SDSS_DR12.GalGroup table
The GalGroup table contains galaxy groups and clusters that are extracted from the Galaxy table using the FoF method.
Table structure
| Column | Type | UCD | Unit | Description |
|---|---|---|---|---|
| groupid | int | meta.id meta.main | group/cluster id | |
| ngal | int | meta.number | richness (number of members) of the group | |
| raj2000 | double | pos.eq.ra | deg | right ascension of the group centre |
| dej2000 | double | pos.eq.dec | deg | declination of the group centre |
| lam | double | pos | deg | SDSS survey coordinate lambda of the group centre |
| eta | double | pos | deg | SDSS survey coordinate eta of the group centre |
| crd_x | double | pos.cartesian.x | Cartesian coordinate of the group centre along the x-axis | |
| crd_y | double | pos.cartesian.y | Cartesian coordinate of the group centre along the y-axis | |
| crd_z | double | pos.cartesian.z | Cartesian coordinate of the group centre along the z-axis | |
| zcmb | double | src.redshift | CMB-corrected redshift of group, calculated as an average over all group/cluster members | |
| groupdist | double | pos.distance | Mpc | comoving distance to the group centre |
| sigma_v | float | stat.stdev phys.veloc | km/s | rms radial velocity deviation (in physical coordinates) |
| sigma_sky | float | stat.stdev pos.distance | Mpc | rms deviation of the projected distance in the sky from the group centre (in physical coordinates); sigma_sky defines the extent of the group in the sky |
| r_max | float | stat.max phys.size.radius | Mpc | maximum radius of the group/cluster in the plane of the sky |
| mass_200 | float | phys.mass | 10+12Msun | estimated mass of the group assuming the NFW density profile |
| r_200 | float | phys.size.radius phys.virial | Mpc | radius in which the group mean density is 200 times higher than the average density of the Universe; radius of the group that contains the mass mass_200 |
| lum_r_group | float | phys.luminosity | 10+10Lsun | observed luminosity, i.e. the sum of luminosities of the galaxies in the group/cluster |
| weight | float | stat.weight | weight factor for the group at the mean distance of the group | |
| den1_5 | float | src.density | normalized environmental density (mean of group galaxy densities) of the group for the smoothing scale of 1.5 Mpc | |
| den3 | float | src.density | normalized environmental density (mean of group galaxy densities) of the group for the smoothing scale of 3 Mpc | |
| den6 | float | src.density | normalized environmental density (mean of group galaxy densities) of the group for the smoothing scale of 6 Mpc | |
| den10 | float | src.density | normalized environmental density (mean of group galaxy densities) of the group for the smoothing scale of 10 Mpc | |
| dist_nearest_cl | float | pos.distance | Mpc | distance to the nearest cluster in comoving coordinates |
| id_nearest_cl | int | meta.id | groupid of the nearest cluster | |
| id_merger | int | meta.id | id of the merging system, zero otherwise |