Hickson Compact Groups

The Hickson Compact Groups (Hickson 1982: HCGs) are characterized by a small number of members (4 to 10) with a low velocity dispersion (σ = 200 km s^-1, Hickson et al. 1992). Their projected galaxy densities are extremely high, similar to the cores of dense clusters, but they are found in low galaxy density environments, as a consequence of the isolation criterion used for their selection (Hickson 1982; Sulentic 1987). This combination of high galaxy density in the low density environment makes them unique and interesting laboratories for studying galaxy interaction and evolution.

The origin and the existence of Hickson compact groups constitute a matter of great interest and debate. Their short crossing times argue for short lifetimes, yet few merger candidates or postcursors are found (Zepf et al. 1991; Moles et al. 1994; Sulentic & Rabaça 1994). Diaferio et al. (1994) proposed that compact groups could form continually out of rich groups. Governato et al. (1996) suggest that primordial merger events with ongoing acquisition of intruders might prevent the groups from merging entirely. These models seem at work in HCG 92 (Moles et al. 1997). On the other hand, well isolated groups with no obvious intruders also exist (e.g.  HCG 96 Verdes-Montenegro et al. 1997). Athanassoula et al. (1997) have shown that compact groups with an appropriate arrangement of luminous and dark matter can persist over several hundred million years, in agreement with the analysis of the dynamics of satellite galaxies (Perea et al. 2000).

The characteristics of the interstellar medium in HCGs are also not well understood. Elevated star formation activity induced by tidal interactions and resulting enhancement in FIR and CO emission are expected but not supported by observations (see  Verdes-Montenegro et al. 1998 and references therein). The highly perturbed distributions of the molecular gas in the most CO deficient groups suggest that strong disruptions of gaseous disks and gas stripping by continuous tidal disruption may suppress star formation ( Yun et al. 1997).A detailed analysis of the morphology and the strength of interactions among the HCG galaxies is needed in order to better understand the way these compact groups form and evolve. Studies of neutral hydrogen (HI) can offer valuable insights into the dynamics of galaxy interaction and evolution (e.g. Yun et al. 1994; Hibbard & van Gorkom 1997), and we were motivated to undertake a survey of HI distribution and content for a large sample of HCGs (Williams et al. 1997, 1999; Verdes-Montenegro et al. 2000a,b; Huchtmeier et al. 2000) in order to analyze the type and effect of the interactions that are taking place.

We have analyzed the total HI content of 72 HCGs and found that nearly 60% of the HI is missing in these groups. We further investigated the detailed distribution and kinematics of HI within a subset of 16 groups using high resolution observations obtained using the VLA ( Verdes-Montenegro et al. 2001). The HI deficiency was found to be larger when galaxies were considered individually (76% HI is missing) in agreement with the efficient gas stripping evidenced by HI tails and bridges. Among the 44 groups of our sample observed in X-ray (Ponman et al., 1996), the detection rate of hot gas was higher for the HI deficient groups with respect to groups with a normal HI content (45% versus 18%), independently of the distances or exposure times. Based on these results, we have proposed an evolutionary scenario in which the interstellar medium is removed from individual galaxies and the total HI content in the group would decrease as the groups evolve (VM01). Borthakur+ (2010,2014) using GBT data found a diffuse HI component missed by the VLA increasing with evolutionary stage, more consistent with tidal stripping than with ram-pressure, as X-ray data support (Rasmussen+ 2008, Desjardins+ 2013).

