In this section, we describe the HST/WFPC2 images and discuss each individual object in the sample. In view of the large volume of the combined ground- plus space-based data set, we opted to only present the Cycle 9 HST/WFPC2 images in the printed version of the present paper (Figs. 3). The electronic version contains the full "pan-chromatic atlas of nearby galaxies", that include UIT and ground-based UBVR(I) and JHK images, wherever available. Of these, we show in the printed version a few example pages (Figs. 4) to clarify the adopted format.
|Figure 3.01-3.02||Figure 3.03||Figure 3.04-3.05||Figure 3.06-3.07||Figure 3.08-3.09|
|Figure 3.10||Figure 3.11a-3.11b||Figure 3.12a-3.12b||Figure 3.13a-3.13b||Figure 3.14|
|Figure 3.15a-3.15b||Figure 3.16-3.17||Figure 3.18a-3.18b||Figure 3.19-3.20||Figure 3.21-3.22|
|Figure 3.23a-3.23b||Figure 3.24-3.25||Figure 3.26||Figure 3.27-3.28||Figure 3.29|
|Figure 3.30-3.31||Figure 3.32-3.33||Figure 3.34a-3.34b||Figure 3.35-3.36||Figure 3.37-3.38|
Figure 3. Atlas of HST WFPC2 observations of 37 nearby galaxies in the mid-UV F255W (only for the 11 galaxies in HST's CVZ) and F300W filters, and in the optical I-band (F814W) filter. The galaxies are sorted in the atlas approximately according to their observed morphological type following the discussion of the individual objects in Section 3. For the convenience of the reader, Table 2 cross-references the available galaxy data with the atlas page numbering. Each of the grey-scale renditions is displayed using a logarithmic stretch and the image scale and orientation on the sky are indicated. For some galaxies we will present images at two different image scales and stretches, in order to emphasize small-scale features discussed in the text (see Section 3). We also present images for some serendipitously observed galaxies.
--- MID-UV-FAINT ELLIPTICAL GALAXIES.
3.01 CGCG 097-094 (top): WFPC2 F300W and F814W. Serendipitous observation of an early-type galaxy. Also visible are an edge-on spiral galaxy and a face-on disk galaxy (both anonymous); 3.02 NGC 1396 (bottom): WFPC2 F300W and F814W.
3.03 NGC 4478 WFPC2 F300W (left) and F814W (right).
EARLY-TYPE GALAXIES WITH SIGNIFICANT AGN EMISSION IN THE MID-UV.
3.04 NGC 3516 (top): WFPC2 F255W, F300W and F814W; 3.05 UGC 03426 (bottom): WFPC2 F255W, F300W and F814W.
3.06 UGC 08823 (top): WFPC2 F300W and F814W.
EARLY-TYPE GALAXIES SHOWING EVIDENCE FOR RECENT MERGING OR STRONG INTERACTION.
3.07 NGC 3921 (bottom): WFPC2 F300W and F814W.
3.08 UGC 05101 (top): WFPC2 F255W, F300W and F814W; 3.09 UGC 08696 (bottom): WFPC2 F255W, F300W and F814W.
EARLY-TYPE GALAXIES DISPLAYING SPIRAL STRUCTURE IN THE MID-UV.
3.10 NGC 2551 WFPC2 F300W (left) and F814W (right).
GRAND-DESIGN SPIRAL GALAXIES.
3.11a NGC 6753 (top): WFPC2 F300W and F814W; 3.11b NGC 6753 (bottom): Nuclear portion of NGC~6753, emphasizing the dust lane crossing the inner spiral arms discussed in the text.
3.12a NGC 7769 (top): WFPC2 F300W and F814W; 3.12b NGC 7769 (bottom): Central portion of NGC~7769, emphasizing the dust lanes crossing the spiral arms and the dust lane crossing its small bulge.
3.13a NGC 3310 (top): WFPC2 F300W and F814W; 3.13b NGC 3310 (bottom): Nuclear portion of NGC 3310, emphasizing the spiral arm and nucleus crossing dust lanes.
3.14 NGC 7685 WFPC2 F300W (left}) and F814W (right}).
SPIRAL GALAXIES WITH INNER RINGS.
3.15a NGC 6782 (top): WFPC2 F255W, F300W and F814W;
3.15b NGC 6782 (bottom): Enlargement of the central
portion of NGC 6782, emphasizing the spectacular ring of hot, young
EDGE-ON SPIRAL GALAXIES.
3.16 UGC 10043 (top): WFPC2 F300W and F814W;
3.17 UGC 06697 (bottom): WFPC2 F300W and F814W.
3.18a IC 3949 (top) WFPC2 F300W and F814W. Note the
four highly inclined systems visible in F814W that have their major axes
pointing toward IC 3949;
3.18b IC 3949 (field) (bottom) Several serendipitously
observed early-type galaxies.
3.19 ESO 033-G022 (top): WFPC2 F300W and F814W;
3.20 IC 4393 (bottom): WFPC2 F300W and F814W.
LATE-TYPE SPIRALS AND SMALLER SPIRALS.
3.21 MCG+03-30-071 (top): WFPC2 F300W and F814W.
Also visible is neighboring early-type galaxy MCG+03-30-067.
Comparison of the F300W and F814W images of these two galaxies gives
the best example of the general trends seen in the present work;
3.22 MCG+06-24-047 (bottom): WFPC2 F300W and F814W.
The bright point source is a galactic (fore-ground) star.
3.23a UGC 05028 (top): WFPC2 F300W and F814W;
3.23b UGC 05029 (bottom): WFPC2 F300W and F814W.
3.24 NGC 386B (top): F300W and F814W;
3.25 ESO 418-G008 (bottom): WFPC2 F300W and F814W.
3.26 NGC 1510: WFPC2 F300W (left) and F814W (right).
MAGELLANIC IRREGULAR GALAXIES.
3.27 NGC 5253 (top): WFPC2 F300W and F814W;
3.28 NGC 1140 (bottom): WFPC2 F300W and F814W.
3.29 UGC 05626; WFPC2 F255W (left), F300W
(middle), and F814W (right).
3.30 UGC 09855 (top): WFPC2 F255W, F300W and F814W;
3.31 NGC 6789 (bottom): WFPC2 F255W, F300W and F814W.
3.32 Mrk 66 (top): WFPC2 F300W and F814W;
3.33 UGC 05189 (bottom): WFPC2 F300W and F814W.
INTERACTING, PECULIAR, AND MERGING GALAXIES:
3.34a NGC 3690/IC 694 (top): WFPC2 F255W, F300W and F814W;
3.34b NGC 3690/IC 694 (bottom): Enlargement of the central
portion of this merging galaxy pair, emphasizing the dusty patches and
star forming knots.
3.35 UGC 06527 (top): WFPC2 F300W and F814W;
3.36 UGC 08335 (bottom): WFPC2 F300W and F814W.
3.37 IC 2184 (top): WFPC2 F255W, F300W, and F814W;
3.38 NGC 5278/79 (bottom): WFPC2 F255W, F300W, and F814W.
SPIRAL GALAXIES WITH INNER RINGS.
3.15a NGC 6782 (top): WFPC2 F255W, F300W and F814W; 3.15b NGC 6782 (bottom): Enlargement of the central portion of NGC 6782, emphasizing the spectacular ring of hot, young stars.
