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

38 2 • The star formation history of NGC 5471
region. In order to visualize this, we have selected four different areas in the CMD and
proceeded to locate spatially the stars in these four boxes. Figure 2.11 displays the resulting
distributions. The youngest and more massive stars (a) are clearly concentrated in the main
star forming emission line knots, implying that massive stars form mainly in clusters. Only
two stars are not directly associated with these knots, one of which belongs to an isochrone
of 20 Myr. Panel (d) displays the distribution of intermediate mass stars (∼ 5−10 M ⊙ ) older
than about 50 Myr; these stars are distributed mainly towards the halo, with only a few in
the core of the region. Panels (b) and (c) show the distribution of intermediate mass stars
(∼ 10 − 15 M ⊙ ), both those which are in the main sequence and those which have already
evolved out of it. The distributions of stars in these two CMD boxes are clearly fairly uniform
throughout the core of NGC 5471, with some of them distributed in the halo, a somewhat
complementary spatial distribution to the older lower mass stars in the bottom-right panel.
Because the stars in these two intermediate boxes are of the same mass but they are found
in a range of evolutionary stages, this implies that star formation has proceeded more or less
uniformly in the core for the last ∼ 20 − 50 Myr.
From this analysis of the spatial distribution of the CMD, a clear picture emerges in
which the star formation in the NGC 5471 complex has proceeded in a general spatiotemporal
sequence from the halo inwards to the core. During the first epoch, ∼ 50−100 Myr
ago, the star formation occurred mainly in the halo, then it moved inwards and occurred
more or less uniformly in the core ∼ 20 − 50 Myr ago, although some of the star formation
was also important in the halo, and finally the current ionizing star forming event is mostly
concentrated in the singular bright clusters well located within the core. At the same time,
we have seen that, both from the integrated photometric analysis and from the CMD, as the
events moved inwards from the halo to the core the amount of mass of gas formed into stars
seems to have increased.
We note that the current event of ionizing clusters is apparently contained within a large
bubble which defines the core of NGC 5471. This bubble, of projected size 400 × 550 pc, is
clearly defined geometrically (cf. Figure 2.2) and kinematically (Munoz-Tunon et al., 1995),
and is likely to have been produced by the stars that formed ∼ 20 Myr ago.
There are other examples that seem to follow the general process of star formation that we
have outlined here for NGC 5471. Walborn and Blades (1997) find a similar spatio-temporal
distribution of stars in 30 Doradus, and Úbeda et al. (2007) also find an inwards sequence
of star formation and a bubble for cluster I-A in NGC 4214.