A spatially resolved study of ionized regions in galaxies at different ...

30 2 • The star formation history of NGC 5471
2.4.1 Cluster Analysis
Analysing the integrated properties of a complex region like NGC 5471 is an exercise
that one has to undertake with care, because the integrated photometry includes a wide
range of regions with different ages and extinction values. Thus the result of synthesizing
the integrated ultraviolet-to-NIR photometry with CHORIZOS, which finds the most likely
single stellar population (SSP), is only of general interest as a guiding signpost. If we use
only the optical-to-NIR photometry, CHORIZOS finds a most likely SSP with relatively high
extinction and a young age ∼ 4 Myr. If we include the FUV and NUV GALEX photometry
then CHORIZOS finds two loci of maximum likelyhood for a single SSP, one that corresponds
to an age of ∼ 8 Myr with moderate extinction and a second maximum at an age around 50
Myr and very low extinction. This result suggests that a significant fraction of the GALEX
flux integrated within the 15 arcsec radius aperture from NGC 5471 is produced by a postnebular
stellar population of B stars (see also Waller et al., 1997).
Three previous studies have modelled the age of NGC 5471 using ultraviolet-to-optical
spectra. Rosa and Benvenuti (1994) use an HST/FOS spectrum through an aperture of
4.3 arcsec around knot A. Mas-Hesse and Kunth (1999) use an IUE spectrum through an
aperture of 10×20 arcsec and the FOS optical spectrum of Rosa and Benvenuti (1994).
Pellerin (2006) uses a FUSE spectrum through an aperture of 4×20 arcsec. All three works
find that the ultraviolet spectrum of NGC 5471 can be explained with a young burst of less
than about 5 Myr and a relatively low extinction. That these previous works are not sensitive
to an older blue population may be due to the fact that they are based on data acquired at
relative shallow flux levels (as indeed is stated in the three papers), and would thus be biased
towards the brightest ultraviolet knots; while the integrated GALEX photometry through a
large aperture is more sensitive to the extended, fainter, more evolved stellar population 5 .
The photometric analysis of the clusters identified in section 2.3.1 has some advantages
and some disadvantages with respect the integrated photometry of the whole NGC 5471
region. On one hand, it is performed on smaller regions, which can be more readily identified
with local star forming events, including a spatially well defined stellar cluster with its surrounding
emission line nebula. Thus their photometric properties pertain to a more coherent
whole, for example, in terms of age and extinction. On the other hand, a smaller cluster
means that the stochastic biases may play a more important role. However, we should not
forget that even so, each of these knots can be regarded as a giant Hii region; for example,
knot A is somewhat more luminous in ionizing power than the giant Hii region 30 Doradus
in the local group.
Figure 2.9 shows a color-color-Hα plot of the individual clusters. The hypernova knot
is not shown, and knots 3 and 4 have been summed into a single knot 34. There is a clear
correlation in the sense that redder knots have less Hα flux. The flux of Hα can decrease
5 Recall that the 1σ detection for the GALEX data is AB FUV = 27.6 mag arcsec −2 .