EDGE-ON SPIRAL GALAXIES.
3.16 UGC 10043 (top): WFPC2 F300W and F814W; 3.17 UGC 06697 (bottom): WFPC2 F300W and F814W.
3.18a IC 3949 (top) WFPC2 F300W and F814W. Note the four highly inclined systems visible in F814W that have their major axes pointing toward IC 3949; 3.18b IC 3949 (field) (bottom) Several serendipitously observed early-type galaxies.
3.19 ESO 033-G022 (top): WFPC2 F300W and F814W; 3.20 IC 4393 (bottom): WFPC2 F300W and F814W.
LATE-TYPE SPIRALS AND SMALLER SPIRALS.
3.21 MCG+03-30-071 (top): WFPC2 F300W and F814W. Also visible is neighboring early-type galaxy MCG+03-30-067. Comparison of the F300W and F814W images of these two galaxies gives the best example of the general trends seen in the present work; 3.22 MCG+06-24-047 (bottom): WFPC2 F300W and F814W. The bright point source is a galactic (fore-ground) star.
3.23a UGC 05028 (top): WFPC2 F300W and F814W; 3.23b UGC 05029 (bottom): WFPC2 F300W and F814W.
3.24 NGC 386B (top): F300W and F814W; 3.25 ESO 418-G008 (bottom): WFPC2 F300W and F814W.
3.26 NGC 1510: WFPC2 F300W (left) and F814W (right).
MAGELLANIC IRREGULAR GALAXIES.
3.27 NGC 5253 (top): WFPC2 F300W and F814W; 3.28 NGC 1140 (bottom): WFPC2 F300W and F814W.
3.29 UGC 05626; WFPC2 F255W (left), F300W (middle), and F814W (right).
3.30 UGC 09855 (top): WFPC2 F255W, F300W and F814W; 3.31 NGC 6789 (bottom): WFPC2 F255W, F300W and F814W.
3.32 Mrk 66 (top): WFPC2 F300W and F814W; 3.33 UGC 05189 (bottom): WFPC2 F300W and F814W.
INTERACTING, PECULIAR, AND MERGING GALAXIES:
3.34a NGC 3690/IC 694 (top): WFPC2 F255W, F300W and F814W; 3.34b NGC 3690/IC 694 (bottom): Enlargement of the central portion of this merging galaxy pair, emphasizing the dusty patches and star forming knots.
3.35 UGC 06527 (top): WFPC2 F300W and F814W; 3.36 UGC 08335 (bottom): WFPC2 F300W and F814W.
3.37 IC 2184 (top): WFPC2 F255W, F300W, and F814W; 3.38 NGC 5278/79 (bottom): WFPC2 F255W, F300W, and F814W.
|Figure 4.01||Figure 4.02||Figure 4.03||Figure 4.04||Figure 4.05|
|Figure 4.06||Figure 4.07||Figure 4.08||Figure 4.09||Figure 4.10|
|Figure 4.11||Figure 4.12||Figure 4.13||Figure 4.14||Figure 4.15|
|Figure 4.16||Figure 4.17||Figure 4.18||Figure 4.19||Figure 4.20|
|Figure 4.21||Figure 4.22||Figure 4.23||Figure 4.24||Figure 4.25|
|Figure 4.26||Figure 4.27||Figure 4.28||Figure 4.29||Figure 4.30|
|Figure 4.31||Figure 4.32||Figure 4.33||Figure 4.34||Figure 4.35|
|Figure 4.36||Figure 4.37||Figure 4.38|
Figure 4. Pan-chromatic atlas of far- and mid-UV (UIT), mid-UV (HST / WFPC2), and ground-based optical UBVR and near-IR JHK (ground or HST / NICMOS) observations of the 37 nearby galaxies in the present sample. As in Fig. 3, the galaxies are sorted and numbered according to morphology, and each of the grey-scale renditions is displayed using a logarithmic stretch. Unlike Fig. 3, all images have been rotated to have North up and East to the left, and are presented on a common plate scale. The printed version contains only a few example pages; for the full atlas we refer the reader to the electronic version.
|MID-UV-FAINT ELLIPTICAL GALAXIES.|
|4.01 CGCG 097-094 (top): UIT 150nm and 250nm; (middle): HST / WFPC2 F300W, and ground-based U and B; (bottom): ground-based V and R, and HST / WFPC2 F814W.|
|4.02 NGC 1396 (top): UIT 150nm and 250nm, and HST / WFPC2 F300W; (bottom): photographic J (blue) and IIIaF+RG610 (red), and HST/WFPC2 F814W.|
|4.03 NGC 4478 (top): UIT 150nm and 250nm; (middle): WFPC2 F300W, and ground-based U and B; (bottom): ground-based V and R, and HST / WFPC2 F814W.|
|EARLY-TYPE GALAXIES WITH SIGNIFICANT AGN EMISSION IN THE MID-UV.|
|4.04 NGC 3516 (top): UIT 150nm, and HST / WFPC2 F255W; (2nd row): HST / WFPC2 F300W, and ground-based U and B; (3rd row): ground-based V and R, and HST / WFPC2 F814W; (bottom): HST / NICMOS F160W ($H$).|
|4.05 UGC 03426 (top): UIT 150nm and HST / WFPC2 F255W; (middle): HST / WFPC2 F300W, and ground-based U and B; (bottom): ground-based V and R, and HST / WFPC2 F814W.|
|4.06 UGC 08823 (top): UIT 150nm; (middle): HST/WFPC2 F300W, and ground-based U and B; (bottom): ground-based V and R, and HST / WFPC2 F814W.|
|EARLY-TYPE GALAXIES SHOWING EVIDENCE FOR RECENT MERGING OR STRONG INTERACTION.|
|4.07 NGC 3921 (top): HST / WFPC2 F300W, and ground-based B; (middle): ground-based V and R, and HST / WFPC2 F814W; (bottom): HST / NICMOS F110W (J) and F160W (H), and ground-based K.|
|4.08 UGC 05101 (top): HST / WFPC2 F255W; (2nd row): HST / WFPC2 F300W, and ground-based U and B; (3rd row): ground-based V and R, and HST/WFPC2 F814W; (bottom): HST/NICMOS F110W (J), F160W (H), and F220M (K).|
|4.09 UGC 08696 (top): HST/WFPC2 F255W; (2nd row): HST / WFPC2 F300W, and ground-based U and IIIaJ+GG385; (3rd row): ground-based R, and HST / WFPC2 F814W; (bottom): HST/NICMOS F110W (J), F160W (H), and F222M (K).|
|EARLY-TYPE GALAXIES DISPLAYING SPIRAL STRUCTURE IN THE MID-UV.|
|4.10 NGC 2551 (top): UIT 150nm and 250nm; (middle): HST / WFPC2 F300W, and ground-based U and B; (bottom): ground-based V and R, and HST / WFPC2 F814W.|
|GRAND-DESIGN SPIRAL GALAXIES.|
|4.11 NGC 6753 (top): HST / WFPC2 F300W, and ground-based U and B; (middle): ground-based V and R, and HST / WFPC2 F814W; (bottom): ground-based J, H, and K.|
|4.12 NGC 7769 (top): HST / WFPC2 F300W, and ground-based U and B; (middle): ground-based V and R, and HST / WFPC2 F814W; (bottom): ground-based I and H.|
|4.13 NGC 3310 (top): UIT 150nm; (2nd row): HST / WFPC2 F300W, and ground-based U and B; (3rd row): ground-based V and R, and HST / WFPC2 F814W; (bottom): HST / NICMOS F160W (H).|
|4.14 NGC 7685 (top): HST/WFPC2 F300W, and ground-based U and B; (middle): ground-based V and R, and HST / WFPC2 F814W; (bottom): ground-based I and K.|
|SPIRAL GALAXIES WITH INNER RINGS.|
|4.15 NGC 6782 (top): HST/WFPC2 F255W; (2nd row): HST/WFPC2 F300W, and ground-based U and B; (3rd row): ground-based V and R, and HST / WFPC2 F814W; (bottom): ground-based J, H and K.|
|EDGE-ON SPIRAL GALAXIES.|
|4.16 UGC 10043 (top): HST / WFPC2 F300W, and ground-based U and B; (bottom): ground-based V and R, and HST/WFPC2 F814W.|
