Disentangling the physical parameters of gaseous nebulae and galaxies

Abstract

We present an analysis to disentangle the connection between physical quantities that characterize the conditions of ionized H ii regions-metallicity (Z), ionization parameter (U), and electron density (ne)-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 ≤ z ≤ 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 [O iii]λ4363 and [O ii]λλ7320,7330. The derived metallicities cover a range of 12 + log O/H ∼7.6-8.9. It is found that the three nebular parameters, Z, U, and ne, 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 multiregression analysis. In particular, we find that U is primarily controlled by sSFR, as U∝sSFR0.43, but also depends significantly on both Z and ne. The derived partial dependence of U∝Z-0.36 is weaker than the apparent correlation (U∝Z-1.52). The partial dependence of U on ne is found to be U∝ne-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.