The groups observed with OVRO were HCG40 and HCG79. These two groups are the ones with highest compactness (as defined by Hickson 1982, who found an average surface bright- ness of 21.2 and 20.5 mag arcsec−2 for HCG40 and HCG79, respectively), and show a low ve- locity dispersion, below the average of our HCGs sample, leading the galaxies within the group to experience a strong interaction between them. In Fig. 3.1 we display the distribution of the surface brightness averaged over the area of the full group for the 20 HCGs in our sample, to show that HCG79 and HCG40 have the highest compactness. Furthermore, these two groups have been found to show a high degree of isolation from the galaxies in their environment (Durbala et al. 2008, Verdes-Montenegro et al., in prep). Thus, the features that make HCGs interesting targets to study (high density of galaxies, isolation from the environment and low
41
velocity dispersion) are present in these 2 groups in an extreme way, making them particularly interesting objects out of our full sample of 20 HCGs.
Figure 3.1: Distribution of the total magnitude of the HCGs averaged over the area of the group (in arcsec) for the HCGs in our full sample. The data are taken from Hickson (1982).
3.1.1 HCG 40
In Fig. 3.2 we display an optical image of HCG40 from the Subaru telescope, which is prob- ably the best optical image available of this group. HCG40 is composed of five galaxies. Three of them are classified as early-type galaxies, as can be seen in Tab. 3.1, where the compilation of the morphologies and nuclear activity of the galaxies in HCG40 is displayed. The median velocity of the group is 6572 kms−1, and the velocity dispersion of the galaxies is 177 kms−1. Martínez (2008), who studied the nuclear activity in a sample of 65 HCGs using the Kewley et al.
(2006) criteria, found 40e to host an AGN and 40c and 40d to be transition objects, with the other galaxies in the group showing no signs of nuclear activity.
Although VM01 found HCG40 to be the most HI deficient group in their sample, new obser- vations of the group (Borthakur et al. 2010) using a larger bandwidth have detected a HI mass almost 5 times larger, so the group is finally classified as slightly HI deficient. In the evolu- tionary sequence model of Verdes-Montenegro et al. (2001), it was initially classified as very evolved (Phase 3a) due to the low HI content detected in the disks of the galaxies, but new VLA observations performed with a wider velocity range (Verdes-Montenegro et al., in prep.) have detected a larger amount of atomic gas in the galaxies, which set HCG40 in a less evolved stage.
Rasmussen et al. (2008) suggested, based on the extended intense X-ray emission they observed in HCG40, that it could be one of the spiral-rich groups with a hot IGM, which are not very com- mon. Nevertheless, the high compactness of the group makes it hard to exclude the individual galaxies as the only sources of the X-ray radiation.
3.1 The groups 43
Table 3.1: Basic data, Hubble type and nuclear activity classification of the galaxies in HCG40 and HCG79
Galaxy Vel. BTc B-R Hubble type Nuclear activity
(kms−1) (mag) (mag) NED LEDA Sulentic Durbala Martinez Durbala
40a 6481 13.38 1.75 E E S0
40b 6676 14.62 1.84 SA0 E-S0 S0
40c 6264 14.60 2.00 SBb Sab Sb TO
40d 6429 14.53 1.56 SB0a S0a S0a TO
40e 6305 16.58 1.84 SAB Sa Sbc AGN
79a 4115 14.49 1.60 Sa S0 E/S0 E3 TO AGN
79b 4369 14.09 1.44 S0 S0 S0 S0 TO AGN
79c 4352 14.98 1.27 S0 S0 S0 S0
79d 4501 15.16 0.85 SBc Sc Sd Sd AGN
The morphologies listed are the ones compiled from NED and LEDA, the one set by J. Sulentic (private communication) and the one from Durbala et al. (2008). The nuclear activity classifications are taken from Martínez (2008) and Durbala et al. (2008), where TO stands forTransition Object.
3.1.2 HCG 79
HCG79 is not only the HCG with the highest compactness, but also one of the most isolated groups of galaxies. An optical image of the group taken by the Hubble Space Telescope is pre- sented in Fig. 3.3 for a better understanding of the description of the group. In spite of being known as Seyfert Sextet, HCG79 has only 4 members. 79f, not included in the original Hickson catalogue (1982), is not considered as a galaxy, but as a tidal tail that lies to the NE of 79b. Even though Hickson (1982) did not classify it as a galaxy, the RC3 catalog (de Vaucouleurs et al. 1991) lists it as a galaxy with designation NGC6027e. Thus, it is usually referred to as 79f, indepen- dently of whether the authors consider it to be a real galaxy or not. Following this convention, we design it as 79f, even though we are not considering it to be a real galaxy. 79e, included by Hickson (1982), is an almost face-on spiral located at the southern part of 79b. It was discarded as a member of the group after the redshift measurements of Hickson et al. (1992), who found it to be a background galaxy with v∼20000 kms−1, in contrast to a group average velocity of 4398 kms−1. The velocity dispersion of the group is 128 kms−1. The galaxies in HCG79 are em- bedded in a massive luminous halo, which contains a significant fraction of the luminosity and stellar mass of the group and implies strong former stripping processes. The 4 real members of HCG79 are classified into three early-type lenticular galaxies and one late-type galaxy (see Tab.
3.1).
Durbala et al. (2008) found 79a and 79c to show evidence for nuclear activity at radio, mid- infrared and optical wavelengths. Martínez (2008), based in the Kewley diagnostic criteria al- ready mentioned, set these two galaxies as transition objects and found 79d to host an AGN.
The properties of HCG79 have been studied in detail by Durbala et al. (2008). Based on the higher luminosity fraction contained in the group halo, they set HCG79 as a highly evolved compact group, with evidences for past accretion events and a low atomic gas fraction. They found 79a, 79b and 79c to have been physically related for a long time and to be the result of a sequential acquisition process. The last galaxy to enter the group is, according to them, 79d, which is at the present entering the group and shows no clear signs of interaction with the other members of the group. As a result, almost all the HI detected in HCG79 is contained in the disk of this new-entering galaxy 79d. The model by VM01 detailed in Chap. 2 establishes the evolutionary stage of a group as a function of the ratio of atomic gas contained in the disks of the
40d
40a
40c
40b 40e
Figure 3.2: Optical image (combined J and K’ bands) of HCG40 taken with the the Subaru telescope. Field of view is 1.9×2.9 arcmin. The Subaru telescope is operated by the National Astronomical Observatory of Japan.
galaxies with respect to the full content of HI of the group. Thus, following this classification, HCG79 is classified in the first evolutionary phase.