|4.17 UGC 06697 (top): UIT 150nm and 250nm; (middle): HST / WFPC2 F300W, and ground-based U and B; (bottom): ground-based V and R, and HST / WFPC2 F814W.|
|4.18 IC 3949 (top): UIT 150nm; (middle): HST/WFPC2 F300W, and ground-based U and B; (bottom): ground-based V and R, and HST / WFPC2 F814W.|
|4.19 ESO 033-G022 (top): HST / WFPC2 F300W, and ground-based B; (bottom): ground-based V and I, and HST / WFPC2 F814W.|
|4.20 IC 4393 (top): HST/WFPC2 F300W, and ground-based U and B; (middle): ground-based V and I, and HST / WFPC2 F814W; (bottom): ground-based I and K.|
|LATE-TYPE SPIRALS AND SMALLER SPIRAL GALAXIES.|
|4.21 MCG+03-30-071 (top): UIT 150nm and 250nm; (middle): HST/WFPC2 F300W, and ground-based U and B; (bottom): ground-based V and I, and HST / WFPC2 F814W.|
The galaxies are presented roughly in order of their observed morphological type. We start with the regular early-type galaxies in Section 3.01-3.03 (and correspondingly numbered figures Figs. 3.01-3.03 and 4.01-4.03). Subsequent sections present early-type galaxies with significant AGN in the mid-UV (Section 3.04-3.06), and merger products which are likely to evolve into early-type remnants (Section 3.07-3.09). Next, we discuss early- through mid-type spiral galaxies showing spiral structure in the mid-UV (Section 3.10), grand design spirals (Section 3.11-3.14), a spiral with an inner ring (Section 3.15), and edge-on spiral galaxies (Section 3.16-3.20). We discuss the late-type- and dwarf spiral galaxies in Section 3.21-3.27, and the Magellanic Irregulars in Section 3.27-3.33. We finish with the peculiar galaxies and merging/interacting systems (Section 3.34-3.38).
Although they are not part of the selected sample, other galaxies that appear within the WFPC2 FOV will be discussed below (including their types), in context with the targeted galaxies of similar morphological type. In a few cases, our HST and ground-based images show the type listed in the RC3 to be incorrect, for various reasons explained below.
Most of our Cycle 9 mid-UV galaxies, except for some of their outskirts, fit inside a single WFPC2 CCD, and so the current paper shows only a single WFPC2 CCD in most Figures 3.nn-4.nn. However, a few edge-on galaxies are too large to fit on a single WFPC2 CCD. For these galaxies, the relevant parts of a full WFPC2 CCD mosaic are therefore shown instead in the current paper (see UGC 10043 in Fig. 3.16). Since the number of pixels we can display in this manner on one ApJ page does not do full justice to the high quality of the WFPC2 data, we will display in a sequel paper (Odewahn et al. 2002b) the multi-color WFPC2 mosaics in full-page size for the two remaining large edge-on galaxies (i.e., ESO 033 - G022 in Fig. 3.19 and IC 4394 in Fig. 3.20) and possibly for a few other large objects, as needed. This sequel paper will dither all WFPC2 mosaics for our sample of 37 objects, and do a detailed sky-subtraction across the CCD's, which is needed for reliable determination of the mosaic-wide multi-color light-profiles, as well as the subsequent quantitative analysis. This paper will thus show a few additional images not shown in the current paper.
The HST images, in general, have a very high dynamic range. This makes it difficult to properly display the full range of structures within a galaxy. Each of the grey-scale renditions in the atlas is displayed using an inverse logarithmic stretch with lower and upper display limits approximately equal to the sky-background (displayed as white) and the 3 times the maximum data value in the galaxies' brightest parts (black). This compromise allows one to discern individual high-SB structures within a galaxy while still being able to see much of the faint outer parts within a galaxy. The two unexposed edges of each WFPC2 image allow one to trace the total 80" × 80" (75" × 75" useful) FOV of a WFC CCD. When a galaxy is much smaller than the WFC FOV, or when there is too much morphological information contained within a single WFPC2 CCD, we (also) present the relevant portion of that CCD-frame, but enlarged by a factor of 2 or 4 (i.e., a 37" × 37" or 19" × 19" FOV).
The ground-based images (in Figs. 4) tend to have a much smaller dynamic range (because of the much higher sky-background and lower resolution). Nonetheless, we use a similarly defined stretch and display limits. Often, the extent of a galaxy becomes more obvious in these ground-based images than in the mid-UV images. In Figs. 4.01-4.38 all images are displayed on the same angular scale and at the same orientation, with North up and East to the left.
Mid-UV-faint Early-type Galaxies:
3.01 CGCG 097 - 094 (T = - 5; E) An early-type galaxy in Abell 1367 that is faint in the mid-UV compared to F814W. CGCG 097 - 094 was observed in the field of primary target MCG+03 - 30 - 071. Other cluster members are also visible (Figs. 3.01 and 4.01): the edge-on galaxy is still visible in F300W, suggesting that, despite its dust content, it still transmits some light at 2930Å. Most of this UV light may come just from the near edge of the disk (the side facing us). MCG+03 - 30 - 071 itself is discussed in Section 3.21, and edge-on galaxies are discussed further in Section 3.16-3.20 below.
The vertical image flaws (faint streaks) in the F300W image are due to WFPC2 Charge Transfer Efficiency (CTE) effects, the severity of which has significantly increased over the past years. They are visible in the high contrast stretches that were needed to show the fainter features of this and other objects in the atlas.
3.02 NGC 1396 (T = - 3; S0 - ) A lenticular galaxy that is faint in the mid-UV compared to F814W, indicative of an old stellar population. It has a weak nuclear point source. NGC 1396 is not detected at 150 nm in the UIT far-UV image of its bright neighbor NGC 1399, and is at best barely visible in the 250 nm UIT image. We do not have ground-based images as yet for this galaxy, and so substitute SERC IIIaJ and POSS-II IIIaF plate scans for the B and R images, respectively.
