A spatially resolved study of ionized regions in galaxies at different ...
A spatially resolved study of ionized regions in galaxies at different ... A spatially resolved study of ionized regions in galaxies at different ...
104 3 • IFS of a GEHR in NGC 6946 found for several Hii regions in M101 by McCall et al. (1985) and in NGC 604 by Diaz et al. (1987). According to these authors, this effect can be understood as an excitation effect due to the lower ionization potential of S + with respect to to N + combined with the relative low S + /S ++ ratio. Regarding the S 23 parameter (see map in Figure 3.28), as it can be appreciated in the lower panel of Figure 3.29, despite the large range of values for the excitation, this parameter seems to remain uniform. Nevertheless, due to the low signal-to-noise of the infrared sulphur lines of our data set, more data is needed to test this behaviour.
Chapter 4 Long-slit spectrophotometry of multiple knots of Hii galaxies 4.1 Introduction Hii galaxies are low mass irregular galaxies with, at least, a recent episode of violent star formation (Melnick et al., 1985) concentrated in a few parsecs close to their cores. The intense processes of star formation occurring in the luminous knots of Hii galaxies make them easily detected in surveys based on strong emission lines. In fact, these knots present spectra similar to those of giant extragalactic Hii regions (GEHRs; Sargent and Searle, 1970; French, 1980). Therefore, we can apply the same measurement techniques as for Hii regions. In this way, it is possible to use these knots to ascertain the basic properties of the host galaxy, such as extinction, metallicity, star formation rates, and from them derive the chemical evolution and the star formation history of the whole galaxy. In some cases, it is possible to detect in these objects, intermediate-to-old stellar populations which have a more uniform spatial distribution than the bright and young stellar populations associated with the ionizing clusters (Schulte-Ladbeck et al., 1998). This older population produces a characteristic spectrum with absorption features which mainly affect the hydrogen recombination emission lines (see Diaz, 1988). In some cases, the underlying stellar absorption can severely affect the ratios of Hi line pairs and hence the determination of the reddening constant. Hii galaxies are a subset of blue compact dwarf galaxies (BCDs), which are selected by their compact aspect and blue colour. A considerable number of the objects observed at 105
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104 3 • IFS <strong>of</strong> a GEHR <strong>in</strong> NGC 6946<br />
found for several Hii <strong>regions</strong> <strong>in</strong> M101 by McCall et al. (1985) and <strong>in</strong> NGC 604 by Diaz et al.<br />
(1987). Accord<strong>in</strong>g to these authors, this effect can be understood as an excit<strong>at</strong>ion effect due<br />
to the lower ioniz<strong>at</strong>ion potential <strong>of</strong> S + with respect to to N + comb<strong>in</strong>ed with the rel<strong>at</strong>ive low<br />
S + /S ++ r<strong>at</strong>io.<br />
Regard<strong>in</strong>g the S 23 parameter (see map <strong>in</strong> Figure 3.28), as it can be appreci<strong>at</strong>ed <strong>in</strong> the<br />
lower panel <strong>of</strong> Figure 3.29, despite the large range <strong>of</strong> values for the excit<strong>at</strong>ion, this parameter<br />
seems to rema<strong>in</strong> uniform. Nevertheless, due to the low signal-to-noise <strong>of</strong> the <strong>in</strong>frared sulphur<br />
l<strong>in</strong>es <strong>of</strong> our d<strong>at</strong>a set, more d<strong>at</strong>a is needed to test this behaviour.
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