Abstract
We use Hubble Space Telescope UV and optical spectra of the narrow-line Seyfert 1 (NLS1) galaxy Ark 564 to investigate its internal reddening and properties of its emission-line and intrinsic UV absorption gas. We find that the extinction curve of Ark 564, derived from a comparison of its UV/optical continuum to that of an unreddened NLS1, lacks a 2200 ? bump and turns up toward the UV at a longer wavelength (4000 ?) than the standard Galactic, LMC, and SMC curves. However, it does not show the extremely steep rise to 1200 ? that characterizes the extinction curve of the Seyfert 1 galaxy NGC 3227. The emission lines and continuum experience the same amount of reddening, indicating the presence of a dust screen that is external to the narrow-line region. Echelle spectra from the Space Telescope Imaging Spectrograph show intrinsic UV absorption lines due to Ly?, N V, C IV, Si IV, and Si III, centered at a radial velocity of -190 km s-1 (relative to the host galaxy). Photoionization models of the UV absorber indicate that it has a sufficient column (NH = 1.6 ? 1021 cm-2) and is at a sufficient distance from the nucleus (D > 95 pc) to be the source of the dust screen. Thus, Ark 564 contains a dusty lukewarm absorber similar to that seen in NGC 3227.
Highlights
Narrow-line Seyfert 1 (NLS1) galaxies were Ðrst introduced as a class of active galactic nuclei (AGNs) by Osterbrock & Pogge (1985)
The continuum spectrum of Ark 564 is clearly much Ñatter than that of Mrk 493 in the UV. This suggests that the spectrum of Ark 564 may experience a signiÐcant amount of internal reddening. This is supported by the Ðndings of Walter & Fink (1993), who conclude that Ark 564 is internally reddened based on the large observed Ha/Hb ratio (\4.4) and the small ratio of ultraviolet to X-ray continuum Ñux compared to most Seyfert 1 galaxies in their sample
The predicted column for Si IV is close to the observed value and, the predicted Si III column is somewhat high, it is within a factor of 2 of that observed
Summary
Narrow-line Seyfert 1 (NLS1) galaxies were Ðrst introduced as a class of active galactic nuclei (AGNs) by Osterbrock & Pogge (1985). Their optical spectra show narrow (FWHM \ 2000 km s~1) permitted and forbidden lines, similar to Seyfert 2 galaxies, but their emission-line ratios are more like those of Seyfert 1 galaxies. Like that in the broad-line region (BLR) of Seyfert 1 galaxies. These authors note that the Hb equivalent widths of NLS1s are smaller than typical values for normal Seyfert 1s, suggesting that they are not just normal Seyfert 1s seen at a special viewing angle. We use the HST observations to explore the internal reddening in Ark 564 and the physical conditions in the absorbing gas
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