In these Seyfert 2 galaxies the nebular HI recombination lines can be excited by ionizing radiation from hot stars associated with the starburst and also by the active nucleus. We can estimate the contribution of the starburst to the emission lines by comparing the number of ionizing photons predicted by the UV continuum with that derived from these recombination lines.
The reddening of the gas is derived adding 1-1.5 Å to the observed equivalent width of the Balmer lines in emission (H10#10, H61#61 and H63#63), to account for the contribution of absorption by the underlying stellar population. Then, we fit a slope to the dependence of the ratio of corrected measured flux to the theoretical value versus the reddening curve. Assuming Case B recombination at T=104 K the E(B-V) obtained is 0.75, 0.67, and 0.52 for NGC 7130, NGC 5135 and IC 3639, respectively. These values of the color excess are systematically larger than those derived from the UV continuum slope (see Table 4). This discrepancy between the extinction derived from the two methods has been also found in starburst galaxies (Fanelli, O'Connell, & Thuan 1988; Calzetti, Kinney, & Storchi-Bergman 1994). The log of the H10#10luminosity measured in the central 1.511#113.5 arcsec in each spectrum (corrected for the reddening derived from the Balmer decrement) is 41.12 (erg s-1), 41.08 (erg s-1), and 40.62 (erg s-1) for NGC 7130, NGC 5135 and IC 3639, respectively. These luminosities represent a lower limit to the total luminosity, since the ionized gas is more extended than the spectrograph slit. The required number of ionizing photons is 64#64, 65#65, and 66#66 ph s-1 for NGC 7130, NGC 5135 and IC 3639, respectively. It is also possible that there is a more heavily-extincted (dustier) component of ionized gas associated with the starburst and/or AGN. This may be more apparent at near-IR wavelengths, so we have used the spectra of the Br61#61 emission-lines measured by Goldader et al (1997). In the cases of NGC 7130 and NGC 5135, the implied ionizing photon luminosities are 67#67 and 68#68 ph s-1 (larger than the values based on the Balmer lines by a factor of 69#691.5). Comparing the ionizing photon luminosities derived from the HI recombination lines to those estimated for the starburst (based on UV continuum luminosity), we find that the starburst itself can account for most of the Hydrogen-ionizing photons (but see Section 7.5 below). This information is summarized in Table 5.
The equivalent width of the Balmer recombination emission lines is another indicator of the age of the burst (Copetti, Pastoriza, & Dottori 1986). We have measured the equivalent width of H10#10 in the resulting spectrum after subtracting the contribution of the old bulge component and after having corrected both the line and continuum for extinction (see Table 4). The corrected values are 138 Å, 110 Å, and 131 Å for NGC 7130, NGC 5135 and IC 3639, repectively. These equivalent widths are compatible with an instantaneous burst of about 4 - 5 Myr (Leitherer & Heckman 1995). However, the values are somewhat difficult to interpret. On the one hand, they represent a lower limit to the real equivalent width since the ionized gas is likely to be more spatially-extended than the continuum. On the other hand, they represent an upper limit, since the hidden type 1 Seyfert nucleus will also contribute to the excitation of these lines.