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S.B. Kraemer, D.M. Crenshaw, F.C. Bruhweiler, J.R. Ruiz (Catholic University of America/NASA's GSFC)
We have obtained low-dispersion spectra of NGC~3227 with the Space Telescope Imaging Spectrograph (STIS) to study the intrinsic UV absorption and the reddening of the nucleus in this Seyfert 1 galaxy. The UV spectra show a wealth of absorption lines at the systemic redshift that span a wide range in ionization state (Mg~I to N~V). The equivalent widths of the lines are consistent with our earlier prediction that a ``lukewarm absorber''(Te = 18,000 K at the ionized face) with a substantial column of gas (NH = 2 x 1021 cm-2) is present and likely responsible for the reddening of the nucleus. The lukewarm absorber is also responsible for most of the absorption in the X-rays at energies less than 1 keV, although a more highly ionized ``warm absorber'' is needed to account for the O~VII and O~VIII ionization edges. In addition, we require a small column (NH = 5 x 1019 cm-2) of cold gas to match the strengths of the neutral and singly-ionized lines in the UV spectra. NGC 3227 is the first Seyfert galaxy in which a strong link between the reddening and intrinsic UV absorption has been found. By comparing our STIS UV and optical spectra with those of the unreddened Seyfert NGC 4151, we have determined a reddening curve for the nuclear continuum source in NGC 3227 over the 1150 -- 10,200 Å\ range. The reddening curve does not show a 2200 Å\ bump, and is steeper in the UV than reddening curves derived for the Galaxy, LMC, and SMC, suggesting a preponderence of small dust grains near the nucleus.
The author(s) of this abstract have provided an email address for comments about the abstract: stiskraemer@yancey.gsfc.nasa.gov