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

4.4. Discussion 145
ID A(V) c(Hβ) M ∗ f(age < 10 Myr)
(mag) (M ⊙ ) (%)
IIZw71 Knot A 0.41 0.19 1.4×10 6 0.08
IIZw71 Knot B 0.99 0.46 8.3×10 6 0.11
IIZw71 Knot C 0.09 0.04 3.3×10 7 0.008
IIZw71 Knot D 0.54 0.25 2.9×10 6 0.013
J1657 Knot A 0.18 0.08 2.8×10 8 0.58
J1657 Knot B 0.00 0.00 1.0×10 8 0.21
J1657 Knot C 0.00 0.00 1.4×10 8 0.28
Table 4.9: Values of the extinction, total stellar mass, and fraction of the mass in stars younger than
10 Myr for each knot of IIZw71 and J1657 in the best-fit model predicted by STARLIGHT.
Table 4.9 for all the knots, together with the internal extinction, represented by A(V) (as
given by STARLIGHT) and the reddening constant (using the transformations of Appendix
A), estimated by the model. For IIZw71, the masses were corrected using an aperture factor,
which takes into account the light not collected by the slit and which was calculated as the
weighted mean of the ratios of the light in the whole region as measured in the Hα image and
in the regions covered by the slit. In the case of J1657, no aperture correction factors have
been taken into account for the Hα luminosities, due to the compact nature of the object.
Indeed, the discrepancy factors between our Hα luminosities for Knot A and those measured
in the SDSS catalogue using a 3 arcsec fiber is not larger than 1.3.
Although it is not expected to find the same values of extinction in the gas and the stellar
population, there is a very good agreement between the values of the extinction estimated
by STARLIGHT in knots C and D of IIZw71 in relation to the values obtained from the
decrement of Balmer. In J1657, although the behaviour of the extinction is inverted, being
larger in knot A that in B and C, the opposite from the derived values using the decrement
of Balmer, the general result is consistent with low extinction. In knots A and B of IIZw71,
the values are higher than those derived for the ionized gas. In these two knots the specific
weight of the youngest population is higher. Since STARLIGHT assumes a common source
of extinction for all the stellar populations entering the fit, the degeneracy between age and
extinction tends to overestimate the extinction derived for the optimal fit. On the other
hand, in knots C and D, which are probably contaminated by the light coming from the
host galaxy, the contribution by the young stellar population is much lower, and the light at
longer wavelengths is dominated by the older stellar population and the estimated average
extinction is more representative.