3.03 NGC 4478 (T = - 5; E) An elliptical galaxy that is faint in the mid-UV compared to F814W, although comparatively not as faint in F300W as some of the other early-type galaxies discussed. NGC 4478 is one of the giant ellipticals in the Virgo cluster, many of which have a UV-upturn in their far-UV spectrum (Burstein et al. 1988). This UV-upturn population is expected to be mostly visible below 2000Å and contributes less than 3-30%, and in most cases less than 10% of the total light at 2930Å (see Figs. 6-7 of Burstein et al. 1988). Another point source is seen just below the nucleus. It is very bright at F300W and in the UIT 250 nm image, and is likely a blue foreground star.
Early-type Galaxies with Significant AGN Emission in the Mid-UV:
3.04 NGC 3516 (T= - 2; S0) NGC 3516 is a well known classical Seyfert 1 galaxy (Seyfert 1943; Khachikian 1974; Keel & Weedman 1978; Kent 1985; Filippenko, 1985) that becomes essentially a point source in the HST mid-UV images. The AGN-dominated F300W image shows a strong central point source exhibiting diffraction spikes. A faint extension is seen in this filter, but not in F255W. The faint fuzz seen around the central point-source in the 150 nm UIT image is likely due in a significant part to the PSF caused by the far-UV optics plus photographic film of the UIT camera. The ground-based and F814W images show a faint bar with a position angle PA - 17. NGC 3516 has a companion galaxy with a small velocity difference (Keel 1996). It is also a well-known radio- and X-ray source (e.g., Taylor et al. 1996; White & Becker 1992; Laurent-Muehleisen 1997; Perlman et al. 1998; Radecke 1997), and is a low-luminosity AGN with the jet pointing close to the line-of-sight to the observer. Quantitative PSF-fits of all these AGN dominated objects will be given by Odewahn et al. (2002, in prep.), in conjunction with the HST+ground-based light-profiles for all our Cycle 9 sample.
3.05 UGC 03426 (a.k.a. Mrk 3; T = - 2; S0) An early-type Seyfert 2 galaxy with a blue, nuclear "disk-like" feature that may be associated with a bipolar outflow (Ruiz et al. 2001) rather than a nuclear star-forming disk. The feature is oriented roughly perpendicular to the galaxy major axis in F814W and is visible in both F300W and F255W. Note the apparent change in galaxy position angle blueward of the B filter in Fig. 4.05.
3.06 UGC 08823 (a.k.a. Mrk 279; T = - 2; S0) An early-type galaxy that becomes essentially a point source in the mid-UV. Note that almost no extended light is seen in F300W compared to F814W. It appears to be interacting with its neighbor MCG+12 - 13 - 024, with which it forms a physical pair (Keel 1996). It is a well known Seyfert 1 galaxy (Khachikian 1974; Osterbrock, 1977). For a high-resolution optical study of this object we refer the reader to Knapen, Shlosman, & Peletier (2000).
Early-type Galaxies with Evidence for Recent Merging or Strong Interaction:
The next three systems (sections 3.07-3.09) include morphologically peculiar objects with single stellar systems. We discuss these objects here, at the end of the sequence of early-types, because they will likely evolve into early-types on relatively short time scales ( 1 Gyr; see Barnes & Hernquist 1992), and likely already have developed r1/4 profiles in their inner parts (e.g., van Albada 1982, Windhorst et al. 1994b, 1998). Earlier stage interactions - in which the two galaxies are still separated - are discussed at the end of the sequence (sections 3.33-4.38), after the Spirals and Irregulars, since at high redshift such objects are more likely seen as interacting galaxies or peculiars.
3.07 NGC 3921 (a.k.a. Arp 224, Mrk 430, UGC 08823; T = 0; S0a) NGC 3921 is a disturbed early-type galaxy with a blue nuclear region which is visible in F300W. This object is a well-studied merger remnant (Schweizer 1996, Yun & Hibbard 2001a), and was probably produced by the merger of a gas-rich and gas-poor progenitor (Hibbard & van Gorkom 1996, Schweizer et al. 1996). It lies near the end of the Toomre Sequence of on-going mergers (Too mre 1977), and represents the final stage of merging, where there is a single nucleus but multiple tidal features. It will likely evolve into an elliptical galaxy (Schweizer 1996, Hibbard & Yun 1999a). The ground-based images in Fig. 4.07 show the fainter debris beyond the nuclear regions, including remaining tidal features. The near-IR images show a "wake" to the North of the bulge - possibly also a tidal feature from the same merger.
3.08 UGC 05101 (T = 11; Pec) This object is very faint in F300W and is not detected in F255W. The F814W images show that it has an inclined dusty disk. This object is an Ultra-Luminous Infrared Galaxy (ULIG) with LIR 1012 L (Sanders et al. 1988). Like NGC 3921, it appears to be a late stage merger remnant, with a single nucleus (Scoville et al. 2000) and multiple extended tidal features seen at larger scales in the ground-based images, including a tidal tail and a loop or polar ring-like structure (Sanders et al. 1988; Surace, Sanders & Evans 2000). The latter authors explain the large scale morphology as resulting from a "plunging" collision at non-zero impact parameter of two highly inclined disks.
Structures like these are typical for a merger in progress. The tails are thought to evolve on time-scales of > 1 Gyr while, once the halos overlap, the galaxy nuclei evolve on much shorter time-scales (of order 100 Myr). This results in one visible tidal tail per progenitor disk, while the other side of the disk gets pulled into a "bridge" before it disperses. A dusty disk, such as seen in UGC 05101, is a common feature of gas-rich merger remnants, e.g., NGC 2623 (Bryant & Scoville 1999), NGC 3256 (Zepf et al. 1999), NGC 7252 (Wang, Schweizer & Scoville 1992; Whitmore et al. 1993), and Mrk 273 and NGC 3310 (this paper). Disks have recently been reproduced in numerical simulations of merging galaxies (Naab & Burkert 2001; Barnes 2002), and hence their presence does not rule out a major merger origin, as was once thought.
3.09 UGC 08696 (a.k.a. Mrk 273, VV 851; T = 11; Pec) This object is faint in F300W and barely detected in F255W. The F300W morphology is very similar to the U' morphology presented in Surace & Sanders (2000). It appears to have a dusty disk, visible on one side of the nucleus in F814W. It is a merger product, as attested to by the tidal tail evident in F814W and in the ground-based images (Fig. 4.09). The horizontal streak in the ground-based R-band image is due to a bright star outside the FOV. Like UGC 05101, this system is a well known and well studied ULIG. But unlike the previous two systems, radio and near-IR imaging reveal two distinct nuclei separated by 1" (Majewski et al. 1993; Knapen et al. 1997; Scoville et al. 2000). One of the near-IR and radio nuclei coincides with the brightest peak in the F300W image, but the second near-IR nucleus is obscured, even at F814W. The southern peak in the F300W image has no optical or near-IR counterpart, and is probably a very young star cluster (Surace & Sanders 2000). CO-line mapping reveals the presence of two orthogonal kinematic components (Yun & Scoville 1995; Downes & Solomon 1998), suggesting a highly inclined encounter between two gas-rich system.
