3 Individual notes and results

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 $\mu _{R}=24.5$, 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

3.1 HCG 67

3.1.1 Morphology

According to Ribeiro et al. ([1998]) this group is a loose one with a compactness increasing inwards; their kinematic analysis, which identifies 14 objects at $V_{\rm hel}\approx$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. ([1997]) numbered up to 17 members, which show a bimodal redshift distribution with a separation of $\approx 500$ 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$^\circ$. 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$^\circ$.

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. [1996]; Loyer et al. [1998]). 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:

• With a total of 15 components, the overall quality of fit is lower than expected: although a couple of percent in many areas, the rms residual is as high as 20% in others, distributed into a chaotic pattern with a cellular dimension of $\approx 10''$.
• Despite these limitations, a rather clear picture of "n2'' has emerged: it turned out to be the innermost region of an E3 galaxy with an effective radius of $\simeq 4.5''$; it is both larger and brighter than "d''. Its absolute luminosity is M_B=-21.0 (for h=0.75), and its photometric profile (Fig. 2) does not appear significantly stripped off in the outer parts; we conclude that "n2'' is unlikely to lie at the minimum distance to "n1'', the nucleus of "a'', allowed by the respective lines of sight, i.e., 4'' or $\simeq 2$ kpc.
• A very faint excess of light is seen close to "n2'', as a long, flat structure likely too faint to be a galaxy of its own (its surface brightness is $\approx$ 1% of the central brightness of "n2''); whether it is connected to "n2'', thus suggesting an ongoing merger, or not, is unclear.
• The profile of "a'' (Fig. 2, top panel) is similar to many other galaxies in compact groups (Bettoni & Fasano [1993]: hereafter BF93); these authors suspected such galaxies of harboring a tidal halo; more unexpectedly, the profile of "a'' is also very similar to that of an archetypal cluster cD, NGC 6166 in Abel 2199 (Oemler [1976]; Schombert [1986]), which is twice as bright: this, together with the irregular luminosity distribution, and the presence of diffuse X-rays, is evidence for an evolution marked by mergers.

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$^\circ$, 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''

3.1.2 Spectroscopy

We have obtained kinematical data along a line at PA = 37$^\circ$ cutting the " $\rm n1-\rm n2$'' 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$^\circ$ 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:

• On the NE side, "a'' exhibits a significant rotation, with an amplitude of $\simeq 200$ km s^-1, not inconsistent with the value expected for an E3 elliptical with a central velocity dispersion of 291 km s^-1 and isotropic dynamics.
• On the SW side, between $r\simeq 3$ and 15'', we note a perturbation in the velocity and a rise of the dispersion up to 380 km s^-1 at r=7''. This is in agreement with BB99, whose spectrum at PA = 17$^\circ$ shows the same behavior in a region located at a few arcsec from ours. There is another strong perturbation, between 15'' and 30'' from "a'', toward "d''.
• Simple simulations involving template spectra show that kinematic perturbations similar to those observed can be generated by mixing the light from two objects with differential radial velocities of the order of 100 to 200 km s^-1(this was already noticed by Combes et al. [1995]). V shifted and $\sigma$ is increased, and when the difference in velocity is sufficient, the error bars on V and $\sigma$ are also increased; this reflects the conflict between the actual, twin-peaked line-of-sight velocity distribution and the single Gaussian assumed in the "standard'' reduction. This is likely to explain the observed features in the region of the optical transition between "a'' and "d''. Closer to "a'', the maximum at r=7'' may also be due to the optical superposition of "n2'', although, as an alternative, a physical effect like tidal heating cannot be ruled out.

BB99 have also obtained the gas kinematics of member "c'' (Scd) of HCG 67, which has a rotation curve typical of a spiral, with a maximum velocity of $\approx$ 250 km s^-1. The galaxy also exhibits an asymmetric structure, and a feature resembling a quite open arm which, given the high inclination of the equatorial plane, would be distorted; this may be indicative of an ongoing interaction (see Noguchi [1990]).