IRAC/Spitzer photometry of HCGs shows a gap in colour-colour space (Johnson+ 2007) suggesting rapid evolution from dusty to dust-free systems, correlating with HI-depletion. This agrees with the bimodality in specific SFR found in Tzanavaris+ (2010) and MoHEG (Molecular Hydrogen Emiting Galaxy, Ogle+ 2010) galaxies have been identified as the rare transition objects. Based on detection of warm H2 emission inconsistent with X-ray heating/AGN activity in 23 HCGs we proposed that shocks might exist in many HCGs induced by galaxies interacting with a cold IGM (Cluver et al 2013). An extreme example is the intergalactic shock of Stefan’s Quintet (HCG92, Appleton et al 2006; Cluver et al 2010) with turbulent motions of ~1000 km/s. As part of a recently started mapping of 12 HCGs in CO(1-0) with CARMA maps, in combination with  [C II] and [O I] observations from Herschel, we have studied HCG57  (Alatalo et al 2014) and found the existence of shocks, consistent with peculiar CO kinematics in HCG 57a with evidences of suppressed star formation, falling a factor of 10–30 below normal galaxies on the Kennicutt-Schmidt relation. This supports shock-induced SF suppression as the explanation for the HCG galaxies falling in the mid-infrared green valley.

We observed HCG 91 with the WiFeS integral field spectrograph and found H II regions with abundance and kinematic offsets in HCG 91c (Vogt et al 2015). VLA-HI observations  suggest that HCG 91c is caught early in its interaction with the other members of HCG 91, and find evidence points towards infalling and collapsing extraplanar gas clouds at the disc-halo interface, providing evidence that some of the perturbations possibly associated with the early phase of galaxy evolution in compact groups impact the star-forming disc locally, and on sub-kpc scales.

A proposed evolutionary scenario for HCGs (see Verdes-Montenegro et al. 2001)

The physical mechanisms responsible for these changes in the morphology and gas content of galaxies are unclear. In order to understand the fate of gas in HCGs we have started a study to look for an intragroup medium (IGM). We have considered several possibilities: (a) Neutral gas: interstellar medium stripped from the member galaxies could remain neutral for a significant amount of time (see e.g. reports of HI clouds within Virgo cluster, Oosterloo and van Gorkom, 2005). (b) Hot gas, since the higher X-ray detection rate of HI deficient groups is consistent with a picture where the HI is stripped from spiral galaxies and is then heated to the temperature of the surrounding IGM. Enhanced star formation activity which might be contributing to exhausting the gas supply has been also studied.

We have performed a Spitzer mid-infrared spectroscopy study of a sample of 74 galaxies located in 23 Hickson Compact Groups (Cluver et al 2013), chosen to be at a dynamically-active stage of HI  depletion. We find evidence for enhanced warm H2  emission  in 20% of the galaxies, with 8 galaxies having extreme values, similar to what has been seen previously in the compact group HCG 92 (Stephan’s Quintet,  Appleton et al. 2006; Cluver et al. 2010). In the case of HCG 92 it  was shown to be associated with the dissipation of mechanical energy associated with a large-scale shock caused when one group member collided, at high velocity, with tidal debris in the intragroup medium. This might be the explanation for those  H2 -enhanced systems,  since other sources of heating (UV or X-ray excitation from star formation or AGN) are insufficient to account for the observed emission. These galaxies fall predominantly in a region of mid-infrared color-color space identified as being connected to rapid transformations in HCG galaxy evolution, and are primarily early-type disk systems.

Papers

  Galaxy interactions in compact groups - II. Abundance and kinematic anomalies in HCG 91c
Vogt, Frédéric P. A.; Dopita, Michael A.; Borthakur, Sanchayeeta; Verdes-Montenegro, Lourdes; Heckman, Timothy M.; Yun, Min S.; Chambers, Kenneth C.
2015/07 Monthly Notices of the Royal Astronomical Society, Volume 450, Issue 3, p.2593-2614.

  Strong Far-infrared Cooling Lines, Peculiar CO Kinematics, and Possible Star-formation Suppression in Hickson Compact Group 57
Alatalo, K.; Appleton, P. N.; Lisenfeld, U.; Bitsakis, T.; Guillard, P.; Charmandaris, V.; Cluver, M.; Dopita, M. A.; Freeland, E.; Jarrett, T.; Kewley, L. J.; Ogle, P. M.; Rasmussen, J.; Rich, J. A.; Verdes-Montenegro, L.; Xu, C. K.; Yun, M.
2014/11 The Astrophysical Journal, Volume 795, Issue 2, article id. 159, 16 pp. (2014).