Early-type Galaxies Displaying Spiral Structure in the Mid-UV:
3.10 NGC 2551 (T = 0.2; S0a) An early-type spiral galaxy, whose spiral structure is much more pronounced in F300W than in F814W. In the optical/red, the galaxy would be classified as an S0a, in the mid-UV as an Scd, so its morphological K-correction is significant. NGC 2551's bulge is faint in the mid-UV compared to F814W, although it seems bluer than that of other early-type spirals. The background galaxy seen in F814W does not appear in F300W, but this does not necessarily mean that the disk of NGC 2551 is very dusty at this location - the background galaxy may just be as red as some of the other field galaxies seen in the background of our other images (we chose the exposure times in F300W and F814W to provide relatively high S/N for nearby galaxies, not for higher redshift ones). Fig. 4.10 shows that NGC 2551 was also detected with UIT at 250 nm, but probably not at 150 nm. The UIT resolution (FWHM ~ 3) is much lower than HST's, but on large scales, the UIT 250 nm morphology is comparable to our HST F300W image.
Grand-design Spiral Galaxies:
3.11 NGC 6753 (T = 3; Sb) Grand-design spiral galaxy with very pronounced spiral structure in both F814W and F300W. Dust-lanes are visible crossing the spiral arms in F814W, corresponding to regions of reduced or no light in F300W. A detailed discussion of the HST images of this object is given by Eskridge et al. (2002a). An enlargement of Fig. 3.11a is shown in Fig. 3.11b. In addition to dust lanes crossing the spiral arms in a regular pattern and with a fairly constant pitch angle, there is a large dust-lane crossing the inner spiral arms under a significantly different pitch angle, best seen in the F814W image. This is likely a foreground trail of dust (see also Section 3.38). Fig. 4.11 shows that the morphological K-correction is modest from F300W through the I-band, although in the near-IR the object appears to be of earlier type in ground-based seeing.
3.12 NGC 7769 (a.k.a. Mrk 9005; T = 3; Sb) Grand-design spiral galaxy with very significant spiral structure in F300W, which is more pronounced than that seen in F814W, suggesting active star-formation. In the optical/red, the galaxy would be classified as an Sb, in the mid-UV as an Scd, so its morphological K-correction is significant. The enlargement in Fig. 3.12b shows dust-lanes crossing the spiral arms in F814W, corresponding to regions of reduced or no light in F300W. The object also shows a faint nuclear dust-lane crossing its small bulge. NGC 7769 is interacting with SBa galaxy NGC 7771. H I mapping observations by Nordgren et al. (1997) show NGC 7771 to have an extended gaseous tail, while NGC 7769 counter-rotates with respect to NGC 7771. Therefore, this is thought to be a prograde-retrograde encounter, with NGC 7769 having retrograde kinematics. Note the possible emergence of a bar in the ground-based near-IR J and H-band images.
3.13 NGC 3310 (a.k.a. Arp 217, VV 356; T = 4; Sbc) This mid-type spiral galaxy is well-known to harbor a substantial global star-burst, likely either the remnant of a merger or a significant accretion event (Balick & Heckman 1981; Mulder, van Driel, & Braine 1995; Smith et al. 1996b; Kregel & Sancisi 2001; and references therein). Intense star formation is readily visible in the nuclear ring (Conselice et al. 2000), the spiral arms, and linear "arrow" feature (e.g., Mulder & van Driel 1996). H I mapping of this object shows extended H I tails (Kregel & Sancisi 2001), and it is likely that the spiral "arms" do not lie in a single plane. As a result of the global star-burst, the morphology of NGC 3310 remains similar in the F300W and F814W images. Some differences in structure between the red and mid-UV result from dust features, however. The enlargement in Fig. 3.13b shows that the galaxy nucleus appears displaced from the line of symmetry of the inner spiral arms. Fig. 3.13b also shows a small nuclear dust-lane crossing its small bulge, and dust-lanes crossing the spiral arms (some at significant pitch-angles) in F814W, corresponding to regions of reduced or no light in F300W.
The ground-based images in Fig. 4.13 show that the outer spiral arms are more complicated, and appear to dissolve in the southern part at all wavelengths. The far-UV morphology of NGC 3310 at UIT spatial resolution (Smith et al. 1996b) is also shown for comparison. This is an example of a galaxy that looks rather similar in all filters from the far-UV through the red.
3.14 NGC 7685 (T = 5.3; Sc) Barred late-type spiral with significant spiral structure in F300W and F814W. A small bar and nuclear bulge are visible in F814W. The bulge appears to be bisected by a small nuclear dust lane. Fig. 4.14 shows that the morphological K-correction is modest from F300W through the K-band.
Spiral galaxies with inner rings:
3.15 NGC 6782 (T = 0.8; Sa) Early-type spiral galaxy with a spectacular ring structure in the mid-UV, visible in both F300W and F255W. An enlargement of Fig. 3.15a is shown in Fig. 3.15b. This ring is surrounded by two faint and apparently partially dusty spiral arms as visible in F814W, and an outer spiral structure and outer ring that is visible in F300W and in the optical pass-bands (Fig. 4.15). The inner ring appears to be driven by a small bar visible in F814W and F300W at PA = - 30°. The ground-based images in Fig. 4.15 show that the outer ring encloses an outer bar, with a position angle (PA 0°) that differs from that of the inner bar visible in the HST images. A detailed discussion of the HST images of this object is given by Eskridge et al. (2002a). In depth studies of galaxies with such star-forming rings are given by Buta et al. (e.g., 1996, 1998a, b, 2000, 2001). Fig. 4.15 suggests a significant morphological K-correction from F255W through the K-band, most of which is caused by the ring, but with the bars being more prevalent at the longer wavelengths, as noticed in the ground-based near-IR study of Eskridge et al. (2000).
Edge-on spiral galaxies:
Here, we discuss edge-on galaxies as a single morphological class to allow easier comparison, despite the fact that their intrinsic T-types may span most of the Hubble sequence.
3.16 UGC 10043 (T = 4.0; Sbc pec) An edge-on spiral galaxy that is faint in F300W, although it is still visible in this filter almost throughout the entire disk. The central bulge is relatively small in size and faint in the mid-UV, as for most of the other edge-on galaxies in our sample. The PA of the (major axis of the) bulge is approximately perpendicular to that of the disk, which is most clearly seen in the HST observations when compared to the ground-based images (Fig. 4.16). This may indicate a polar ring, which interpretation we hope to confirm with spectroscopy in a future paper. The F814W images show significant dust lanes, mostly in the inner parts of the edge-on disk. Compared to F814W, the F300W flux shines through better in the outskirts than in the central part of the disk, and in particular, it shines through well in several bright knots. It is also possible that, instead, the knots visible in the mid-UV are in front of most of the dust.
Qualitatively, it appears that the dust extinction - if this relative dimming of the UV-light is indeed due to dust extinction - decreases from the inside outwards (see also Jansen et al. 1994). Examples of these edge-on objects have been studied in the optical by de Grijs (1999). In the mid-UV, edge-on disk galaxies have a large range in brightness. Some are particularly bright in the UV (e.g., UGC 06697, Marcum et al. 2001; NGC 4631, Smith et al. 2001), while others are much fainter, such as some of the edge-on galaxies shown in the current paper. As pointed out by Kuchinski et al. (2001), it is as yet unclear to which degree dust attenuation affects the appearance of highly inclined galaxies at UV wavelengths. This is due to a likely complex dependence on the distribution of dust and actively star-forming regions throughout a galactic disk (Kuchinski et al. 2001). Due to the lack of a statistically significant population of edge-on galaxies observed in mid-UV light, a detailed discussion of this effect is beyond the scope of the present paper, but will be addressed in a future paper when more edge-on objects have been observed in the mid-UV.
