HCG: Help
HCG  Hickson's Compact Groups of Galaxies (HCG) Catalog 
The HCG database table is based on the Hickson Catalog, which is a list of
100 compact groups of galaxies that were identified by a systematic search of
the Palomar Observatory Sky Survey red prints. Each group contains four or
more galaxies, has an estimated mean surface brightness brighter than 26.0
magnitude per arcsec^2 and satisfies an isolation criterion. Dynamical
parameters which were derived for 92 of the 100 groups are also included in
the database. (Note that the Hubble constant was assumed to be Ho = 100
km/s/Mpc.)
This database table essentially contains the information given in Table 1 of
Hickson, P. (1982, ApJ, 255, 382) and Table 3 of Hickson, P. et al. (1992,
ApJ, 399, 353). Consequently, the information on individual galaxies in the
Hickson groups that is also given in these references, e.g., in Table 2 of
Hickson, P. et al. (1992, ApJ, 399, 353), is not in the HCG database table;
however, the latter data can be found in the related HEASARC database table
HCGGALAXY.
1982ApJ...255..382H
Systematic Properties of Compact Groups of Galaxies
Hickson P.
<Astrophys. J. 255, 382 + Erratum 259, 930 (1982)>
=1982ApJ...255..382H
A photometric catalog of compact groups of galaxies.
Hickson P., Kindl E., Auman J.R.
<Astrophys. J. Suppl. Ser. 70, 687 (1989)>
=1989ApJS...70..687H
The luminosity function of compact groups of galaxies
Mendes de Oliveira C., Hickson P.
<Astrophys. J. 380, 30 (1991)>
=1991ApJ...380...30M
Dynamical properties of compact groups of galaxies.
Hickson P., Mendes de Oliveira C., Huchra J.P., Palumbo G.G.C.
<Astrophys. J. 399, 353 (1992)>
=1992ApJ...399..353H
Morphology of galaxies in compact groups.
Mendes de Oliveira C., Hickson P.
<Astrophys. J., 427, 684 (1994)>
=1994ApJ...427..684M
This online catalog was created by the HEASARC in August, 1999, based on
machinereadable tables obtained from the ADC/CDS data centers (CDS catalog
VII/213: files groups.dat and dynamics.dat). The HEASARC refined the
coordinates, corrected the dynamics_flag values, and updated the table's
metadata in August, 2005.
HCG
The HCG catalog entry number, ranging from 1 to 100, for the compact group.
Name
The HCG catalog compact group designation.
RA
The right ascension in the specified equinox. The position given for the
group is actually the center of the circle that encloses the group. The
equinox of the positions in the originating table was B1950, notice, and the
RA was given to a precision of 1 second.
Dec
The declination in the specified equinox. The position given for the
group is actually the center of the circle that encloses the group. The
equinox of the positions in the originating table was B1950, notice, and the
declination was given to a precision of 1 arcsecond.
LII
The galactic longitude of the center of the circle that encloses the
compact group.
BII
The galactic latitude of the center of the circle that encloses the
compact group.
Dynamics_Flag
A flag that is set to 'Y' if dynamical parameter information
is available for the particular compact group.
Group_Type
A twocharacter code (a letter followed by a number) classifying
the group type, based on its two brightest members, according to the following
scheme:
S The brightest galaxy is a spiral galaxy.
E The brightest galaxy is not a spiral galaxy.
1 m(b)  m(a) >= 1.0
2 0.5 <= m(b)  m(a) < 1.0
3 m(b)  m(a) < 0.5
where m(a) and m(b) are the estimated red magnitudes of the brightest
and second brightest members, respectively, of the cluster.
Member_Count
The total number of galaxies in the compact group that are
within 3 magnitudes of the brightest galaxy.
Angular_Size
The angular diameter, in arcminutes, of the smallest circle
containing the geometric centers of all group members.
Tot_Mag_Group
The total estimated red magnitude of those galaxies
counted as group galaxies, i.e., those with individual magnitudes within
3 magnitudes of the brightest galaxy.
Brightest_Mag
The estimated red magnitude of the brightest galaxy in
the compact group.
Brightest_Redshift
The corrected redshift of the brightest galaxy in
the compact group. In all but ten cases this parameter is blank, and the
ADC/CDS documentation recommends that that the value of the group redshift
(the parameter 'redshift' in the HEASARC database) derived from the median
galaxy heliocentric velocity be preferred.
Alt_Name1
An alternate designation for the compact group.
Alt_Name2
An alternate designation for the compact group.
Alt_Name3
An alternate designation for the compact group.
Alt_Name4
An alternate designation for the compact group.
Redshift
The group redshift derived from the median galaxy heliocentric
velocity.
Number_Galaxies
The number of galaxies in the group with accordant
velocities, i.e., that have individual velocities that are within 1000
km/sec of the group median velocity, and are thus assumed to be physically
associated with the group.
Log_Sigma_Vrad
The logarithm (base 10) of the radial velocity dispersion,
in km/sec, of the accordant galaxies.
Log_Vel_Dispersion
The logarithm (base 10) of the estimated intrinsic
threedimensional velocity dispersion, in km/sec, corrected for measurement
errors.
Log_Median_Sep
The logarithm (base 10) of the median projected separation,
in kpc.
Log_Crossing_Time
The logarithm (base 10) of the dimensionless crossing
time parameter, which is the ratio of the crossing time to the approximate
age of the universe, and is independent of the choice of Hubble constant.
Log_Virial_Mass
The logarithm (base 10) of the virial mass of the group,
in grams.
Log_Projected_Mass
The logarithm (base 10) of the projected mass of the
group, in grams.
Log_Average_Mass
The logarithm (base 10) of the average mass of the group,
in grams.
Log_Median_Mass
The logarithm (base 10) of the median mass of the group,
in grams.
Log_Adopted_Mass
The logarithm (base 10) of the adopted mass of the group,
in grams. The adopted mass for the group is the median of the nonzero mass
estimates given in the four parameters log_virial_mass, log_projected_mass,
log_average_mass, and log_median_mass.
Log_Tot_Luminosity
The logarithm (base 10) of the total blue luminosity of
the accordant galaxies, in units of Watts (=10^7 erg/sec).
Log_Mass_To_Light
The logarithm (base 10) of the mass to light (M/L) ratio,
in solar units, i.e., solar mass/solar luminosity.
Log_Mass_Density
The logarithm (base 10) of the mass density, in gm/cm^3.
Questions regarding the HCG database table can be addressed to the
HEASARC User Hotline.
If you have any problems, please consult the help
page or mail ledashelp@star.le.ac.uk
