In this section, we comment on each group, summing up relevant results from the literature, and discussing our own results in this context. Basic morphological characteristics are shown in Fig. 1, which presents isophote maps for the groups.
|Figure 1: Isophote maps from the R-band PH-CFHT frames. North is at the top, East to the left. The outermost isophotes correspond to , 25.0, and 23.5, respectively, for HCG 67, HCG 74, and HCG 79; the interval in magnitude is 0.5. Several stars and background galaxies have been removed; for HCG 67, the contour of the outermost isophotes is perturbed by an imperfectly removed bright star. The arrows indicate the approximate direction of a component out of the graph|
According to Ribeiro et al. () this group is a loose one with a compactness increasing inwards; their kinematic analysis, which identifies 14 objects at 7435 km s-1, cannot reject the possibility that this group is merely an artifact of a projection effect along the line of sight. de Carvalho et al. () numbered up to 17 members, which show a bimodal redshift distribution with a separation of km s-1.
The group contains two dominant galaxies named "a'' and "b'' in the Hickson nomenclature. According to MdOH, member "a'' is a double-nucleated, early-type (E1) galaxy, and "b'' is a Sc spiral seen nearly edge-on. The two central objects have been confirmed on the DB-Asiago image by BB99. Component "a'' has two compact companions designated "c'' and "d''. The northern one, "c'', is a radio source. Infrared and radio emission are detected from the spiral galaxy (H93). H93 estimates a median galaxy separation of 49 h-1 kpc (h is the dimensionless Hubble constant H0/[100 km s-1 Mpc-1]) and a radial velocity dispersion of 209 km s-1.
Our contribution to the description is as follows. On the DB-Asiago and PH-CFHT images, we noticed that the outer isophotes of "a'', although distorted by the presence of "d'', are actually those of an E3 elliptical with a major axis at PA = 23. The unperturbed part of the isophotes have a center coinciding roughly with one of the central luminosity peaks (let us designate it by "n1''). The second nucleus ("n2'') is significantly brighter; their separation is 4'', and their relative position is at PA = 135.
With the aim of investigating the very nature of "n2'', we have expanded the luminosity distribution of the R-band PH-CFHT frame into a series of Gaussian components grouped into three sets centered, respectively, on "a'' (i.e., "n1''), "n2'', and "d''. We have followed a technique successfully applied to E and S0 galaxies (e.g., Emsellem et al. ; Loyer et al. ). The dimension, central brightness, axial ratio of individual components of each set, together with the central coordinates and position angle of these sets, where left as free parameters and were determined in a region of radius 40'' around "a''. Starting with a reduced number of components, inspection of the residuals between the model and the observed distribution allowed us to add more components to each set, and thus to optimize the determination of the parameters. Typically, for early-type galaxies, the final rms residual is smaller than 10% in intensity. In the present case, we found the following:
|Figure 2: HCG 67. R-band photometric profiles of components "a'' ( top panel) and "n2'' ( bottom panel), folded around the center. Both profiles are at PA = 89, which is the major axis of "n2''. The ordinates, in magnitude scale, have been shifted by an arbitrary value, the same for "a'' and "n2''|
We have obtained kinematical data along a line at PA = 37 cutting the " '' system roughly through the apparent barycenter of the light distribution; the slit cuts "d'' at about 4'' of its nucleus, and at an inclination of about 53 with respect to its major axis. On this line, the brighter point in "a'', which is the origin of our abscissae, coincides within one or two arcsec with the distance to "n2''. We found (Fig. 3) the following:
|HCG 67a||E1||12.74||7262||two nuclei - disturbed morphology|
|HCG 67b||Sc||13.89||7644||dust lane across the galaxy|
|HCG 67c||Scd||15.06||7430||asymmetric morphology, close companion|
|HCG 67d||S0||15.25||7071||may have non concentric isophotes|
|HCG 74a||E1||14.06||12255||double nucleus, non concentric isophotes towards E, strong radio source|
|HCG 79a||E0||14.35||4292||Nuclear H - strong dust lane - radio source|
|HCG 79b||S0||13.78||4446||strong bar - tidal tail to the NE|
|HCG 79c||S0||14.72||4146||extended envelope towards 79b|
|HCG 79d||Sdm||15.87||4503||asymmetric disk with a faint extension towards 79a|
|Figure 3: HCG 67. Profiles of velocity dispersion ( top panel) and radial velocity ( middle) along a line at PA = 37 connecting the nuclei of the "a'' and "d'' members. Positions of the two components are indicated. Due to the smoothing of the spectrum in low-flux regions, individual points are not fully independent. Bottom panel: surface brightness I along the same line, as measured on the spectrum|
The group is a quintet composed of early-type galaxies; it is dominated by an elliptical galaxy, member "a'', with two very close companions, "b'' and "c''. The elliptical is a radio source. All members have accordant redshift. H93 gives a median galaxy separation of 39 h-1 kpc, while the radial velocity dispersion is 316 km s-1.