3.17 UGC 06697 (T = 10; Im) Late-type edge-on galaxy with almost no bulge and with significant dust-lanes seen by comparing F300W to F814W. Although this galaxy is bright in the UV and was observed both at 2500Å and 1500Å by UIT, comparison of the F300W and F814W images shows that several regions seem to be completely obscured by dust. They appear to be dust pockets that bisect the disk. The ground-based images in Fig. 4.17 show a significant warp on larger scales, which is possibly related to the two or three companion galaxies visible within the FOV. On balance, though, the overall appearance of this object is very similar from the far-UV through F814W.
3.18 IC 3949 (T = - 2; S0 pec) Edge-on spiral with a very faint bulge, if any, and significant dust-lanes (compare the F300W and F814W image), especially in the lower (western) part of the displayed image. The plane of the dust appears to be warped, and asymmetric in the western part of the image compared to the upper (eastern) part. The biggest dust pocket to the lower right of the center is also visible as a depression in the light in the ground-based UBVR images (Fig. 4.18).
Four faint edge-on galaxies are seen to the left of IC 3949 in F814W (Fig. 3.18a), two of which are barely visible in F300W. It is perhaps noteworthy that all four of these highly inclined galaxies point their major axis towards the central part of IC 3949. Although this is probably a chance alignment and the edge-on galaxies are probably background galaxies, it would be interesting to test this hypothesis with further kinematic data. If these systems turn out to be at a similar distance as IC 3949, they might be smaller dwarf-like objects falling into IC 3949, perhaps disturbing its disk as the irregularly distributed edge-on dust-lanes might suggest. Schwarzkopf & Dettmar (2000) found that edge-on galaxies have on average 60% thicker disks and are ~ 2 × more likely to have warps when they have low mass companions, which will likely soon result in minor mergers.
Fig. 3.18b shows other early-type objects surrounding IC 3949, which, like other early-type galaxies in our HST sample, have dim light distributions in F300W. The RC3 incorrectly classifies IC 3949 as S0 pec, which may be due to confusion with the brighter of these early-type galaxies, although this object is fainter than the edge-on galaxy. These objects were selected as field ellipticals and do not show AGN in the UV (unlike the others in our sample), although the two brightest ones show a small central disk or boxy isophotal structure.
3.19 ESO 033 - G022 (T = 7; Sd) Edge-on late-type spiral galaxy that is faint in F300W, although it is still barely visible in this filter throughout most of the disk. There is almost no central bulge. The F814W images show a thin dust lane in the plane of the galaxy, mostly in the inner parts of the edge-on disk. Compared to F814W, the F300W flux shines through better in the outskirts than in the central part of the disk, and in particular shines through well in several bright knots. In the ground-based images for this low Galactic latitude object from de Grijs, Peletier & van der Kruit (1997) (see Fig. 4.19), the bright stars (which are apparent in the F814W HST image) have been removed to allow for more accurate surface photometry.
3.20 IC 4394 (a.k.a. ESO 446 - G044; T = 6; Scd) Edge-on spiral galaxy that shows a thick dust lane in the plane of the galaxy in F814W, mostly in the inner parts of the edge-on disk. This galaxy is fainter in F300W than F814W as well, but comparatively not nearly as much as most of the other edge-on galaxies. This could be because its inclination is not as close to 90 as some of the others, allowing a more unimpeded view of the bright star forming regions located on the near side of the galaxy plane. Hence, the relative bright appearance in F300W does not imply a smaller dust content than inferred in other edge-ons. Almost no central bulge is visible. Compared to F814W, the F300W flux shines through better in the outskirts than in the central parts of the disk, and in particular in several bright knots, more so than seen in the other galaxies.
Late-type spirals and smaller spirals:
3.21 MCG+03 - 30 - 071 (T = 10; Im) Small late-type spiral galaxy in Abell 1367, displaying a four-armed spiral pattern that dissolves into individual star forming knots in F300W. MCG+03 - 30 - 071 was detected by UIT both at 1500Å and at 2500Å. There are several galaxies at small projected distances to this galaxy, but the smallest difference in velocity is 715 km s - 1 (for the early-type neighbor ~ 40 to the SW, MCG+03 - 30 - 067, which is also visible in the FOV). Comparing these two galaxies from the far-UV to F814W gives the best example of the general trends observed in this work (see the discussion in Section 4).
3.22 MCG+06 - 24 - 047 (T = 4; SBc) Small spiral galaxy with well-developed spiral structure that is more pronounced in F300W, although still clearly visible in F814W. The classification in the RC3 as T = 11/Pec probably resulted from the bright foreground star ~ 7 south of the nucleus. In the mid-UV this galaxy would be classified as a Sd/Irr rather than SBc, so the morphological K-correction is non-zero. The ground-based UBVR images (Fig. 4.22) show a rather modest dependence of the amplitude of the spiral structure on rest-frame wavelength, so most of the morphological K-correction is caused by the stellar population below the Balmer break. A small bulge is visible in F814W. The bright star is blue (it is even marginally detected at 1500Å), so that the F300W appearance is not significantly affected by the filters' red-leak.
3.23 UGC 05028/29 (a.k.a. Arp 300, VV 106; T = 3+9; Sbc+SBdm/Pec) Fig. 3.23a shows the small late-type/peculiar spiral galaxy UGC 05028 (Mrk 111) of the pair Arp 300 (see Fig. 4.23). It has spiral structure that is more pronounced in F300W although still clearly visible in F814W. Ground-based seeing hides most of the irregular nature of this object. No bulge is visible in F814W, although a small central bar-like structure is visible in both filters. The brightest "knot" in F300W and particularly in F814W, south of the center, is well resolved. Although reddening by dust plays a significant role, this "knot" may be the remnant of another small late-type galaxy that is merging with UGC 05028, as its color is similar to that of the small bulge of UGC 05028's larger companion UGC 05029. If so, this knot and the bar will eventually merge and form a central bulge. As presently observed UGC 05028 is extremely asymmetric.
Fig. 3.23b shows the second member of the Arp 300 system, Sbc spiral galaxy UGC 05029, which is a physical companion to UGC 05028. The spiral structure in this galaxy is more pronounced in F300W, although it is clearly also visible in F814W, like for some of the other early-mid type spirals discussed above (e.g., NGC 2551). Most of the blue OB associations are located on the side facing UGC 05028. Another edge-on field galaxy is seen in Fig. 4.23, but is too faint to be resolved into star-forming regions. The five rather red objects between this edge-on galaxy and the bright mid-type spiral are likely a group of background galaxies - they are not seen in F300W at all (see the discussion of other background objects above).
3.24 NGC 3860B (a.k.a. CGCG 97 - 114; T = 10; Im) Small late-type spiral galaxy with spiral structure that is more pronounced in F300W, although it is still visible in F814W. Its rather late classification probably resulted from the fact that the spiral structure is only barely visible in the ground-based images. The UIT, HST and ground-based images show only a modest dependence of morphology on rest-frame wavelength. A very small bulge is visible in F814W (Fig. 3.24).
