Photometric Classification of Galaxy Groups

GONZÁLEZ, E. P.; ALACORIA, J.; TAPIA VEGA, R.; ALONSO, M. S.; LAMBAS, D. G.

Pucón, Chile

Congreso; XI IAU Regional Latin American Meeting of Astronomy; 2005

Sociedad Chilena de Astronomía

We present a photometric classification of galaxy groups obtained by Merchan & Zandivarez (2005) from the Third Data Release of the Sloan Digital Sky Survey (SDSS-DR3). We divide the groups into Equal (E), Binary (B) and Central Dominant (cD) systems, by comparing r - band member galaxy magnitudes. cD groups contain one galaxy in the center with higher luminosity than the rest of the members, B groups have two central luminosity galaxies while in E groups all galaxies have a comparable luminosities. The number of members in the groups have a range between 6 and 10 members. We classified the galaxy groups according to their luminosity distribution using the chi parameter: chi={(N-1)M_1}/{sum^{N}_{i=2}M_i} Where M and N are the luminosity and the number of member in the group respectively. This effectively quantifies the difference between the most luminous galaxy and the rest of the members. The values of adopted to classified different groups are: chi ≤ 1.025 (E groups), 1.025 > chi ≥ 1.080 (B groups) and chi > 1.080 (cD groups). We find that in cD groups all members are classified as bulges (compacticy parameter C > 2.5) while E groups shows both bulges and disk galaxies. We also analyze star formation rates of members using the eClass´ spectral parameter where eClass´ > 0 indicates strong star formation activity. The distribution of the eClass´ parameter for bulge galaxies es similar for members of either cD, B or E groups. On the contrary, for disk galaxies this distrution shows a peak around -0.1, for cD groups while in B and E groups the eClass´ parameter is distributed homogenously over the range -0.15 and 0.15, indicating that star formation of disk is strongly modified by environment and that our classification correlates to galaxy properties.