The detection of stellar features in the optical spectra of starburst galaxies is a difficult business. The reason is that the optical spectra of hot stars are dominated by absorption lines of H and He, with only very weak metallic lines (Walborn & Fitzpatrick 1990). In starbursts, the H and He absorption features are coincident with the nebular emission lines that mask any absorption signatures. However, the high-order Balmer series are detected in absorption in many starburst galaxies (e.g. NGC 7714, González-Delgado et al 1995) or even in the spectra of giant HII regions (e.g. NGC 604, Terlevich et al 1996). These features can be seen in absorption because the strengths of the Balmer emission lines decrease quickly with decreasing wavelength, while the stellar absorption lines have nearly constant equivalent widths. Thus, the net effect is that H9#9, H10#10 and H61#61 are seen in emission while the high-order lines such as H8 to H12 can be seen in absorption.
The spectra of the three Seyferts show clearly the high order lines of the Balmer series in absorption, H9 (3835 Å), H10 (3798 Å), H11 (3771 Å), H12 (3750 Å) (Figure 16). H10#10 to H62#62 also show some absorption wings. The spectra also show HeI lines in absorption, the stronger ones are at 1#14921, 1#14387, 1#14143, 1#14026 and 1#13819. Note that one of the strongest lines, HeI 1#14471, is not detected in absorption. The reason is that the corresponding nebular line is also very strong in emission (see for example the emission line spectrum of the Orion nebula published by Osterbrock, Tran, & Veilleux, 1992) diluting the absorption feature. Note also that the spectra of A stars are rich in HI lines, which are even stronger and wider than in B stars, but they do not show any He lines in absorption. Thus, the detection of these HeI absorption lines is strong evidence for the presence of very hot, massive, and young (O and early B) stars in these Seyfert 2 nuclei.
However, the spectra also show features from an old population, incluiding the G band (4284-4318 Å, as defined by Bica & Alloin 1986) and the MgI+MgH feature (5156-5196 Å, as defined by Bica & Alloin 1986). These are strong in G and K stars, with equivalent widths of around 6 to 7 Å. These features come from the bulge component of these galaxies that contributes to the optical light. In fact, a simple combination of a B and G star can reproduce most of the stellar features seen in the nuclear spectra of these Seyferts (Figure 17). The equivalent width of MgI+MgH is 1.8 Å, 1.9 Å and 4.0 Å, and the equivalent width of the G band is 1.9 Å, 1.9 Å, and 3.7 Å, for NGC 7130, NGC 5135 and IC 3639 respectively. These values are weaker than in K stars and also weaker than in the normal nuclei of early-type spirals galaxies (e.g. the equivalent width of the G band is 619#191 Å, Heckman, Balick, & Crane 1980). From the dilution of the MgI+MgH feature and the G band we infer that 30% (in NGC 7130 and NGC 5135) and 60% (in IC3639) of the blue light in our optical spectra comes from the old bulge stars.
As part of this program we have also observed, to be used as templates, three galaxies that are known to be in different evolutionary stages. They are NGC 4339, NGC 205, and NGC 1569. The spectrum of NGC 4339 shows the features typical of an old bulge stellar population (Ho, Filippenko, & Sargent 1995). Its spectrum is well fitted by a K0III star. NGC 205 is a galaxy that shows the typical signatures of an intermediate age population. Population synthesis of the optical continuum (Bica & Alloin1990) indicates that the optical light is dominated by a population of stars with age 0.1-1 Gyr. The spectrum is well described by a combination of A2I and G0V Stars. NGC 1569 is an example of a galaxy with very young stars. Our evolutionary synthesis analysis (González Delgado et al 1997) indicates that O and B stars are the dominant contributors to the optical light. The spectrum of a B0V star combined with a small fraction of the light from a G0V star reproduces well most of the stellar features in the spectrum of the cluster B in NGC 1569.
We have compared the normalized spectra of the Seyferts with our template galaxies. The stellar features in NGC 7130 and NGC 5135 are very well matched by NGC 1569 (Figure 18). The stellar features of the spectrum of IC 3639 could be fitted by the spectrum of NGC 205; however, we note that after subtracting NGC 205 from IC 3639 the resulting spectrum shows all the Balmer lines apparently in emission with broad wings. This indicates that the Balmer absorption lines in IC 3639 are narrower than in NGC 205, and the population responsible for the H absorption features is probably dominated more by B stars than by A stars. We have measured the width of H63#63 in the stars from the atlas of Jacobi, Hunter, & Christian (1984), and the FWHM of a A2V star is 23 Å and B0V star is 11 Å. Thus, the broad wings in the Balmer emission lines results from an oversubtraction of the absorption features. In fact, these wings do not show up if we subtract from IC 3639 a combination of the spectrum of NGC 1569 with NGC 4339; 60% of the continuum light is accounted for by NGC 1569 (starburst) and 40% by NGC 4339 (old stars), as seen in Figure 19.
Our conclusion is that massive stars (O and early-B) are the responsible for producing the high-order Balmer series and the HeI absorption lines in the spectra of these Seyfert 2 galaxies. However, a larger contribution is made by the old population in IC 3639 compared to NGC 7130 and NGC 5135. The strong nebular emission lines in IC 3639 likewise conspire against the detection of some weaker stellar signatures of massive stars at optical wavelengths that are more clearly detected in NGC 7130 and NGC 5135.
In view of the relative ease with which our optical spectra directly demonstrate the presence of hot stars in the nuclei of these three Seyfert galaxies, it is worthwhile considering why the `featureless continuum' in Seyfert nuclei was ever given such an (apparently) inaccurate name. We believe there is a reasonable explanation for this. The three galaxies discussed in this paper are (along with Mrk 477) the Seyfert 2 nuclei with the highest UV fluxes (thereby enabling spectroscopy with GHRS). This means that these galaxies also have optical `featureless continua' that are unusually bright relative to both the old underlying stellar population and the nebular emission (line and continuum). Thus, rather than simply being a mysterious and rather minor contributor to the optical light, the `featureless continuum' actually dominates the near-UV light, and can then be seen to be anything but `featureless'. This leaves unanswered the question as to whether the continuum generically has a starburst component in all type 2 Seyfert nuclei. We will return briefly to this issue in section 8 below.