Modeling the physical properties in the ISM of the low-metallicity galaxy NGC 4214^⋆

¹ Institut für theoretische Astrophysik, Zentrum für Astronomie der Universität Heidelberg, Albert-Ueberle Str. 2, 69120 Heidelberg, Germany
e-mail: diane.cormier@zah.uni-heidelberg.de
² Laboratoire AIM, CEA/DSM – CNRS – Université Paris Diderot, Irfu/Service d’Astrophysique, CEA Saclay, 91191 Gif-sur-Yvette, France

Received: 30 April 2015
Accepted: 12 June 2015

Abstract

We present a model for the interstellar medium of NGC 4214 with the objective to probe the physical conditions in the two main star-forming regions and their connection with the star formation activity of the galaxy. We used the spectral synthesis code Cloudy to model an H ii region and the associated photodissociation region (PDR) to reproduce the emission of mid- and far-infrared fine-structure lines from the Spitzer and Herschel space telescopes for these two regions. Input parameters of the model, such as elemental abundances and star formation history, are guided by earlier studies of the galaxy, and we investigated the effect of the mode in which star formation takes place (bursty or continuous) on the line emission. Furthermore, we tested the effect of adding pressure support with magnetic fields and turbulence on the line predictions. We find that this model can satisfactorily predict (within a factor of ~2) all observed lines that originate from the ionized medium ([S iv] 10.5 μm, [Ne iii] 15.6 μm, [S iii] 18.7 μm, [S iii] 33.5 μm, and [O iii] 88 μm), with the exception of [Ne ii] 12.8 μm and [N ii] 122 μm, which may arise from a lower ionization medium. In the PDR, the [O i] 63 μm, [O i] 145 μm, and [C ii] 157 μm lines are matched within a factor of ~5 and work better when weak pressure support is added to the thermal pressure or when the PDR clouds are placed farther away from the H ii regions and have covering factors lower than unity. Our models of the H ii region agree with different evolutionary stages found in previous studies, with a more evolved, diffuse central region, and a younger, more compact southern region. However, the local PDR conditions are averaged out on the 175 pc scales probed and do not reflect differences observed in the star formation properties of the two regions. Their increased porosity stands out as an intrinsic characteristic of the low-metallicity ISM, with the PDR covering factor tracing the evolution of the regions.