3.25 ESO 418 - G008 (T = 8; Sdm) Small late-type spiral galaxy that is resolved in at least its brighter star-forming regions (Fig. 3.25), which are mostly distributed along its outer perimeter, and perhaps constitute the beginning of spiral structure. The F814W image also suggests a faint bar running along the minor axis that may be connected to this spiral structure. The B and V images of Matthews & Gallagher (1997) show that ESO 418 - G008 has a bright, prominent bar (see also Fig. 4.25). A faint low-SB structure is visible in F814W that is spread more smoothly throughout the galaxy, presumably its unresolved older stellar population. A color map made from our HST data (oposite.stsci.edu/pubinfo/pr/2001/04/) reveals a very faint red nucleus. Such compact nuclei are common features of extreme late-type spirals (Matthews & Gallagher 1997; Matthews et al. 1999; Böker et al. 2002). The nucleus of ESO 418-G008 was not previously visible in ground-based images (Matthews & Gallagher 1997), but Böker et al. (2002) have recently imaged it in the F814W filter with the WFPC2 PC. They measure MI = - 10.24 mag, assuming a distance of 14.1 Mpc.
The galaxy appears to have sharp "edges" in both filters, but this may be largely a visual impression caused by the single star-forming region to the lower left (north) in Fig. 3.25. The ground-based UBVR images show similarly an outer spiral-like structure that becomes less and an inner bar that becomes more pronounced at longer wavelengths. A late-type galaxy with a modest morphological K-correction.
3.26 NGC 1510 (T = -2.3; S0 pec) Small blue, amorphous galaxy (Eichendorf & Nieto 1984). NGC 1510 is a physical companion to the much larger SB galaxy NGC 1512, with which it is interacting (Hawarden et al. 1979). The early-type classification in the RC3 probably resulted from ground-based seeing hiding most of the irregular nature of this object. No clear bulge is visible. NGC 1510 has fairly similar appearance in F814W and F300W, but the F814W image shows more of a diffuse component, while the F300W resembles more a disk-like component seen edge-on, and shows some linear dust features. The object is resolved in at least its brighter star-forming regions. Scans of ground-based photographic plates in B (from Lauberts & Valentijn 1989, as obtained through NED) and in RF (from the second generation Digital Sky Survey) are substituted for CCD images in Fig. 4.26.
3.27 NGC 5253 (a.k.a. Haro 10; T = 10; Im) NGC 5253 is a peculiar I0 galaxy or type II irregular. This object has a very similar appearance in F300W and F814W, and is clearly resolved into individual bright star-forming regions or stars. Table 1 shows that it is one of the closest galaxies in our sample. The dust lane that appears to emerge from one side of the center outwards is visible at all wavelengths, including in the lower-resolution ground-based UBVR images (Fig. 4.27). Overall, this object has a largely similar appearance from UIT 150 nm through the ground-based K-band, with some mild wavelength dependence modulations due to dust patches.
This object was first described after discovery of a nearby nova (Hubble & Lundmark, 1923; Payne-Gaposchkin, 1936). NGC 5253 is a a well studied nearby star-bursting galaxy at a distance of 4.1 Mpc (Sandage et al. 1994). It is typically classified as an amorphous or irregular galaxy, but has outer isophotes reminiscent of a dwarf elliptical (Sersic et al. 1972; Caldwell & Phillips 1989). It has a star-burst located at its center, with dozens of intense blue star forming clusters (Caldwell & Phillips 1989; Storchi-Bergmann et al. 1995; Calzetti et al. 1995). NGC 5253 also contains an unusual H morphology (Marlowe et al. 1995; Calzetti et al. 1995) with loops and filaments and a diffuse component that is at least partially produced from shocks (Calzetti et al. 1999).
Recent HST images in optical broad-band and line-emission show that this object is a small star-bursting galaxy (Beck et al. 1996; Calzetti et al. 1997, 1999; Gorjian, 1996). The galaxy was detected by IRAS (IRAS 13370-3123 in the IRAS Point Source Catalog; Beichman et al. 1988), and is a weak radio source in the Parkes-MIT-NRAO 4.85 GHz Survey (Wright et al. 1996), and also had a FAUST Far-UV point source 0.5 from the galaxy (Bowyer et al. 1995). Other recent studies include a systematic H I study (Kobulnicky & Skillman 1995), radio observations of a star-bursting knot that has no optical counterpart (Turner et al. 1998, 2000), as well as a CO-map (Turner et al. 1997), ROSAT X-ray studies of multiple super-bubbles in its star-burst nucleus (Strickland 1999), and ISO observations of its hot stars and Wolf-Rayet like outflow (Crowther et al. 1999), amongst others.
Only 130 kpc away is the larger spiral galaxy NGC 5236 (M 83) which also experiences a star-burst in its nuclear regions. NGC 5253 has a large outer distribution of H I gas that could be feeding the central star-burst in M 83 (Kobulnicky & Skillman 1995). Also, the bar in M 83 might be triggering its central star-burst. However, there is evidence that the star-bursting properties in this system are produced in part by a dynamical interaction between the two objects (Caldwell & Phillips 1989).
3.28 NGC 1140 (a.k.a. Mrk 1063, VV 482; T = 10; Im) Irregular galaxy. This object has a fairly similar appearance from the mid-UV through the I-band (Fig. 4.28). In F300W and F814W (Fig. 3.28) the object is resolved in at least its brighter star-forming regions. The "super star clusters" visible here were studied using HST optical-band imaging by Hunter, O'Connell, & Gallagher (1994). The larger field-of-view in the ground-based images also shows a trail of knots to the south-west, possibly an infalling dwarf companion.
3.29 UGC 05626 (T = 10; Im) Irregular galaxy. The object is resolved in at least its brighter star-forming regions which are mostly draped around its edges as seen in F300W. The bluest and brightest star-forming knots to the left (north) are also visible in F255W. In F814W, the object also shows a faint low-SB structure which is spread more smoothly throughout the galaxy, presumably its unresolved older stellar population. No central bulge is visible at any wavelength. The structure in the ground-based UBVR images (Fig. 4.29) is similarly independent of wavelength, and shows the outer rings of knots, resembling the beginning of spiral structure. It is curious that such spiral structure would start before even a small central bulge or bar has formed, and leads one to wonder about the dark matter content of (the central parts of) this galaxy.
3.30 UGC 09855 (T = 10; Im) Magellanic type irregular. The object is resolved in at least its brighter star-forming regions. The brightest of these regions are also visible in F255W. The F814W shows a faint central bulge that is not quite aligned with the geometrical center of the object (see also the ground-based images in Fig. 4.30, and the disk of UGC 05626 in Section 3.29).
3.31 NGC 6789 (T = 10; Im) Magellanic type irregular. The object is resolved in at least its brighter star-forming regions. The brightest ones of these are also (but barely so) visible in F255W. The structure in the ground-based UBVR images (Fig. 4.31) is rather independent of wavelength, and no central bulge is visible at any wavelength.
