Measurements of optical recombination lines

Measurements of optical recombination lines - Measurements...

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Measurements of optical recombination lines (again, Hα is strongest and easiest to deal with) may use either narrow-band aperture photometry or imaging. Imaging is a bigger reduction headache, but can also tell you where the action is going on. Reddening is always a potential problem across some regions, if seldom on galaxy-wide scales. One also needs to allow for the contribution of the adjacent [N II] forbidden lines in deriving pure Hα intensities. There are now hundreds of integrated Hα fluxes for galaxies, spanning Hubble types, luminosity, and environment. Here again, it is frequently useful to deal with an equivalent width for the line, giving a normalized SFR compared to its past average. Kennicutt and Kent (1983, AJ 88, 1094) show this relation between Hubble type and EW(Hα) (as usual, courtesy of the AAS): Not surprisingly, the SFR per unit starlight climbs to later Hubble types (toward Sc). To some extent, this reflects the dominant bulges of earlier types, and without information on the gas content it is not so clear what happens within the disk itself. Models of an evolving population with various SFR histories can also predict broad-band colors as well as EW(Hα); it turns out the Hα is vastly more sensitive to bursts of small relative mass. These figures from Kennicutt in Stellar Populations , and Kennicutt, Tamblyn, and Congdon 1994 (ApJ 435, 22) compare models with various SFR(T) to spiral galaxies; the dispersion and thus discrimination among models is much less for continuum colors. One way to think of this is that the Hα equivalent width is a color index with baseline running all the way from 6563 Å to the Lyman continuum in the far UV.
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The curves are models with different (declining) star-formation histories. The uppermost systems in their Fig. 2 (above, courtesy AAS) are those in which transient bursts of star formation must be taking place. There will be more to say about starburst systems later. Emission-line images allow mapping of the distribution in both space and luminosity of large numbers of individual H II regions (more properly complexes with unresolved or barely resolved structure). The example at right, NGC 5427, shows numerous H II regions and clumps of them in a continuum-subtracted Hα+[N II] image. The luminous H II regions often align very well with optical spiral features, but not usually so well that the arms are visible in the H II regions alone. Rings of star formation are not uncommon. There is a range of concentration to spiral arms; Hodge (1985 PASP 97, 688) has quantified this with a mean vs. variance statistic. The luminosity function of H II regions is frequently close to a power-law form (Kennicutt et al. 1989, ApJ 337, 761) but there are variations among galaxies such that it is not clear whether the luminosity
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Measurements of optical recombination lines - Measurements...

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