No abstract

Aims. We study the environmental dependence of local luminous infrared galaxies (LIRGs) and ultraluminous infrared galaxies (ULIRGs) found in the Sloan Digital Sky Survey (SDSS) data. Methods. The LIRG and ULIRG samples are constructed by cross-correlating spectroscopic catalogs of galaxies of the SDSS Data Release 7 and the Infrared Astronomical Satellite Faint Source Catalog. We examine the effects of the large-scale background density (Σ 5 ), galaxy clusters and the nearest neighbor galaxy on the properties of infrared galaxies (IRGs). Results. We find that the fraction of LIRGs plus ULIRGs among IRGs ( f (U)LIRGs ) and the infrared luminosity (L IR ) of IRGs strongly depend on the morphology of and the distance to the nearest neighbor galaxy: the probability of an IRG being a (U)LIRG ( f (U)LIRGs ) and its L IR both increases as it approaches a late-type galaxy, but decreases as it approaches an early-type galaxy (within half the virial radius of its neighbor). We find no dependence of f (U)LIRGs on the background density (surface galaxy number density) at a fixed stellar mass of galaxies. The dependence of f (U)LIRGs on the distance to galaxy clusters is also found to be very weak, but in the highest density regions, such as the center of galaxy clusters, few (U)LIRGs are found.Conclusions. This environmental dependence of LIRGs and ULIRGs and the evolution of the star formation rate (SFR)-environment relation from high redshifts to low redshifts seem to support the idea that galaxy-galaxy interactions and merging play a critical role in triggering the star formation activity of LIRGs and ULIRGs.

We present a study on four new star clusters discovered in the halo of the intriguing dwarf irregular galaxy NGC 6822 from a wide field survey covering 3 • × 3 • area carried out with MegaCam at Canada-France-Hawaii Telescope (CFHT). The star clusters have extended structures with half-light radii R h ≈ 7.5-14.0 pc, larger than typical Galactic globular clusters and other known globular clusters in NGC 6822. The integrated colors and color magnitude diagrams (CMD) of resolved stars suggest that the new star clusters are 2 -10 Gyr old and relatively metal poor with Z=0.0001-0.004 based on the comparison with theoretical models. The projected distance of each star cluster from the galaxy center ranges from 10.7 ′ (≈ 1.5 kpc) to 77 ′ (≈ 11 kpc), far beyond the optical body of the galaxy. Interestingly, the new star clusters are aligned along the elongated old stellar halo of NGC 6822, which is almost perpendicular to the HI gas distribution where young stellar populations exist. We also find that the colors and half-light radii of the new clusters are correlated with the galactocentric distance: clusters farther from the galaxy center are larger and bluer than those closer to the galaxy center. We discuss the stellar structure and evolution of NGC 6822 implied by these new extended star clusters in the halo. We also discuss the current status of observational and theoretical understandings regarding the origin of extended star clusters in NGC 6822 and other galaxies.

We investigate the connection between the presence of bars and AGN activity, using a volumelimited sample of ∼9,000 late-type galaxies with axis ratio b/a > 0.6 and M r < −19.5 + 5logh at low redshift (0.02 ≤ z 0.055), selected from Sloan Digital Sky Survey Data Release 7. We find that the bar fraction in AGN-host galaxies (42.6%) is ∼2.5 times higher than in non-AGN galaxies (15.6%), and that the AGN fraction is a factor of two higher in strong-barred galaxies (34.5%) than in non-barred galaxies (15.0%). However, these trends are simply caused by the fact that AGN-host galaxies are on average more massive and redder than non-AGN galaxies because the fraction of strong-barred galaxies (f SB ) increases with u − r color and stellar velocity dispersion. When u − r color and velocity dispersion (or stellar mass) are fixed, both the excess of f SB in AGN-host galaxies and the enhanced AGN fraction in strong-barred galaxies disappears. Among AGN-host galaxies we find no strong difference of the Eddington ratio distributions between barred and non-barred systems.These results indicate that AGN activity is not dominated by the presence of bars, and that AGN power is not enhanced by bars. In conclusion we do not find a clear evidence that bars trigger AGN activity.