Ultraviolet-optical observations of the Seyfert 2 galaxies NGC 7130, NGC 5135 and IC 3639:
Implications for the Starburst-AGN connection. *

next up previous       1998, Astrophysical Journal 505, 174
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Gonzalez Delgado1,2, R.M., Heckman3, T.M., Leitherer1, C., et al.
1Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218
2Instituto de Astrofísica de Andalucía, Apdo. 3004, 18080 Granada, Spain
3Department of Physics & Astronomy, JHU, Baltimore, MD 21218
*Based on observations with the NASA/ESA Hubble Space Telescope obtained at the Space Telescope Science Institute, which is operated by AURA, Inc., under NASA contract NAS5-26555.


We present and discuss HST (WFPC2 and FOC) images and ultraviolet (GHRS) spectra plus ground-based optical spectra of three Seyfert 2 nuclei (NGC 7130, NGC 5135 and IC 3639). These galaxies together with Mrk 477 (Heckman et al 1997) were selected on the basis of ultraviolet-brightness from a bigger sample that comprises the 20 brightest Seyfert 2 nuclei, with the goal of studying the Starburst-AGN connection and the origin of the so-called `featureless continuum' in Seyfert 2 nuclei.

The data provide direct evidence of the existence of nuclear starbursts 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 (SV 1501, CIII 1426,1428, SiIII 1417, and SiIII+PIII 1341-1344) and in the stellar winds (CIV 1550, SiIV 1400, and NV 1240) of massive stars. Signatures of massive stars are also clearly detected in their optical and near-UV spectra where the high order Balmer series and HeI lines are observed in absorption. These lines are formed 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 these Seyfert 2 nuclei. Interstellar absorption lines similar to those formed in the interstellar medium of starbursts are also observed. They are blueshifted by a few hundred km s-1 with respect to the systemic velocity, indicating that the interstellar gas is outflowing. These outflows are most likely driven by the nuclear starburst.

These starbursts are dusty, compact, and powerful. They have sizes ranging from less than 100 pc to a few hundred pc (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 magnitudes in the UV), and the implied bolometric luminosities are of-order 1010 L2#2. The bolometric luminosities of these starbursts are similar to the estimated bolometric luminosities of their obscured Seyfert 1 nuclei. The data on this small sample suggest that more powerful AGN may be related to more powerful central starbursts. Comparing the HST spectra to IUE spectra obtained through apertures with projected sizes of 3 to 11 kpc (and to IRAS far-IR data) we estimate the nuclear starbursts account for 6 to 25% of the total intrinsic UV luminosity of the entire galaxy.

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Rosa Gonzalez