The other absorption features stronger than the photospheric lines seen in the spectra are the resonance lines CIV 1#11550, SiIV 1#11400 and NV 1#1 1240. They can form in the photospheres of O and B stars; however, they develop a P Cygni profile if they form in the stellar wind of O stars. In that case, the absorption feature shows a velocity blueshift of the order of a thousand km s-1. These lines are very prominent if the spectrum is dominated by a starburst with age between 1 to 10 Myr. The profiles of these lines have been used to derive the stellar content and star formation history in starburst galaxies (Robert, Leitherer & Heckman 1993; Leitherer et al 1996; Conti et al 1996; González Delgado et al 1998). The UV spectrum of these Seyfert 2 galaxies is very similar to the starburst galaxy NGC 1741 (Conti et al 1996). This similarity suggests that the stellar content in these Seyfert galaxies is similar to typical starburst galaxies.
To prove this conclusion we use the line profile synthesis technique for the wind lines. Evolutionary synthesis models of a stellar population for these lines has been computed by Robert et al (1993) and Leitherer et al (1996) based on an IUE library of O and B stars. The models assume solar metallicty, and the lines are synthesized for different star formation histories (instantaneous and continuous star formation) and different assumptions for the IMF.
The wind line least affected by the AGN contribution is SiIV, so we now use only this line for the fit. However, later we will check that the best solutions found with SiIV are compatible with the absorption part of the wind profiles of the CIV and NV lines. Because of a possible contribution from the Seyfert nebular emission to the SIV, to perform the fit we used only a wavelength window from 1381 Å to 1387 Å on the blue side of the wind profile. To constrain the solutions we have computed the 55#55 parameter between the observations and every model as a function of the age, IMF upper mass cut-off (M56#56), and IMF slope for instantaneous burst or continuous star formation (csf) models. The results are plotted for NGC 7130 (Figure 10), NGC 5135 (Figure 11) and IC 3639 (Figure 12) as bubble diagram, where the size of the bubble is proportional to 55#55. The smallest deviations between observations and models are given by the smallest bubbles.
SiIV shows a strong P Cygni profile if the starburst is dominated by O supergiants. A conspicuous profile indicates that the star formation occurs in a short period of time, due to the fact that the relative number of these stars with respect to the total O stars is low because their relative life time is short. Therefore, the SiIV line is very useful to discriminate between models of instantaneous bursts and continuous star formation (csf).
Figure 10 shows clearly that NGC 7130 has experienced a burst of star formation. The best fit indicates that stars more massive than 60 M16#16 are present in the burst (Figure 10a), and the mass distribution follows a Salpeter (9#9=2.35) or flatter IMF (Figure 10b). The best fits indicate an age of 3-4 Myr.
In NGC 5135, the fit favors burst models with M 57#57M16#16 (Figure 11a), Salpeter or steeper (9#9=3.0) IMF (Figure 13b), and age 3-5 Myr. However, we cannot exclude csf models with M 58#58 M16#16 (Figure 11c), and Salpeter or flatter IMF's (Figure 13d).
As for NGC 5135, the SiIV line profile in IC 3639 is compatible with both burst and csf models. Burst models with Mup=60 M16#16 and age 3-4 Myr and csf models with star formation that began 4-5 Myr ago provide good descriptions of the data.
The results show clear evidence that massive stars are present in the inner 1.7411#111.74 arcsec2 (few hundred pc) of these Seyfert 2 galaxies. The stellar content and age of the burst in these nuclear starbursts seem to be similar. However, we cannot exclude the possiblity that continuous star formation is taking place in NGC 5135 and IC 3639. The UV continuum morphology of NGC 5135 (see Figure 6b) suggests that the different knots seen in the FOC image are super-star clusters. If this is correct, then it favors a multiple burst scenario.