5. APPLICATIONS TO ASTROPHYSICAL ENVIRONMENTS
Since the first observations of the emission-line spectra of nuclei of galaxies by Seyfert in 1943 , the development of the observational techniques helped to draw a picture of a large class of objects called Active Galactic Nuclei, from the central engine to the extended emission-line region, hundreds of kpc away. At present, the scenario, improved thanks to available emission-line and continuum spectral data from radio to x-rays, shows an interplay among different physical processes. These include the radiation from the central engine, star forming regions, dust, and jets, all of them contributing to the observed spectra.
More than 20 years ago, diagnostic diagrams of emission-lines of samples of Seyfert 2 galaxies indicated that photoionization could be the main process powering the gas, although some features required an additional energy source. The narrow line region (NLR) velocity field suggested that shocks, due to the motion of the emitting clouds throughout a low density inter-cloud gas, could be the solution. Some authors, using diagnostic models for supernova remnants (SNR), discarded the effect of shocks in AGN. Since these models only account for the effect of low velocity shocks, they are not appropriate to study the NLR, where the velocities are larger than 100 km/s. In addition, it is not possible to turn off the central engine and forget about the effect of the central radiation on the shocked region. SUMA was born from these considerations - a code which accounts for both photoionization and shock powering the gas.
At the beginning, general models which used diagnostic diagrams for samples of Seyfert 2 galaxies helped to set the basis for more sophisticated models, more appropriate to study particular galactic nuclei accounting for the observations on different ranges of the emission-line spectra. Improved with the calculations for the resulting continuum emission and accounting for the effect of dust, the actual version of SUMA proved to be a major tool to understand the interplay of the physical processes occurring in AGN as well as in explaining the emission-line spectra of many individual objects.
5.2 Luminous Infrared Galaxies
Observations in the infrared revealed luminous, ultra-luminous, and hyper-luminous galaxies (LIRG, ULIRG, HLIRG, respectively), with infrared luminosities up to 1013 L¤ The origin of this high IR luminosity relates to the galaxy type (such as HII galaxy, starburst nucleus galaxy, or AGN), the source of dust illumination and heating by a photoionizing flux and/or collisional processes, the composition of dust ranging from relatively large silicate grains to small graphite, PAH, ices, etc.
In starburst environments and in the outflow regions of AGN, dust dominates the IR emission by reprocessing radiation from the primary source (stars and/or an active nucleus). Collisional processes, which were found to be so important to explain the emission-line and continuum spectra (e.g. the strong high ionization level line fluxes, the soft X-ray emission, strong heating of dust, radio synchrotron emission, etc.) from the nuclear and circumnuclear regions of AGNs, starbursts and HII regions, dominate the emitted radiation flux in mergers. Particularly, it was shown that collisional heating of dust grains by gas in supersonic velocity regimes leads to relatively high temperatures which could explain the IR emission between 3 and 1000 μm (the "IR bump") in the SED of starburst galaxies  and AGNs . The IR luminosity of galaxies depends on both dust reprocessed radiation in the IR and bremsstrahlung from cool gas. Dust IR emission is usually analysed independently of bremsstrahlung which covers the whole radio - X-ray frequency range. Indeed, in the IR domain dust emission dominates, but the modeling of the SED requires the consistent calculation of gas and dust spectra. Previous modeling of AGNs and starburst galaxies  has shown that many different conditions coexist in each galaxy. The observed continuum and emission-line spectra account for all of them. The analysis of dust-to-gas ratios in galaxies with different IR luminosities , shows that relatively high d/g are found in the LIRG clouds, higher than in the ISM by about a factor 100. About half of the LIRGs contain an AGN, which is found in almost all ULIRGs and in none of the HLIRGs. However, high infrared luminosities as those observed in HLIRGs do not correspond to higher dust-to-gas ratios. On the contrary, the highest dust-to-gas ratios appear in LIRGs. The modeling of Arp 220  is shown in Fig. 5.
5.3 Symbiotic Stars
Symbiotic systems (SSs) are currently understood as interacting binaries composed by a compact star, generally but not necessarily a white dwarf (WD) which is the source of the ionizing radiation; a cool giant star, which is at the origin of dust formation and ejection episodes; and different emitting gas and dust nebula. Because of the associated circumstellar matter and energetic activity, many SSs emit detectable radiation across nearly the entire electromagnetic spectrum. On the basis of near-infrared colours, SSs were classified in S and D types  according to whether the cool star (S-type) or dust (D-type) dominates the 1 to 4 μm spectral range.
In the past years, theoretical models  as well as observations  have categorically shown that in SSs both the hot and cool stars lose mass through strong stellar winds which collide within and outside the system, hence creating a complex network of wakes and shock fronts which result in a complicated structure of gas and dust nebula . This confirmed the primary role of shocks in the nowadays accepted interpretation of SSs as colliding-wind binary systems.
We refer to two main shocks: the shock between the stars facing the WD, which is a head-on shock (the reverse shock), and the head-on-back shock, which propagates outwards the system (the expanding shock). Both the nebulae downstream of the shock fronts are ionized and heated by the radiation flux from the hot star and by shocks. The photoionizing radiation flux reaches the very shock front of the reverse shock, while it reaches the edge opposite to the shock front downstream of the expanding shock. The derived models have been successfully applied to several SSs ( and references therein). In Fig. 6 the comparison of observed line ratios with models calculated by SUMA, as well as the comparison of the continuum spectrum, are given for instance for the H1-36 symbiotic star .