Compact Galaxy Groups Reveal Details of Their Close Encounters

HCG07 Hickson Compact Group Galaxies

Galaxies - spirals laced with nests of recent star formation, quiescent ellipticals composed mainly of old red stars, and numerous faint dwarfs - are the basic visible building blocks of the Universe. Galaxies are rarely found in isolation, but rather in sparse groups – sort of galactic urban sprawl.  But there are occasional dense concentrations, often found in the center of giant clusters, but also, intriguingly, as more isolated compact groups  (and yes, called Compact Galaxy Groups or CGs).  The galaxies in these Compact Groups show dramatic differences in the way they evolve and change with time compared with galaxies in more isolated surroundings. Why is this? Collisions between galaxies in these dense groups are common, leading to rapid star formation, but there seems to be more to the puzzle.

A team led by Dr Iraklis Konstantopoulos of the Australian Astronomical Observatory (AAO) have now obtained spectacular images of some CGs with the Dark Energy camera attached to the Blanco 4-meter telescope at the Cerro Tololo Inter-American Observatory (CTIO). This camera is able to image large areas of the sky to unprecedented faint limits. The team aims to combine these images with spectroscopic data taken with facilities of the Australian Astronomical Observatory that will reveal the velocities of the galaxies, leading to a much better understanding of their gravitational interactions.

The new images reveal faint streams of gas and stars called tidal tails, created in the mutual gravitational interaction when two galaxies suffer a close encounter. The tails, one preceding and one trailing the galaxy, persist long after the encounter, and allow the astronomers to calculate how long ago the event took place. The Dark Energy Camera, which can image a field four times the size of the full moon, is able to record these faint tidal tails, and its wide field will uncover unexpected surprises.

“The imagery reveals the assembly history of these galaxies living so close to each other via their previous interactions,” Dr Konstantopoulos said.  “We look for stretched out tidal debris tails and roughly determine their ages.  The time when interactions created the tidal debris and the arrangement of those 'fossils' tell us which galaxies interacted, and when.”

Not all CGs are alike: in some, the gas is contained within the individual galaxies, while in other groups the gas spreads out among the galaxies. These new data will allow astronomers to untangle the physical mechanism that leads to such differences.

Another new exploration is the census of faint dwarf galaxies. As their name implies, these are minor galaxies in comparison with giant ellipticals and spirals, but they are very numerous, and the new data will reveal how many are lurking in these Compact Groups.

The international team consists of astronomers at the Australian Astronomical Observatory, the National Optical Astronomy Observatory (US), and Monash University in Melbourne.

CTIO is a division of the National Optical Astronomy Observatory, operated by Association of Universities for Research in Astronomy Inc. under a cooperative agreement with the National Science Foundation. This research is supported by the Science and Industry Endowment Fund.

Media contact: 
Dr Amanda Bauer (AAO) 
Phone: +61 4 4702 9368 
105 Delhi Road, North Ryde NSW 2113

Science Contact:
Iraklis Konstantopoulos (AAO)

Image Gallery 

HCG07 Hickson Compact Group Galaxies

HCG 07: Galaxies in this cluster are undergoing a burst of star formation, but no tidal tails. How many dwarf galaxies are hidden here? (This image covers an area about a third the size of the full moon.)


HCG48 Hickson Compact Group Galaxies

HCG 48: This group is dominated by a massive elliptical galaxy that has presumably formed by ingesting (astronomers refer to this as accreting) all of its neighbors.


HCG31 Hickson Compact Group Galaxies

HCG 31: The tidal tails are clues to recent interactions, but no evidence of heated gas between the galaxies, as would be expected.


HCG59 Hickson Compact Group Galaxies

HCG 59: Two interacting giants have released a giant stellar stream in this Compact Group, which also hosts a bursting irregular galaxy.


HCG62 Hickson Compact Group Galaxies

HCG 62: The brightest Compact Group in the X-ray spectrum, astronomers seek to understand how the galaxies which share a common halo will evolve.


HCG79 Hickson Compact Group Galaxies

HCG 79: Known as Seyfert’s Sextet, four of these galaxies are involved in an ongoing interaction. The fifth galaxy is in the background and the sixth is actually material released in the interaction, the best candidate for a tidal dwarf galaxy in the local Universe.

Images Credit: Dane Kleiner