– 1 –1. Emission Lines in Nearby Galaxies1.1. Models of Integrated Light of GalaxiesModels of galactic evolution including chemical evolution can be used to predict thecontinuous spectrum, absorption lines, and emission lines from the ISM as a functionof assumed star formation history, IMF, age, and, with less importance (at least formassive g alaxies), the time dependence of mass loss via galactic winds. Inputs also includeevolutionary tracks (function of stellar mass, age, and metallicity, as well as mass losshistory), nucleosynthesis yields (ranging from just studying some total metallicity tofollowing each element individually). See the earlier discussion of chemical evolution models,both analytical and numerical, as well as the semi-analytical models which combine thesefactors with a cosmological hierarchical formation scenario.1.2. Integrated Light of Galaxies – HII Region Emission LinesThe luminosity of massive stars is very high (L ∝ M3), and they tend to be clustered.Massive young star clusters (where high mass stars tend t o reside) are detectable even indistant galaxies.Interpreting the emission line spectrum in the integrated light requires understandthe ionization field, both its total flux and its spectrum, both of which may be functionsof position within the galaxy. The dominant contribution to the ionizing continuum instar-forming galaxies is massive stars. Knowledge of the IMF, the star formation history,metallicity, spatial distribution of dust and its reddening curve, are all needed to computethe ratios of the strength of va r io us emission lines from the ISM occuring at different r estwavelengths.– 2 –Before proceeding to the emission lines themselves, we address the reddening(absorption of light by the intervening material, both through our galaxy and the IGMalong the line sight, and within the g alaxy of interest itself). Both of these a r e functions ofwavelength, with more absorption at bluer wavelengths. Assuming the galaxy of interest isnot close to the plane of the Milky Way, the first term is relatively small. The absorptionwithin the source galaxy is a function of of the chemical composition and grain sizedistribution of the dust within the galaxy, of its spatial distribution, and of the angle of thegalaxy with respect to the line of sight. Since absorption depends on dust, the reddeningcurve (the extinction as a function of wavelength) depends on the mean metallicity of thegalaxy.Complex scenarios arise. For example, in a starburst there are so many SN that theybubbles they each blow in the ISM merge together a nd remain relatively clear of gas.However, in normal star-forming galaxies, the star formation is less vigorous, it takes placein individual HII regions, not huge global complexes, and the HII regions individually donot output as much energy, and thus cannot clear the ISM around t hem. In this case,they will remain dust-enshrouded for a longer time than would be the case of a starburst.Because so much of the stellar emitted energy is absorbed by the dust, which then reradiatesit in the IR, the bolometric IR emission from galaxies integrated in wavelength (from 5 to1000 µ for example) is often used as an indicator of star formation. Another problem thatis encountered in calculating interstellar absorption is that the stars that are responsiblefor the UV photoionizing flux are often physically separate rom the g as that produces theemission lines. See papers by Daniella Calzetti and collaborators, including her early workCalzetti, Kinney Storchi-Bergmann (1994, ApJ, 42 9, 582).Emission lines from the hottest stars themselves (in addition to those from the ISM)can also sometimes be seen in the integrated light. An example is the Wolf-Rayet population– 3 –(Wolf-Rayet stars are highly evolved stars which have lost almost all o f their outer envelope,so that nuclear processed material is exposed) with its broad emission features from stellarwinds (“bumps”); the primary features are broad emission from HeII at 4686˚A and a redbump aro und 5700˚A.The overall scheme is to use the stellar continuum spectrum to get the age of the clusterand derive the number of ionizing photons. Then subtract out the stellar component ofintegrat ed galaxy spectrum; the residual is the nebular spectrum. Then one fits the relativeintensities of various emission lines to get the metallicity of the species giving rise to eachtransition. The one must correct the species abundance to the element abundance throughknowledge of the ionization. These models have many parameters, inner and outer r adiiof HII regions, etc. Codes such a s STARBURST99 by Claus Leitherer or the Pegase codeare often used for this purpose to determine t he stellar spectrum, while the photoionizationcode CLOUDY by Gary Ferland analyzes the emission line spectrum. More sophisticatedcodes are under development that couple the chemical to the spectral energy distributionof starburst galaxies by incorporating a star formation and a metallicity evolution givenby a chemical evolution code (see, e.g. Martin-Manjon, Molla, Diaz & Terlevich, 2008,arXiv:0810.2162).An observational issue is correctly finding the emission line luminosity, since anyunderlying stellar continuum emission or line a bsorption must be removed; the latter isparticularly troublesome for the Balmer lines, which are strongly in absorption in the stellarcomponent, and in emission from the gas. Disentangling the two contributions is sometimesdifficult.The first thing to determine is whether the emission spectrum arises from gasphotoionized by dilute starlight or by an AGN with a much harder ionizing spectrum.Of course, if the emission lines are very broad, then they must arise via an AGN. But– 4 –for narrow lines, we need some way to separate the two. The usual AGN diag nostic isbased on line ratios which are sensitive to the hardness of the photoionizing spectrumsuch as [NII]/Hα (or [SII]/Hα) vs [OIII]/Hβ (the “Ba ldwin diagram”, Baldwin, Phillips &Terlevich, 1981, PASP, 93, 5). Note that these line pairs are close together in wavelengthso that the ratio is at least approximately indep endent of reddening.For distant galaxies, the detectability of AGN emission in the integrated light may bereduced due to an aperture effect. Spectrographs usually operate with a slit approximatelymatched to the seeing. For a more distant galaxy, the spatial resolution at the source willcorrespond to a la r ger physical size,