8 Properties of the burst region

Huchra ([1977]) was the first to propose that the starburst galaxy can be thought of as made up of stellar populations of two ages: the underlying galaxy which is the old galaxy, superimposed on which is the young burst component. To estimate the burst component, the contribution of the underlying galaxy has to be subtracted. This is not an easy task. Aperture photometry of the region in which the young population is present gives the total intensity in that region. The contribution of the galaxy can be subtracted by estimating the galaxy from an annular region around the burst region. However, this approach suffers from a major drawback. The galaxy contribution estimated by this method gives the underlying galaxy value outside the region and not at the position of the burst. Such an approach would underestimate the galaxy contribution especially when the underlying galaxy luminosity profile is an exponential disk as described in the previous section. This would lead to erroneous values for the colour of the burst. To overcome this difficulty, we use the disk component estimated in the last section to derive the burst values. The exponential disk is extrapolated right up to the central regions. We construct a disk model of the galaxy. This is then subtracted from the galaxy. The residual intensity in the central region gives us the contribution of the burst component. This approach is used only for the central region in each galaxy since non-axisymmetric structures like bars can contribute to the residual in the outer regions. The burst colours derived after subtracting the disk component as described above are presented in Table 5. Mrk 363 shows globally enhanced star formation. This makes it impossible to separate the young and the old stellar components in this galaxy and hence estimate the burst colours from the results of the disk fitting.

 

Table 5: Colours of the burst component