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

94 3 • IFS of a GEHR in NGC 6946
Radius F(Hα) L(Hα) Q(H 0 ) M(Hii)
(pc) (erg cm −2 s −1 ) (erg s −1 ) (photon s −1 ) (M ⊙
Knot A 190 1.93 × 10 −12 8.03 × 10 39 7.9 × 10 51 2.5 × 10 5
Knot B 95 1.08 × 10 −13 4.52 × 10 38 4.5 × 10 50 1.5 × 10 4
Knot C 115 3.76 × 10 −13 1.55 × 10 39 1.5 × 10 51 5.0 × 10 4
Knot D 100 1.51 × 10 −13 6.28 × 10 38 6.2 × 10 50 2.0 × 10 4
Table 3.10: Hα flux and derived parameters for the (mosaic) integrated spectra of the four knots.
M HII = Q(H 0 ) m p
n e α B
The values are also listed in Table 3.10. All of them, given the assumptions of no dust
absoption or photon leakage, represent lower limits.
Table 3.10 also gives the estimated radius, in parsecs, of a circular aperture covering the
spatial distribution of the fibers used in the integrated spectra of each knot. This value is
not intended to be accurate, but to give an order of magnitude of the sizes involved.
As it can be seen, the luminosities and masses of H + from this four knots are typical
of giant Hii regions and can be compared to the case of NGC 5471. Knot A of NGC 6946
is about a factor of 2 more luminous than Knot 1 in NGC 5471, while Knot C is similar
to Knot 2 and the other two knots are of the same order of magnitude as Knot 4 or 10. It
should be noted that the spatial scales involved in each case are very different. While the
effective radius of the knots of NGC 5471 are around 60 pc, in NGC 6946 the radii can be
2 o 3 times larger.
3.5.2 Derived properties of the WR population
The detection and measurement of the WR blue bump in three of the four knots can be
very useful for the study of the ionizing stellar population.
In this way, it is possible to compare the observed relative intensities and equivalent
widths of the WR bumps with the predictions of Starburst 99 (Leitherer et al., 1999) population
synthesis models as a function of the metallicity, age and star formation law of the
cluster. For this purpose, we have run a Starburst 99 model based on stellar model atmospheres
from Smith et al. (2002), Geneva evolutionary tracks with high stellar mass loss
(Meynet et al., 1994), a Kroupa Initial Mass Function (IMF; Kroupa, 2002) in two intervals
(0.1-0.5 and 0.5-100 M ⊙ ) with different exponents (1.3 and 2.3 respectively), a metallicity
of Z = 0.02, the theoretical wind model (Leitherer et al., 1992) and a supernova cut-off of 8
M ⊙ .
In Figure 3.26 we show the predicted equivalent width and relative intensity to Hβ for