   

Table 2: Basic properties of member galaxies from the literature

Member	Type	$B_{\rm T}$	$V_{\rm hel}$	Notes
ident.		(mag)	(km s-1)
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 74b	E3	15.07	12110
HCG 74c	S0	16.10	12226
HCG 74d	E2	16.32	11681
HCG 74e	S0	17.80	11489
HCG 79a	E0	14.35	4292	Nuclear H$\alpha$ - 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

Notes. The values of the total B-mag and the systemic velocities of single members of the groups are obtained from Hickson ([1993]). Notes on morphological peculiarities are reported from Mendes de Oliveira & Hickson ([1994]).

Figure 3: HCG 67. Profiles of velocity dispersion $\sigma$ ( top panel) and radial velocity $V_{\rm hel}$ ( middle) along a line at PA = 37$^\circ$ 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

3.2 HCG 74

3.2.1 Morphology

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'' ( $1.10\pm0.05''$ FWHM), and found it slightly wider than stars ( $0.85\pm0.10''$ 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.

3.2.2 Spectroscopy

Our kinematic measurements show a visible transition between "a'' and "b'' between $\simeq$ 8 $^{\prime \prime }$ and $\simeq$ 15 $^{\prime \prime }$ (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 $\sigma$, $V_{\rm hel}$, and I along a line at PA = 21$^\circ$ connecting the nuclei of the "a'' and "b''. The faint outskirts of the component "e'' are visible at $\approx$19 $^{\prime \prime }$ in the NE direction

Figure 5: HCG 79. Profiles of $\sigma$, $V_{\rm hel}$, and I along a line at PA = 79$^\circ$ connecting the nuclei of the "c'' and "b''

Figure 6: HCG 79. Profiles of $\sigma$, $V_{\rm hel}$, and I along a line at PA = 65$^\circ$ connecting the nuclei of "a'' and "b''

   

Table 3: Main kinematic parameters from this study

Member	V $_{0,{\rm hel}}$	$\sigma_0$
ident.	(km s-1)	(km s-1)
HCG 67a	$7246\pm 24$	$291\pm 23$
HCG 67d	$7247\pm 36$	$112\pm 49$
HCG 74a	$12208\pm 32$	$312\pm 31$
HCG 74b	$11976\pm 21$	$212\pm 21$
HCG 79a	$4237\pm 18$	$155\pm 16$
HCG 79b	$4447\pm 11$	$130\pm 13$
HCG 79c	$4053\pm 08$	$59\pm 20$

3.3 HCG 79

3.3.1 Morphology

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 $\approx 30''$ 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.

3.3.2 Spectroscopy

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. ([1989]) 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 $^{\prime \prime }$ towards the SW. We can make the following comments.

• In "b'', at distances less than $\simeq 12''$, the rotation and dispersion profiles are typical of a normal S0 galaxy observed almost edge-on. The velocity profile is very perturbed towards the SW just starting at r=10 $^{\prime \prime }$. Whether this is due to the sole light mixing between "b'' and "c'' is still unclear.
• To the NE, the rotation of "b'' falls to zero at $r\simeq 12''$, and shows unambiguous counter-rotation beyond; this is likely due to the low-luminosity extension. It is unclear whether this is just the effect of an optical superposition, or is actually revealing the movement of some tidally perturbed material (from "b'' itself or from the low-luminosity companion).
• Member "c'' shows no rotation along the direction of the slit, which is at $\approx$ 45$^\circ$ from its major axis. Since "c'' is significantly flattened, this lack of rotation indicates that, either: i) it is oblate and inclined with respect to the line of sight, with an anisotropic dynamics, or ii) it is prolate, and it is rotating (or tumbling) around an axis coincident with the line of sight. Both cases would be, statistically, expected for a galaxy much more massive than this one ( M_B=-19.04). This off-average behavior could be regarded as a (mid) evidence of physical interaction within the group.
• The kinematics of component "a'' has also been obtained along the direction PA = 65$^\circ$ (Fig. 6), which corresponds to the galaxy major axis (see also BF93). While the "c'' and "b'' members are S0 galaxies, "a'' is classified as elliptical. The rotation curve seems nearly unperturbed in the SW side while the NE side, i.e., towards the rest of the group, shows a steep rise.