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

148 4 • Long-slit spectrophotometry of multiple knots of Hii galaxies
1400
1350
B
C
Hα velocity
[SII] 6717 Å velocity
1,2e-15
1e-15
velocity (km/s)
1300
1250
A
D
8e-16
6e-16
4e-16
Flux Hα (erg s -1 cm -2 Å)
1200
2e-16
1150
-20 -10 0 10
0
20
spatial distribution (arcsecs)
Figure 4.21: Rotation curve of IIZw71. Filled circles represent heliocentric velocity derived from Hα
and open squares from [Sii] 6717 Å. Solid line represents the spatial distribution of flux of Hα to be
compared with the rotation curve.
gas surrounding the knots of star formation which reaches 60 km/s in the case of knot C.
Assuming that we see the polar ring of the galaxy edge-on, with a radial velocity of 85
km/s, and considering an optical radius of 20 arcsec, which is where we can measure the
emission lines with a good enough signal-to-noise ratio, we calculated a dynamical mass
inside this radius of (2.8 ± 0.2) ×10 9 M ⊙ . We then calculated the L B luminosity inside
the radius of 20 arcsec, internal to the polar ring, from the B brightness distribution given
in Cairós et al. (2001), obtaining L B = 7.2×10 8 L ⊙ . Therefore, we obtain a value of 3.9
for the M/L B ratio inside the star forming-ring. This is close to the value of 2.8 found by
Reshetnikov and Combes (1994) from optical observations within a distance of 30 arcsec
from the centre. Considering that their blue light probably encompasses the light coming
from the emission knots in the ring, that luminosity probably constitutes an upper limit and
provides a lower limit to the M/L B .