The Nuclear Activity of Galaxies in the Hickson Compact Groups

Abstract

In order to investigate the nuclear activity of galaxies residing in compact groups of galaxies, we present results of our optical spectroscopic program made at Okayama Astrophysical Observatory. We have performed optical spectroscopy of 69 galaxies which belong to 31 Hickson Compact Groups (HCGs) of Galaxies.Among them, three galaxies have discordant redshifts. Further, spectral quality is too poor to classify other three galaxies. Therefore, we describe our results for the remaining 63 galaxies.Our main results are summarized below. (1) We have found in our sample; 28 AGN, 16 H II nuclei, and 19 normal galaxies which show no emission line.We used this HCG sample for statistical analyses.(2) Comparing the frequency distributions of activity types between the HCGs and the field galaxies whose data are taken from Ho, Filippenko, & Sargent (382 field galaxies), we find that the frequency of H II nuclei in the HCGs is significantly less than that in the field. However, this difference may be due to selection bias that our HCG sample contains more early-type galaxies than the field, because it is known that H II nuclei are rarer in early-type galaxies than in later ones.(3) Applying correction this morphological bias to the HCG sample, we find that there is no statistically significant difference in the frequency of occurrence of emission-line galaxies between the HCGs and the field. This implies that the dense galaxy environment in the HCGs does not affect triggering both the AGN activity and the nuclear starburst. We discuss some implications on the nuclear activity in the HCG galaxies.

A mid-IR study of Hickson compact groups

“…According to Lacy et al (2004), galaxies located within the shaded area are AGN candidates. A total of 16 galaxies in our sample (∼23%) are AGN candidates based on this criterion, almost half of which are found in previous optical spectroscopy studies (Shimada et al 2000;Coziol et al 2000). These are 11 late-type systems (HCG 37b,38b,38c,47c,54abcd,57d,71a,79a) and 5 early-type systems (HCG 40f, 56b, 56e, 71b, and 95b).…”

“…Similarly, Stern et al (2005) propose using of the IRAC [3.6−4.5] vs. [5.8−8.0] colors to identify their AGN candidates. Among our galaxies optical spectra were available for 24 Shimada et al (2000), with 15 classified as H ii galaxies and 9 as AGNs, (3 as Sy2 and 3 as LINERs).…”

“…However, the molecular gas content of their individual galaxies, as traced by the CO emission, is similar to that of interacting pairs and starburst galaxies (Leon et al 1998). Nearly 40% of the HCG members for which nuclear spectroscopy has been obtained display evidence of active galactic nuclei (AGN), and it has been argued that those contain copious amounts of dust (Shimada et al 2000). Up to now, however, there is scant information on the far-IR colors and luminosities of the individual galaxies of the groups, because the available IRAS measurements have typically not resolved the groups, especially at 60 and 100 μm, providing only a single value and color for the ensemble of their galaxies.…”

Kinematics of galaxies in compact groups

“…HCG 47a is an SBb (Hickson 1993) galaxy, showing a ring-shape structure on its southwest side (Fasano et al 1994). Shimada et al (2000) classify HCG 47a as an absorption-line galaxy. Hickson classify HCG 47b, HCG 47c and, HCG 47d as E3, Sc and Sd galaxies.…”

Star formation in the intragroup medium and other diagnostics of the evolutionary stages of compact groups of galaxies

“…Bournaud & Duc (2006) performed simulations in order to relate TDGs and Allam et al (1996), Verdes-Montenegro et al (1998) and Gallagher et al (2008). The indicator "Central Activity" was taken from: Shimada et al (2000), de Carvalho et al (1997), Coziol et al (1998 and Xu et al (2003). b The disagreement between optical and kinematic position angle could be associated with the presence of a bar.…”