Hickson Compact Group
Hickson Compact Group ( HCG abbreviation ) is a catalog of 100 galaxy groups, edited by Paul Hickson in 1982.
The most well-known group of galaxies in the catalog is Stephan's Quintet.
Search criteria
Paul Hickson used the red plates of the Palomar Observatory Sky Survey to search specifically for compact groups of galaxies. He presented on three criteria, which are still used in general for finding new compact groups of galaxies:
Populationskriterion: N ≤ 4
Isolationskriterion: Θ ≥ 3θG
Kompaktheitskriterion: ug < 26.0
In these formulas, N is the total number of galaxies within 3 magnitudes of the brightest galaxy in the group. ug than the average Gesamtmagnitude on just these galaxies per square arc seconds (via the angular diameter? G ) defined.? G describes the angular diameter of the largest concentric circle contains no other galaxies within Grenzmagnituden.
Using these criteria, clusters of galaxies and vast galaxy groups can be separated quickly.
The groups were then classified on the basis of morphological characteristics nor the brightest galaxy and the luminosity difference between the brightest and second brightest galaxy.
S: the brightest galaxy in the group is a spiral galaxy. E: the brightest galaxy in the group is an elliptical or S0 galaxy. I: the difference in brightness between the brightest and the second brightest galaxy of the group is more than a magnitude. II: The difference in brightness between the brightest and the zweithellten group of galaxies is between a half and a magnitude. III. The difference in brightness between the brightest and the second brightest galaxy is less than half a magnitude.
The definition of the catalog according to Hickson:
Most compact groups containment a high fraction of galaxies having morphological or kinematical Peculiarities, nuclear radio and infrared emission, and active galactic nuclei or starburst (AGN ) activity. They containment large quantities of diffuse gas and are dynamically dominated by dark matter. They most likely form as subsystems within looser associations and evolve by gravitational processes. Strong galaxy interactions and merging result is expected to lead to the ultimate demise of the group. Compact groups are surprisingly Numerous, and play a significant role in june galaxy evolution.
" The most compact galaxies Grupen contain high levels of galaxies with morphological or kinematic irregularities; nuclear radio emission and infrared emission and strong star formation or active galactic nuclei (AGN ). They contain large quantities of diffuse gas and are dynamically dominated by dark matter. They usually form a subsystem within a larger group and develop under the influence of gravity. Strong interactions of galaxies are the result of mergers of galaxies and are expected, to the final demise of the group. Compact groups of galaxies are surprisingly common and play an important role in the evolution of galaxies. "
Further investigations of the catalog
Since it is the brightest and nearest known for compact groups of Hickson, they have been repeatedly investigated since the eighties. Particular attention was paid to the environment in which they are located, their dwarf galaxy population, the high proportion of dark matter and its morphological composition.
Thus, in 1992 99 of the 100 groups of Hickson were detected by spectroscopy to measure the velocities and the redshift of each group member. It turned out the one that the velocity dispersion of the group members was very low to form ( at about 250 km / s) and, secondly, that eight groups only by projection effects came about without a real group.
List of HCG galaxy groups
The catalog is sorted in ascending order of right ascension. The individual group members are designated by the suffix A, B, C, etc.. The group type according to Hickson means: S The brightest galaxy is a spiral galaxy. E The brightest galaxy is an elliptical galaxy. I - III difference in the brightnesses of the two brightest galaxies.