10 Discussion

Figures 6 to 11 show the continuum and H$_\alpha$ maps of some of the HCG galaxies of our sample. A isocontour plot is also shown for the largest galaxies of these groups. In the H$_\alpha$ images we have removed residuals of field stars for clarity and we display the galaxy flux one sigma above the background. The scale of the axes are in pixel units and the field of view is $5.12\times 5.12$ arcmin. East is on the top and North on the left. For each image the name of HCG is shown in the caption, while the name of the galaxies are reported in the figure. In the following we present a brief description of the groups and galaxies in Figs. 6 to 11. The values regarding the flux of the lowest isocontours are corrected for Galactic Extinction.

 

Figure 6: Continuum (up), H$_\alpha$ map (middle) and zoomed isocontour map (down) of galaxies A and B of HCG2. The lowest contour is at 1$\sigma$ ($6.27 \ 10^{-17} {\rm erg~cm}^2~{\rm s}^{-1}~{\rm arcsec}^{-2}$) above the background. The interval among the contours is 3$\sigma$

  

Figure 7: Continuum (up), H$_\alpha$ map (middle) and zoomed isocontour map (down) of HCG37. The isocontour plots are given for the largest galaxies only (see Table 7). The lowest contour is at 1$\sigma$ ($3.64 \ 10^{-17}~{\rm erg~cm}^2~{\rm s}^{-1}~{\rm arcsec}^{-2}$) above the background. The interval among the contours are 3$\sigma$

 

Figure 8: Continuum (up), H$_\alpha$ map (middle) and zoomed isocontour map (down) of HCG38. The lowest contour is at 1$\sigma$ ($7.22 \ 10^{-17}~{\rm erg~cm}^2~{\rm s}^{-1}~{\rm arcsec}^{-2}$) above the background. The interval is 1$\sigma$

 

Figure 9: Continuum (up) and H$_\alpha$ map (down) of HCG46

 

Figure 10: Continuum (up), H$_\alpha$ map (middle) and zoomed isocontour map (down) of HCG49. The lowest contour is at 1$\sigma$ ($9.44 \ 10^{-17}~{\rm erg~cm}^2~{\rm s}^{-1}~{\rm arcsec}^{-2}$) above the background. The interval among the contours is 2$\sigma$

 

Figure 11: Continuum (up), H$_\alpha$ map (middle) and zoomed isocontour map (down) of HCG74. The isocontour plots are given for A, B and C galaxies. The lowest contour is at 1$\sigma$ ($7.52 \ 10^{-17}~{\rm erg~cm}^2~{\rm s}^{-1}~{\rm arcsec}^{-2}$) above the background. The interval among the contours is 1$\sigma$

HCG2- This group consists of a triplet of galaxies with accordant redshifts (galaxies a, b and c) plus a fainter member (galaxy d) which has a higher redshift. In Fig. 6 are included only the galaxies a and b, for which we have estimated the H$_\alpha$ fluxes. Galaxy a (late type barred spiral) is brighter in H$_\alpha$ than b (compact irregular), which is also a infrared source. In the H$_\alpha$ map some knots are resolved in the disk of galaxy a. For galaxy b we detect a strong H$_\alpha$emission in the center and no emission in the outer part, as previously noted by Vilchez & Iglesias Paramo (1998a). The estimated SFR$_{\rm iso}$ for a and b are respectively 1.45 and 1.16 $M_\odot$ yr^-1. In the lowest panel of Fig. 6 the isocontour plot, showing the shape and the orientation of the H$_\alpha$ emission, is given. The lowest contour is at 1$\sigma$ above the background, corresponding to an H$_\alpha$ emission of $6.27 \ 10^{-17}$ erg cm² s^-1 arcsec^-2. The interval among the contours is 3$\sigma$. The H$_\alpha$ emitting areas have an extension of about 1643.6 and 315.8 arcsec² for a and b galaxies respectively.

HCG37- This is a compact group with five accordant galaxies: a and b are the dominant galaxies of the group. They are radio sources, as galaxy d. The group has a high velocity dispersion (398.1 km s^-1) and mass-to-light ratio (123 $M_\odot L_\odot$), and a short crossing time (0.0054 ${\rm Ht_c}$). The H$_\alpha$brightest source of the group is galaxy a. This is a blue elliptical galaxy with a rapidly rotating central disk of ionized gas (Rubin et al. 1991). Galaxy b is an edge-on spiral with an intensive H$_\alpha$ emission in the center. This galaxy is also an infrared source. Galaxies c (SOa), d (SBdm) and e (E0) are all fainter H$_\alpha$ emitters than galaxies a and b. The SFR$_{\rm iso}$ estimated for a, b, c, d and e galaxies are respectively 0.26, 0.13, 0.03, 0.08 and 0.02 $M_\odot$ yr^-1. The lowest panel of Fig. 7 presents the isocontours for the largest galaxies of the group: the extensions of H$_\alpha$ emission are of about 235.2, 217.1, 46.7, 98.4 and 28.4 arcsec² for a, b, c, d and e galaxies respectively. The lowest contour is at 1$\sigma$ above the background, that is H$_\alpha$ emission higher than $3.64 \ 10^{-17}~{\rm erg~cm}^2~{\rm s}^{-1}~{\rm arcsec}^{-2}$, while the interval among the contours is 3$\sigma$.

