Volume 569, September 2014
|Number of page(s)||18|
|Published online||16 September 2014|
We supplement our morphological remarks of Paper I by calling attention to structures that have either not been noted or that have been mentioned with an unclear interpretation. In the following, we use the designations of Schweizer (1980, see Fig. A.1. Additionally, we introduce O1 as “object 1”.
Already Mackie & Fabbiano (1998) showed residuals from an elliptical model, based on a photographic B-plate. Here we see in more detail the complex structures which become visible after the subtraction of the elliptical model from Paper I. Figure A.1 shows the wide field, while Fig. A.2 demonstrates the structure in the inner parts. The shell system was first described by Schweizer (1980). He identified two ripples on the south-western part, we see at least four. Striking is the L2-structure in its full extension. In the epoch of Schweizer’s paper, computer simulations of galaxy interactions were just at their very beginning. Today we identify L2 as the long tidal tail of an infalling dwarf galaxy. Morphologically, it might be connected either to NGC 1317 or to Schweizer’s ripple R2, which Schweizer suggested, but if NGC 1317 were related to this tidal structure, we would not expect such a seemingly undisturbed spiral structure. The physical link to R2 is also doubtful, given the quite different widths of the structures in the area of overlap.
Shells appear in simulations as caustics in phase space after the infall of a dwarf galaxy on a radial orbit into the potential of a larger galaxy (e.g. Sanderson & Helmi 2013). A morphological characteristic of these caustics are the sharp outer boundaries which are the turn-around points of stellar orbits. Indeed, we find these sharp boundaries in NGC 1316 at the well-known southern L1-feature, which accordingly has to be interpreted as the remnant of a smaller galaxy, but we also find them at some locations in the shell system, most strikingly in the region of the plume. It is difficult to see how radial orbits can play a role in this case. It might be of significance that the plume itself (which is probably an infalling dwarf given its population properties, see Paper I) has a radial structure.
The ripples are suprisingly coherent. Following the outer shell clockwise, starting at the plume position, one is led on a spiral-like pattern to the inner region. Interestingly, this path avoids the ripple R2.
Another pattern, which occurs in simulations (e.g. Sanderson & Helmi 2013), are the T-like features. One conspicious example is located between L2 and L4.
Besides the plume region, the brightest residual is found at 50′′ west, 90′′ south, labelled “O1”. Figure A.3 shows the region of this object. It is striking that O1 exhibits a larger density of sources than are found in its environment.
Residuals in R after subtraction of a smooth elliptical model. North is up, east to the left. The designations are from Schweizer (1980). This global view shows Schweizer’s L2 structure as a tidal tail.
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Residuals in R of the inner part. The dynamical range is chosen to make the shells better visible. We note particularly the sharp boundary of the shell R2.
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Amplified region around O1. We note the apparently larger number of sources projected onto O1, which may be star clusters. One may speculate that O1 is the remnant of a dwarf galaxy.
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To our knowledge the companion galaxy had never been the topic of a dedicated publication, although many highly interesting features can be identified. Morphological and stellar population aspects have been discussed by e.g. Schweizer (1980), Marcum et al. (2001), Papovich et al. (2003) and particularly by Laurikainen et al. (2006), who used near-infrared filters. Here we briefly add a few morphological remarks on NGC 1317 on the basis of a colour map, as has been done for NGC 1316 in Paper I, and HST-images, which to our knowledge have not yet been shown in the literature. NGC 1317 is a double-barred spiral galaxy with star formation occurring within a ring-like area. In Fig. A.4, the upper panel is a C-image, showing the dust structures more clearly, while the lower panel is a C − R colour image (compare the colour image of NGC 1316 in Paper I). It is striking that the ellipticity of these two images is so different. We attribute this to the outer secondary bar (Laurikainen et al. 2006), which produces the ellipticity, even though it is not distinct in colour from the overall population.
Upper panel: our C-image of NGC 1317, where because of the higher extinction the dust features are much more visible than in the R-image. North is up, east to the left. The scale is valid for both panels. Lower panel: colour map using Washington C and Harris R (Paper I). The dynamical range of the colour wedge is 1.3 < C − R < 2. Blue is dark, red is bright. We note the inner star forming ring, which appears black. The bright features denote dust patterns. Outside the ring, additional young populations are visible as black spots arranged in a ring-like fashion on the eastern side.
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At higher HST/WPC2 resolution, the ring is resolved into a spiral structure with many tightly wound arms, a point already noted by e.g. Piner et al. (1995) and Lin et al. (2008) for their simulations of star-forming rings in galaxies. The inner radius is about 7′′, the outer about 16′′, corresponding to 604 pc and 1380 pc, respectively, if we adopt for NGC 1317 the distance of NGC 1316. This is also the region of Hα-emission (Marcum et al. 2001) and it appears black (C − R ≈ 1.2) on our colour image. Outside this radius one cannot find coherent regions of star formation, but sequences of blueish blobs/spots in the north-eastern and south-western sectors. Because they trace the overall curvature, they very probably represent smaller scale HII-regions, indicating star formation on a lower level than in the inner region. However, they are outside the Hα-map of Marcum et al. (2001). Interestingly, these two sectors build part of a ring. Brighter colours in Fig. A.4 denote dust patterns. It is intriguing
that the dust, in the form of filaments, fills an area with a radius of about 5 kpc, but apparently without much star formation, if any. In the very outer parts, NGC 1317 has the appearance of a grand design spiral with two spiral arms dominating. These spiral arms, however, have colours comparable to the bulge colour of NGC 1316, corresponding to populations with ages of about 2 Gyr. NGC 1317 might thus be a case for the longevity of spiral structures. Struck et al. (2011) showed how fast fly-by encounters can produce long-living density waves. Their simulations resemble very closely the appearance of the outer structure of NGC 1317. Moreover, we point out the similarity with the multiple-ring galaxy NGC 6782, which also presents dust lanes resembling spiral arms. The dust is found between two rings of star formation. In the model of Lin et al. (2008) for NGC 6782, the outer ring appears between the corotation radius and the outer Lindblad resonance. In the case of NGC 1317, star formation in the outer ring might have died out and the remaining HII-regions are only the debris of a previous prominent ring. Horellou et al. (2001) note the unusually small HI-disk and mention the possibility that it has been affected by ram pressure, when transversing through the intergalactic hot medium. This then would have happened when the present intermediate-age population was young. Finally, we remark that if N1317 was at exactly the same distance as N1316, one would expect strong tidal forces which would not leave the disk intact and the dust and probably molecular gas quiet with regard to star formation.
HST/WPC2 image of NGC 1317, F606W. Program: 5446, PI: Illingworth. The left panel shows how the star-forming “ring” is resolved in many tightly wrapped up spiral arms. The right panel shows the visible inner bar. North is up, east to the left.
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Table B.1 lists all GCs that were selected as point sources in the photometry and thus have an entry in the photometric list of Paper I (Richtler et al. 2012a). The columns are the catalogue number, the coordinates (J2000), the R-magnitude, the colour C − R, the heliocentric radial velocity, and its uncertainty. Table B.2 continues the GC list with those objects that are not in the point-source list of Paper I. Their catalogue number derives from the internal catalogue in use and the photometric values of these objects appear only here. The coordinates serve for identification purposes only. Unknown magnitudes and/or colours appear as 99.99. The six double measurements (see Sect. 2.5) are included in Table B.1.
Identification and radial velocities of globular clusters appearing in the catalogue of Paper I.
Identification and radial velocities of globular clusters without entry in the photometric catalogue in Paper I.
© ESO, 2014
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