CATEQUESIS DE LOS ANCIANOS

Dust lanes and spirals may play an important role in the reddening of the BLR in interme- diate types. In their HST broadband WFPC2 imaging survey of nearby Seyferts, Malkan et al. (1998) show that Seyfert 2 nuclei in their sample are more frequently blocked by lanes/patches of host galaxy dust than Seyfert 1 nuclei, and propose these galactic dust structures on scales of hundreds of parsecs could be a viable alternative to the classical parsec-scale torus model. Pogge & Martini (2002), using the technique of “structure map- ping” to enhance image contrast and draw out fine dust structures in the circumnuclear environment, show that essentially all of the Seyferts in the CfA Redshift Survey Sample have circumnuclear dust structures on scales of 100₋1000 pc scales (i.e. the scale size of the NLR). However, they find very few Seyfert 2 nuclei obscured by large-scale (> 1 kpc) dust structures, and do not see any significant differences in the circumnuclear dust morpholo- gies of Seyfert 1s and 2s. In any case, both these studies highlight the prevalence of dust structures on the size scale of the NLR, and emphasize that they are important sources of reddening to consider.

To identify hundred-parsec scale or larger sources of obscuration and determine their effect on a Seyfert’s 1.8/1.9 status, we downloaded any available WFPC2 broadband images of the galaxies in our sample. Twenty-nine galaxies in our sample had images taken with the F606W filter available, many of these observed for the snapshot survey of Malkan et al. (1998). The F606W filter is wide enough to cover several bright emission lines from high

surface brightness regions, and provide sufficient depth to reveal structure at subarcsecond resolution. For NGC 4639, no F606W data was available, so we downloaded and include an image taken with the F547M filter instead. Only five objects had no WFPC2 images available: Mrk 728, Mrk 883, Mrk 1018, Mrk 1179, and Mrk 1320. Those objects that were observed were taken using the PC1 camera (0.00046 resolution), with the exception of UGC 7064, whose nucleus slid off onto the WF 4 chip (0.00_{1 resolution).}

The downloaded images were already reduced by the standard STScI processing pipeline. The only further processing necessary was the removal of cosmic ray hits, which we performed using the IDL procedure IMGCLEAN to flag pixels with values 6σ above the sky value, and replace them with the average value of the surrounding 10 pixels.

To increase the contrast of these images and draw out dusty structures in the nucleus, we employed the “structure mapping” method of Pogge & Martini (2002). This method, based upon the Richardson-Lucy image restoration, removes most of the large-scale, smooth background light to bring out marginally resolved structures (i.e., structures on the same scale as the PSF). A copy of the original image is convolved with a model of the PSF, and the original image is then divided by this PSF-smoothed image. This ratio is then further con- volved with the transpose of the model PSF. Mathematically, the structure map is produced by the procedure

Structure Map = Image

Image⊗PSF⊗ PSF

t _(5.1)

where _⊗is the convolution operator and PSFt _{indicates the transpose of the PSF.}

We used IMEXAMINE routine in IRAF to fit 2-D Gaussians the point sources at the centers of the galaxies, to determine the location of the central source on the detector, and

then generated the PSF at each of these locations using the TinyTim software. These PSFs were then used in the above convolutions to generate the final structure map.

The final processed structure maps are shown in Appendix D. The dark regions show the location of dust obscuration, while the bright regions show areas of concentrated stellar light or emission line regions of the AGN.

The 1.8 galaxies Mrk 334 and UM 146 both show an unobscured central nucleus, but have surrounding dark dust spirals on the same scale as the NLR, in agreement with the higher NLR reddening compared to BLR reddening in these objects. Such sources of dust are also apparent in all the other objects in which the reddening of the NLR is greater than that of the BLR (Mrk 993, NGC 7314, UGC 12138, and UM 146).

For the cases where the reddening of the BLR is greater than the reddening of the NLR, we checked the images for sources of reddening in the NLR. In Mrk 423 (type 1.5), although lanes of dust can be traced into the central 500 pc, there is no obvious dust structure crossing the nucleus. In Mrk 915 (type 1.9), there is a dark lane/bar that runs across the nucleus, and could be the source of the higher reddening of the BLR. NGC 5033 (type 1.2) is in a highly inclined host, and the dust near the central point source may partially cover it.

NGC 3786 and have NGC 7314 (both 1.9s) have NLR/BLR reddening estimates consis- tent to within their (large) errors. In NGC 7314, the reddening of the BLR must be due, at least in part, to the dark lane that crosses over the nucleus seen in Fig. D.24. There is no obvious dust structure crossing the nucleus of NGC 3786.

To summarize, none of the 1.8s have dark lanes crossing their nuclei, although they all show signs of circumnuclear dust. Of the 1.9s, Mrk 915 and NGC 7314 have dark lanes that cross their nuclei, but in Mrk 471, NGC 1365, NGC 2622, NGC 3786, NGC 5252, and UGC

7064 there is no obvious dust structure crossing the nucleus. NGC 2622, NGC 5252 and UGC 7064 are classified as “internally reddened”, (see Chapter 6); if they are are internally reddened, it must be on a smaller size scale than the 100₋1000 pc structures seen in these images (possibly the torus).