3.32 Mrk 66 (T = 11; Pec) This object shows a significant number of blue star-forming knots, the one to the left (west) of the center (Fig. 3.31, F814W) may be part of a faint small bulge. The knots to the right (east) are isolated from the main object in both filters, and may be separated from the main body by a dust-lane or pocket, as is also seen in the ground-based images (Fig. 4.32). It is not clear whether this galaxy is a small linear object or a late-type galaxy seen edge-on.
The ground-based images provide some additional clues, since in all UBVR filters, a low-SB disk or halo is seen surrounding the rather linear structure seen in F300W. If this rounder feature were a halo, then Mrk 66 could be a tri-axial amorphous galaxy, although the fact that this feature is also visible in the ground-based U-band filter below the 4000Å-break would imply a rather young halo age of 1-2 Gyrs. Alternatively, if this rounder feature is a face-on disk, then the linear feature that is primarily visible in F300W could be a star-forming bar. In that case, the rounder feature would have to be a rather old disk (~ 1 Gyr), since no spiral arm structure is visible at the same location in the F300W image. Spatially resolved spectroscopy and kinematics will be necessary to distinguish between these possibilities.
In combination with the HST F300W and F814W images, the ground-based UBVR images confirm that, to first order, this object has little morphological K-correction, i.e., its appearance is rather similar at most wavelengths. However, significant SB-dimming at high redshifts could change a roundish object like Mrk 66 as seen in visible light into a more linear object in the mid-UV, and so help explain the appearance of some of the "chain" galaxies seen at high redshifts (Cowie, Hu, & Songaila 1995; see also Dalcanton & Shectman 1996).
3.33 UGC 05189 (a.k.a. VV 547; T = 10; Im) Like Mrk 66, the HST images of this object show a significant number of blue star-forming knots. The ground-based UBVR images (Fig. 4.33) show that the arc imaged with WFPC2 is in fact part of a much larger, low-SB structure. This is clearly a very dynamically disturbed system. There is no obvious central bulge. In combination with the HST F300W and F814W images, the ground-based UBVR images confirm that this object has almost no morphological K-correction, i.e., its appearance is about the same at all wavelengths. Its light must be dominated by young hot stars throughout.
Interacting, Peculiar, and Merging Galaxies:
3.34 NGC 3690/IC 0694 (a.k.a. Arp 299, VV 118; T = 9; Sm) Merger of two late-type disk galaxies. The object has a very similar morphology in F814W, F300W, and F255W, although the sensitivity in F255W is much reduced. Comparison of the F300W and F814W images reveals significant dust patches, most of which are irregularly distributed, as shown in the enlargements in Fig. 3.34b. This figure also shows the numerous young super star clusters spread throughout the system (e.g., Meurer et al. 1995).
The objects somewhat resemble the Antennae Galaxies (NGC 4038/39), but the encounter geometry is more complicated and the merger is slightly more advanced in Arp 299. H I and CO kinematics suggest a prograde-retrograde or prograde-polar encounter (Hibbard & Yun 1999b, Casoli et al. 1999). There are no obvious bulges in the mid-UV-optical images, but NIR imaging reveals a bulge in at least the easternmost system (Smith et al. 1996a), which is quite hidden by dust in all filters blueward of ~ 1. Several of the bluest star-forming knots shine through clearly in F300W and F255W, although most of the galaxy shows significantly dusty regions, where such star-bursting knots would be much more reddened.
3.35 UGC 06527 (a.k.a. Arp 322, Mrk 176, VV 150, HCG 056; T = 11; Pec) Spiral galaxy in a Hickson compact group that is undergoing a merger with at least one of its companions (e.g., Fasano & Bettoni 1994, Allam et al. 1996). Its spiral disk and arms are distorted into tidal tails because of this encounter, as also implied by UV+photographic numerical simulations of such events (e.g., Barnes & Hernquist 1992). When comparing F814W to F300W, the inner bulge shows a dusty inner disk-like structure. The galaxy is not resolved into its brightest star-forming regions or stars, due to its larger distance. The ground-based images in Fig. 4.35 suggests a small morphological K-correction, if any.
3.36 UGC 08335 (a.k.a. Arp 238, VV 250; T = 4; Sbc) Two strongly interacting disk galaxies of approximately equal luminosity. Both disks are distorted during this encounter and display tidal tails (Fig. 4.36), suggesting a prograde-prograde encounter (e.g., Barnes & Hernquist 1992). The RC3 morphological type probably results from confusing the tidal arms with spiral structure or the two nuclei with a central bar (Fig. 4.36) in ground-based images. When comparing F814W to F300W, the inner bulges of both systems show significant dust-lanes, possibly from gas-rich material raised during the encounter. The system is too distant to be resolved into its brightest star-forming regions or stars.
3.37 IC 2184 (a.k.a. Mrk 8, VV 644; T = 9; Sm) The "flying V" is probably a merger of two disk galaxies seen nearly edge-on, as indicated by the presence of pronounced tidal tails in deep ground-based images (Gallagher et al. 2000). Star formation is active in most of both of the disks, and in the F255W image only the brightest star-forming knots are seen due to the reduced sensitivity of WFPC2 in this filter. Dust patches are visible in F814W, corresponding to reduced or no light in F300W and F255W at those locations. The ground-based UBVR images (Figs. 4.37) similarly show a very modest dependence of the morphology on rest-frame wavelength.
3.38 NGC 5278/79 (a.k.a. Arp 239, Mrk 271, VV 19; T = 3; Sb) This is one of the most curious objects in our sample. Like UGC 08335, this system consists of two strongly interacting spiral galaxies. The encounter clear distorts both galaxy disks, resulting in tidal tails and exciting a strong m=1 mode in the larger of the two galaxies, similar to features seen in numerical simulations of such events (Zeltwanger, Comins, & Lovelace, 2000; see also Phookun et al. 1992). When comparing F814W to F300W, both the inner bulges show significant dust-lanes. Part of the dust appears to be pulled out along with one of the spiral arms in the tidal encounter, possibly due to a shock related to this encounter.
Most remarkable is the very curved thin dust-lane, best seen in R and F814W, that drapes across the bigger galaxy. This dust-lane starts somewhere in the southern spiral arm of the bigger galaxy (upper or southern part in the WFPC2 CCD as shown in Fig. 3.38), curves all the way across the small nuclear bulge of this galaxy, and makes it into the spiral arm to the lower left that appears to connect to the smaller galaxy. Careful comparison of the F300W and F814W images shows that the dust-lane starts in what appears to be at least two funnel-like regions from the southern arm of the bigger galaxy. The dust in the southern arm appears to be distributed non-uniformly along the arm in several (3-4) sizeable "dust pockets or patches", which appear to merge into the dust lane and tidal tail that wraps around the bigger galaxy's nuclear bulge. A somewhat similar but much thinner dust trail was seen in HST images of the colliding nearby galaxy pair NGC 1409/1420 (Keel 2000), with material being pulled out between the two galaxies.
These "dust pockets or patches" are clearly visible in the data of the F300W images, and at the same location, the F255W images are entirely devoid of flux, leading credence to the conjecture that dust pockets cause these depressions in the light distribution. The "patches" are best seen by blinking the F255W, F300W, and F814W images in rapid succession, each displayed at an appropriate logarithmic stretch. The bluer ground-based images (Fig. 4.38) show a similarly diminished surface brightness at the location of the dust patches.