  Enhanced Warm H2 Emission in the Compact Group Mid-Infrared "Green Valley".
M.E. Cluver, P.N. Appleton, P. Ogle, T.H. Jarrett, J. Rasmussen, U. Lisenfeld, P. Guillard, L. Verdes-Montenegro, R. Antonucci, T. Bitsakis, V. Charmandaris, F. Boulanger, E. Egami, C.K. Xu, M.S. Yun.
2013 in press in Apj.

Molecular gas content and SFR in Hickson Compact Groups: enhanced or deficient?
V. Martinez-Badenes, U. Lisenfeld, D. Espada, L. Verdes-Montenegro, S. García-Burillo, S. Leon, J. Sulentic, and M. S. Yun
2012 A&A 540, A96

Detection of Diffuse Neutral Intragroup Medium in Hickson Compact Groups.
S. Borthakur, M. S. Yun, and L. Verdes-Montenegro.
2010 ApJ 710, 385. Erratum.

Galaxy evolution in Hickson compact groups: the role of ram-pressure stripping and strangulation.
J. Rasmussen, T. J. Ponman, L. Verdes-Montenegro, M. S. Yun, and S. Borthakur
2008 Monthly Notices of the Royal Astronomical Society 388, 1245

Seyfert's Sextet: A Slowly Dissolving Stephan's Quintet?.
DURBALA, A., DEL OLMO, A., ROSADO, M., SULENTIC, J. W., PLANA, H., IOVINO, A., PEREA, J., VERDES-MONTENEGRO, L., FUENTES-CARRERA, I
Astronomy and Astrophysics, 2008 AJ 135, 130

 The evolution of HCG 31: Optical and high-resolution HI study
VERDES-MONTENEGRO, L., DEL OLMO, A., YUN, M.S., J., PEREA, J
Astronomy and Astrophysics, 430, 443-464 (2005)

 Ripples and tails in the compact group of galaxies Hickson 54
VERDES-MONTENEGRO, L., DEL OLMO, A., IGLESIAS-PÁRAMO, J., PEREA, J., VILCHEZ, J. M., YUN, M.S., HUCHTMEIER , W. K. Astronomy and Astrophysics, 396, 815-832 (2002).

 The VLA H I Observations of Stephan's Quintet (HCG 92)
WILLIAMS, B. A., YUN, M.S., VERDES-MONTENEGRO, L.
The Astromomical Journal, 123, 2417(2002).

 A Multiwavelength Study of Stephan's Quintet.
SULENTIC, J. W., ROSADO, M., DUTZIN-HACYAN, D., VERDES-MONTENEGRO, L., TRINCHIERI, G., XU, C., PIETSCH, W.
The Astromomical Journal, 122, 2993 (2001)

 Where is the Neutral Atomic Gas in Hickson Groups?
VERDES-MONTENEGRO, L., YUN, M.S., WILLIAMS, B. A., HUCHTMEIER , W. K. , DEL OLMO, A., PEREA, J.
Astronomy and Astrophysics, 377, 812 (2001).

 Effects of interaction induced activities in Hickson compact groups: CO and FIR study
VERDES-MONTENEGRO, L., YUN, M.S., PEREA, J., DEL OLMO, A., HO, P. T. P.
Astrophys. J. 497, 89 (1998).

 Non-linear Dependence of L_B on L_FIR and M_H_2 among Spiral Galaxies and the Effect of Tidal Interaction and Environment.
PEREA, J., DEL OLMO, A., VERDES-MONTENEGRO, L., YUN, M.S.
Astrophys. J. 490, 166-172 (1997).

 Molecular gas and infrared emission in HCG 31 and HCG 92 (Stephan's Quintet) and tidal interactions in compact group environment.
YUN, M.S,  VERDES-MONTENEGRO,  VERDES-MONTENEGRO, L.
Astrophys. J. 475, L21-L24 (1997).

 Hickson 96: a physical compact group
VERDES-MONTENEGRO, L., DEL OLMO, A., PEREA, J., ATHANASSOULA, E.,MARQUEZ, I., AUGARDE, R.
Astron. Astrop. 321, 409-423 (1997)