HCG38- This group contains the interacting pair Arp 237 (galaxies b and c) with one other galaxy at a similar redshift (galaxy a), plus a fainter high-redshift galaxy (d). Galaxy a is a spiral showing an H$_\alpha$ emission more intense in the center than in the outer disk. The H$_\alpha$brightest galaxy b (late type barred spiral) is in the interacting pair and it is an infrared source. In galaxy b we reveal a strong H$_\alpha$ emission in the central part of the galaxy and some resolved knots throughout its arm placed in the direction opposite to galaxy c. This last galaxy is of irregular type and it is the H$_\alpha$dimmest galaxy of the group. The estimated SFR$_{\rm iso}$ for a, b and c are respectively 0.34, 0.39 and 0.19 $M_\odot$ yr^-1. The isocontour plots in Fig. 8 show the H$_\alpha$ emission higher than $7.22 \ 10^{-17}~{\rm erg~cm}^2~{\rm s}^{-1}~{\rm arcsec}^{-2}$ (1$\sigma$ above the background). The interval among the contours is 1$\sigma$. The extensions of H$_\alpha$emission are of about 156.3, 150.4 and 60.0 arcsec² for a, b and c galaxies respectively.

HCG46- This group consists of four early-type galaxies. The velocity dispersion and mass-to-light ratio of the group is relatively high (respectively 323.6 km s^-1 and 478.6 $M_\odot L_\odot$). Galaxies b and c appear to be in contact in the continuum image, but not in the H$_\alpha$ map. Two features are in common to all the galaxies of the group: they show a faint H$_\alpha$emissions that seems confined to the bulge of galaxies. The estimated SFR$_{\rm iso}$for a, b, c and d are respectively 0.04, 0.03, 0.01 and 0.002 $M_\odot$ yr^-1. The extensions of H$_\alpha$ emission at 1$\sigma$ above the background are reported in Table 7.

HCG49- This is a small and very compact group with four accordant galaxies. Its median galaxy separation is only of 12.3 h^-1 kpc and its velocity dispersion is so low (lower than the uncertainties in the velocity measurements) that no estimate can be made of its mass-to-light ratio (Hickson 1993). Galaxies a and b are spiral, while c is an irregular and d an elliptical. Galaxy b is the H$_\alpha$ brightest source of the group, while galaxies a, c and d have comparable H$_\alpha$ emission among them. The estimated SFR$_{\rm iso}$ for a, b, c and d are respectively 0.56, 1.3, 0.26 and 0.14 $M_\odot$yr^-1. In the lowest panel of Fig. 10 the isocontour plot is shown. The lowest contour is at 1$\sigma$ above the background, corresponding to an H$_\alpha$ emission of $9.44 \ 10^{-17}~{\rm erg~cm}^2~{\rm s}^{-1}~{\rm arcsec}^{-2}$. The interval among the contours is 2$\sigma$. The extensions of H$_\alpha$ emission are of about 72.8, 96.3, 48.9 and 27 arcsec² for a, b, c and d galaxies respectively.

HCG74- This group contains five early-type accordant galaxies: a, b, d are elliptical and c and e are lenticular. Galaxy a is the dominant one with two very close companions (b and c) and it is also a radio source. All galaxies show an H$_\alpha$ emission confined to their center. We have not revealed e galaxy for which we have estimated the upper limit. The estimated SFR$_{\rm iso}$ for a, b, c and d detected galaxies are respectively 0.51, 0.12, 0.1 and 0.1 $M_\odot$ yr^-1. In the lowest panel of Fig. 11 the isocontour plots for a, b and c galaxies are shown. The lowest contour is at 1$\sigma$ above the background ($7.52 \ 10^{-17}~{\rm erg~cm}^2~{\rm s}^{-1}~{\rm arcsec}^{-2}$), while the interval among the contours is 1$\sigma$. The extensions of H$_\alpha$ emission are 61.38, 40.59, 21.96 and 23.04 arcsec² for a, b, c and d galaxies respectively.