SDSS IV MaNGA: Metallicity and ionisation parameter in local star-forming galaxies from Bayesian fitting to photoionisation models

¹ Dipartimento di Fisica e Astronomia, Università di Bologna, Via Piero Gobetti 93/2, 40129 Bologna, Italy
e-mail: matilde.mingozzi@inaf.it
² INAF – Osservatorio Astrofisico di Arcetri, Largo E. Fermi 5, 50157 Firenze, Italy
³ INAF – Osservatorio Astrofisico di Padova, Vicolo dell’Osservatorio 5, 35122 Padova, Italy
⁴ European Southern Observatory, Karl-Schwarzschild-Str. 2, Garching bei München 85748, Germany
⁵ University of California Observatories, University of California Santa Cruz, 1156 High St., Santa Cruz, CA 95064, USA
⁶ Department of Astronomy, University of Wisconsin-Madison, 475N. Charter St., Madison, WI 53703, USA
⁷ South African Astronomical Observatory, PO Box 9, Observatory 7935 Cape Town, South Africa
⁸ Apache Point Observatory and New Mexico State University, Sunspot, NM 88349, USA
⁹ Sternberg Astronomical Institute, Moscow State University, Moscow, Russia
¹⁰ Observatories of the Carnegie Institution for Science, 813 Santa Barbara St., Pasadena, CA, USA
¹¹ Departamento de Astronomía, Universidad de Chile, Camino del Observatorio 1515, Las Condes, Santiago, Chile
¹² Centro de Astronomía (CITEVA), Universidad de Antofagasta, Avenida Angamos 601, Antofagasta, Chile
¹³ McDonald Observatory, The University of Texas at Austin, 1 University Station, Austin, TX 78712, USA
¹⁴ Department of Physics & Astronomy, The University of Iowa, 203 Van Allen Hall, Iowa City, IA 52242, USA
¹⁵ Cavendish Laboratory, University of Cambridge, 19 J. J. Thomson Avenue, Cambridge CB3 0HE, UK
¹⁶ Kavli Institute for Cosmology, University of Cambridge, Madingley Road, Cambridge CB3 0HA, UK
¹⁷ Laboratório Interinstitucional de e-Astronomia – LIneA, Rua General José Cristino 77, Rio de Janeiro, RJ 20921-400, Brazil
¹⁸ Universidade Federal do Rio Grande do Sul, IF, 15051, Porto Alegre, RS 91501-970, Brazil
¹⁹ Observatório Nacional, Rua José Cristino, 77, Rio de Janeiro, RJ 20921-400, Brasil
²⁰ Center for Astrophysical Sciences, Department of Physics and Astronomy, Johns Hopkins University, 3400 North Charles Street, Baltimore, MD 21218, USA
²¹ Lawrence Berkeley National Laboratory, 1 Cyclotron Road, Berkeley, CA 94720, USA

Received: 28 November 2019
Accepted: 24 February 2020

Abstract

We measured gas-phase metallicity, ionisation parameter, and dust extinction for a representative sample of 1795 local star-forming galaxies using integral field spectroscopy from the SDSS-IV MaNGA survey. We self-consistently derive these quantities by comparing observed line fluxes with photoionisation models using a Bayesian framework. We also present the first comprehensive study of the [S III]λλ9069,9532 nebular lines, which have long been predicted to be ideal tracers of the ionisation parameter. However, we find that current photoionisation model predictions substantially over-predict the intensity of the [S III] lines, while broadly reproducing other observed optical line ratios. We discuss how to nonetheless make use of the information provided by the [S III] lines by setting a prior on the ionisation parameter. Following this approach, we derive spatially resolved maps and radial profiles of metallicity and ionisation parameter. The metallicity radial profiles derived are comparable with previous works, with metallicity declining toward the outer parts and showing a flattening in the central regions. This is in agreement with infall models of galaxy formation, which predict that spiral discs build up through accretion of material, leading to an inside-out growth. On the other hand, ionisation parameter radial profiles are flat for low-mass galaxies, while their slope becomes positive as galaxy mass increases. However, the ionisation parameter maps we obtain are clumpy, especially for low-mass galaxies. The ionisation parameter is tightly correlated with the equivalent width of Hα [EW(Hα)], following a nearly universal relation, which we attribute to the change of the spectral shape of ionising sources due to ageing of H II regions. We derive a positive correlation between ionisation parameter and metallicity at fixed EW(Hα), in disagreement with previous theoretical work that predict an anti-correlation.