Theoretical modelling of emission-line galaxies: new classification parameters for mid-infrared and optical spectroscopy
Abstract
We have carried out extensive and detailed photoionization modelling to successfully constrain the locations of different emission-line galaxies in optical and mid-infrared diagnostic diagrams. Our model grids cover a wide range in parameter space for the active galaxy continuum and starburst galaxies with different stellar population laws and metallicities. We compare the predicted AGN and star formation mid-infrared line ratios [Ne III]15.56 μm/[Ne II]12.81 μm and [O IV]25.89 μm/[Ne III]15.56 μm to the observed values, and find that the best fit for the active galactic nucleus (AGN) is via a two-zone approximation. This two-zone approximation is a combination of a matter-bounded component, where [Ne III] and [O IV] are emitted efficiently, and a radiation-bounded component that maximizes [Ne II] emission. We overlay the predictions from this two-zone approximation on to the optical [O III]λ5007/Hβ and [N II]λ6583/Hα diagnostic diagram derived from the Sloan Digital Sky Survey, to find that the high-density and low-ionization radiation-bounded component in our two-zone AGN approximation model provides a good lower limit for [N II] emission. This establishes a new theoretical demarcation line for the minimum AGN contribution in this diagram. This new classification results by a factor of ∼1.4 in a higher AGN population than predictions derived from previous divisions of star-forming galaxies. Similarly, we define a maximum AGN contribution in the [O III]/Hβ and [N II]/Hα diagram by using a two-zone approximation within a parameter range typical of the narrow-line region.
- Publication:
-
Monthly Notices of the Royal Astronomical Society
- Pub Date:
- September 2014
- DOI:
- 10.1093/mnras/stu1242
- arXiv:
- arXiv:1406.5563
- Bibcode:
- 2014MNRAS.443.1358M
- Keywords:
-
- galaxies: active;
- galaxies: Seyfert;
- galaxies: starburst;
- galaxies: star formation;
- infrared: galaxies;
- Astrophysics - Astrophysics of Galaxies;
- Astrophysics - Cosmology and Nongalactic Astrophysics
- E-Print:
- Accepted for publication in Monthly Notices of the Royal Astronomical Society Main Journal. The paper contains 13 pages, 16 figures