Ultraviolet-optical observations of the Seyfert 2 galaxies NGC 7130, NGC 5135, and IC 3639: Implications for the starburst-active galactic nucleus connection

We present and discuss Hubble Space Telescope (HST) (WFPC2 and FOC) images and ultraviolet (GHRS) spectra plus ground-based optical spectra of three S eyfert 2 nuclei (NGC 7130, NGC 5135 and IC 3639). These galaxies, together with Mrk 477 (Heckman et al.), were selected on the basis of ultraviolet br ightness from a bigger sample that comprises the 20 brightest Seyfert 2 nuc lei, with the goal of studying the starburst-active galactic nucleus (AGN) connection and the origin of the so-called "featureless continuum" in Seyfe rt 2 nuclei. The data provide direct evidence of the existence of nuclear s tarbursts that dominate the ultraviolet light and that are responsible for the featureless continuum in these type 2 Seyfert nuclei. The GHRS spectra show absorption features formed in the photospheres (S V lambda 1501, C III lambda lambda 1426, 1428, Si III lambda 1417, and Si III + P III lambda 13 41-1344) and in the stellar winds (C IV lambda 1550, Si IV lambda 1400, and N V lambda 1240) of massive stars. Signatures of massive stars are also cl early detected in their optical and near-UV spectra where the high-order Ba lmer series and He I lines are observed in absorption. These lines are form ed in the photospheres of O and B stars, and thus they also provide strong independent evidence of the presence of massive stars in the nuclei of thes e Seyfert 2 nuclei. Interstellar absorption lines similar to those formed i n the interstellar medium of starbursts are also observed. They are blueshi fted by a few hundred km s(-1) with respect to the systemic velocity, indic ating that the interstellar gas is outflowing. These outflows are most like ly driven by the nuclear starburst. These starbursts are dusty, compact, an d powerful. They have sizes ranging from less than 100 pc to a few hundred parsecs (much smaller than that seen in the prototype Seyfert 2 galaxy NGC 1068). Their UV colors imply that they are heavily reddened (by 2 to 3 mag in the UV), and the implied bolometric luminosities are of order 10(10) L.. The bolometric luminosities of these starbursts are similar to the estimat ed bolometric luminosities of their obscured Seyfert 1 nuclei. The data on this small sample suggest that more powerful AGNs may be related to more po werful central starbursts. Comparing the HST spectra to IUE spectra obtaine d through apertures with projected sizes of 3-11 kpc (and to IRAS far-IR da ta) we estimate that the nuclear starbursts account for 6%-25% of the total intrinsic UV luminosity of the entire galaxy.