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arXiv:astro-ph/0612309v1

12 Dec

2006 **FULL TITLE** ASP Conference Series, Vol.

**VOLUME**, **YEAR OF PUBLICATION** **NAMES OF EDITORS** The Narrow Line Region: Current Models and Future Questions Brent Groves1 Max Planck Institute for Astrophysics, Karl-Schwarzschild Str. 1,

85741 Garching, Germany Abstract. I present a broad overview of modelling of the Narrow Line Re- gion (NLR) of active galaxies, and discuss some of the more recent models we currently have for the emission from the NLR. I show why the emission line ratios from the NLR are constrained to certain observed values, and describe what physical parameters we can derive from observations using emission line models. Also presented are some examples of this, looking at the metallicity and excitation mechanism of active galaxies. As a ?nal point, the limitations of the current models are discussed, and how how the combination of modelling and theory can help us solve some of the questions that still remain within the NLR. 1. Introduction The Narrow Line Region (NLR) is the region of extended interstellar gas ionized and heated by the active galactic nucleus (AGN). This region is classi?ed as narrow-line as the gas lies outside the dominating in?uence of the central black hole, and the observed line velocity widths are much less than found in typical Broad Line Regions (discussed earlier in these proceedings). Even so, typical NLR velocity widths are in the range

200

150 km s?1) would produce ionizing photons (Dopita &

Sutherland 1995, 1996). As post- shock gas cools it produces ionizing photons which di?use up- and down-stream, and ionize the pre-shock gas. Thus the ionization in fast shocks is actually a combination of both shock and photo-ionization, where the photoionization is determined by the shock velocity. These shock+precursor models are able to reproduce the observations quite well, as shown in ?gure 2, and can also produce some of the higher ioniza- tion lines. However, the problem with these models is that they require shocks throughout the NLR, meaning shock signatures should always be visible. So by themselves these models cannot explain all NLR emission. Narrow Line Region Models

5 3.4. Multi-Component Photoionization The next level of complexity in terms of photoionization models is multi-component or multi-cloud photoionization, where the combination of two or more photoion- ization models is used to reproduce both the high and low ionization lines in the NLR. Generally, most models limit themselves to two components to minimize the number of free parameters. These models have been used to examine spe- ci?c galaxies (e.g. Kraemer &

Crenshaw 2000;

Morganti et al. 1991) and more generally trying to explain speci?c line ratios (e.g. Komossa &

Schulz 1997;

Murayama &

Taniguchi 1998) or line strengths (Baskin &

Laor 2005). The main problem with these models is that as you increase the number of clouds, the problem becomes unconstrained. A way around this is to provide a physical basis for the multiple clouds, and hence physical constraints. Here I mention three current models that try to deal with this problem. 3.5. AM/I Models The AM/I models of Binette et al. (1996) consider the NLR to be made up of hot, highly ionized, matter bounded1 clouds and, lower ionization, ionization bounded clouds. In their model, the ionization bounded (I-) clouds see the absorbed spectrum from the matter bounded (M-) clouds, and the I-clouds are also a higher density. The resulting emission line spectrum from these models is the controlled by the ratio of these two components, AM/I. Two possible physical pictures of their model are shown in ?gure