Our image confirms the detection by MdOH and BF93 of a second nucleus (hereafter "n2'') embedded in "a'', at 5'' West of the main nucleus ("n1''). For this, on the R-band PH-CFHT image, we have expanded the light distribution of "a'' into a multi-Gaussian function, and we have subtracted this model from the observed frame. On the resulting image, flat to better than 1% in intensity, we have measured the profile of "n2'' ( FWHM), and found it slightly wider than stars ( FWHM); thus, there is a fair level of confidence that it is not stellar. But on the map of the residuals, we did not see any pattern (at a contrast level of 1%) that would reveal ripples or other evidence for a physical interaction, and the possibility remains that it is rather a foreground or background object. Because of the fairly circular isophotes of the model, we would classify "a'' as E0 rather than E1.
Our kinematic measurements show a visible transition between "a'' and "b'' between 8 and 15 (see Fig. 4), with an increase of the velocity dispersion due to the mixing of the light from the two objects. The rotation of the two components is marginal, because the slit position is close to the minor axis for both; in addition, "a'' is an E0 with no expected intrinsic rotation. For "b'', a slightly concave shape for the inner rotation is due to the fact that the slit narrowly misses the nucleus.
|Figure 4: HCG 74. Profiles of , , and I along a line at PA = 21 connecting the nuclei of the "a'' and "b''. The faint outskirts of the component "e'' are visible at 19 in the NE direction|
|Figure 5: HCG 79. Profiles of , , and I along a line at PA = 79 connecting the nuclei of the "c'' and "b''|
|Figure 6: HCG 79. Profiles of , , and I along a line at PA = 65 connecting the nuclei of "a'' and "b''|
|ident.||(km s-1)||(km s-1)|
This group is known as Seyfert's Sextet and is the most compact one in Hickson's catalog, having a median intergalaxy separation of only 6.8 h-1 kpc. It has a relatively low radial velocity dispersion of 138 km s-1. H93 catalogs 5 members in this group, four of which have an accordant redshift. Member "e'' (not reported in Table 2) has in fact a systemic velocity of 19809 km s-1. Inspection of the PH-CFHT images led to the following remarks.
The symmetric strong dust lane of "a'' reveals a perfectly edge-on inclination; the inner isophotes are those of an E3 rather than an E0 elliptical, but it is embedded into an envelope which becomes gradually spherical outwards: this galaxy is thus similar in structure to a scaled-down M104.
Component "b'' is clearly an edge-on S0 with an extended, flat disk. The object at to the NE, designated by "b1'' in BF93, is of uncertain nature. It could be a warped extension of "b'' or, alternatively, another galaxy, so severely stripped off by tidal effects that it has lost most of its symmetric central condensation, and has adopted a comet-like appearance. Component "c'' is also an edge-on S0, with a barely detected dust lane, and with slightly boxy outer isophotes; it is unclear whether the faint luminous bridge between "b'' and "c'' is anything more than the superposition of their individual brightnesses.
We also note the warped disk of the edge-on spiral "d'' (already noted by MdOH), which we regard as evidence of tidal interaction.
RFH give the rotation curve for "d'', whose linear form is consistent with its edge-on orientation. A H I map obtained by Williams et al. () shows the same range of velocities across the H I disk associated with the "d'' galaxy as is seen in the RFH optical spectrum. These authors detect only weak nuclear emission for "a'' and "b''. RFH suggest that the components of HCG 79 are interacting also on the basis of another H I observation (Williams et al. 1989), which detected a tidal tail extending from "d'' towards "a''.
We succeeded in determining the velocity and velocity dispersion profiles of "b'' up to the fainter "c'' component visible at about 30 towards the SW. We can make the following comments.
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