2019MNRAS.483.3322P

In this work we explore accurate new ways to derive the ionization parameter (U) and the equivalent effective temperature (T_*) in H II regions using emission-line intensities from the ionized gas. The so-called softness parameter (η), based on [O II], [O III], [S II], and [S III], has been proposed to estimate the hardening of the ionizing incident field of radiation, but the simplest relation of this parameter with T_* also depends on U and metallicity (Z). Here we provide a Bayesian-like code (HCM-TEFF) that compares the observed emission lines of η with the predictions of a large grid of photoionization models giving precise estimations of both U and T_* when Z is known. We also study the radial variation of these parameters in well-studied disc galaxies observed by the CHAOS collaboration. Our results indicate that the observed radial decreasing of η can be attributed to a radial hardening of T_* across galactic discs as in NGC 628 and NGC 5457. On the other hand NGC 5194, which presents a positive slope of the fitting of the softness parameter, has a flat slope in T_*. On the contrary the three galaxies do not seem to present large radial variations of the ionization parameter. When we inspect a larger sample of galaxies, we observe steeper radial variations of T_* in less bright and later-type galaxies, mimicking a similar trend observed for Z, but the studied sample should be enlarged to obtain more statistically significant conclusions.