5 Discussion

As shown in Sect. 3 on the basis of radial velocity measurements, 6 out of the 7 galaxies grouped around the barycentric position $\alpha$(J2000) = $02^{\rm h}47^{\rm m}36^{\rm s}.3$, $\delta$(J2000) = 44$^\circ$51$^\prime$39 $^{\prime \prime }$, form a physical system with a median radial velocity V=11 730 km s^-1. The group, whose ellipticity calculated following Rood (1979) is $\epsilon$ = 0.68, shows an elongated shape, a property that have been found to be typical of galaxy groups and interpreted as indication of three-dimensional shapes intrinsically prolate (Hickson 1997, and references therein).

Although the group does not exhibit a particularly compact configuration, we find that its properties fulfill all criteria applied by Hickson (Hickson 1982) to define compact groups of galaxies (CGs). In fact the median projected separation of our group, R=121.7 kpc, is significantly higher than the values observed in the most compact systems, like Seyfert's Sextet or HCG 31 (Hickson 1997), but still within the range of R measured in the Hickson's sample (HCGs). The average surface brightness in R band, $\mu_R= 24.4~\rm mag~arcsec^{-2}$, evaluated within the minimum circle containing the centers of all galaxy members, having radius $\theta_{R} \sim 2\hbox{$^\prime$ }.6$, satisfies the compactness criterion, requiring $\mu_{R} < 26.0~\rm mag~arcsec^{-2}$. The isolation of the group is assured by the fact that the nearest cataloged galaxy of comparable magnitude (2MASXi J0246441+444820) is at a distance 3.7 $\theta_{R}$ from the group's center, therefore well outside the isolation radius $\theta_N=3\theta_{R}$.

 

 

Table 6: Emission line fluxes of CG J0247+449e.

Line	$F_{\lambda}$/$F_{\beta}$	$I_{\lambda}/I_{\beta}$
[O II$]\lambda$3727	5.56 $\pm$ 1.18	9.70
[O III$]\lambda$4959	0.30 $\pm$ 0.08	0.29
[O III$]\lambda$5007	0.75 $\pm$ 0.11	0.70
He I$\lambda$5876	0.21 $\pm$ 0.06	0.15
[O I$]\lambda$6300	0.11 $\pm$ 0.04	0.07
[N II$]\lambda$6548	0.50 $\pm$ 0.09	0.30
H$\alpha$	4.77 $\pm$ 0.36	2.86
[N II$]\lambda$6583	1.86 $\pm$ 0.18	1.11
[S II$]\lambda$6716	0.84 $\pm$ 0.12	0.48
[S II$]\lambda$6731	0.71 $\pm$ 0.11	0.41
$E(B-V) = 0.52 \pm 0.08$
$I_{\beta} = 6.04\times10^{-14}$	(erg cm-2 s-1)

Moreover, as we have shown in Sect. 4, CG J0247+44.9 exhibits a wide range of activity types, being characterized by the presence of a Seyfert 2 galaxy, a LINER, a HII galaxy and a strongly interacting starburst galaxy. The two active nuclei are hosted by early-type spirals. This is in agreement with Coziol et al. (1998a,b), who found in a sample of 17 HCGs that the AGNs are preferentially located in the most early-type galaxies and that 50% of them are low-luminosity ones, like Seyfert 2 or LINER. They also pointed out that AGNs are systematically concentrated toward the central parts of the groups, while starburst galaxies are distributed in the external parts. This is not verified in our case, as the two active galaxies of CG J0247+44.9 are located in the outer parts of the group, while the starburst galaxy is near the geometrical central position. It is interesting to note that the closest object to the geometrical center of the group is the non-active, early-type galaxy "b'', whose internal structures (see Sect. 4) might be the outcome of an unequal mass, disk galaxy merger.

A good agreement has also been found between other properties of CG J0247+449 and those of HCGs. In particular the velocity dispersion $\sigma_{\rm V}$ = 458 km s^-1 is lower than the maximum observed value for HCGs, albeit higher than the median of 200 km s^-1 (Hickson et al. 1992). The intrinsic three-dimensional velocity dispersion $\sigma_{\rm 3D} = 709.5$ km s^-1 calculated taking into account the velocity errors, has been used to estimate the dimensionless crossing time $H_0t_{\rm c}$, which gives an indication of the dynamical state of the group. We have found a value of 0.016 that, when compared with the fraction of late-type spirals in the group ( $f_{\rm s} = 0.34$), results in good agreement with the $\log H_0t_{\rm c}-f_{\rm s}$ relationship found by Hickson et al. (1992). According to this relationship, the spiral fraction is lower on average in groups with small crossing times, as expected in case of hierarchical evolution within galaxy groups. Since numerical simulations (Perea et al. 1990) have shown that a group should approach the virial equilibrium condition after about three crossing times and our group has a relatively low $t_{\rm c}$, we calculated the virial mass following Perea et al. (1990) and found a value of $\sim$ $1.5\times10^{13}$ $M_{\odot}$, comparable with the virial masses of HCGs, which reach a maximum value of $7.3\times10^{13}$ $M_{\odot}$. It must be stressed that the assumption of virial equilibrium and the neglect of the mass spectrum of the galaxies could lead to a large overestimate of the virial mass. Assuming as B luminosity of the group the luminosity of all the accordant galaxies, $L_{B} = 1.95\times10^{11}~L_{\odot}$, we found a mass-to-light ratio $M/L_{B}\sim 77$, somewhat higher than the median value for HCGs ( M/L_B = 37.5), but significantly smaller than typical values for loose groups (Hickson et al. 1992, and references therein).

On the basis of the above observational properties we suggest that CG J0247+44.9 is a dynamically old but still evolving compact galaxy group, as indicated by the strong signs of interaction in the close pair IRAS 02443+4437 belonging to the system. This idea is in agreement with the relatively high velocity dispersion of the group, typical of groups having a high fraction of early type galaxies (Hickson 1997). These systems often show an extended soft X-ray halo (Mulchaey 2000, and references therein) suggesting that they are physically bound and - at least in case of symmetric X-ray morphologies - already relaxed, therefore dynamically old groups. The ROSAT All-Sky Survey (RASS) shows no obvious excess of X-ray emission at the location of CG J0247+44.9. To check whether this is consistent with the presence of a galaxy group, we have estimated the expected soft X-ray luminosity and temperature of our group by means of the empirical $L_{\rm X}$- $\sigma_{V}$ and T- $\sigma_{V}$ relations from Mulchaey & Zabludoff (1998) obtaining $L_{\rm X} = 5.96\times10^{42}$ erg s^-1 and T = 1.38 keV. At the distance of the source ( $D \sim 156.4$ Mpc) this luminosity corresponds to a flux of $\sim$ $2.03\times10^{-12}$ erg cm^-2 s^-1, a value approaching the 3$\sigma$ upper limit to the X-ray flux derived from RASS data within a 250 h^-1 kpc ( h = H₀/100) aperture after correction for the Galactic neutral hydrogen column density in the direction of the group (H. Ebeling, private communication). Therefore the lack of cataloged RASS X-ray sources at the location of CG J0247+44.9 is not inconsistent with our results.