytic models.

arXiv comment19 pages, 12 figures, submitted to MNRAS. Comments welcome

Disentangling the physical parameters of gaseous nebulae and galaxies

Abstract

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