Publication: Photoionization models of the CALIFA HII regions I. Hybrid models
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2016-10
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Abstract
Photoionization models of HIT regions require as input a description of the ionizing spectral energy distribution (SED) and of the gas distribution, in terms of ionization parameter U and chemical abundances (e.g., O/H and N/O). A strong degeneracy exists between the hardness of the SED and U, which in turn leads to high uncertainties in the determination of the other parameters, including abundances. One way to resolve the degeneracy is to fix one of the parameters using additional information. For each of the 20 000 sources of the CALIFA HIT regions catalog, a grid of photoionization models is computed assuming the ionizing SED to be described by the underlying stellar population obtained from spectral synthesis modeling. The ionizing SED is then defined as the sum of various stellar bursts of different ages and metallicities. This solves the degeneracy between the shape of the ionizing SED and U. The nebular metallicity (associated with O/H) is defined using the classical strong line method O3N2 (which gives our models the status of "hybrids"). The remaining free parameters are the abundance ratio N/O and the ionization parameter U, which are determined by looking for the model fitting [N II]/H alpha and [O III]/H beta. The models are also selected to fit [O II]/H beta. This process leads to a set of 3200 models that reproduce the three observations simultaneously. We find that the regions associated with young stellar bursts (i.e., ionized by OB stars) are affected by leaking of ionizing photons, the proportion of escaping photons having a median of 80%. The set of photoionization models satisfactorily reproduces the electron temperature derived from the [O III]lambda 4363/5007 line ratio. We determine new relations between the nebular parameters, like the ionization parameter U and the [OH]/[ III] or [S II]/[S III] line ratios. A new relation between N/O and O/H is obtained, mostly compatible with previous empirical determinations (and not with previous results obtained using photoionization models). A new relation between U and O/H is also determined. All the models are publicly available on the Mexican Million Models database 3MdB.
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© ESO, 2016. Artículo firmado por 9 autores. C.M. took advantage of the useful discussions with G. Hagele and his co-workers while a guest at La Plata University. We thank the anonymous referee for the careful review of the paper and the valuable comments. We want to thank Yago Ascasibar and Polychronis Papaderos, the internal CALIFA referee for this paper, for their useful comments. Part of the results presented here have been obtained using computers from the CONACyT project CB2010/153985 and the UNAM-PAPIIT project 107215. The computers "Tychos" (Posgrado en Astrofísica-UNAM, Instituto de Astronomía-UNAM and PNPC-CONACyT) have been used for this research. G.D.I. gratefully acknowledges support from the Mexican CONACYT grant CB-2014-241732. S.F.S. thanks the CONACYT-125180 and DGAPA-IA100815 projects for providing support during this study. Support for L.G. is provided by the Ministry of Economy, Development, and Tourism's Millennium Science Initiative through grant IC120009, awarded to The Millennium Institute of Astrophysics, MAS. L.G. acknowledges support by CONICYT through FONDECYT grant 3140566. This study uses data provided by the Calar Alto Legacy Integral Field Area (CALIFA) survey (http://califa.caha.es/). CALIFA is the first legacy survey performed at Calar Alto. The CALIFA collaboration would like to thank the IAA-CSIC and MPIA-MPG as major partners of the observatory, and CAHA itself, for the unique access to telescope time and support in manpower and infrastructures. The CALIFA collaboration also thanks the CAHA staff for the dedication to this project. Based on observations collected at the Centro Astronómico Hispano Alemán (CAHA) at Calar Alto, operated jointly by the Max-Planck-Institut fur Astronomic and the Instituto de Astrofísica de Andalucía (CSIC).