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Astron. Astrophys. 325, 473-478 (1997)

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4. Discussion

For the discussion of the compact group phenomenon we combine our present sample with that of Williams and Rood (1987). There is some overlap in sources with fine agreement in measured quantities (i.e. radial velocities, fluxes, and linewidths). There is a total of 75 HCGs observed with 61 detections and 14 upper limits.

The (distance-independant) [FORMULA] ratio often has been used to discuss the relative HI content of individual galaxies. In this discussion we will use the integrated HI mass of Hickson Compact Groups and relate it with their integrated luminosities. These values will be compared with the [FORMULA] values of a sample of nearby (individual) galaxies and the integrated values for a selected amount of nearby groups of galaxies.

Assuming elliptical galaxies to possess only few neutral gas by their own we will exclude them when accumulating blue luminosities of all other group members (as given by Hickson 1993). As the HI content of S0 galaxies scatter a lot we marked those groups with at least 50% content in S0 galaxies and those exclusively consisting of S0 galaxies. The result is displayed in Fig. 2 where the integrated [FORMULA] of HCGs is plotted versus their integrated luminosities1. Filled symbols indicate detections while open symbols are used for upper limits. Black dots mark spiral groups, triangles(up) mark mixed S0-spiral groups, and triangles(down) mark those groups consisting of S0 galaxies completely. The solid line represents the sample of nearby galaxies (HR1). Solid circles mark some nearby groups (M81-, M101, CnV I-, and CenA-group, data from RH1). The solid square represents the Coma I group (e.g. Garcia-Barreto et al. 1994), the open square only the seven highly HI-deficient galaxies of this group. The filled rhombus represents the disk galaxies of the Virgo cluster (e.g. HR2), while the open rhombus represents Virgo spirals from the inner region (radius of [FORMULA]) of the cluster. The example of the Coma I group nicely demonstrates the strong HI-deficiency of several HCGs. The four nearby galaxy groups (solid circles) agree well with the regression line of the nearby galaxy sample (solid line) which is not a surprise as these groups are part of the nearby galaxy sample. The scatter of the HCGs in this diagram is enormous, comparable to the scatter of all known galaxies (including elliptical galaxies, which had been excluded from the integrated luminosities of the HCGs).

Among nearby galaxies those with high [FORMULA] values are not frequent but not uncommon either (e.g. HR1). There are a few HI rich HCGs. For compact groups cannibalism and merger events might increase the [FORMULA] values for some galaxies. One could imagine sling-shot events leaving most of the neutral gas within the compact group but not with the expelled galaxy. Accretion of gas from bypassing galaxies or simply additional (not yet detected) HI-rich dwarf members of the HCGs might influence the total [FORMULA] balance of the group.

In Fig. 2 we realize a certain number of groups close to the solid line, their expected place in this diagram. But what is the reason for all those objects with very small [FORMULA] values? For some HCGs we observe an upper limit of about one order-of-magnitude lower than expected! Here we find many of the mixed S0 groups as expected - assuming a rather low [FORMULA] ratio for S0 galaxies. Unfortunately the relatively high upper limits for pure S0 groups are not restrictive enough - higher sensitivity is required in these cases.

In Fig. 3 we compare the [FORMULA] ratio with the relative content of spiral galaxies for our combined sample of HCGs, filled circles mark detections, open circles mark upperlimits. Most compact groups rich in spirals are detected in HI. The detection rate is reduced with decreasing spiral content of the compact groups ([FORMULA], where [FORMULA] is the number of spirals, N the total number of galaxies within the three brightest magnitudes, Table 1). The dotted line shows 'expected values'. It has been constructed assuming average [FORMULA] ratios for spirals (0.65) and non spirals (0.03 for S0 and E galaxies). This line of expected values would not change much by changing the [FORMULA] values (0.65, 0.03) to (0.46, 0.01) for example. This line is a kind of upper envelope to the measured values except for the spiral-poor groups. Fig. 3 demonstrates that there is a general tendency for HCGs to be HI-deficient, albeit those spiral-poor groups which are greatly overabundent in HI.

[FIGURE] Fig. 3. The relative content of neutral hydrogen [FORMULA] versus relative content of spiral galaxies [FORMULA] (Table 1) is given for the combined sample of HCGs. Dots are detections, open circles are upper limits. The dotted line was generated by assuming average [FORMULA] ratios for spirals (0.65) and non-spirals (0.03).


Table 1. Observational data

Very interesting are the low upper limits in the population of HCGs. The strong tidal forces in compact groups might lead to enhanced star formation rates using up much the neutral gas to a high degree and/or yielding a high degree of ionisation by enhanced radiation from the star forming regions. At this point another effect visible with high resolution observations should be mentioned. VLA observations of HCG 95 (Verdes-Montenegro et al. 1997, in preparation) show that most of the observed HI in this group is due to dwarf galaxies that do not fulfill the magnitude restriction (within [FORMULA] + 3) of Hicksons (1982) group definition. Whereas dwarf galaxies do not contribute much to the integrated luminosity of groups their HI-mass might influence the [FORMULA] ratio once the bright group galaxies are of early type. High resolution observations of HCGs will bring more light into the open questions about compact groups of galaxies.

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© European Southern Observatory (ESO) 1997

Online publication: April 28, 1998