Transcript 슬라이드 1
Properties of Barred Galaxies in SDSS DR7 - OPEN KIAS SUMMER INSTITUTE Gwang-Ho Lee, Changbom Park, Myung Gyoon Lee & Yun-Young Choi 0. Abstract We investigate the dependence of barred galaxy fraction on the physical parameters of galaxies and on environmental conditions. A volume – limited sample of this study consists of 17,510 late type galaxies, which is a subset of DR7, brighter than Mr = -19.5 and with redshift 0.025 < z < 0.05485. We find that there are 2,201 barred galaxies (fbar = 12.6%) by visual inspection and by ellipse fitting, and that fraction of barred galaxy rise to 17.6% when consider only moderately inclined systems with b/a≥ 0.65. The fraction of barred galaxy can be described as functions of several physical properties like as u-r color, r-band absolute magnitude, equivalent width of the H-alpha line, and so on. We find u-r color is most important factor in changing the bar fraction, and that bar fraction is higher as u-r becomes redder. It is interesting that barred galaxies have high central star formation activity relative to normal galaxies which have no bar structures. 3. Result 1. Introduction Understanding the role that environment plays in the process of galaxy formation and evolution is one of the most important problems. Moreover, Bar structures are believed to be important with regard to the evolution of galaxies. However, it remains an open question. Fig. (1) Fig. (2) Environmental dependences of galaxy properties and morphologies have been studied extensively. The morphology-density relation was confirmed by Dressler (1980). Recently, Park et al. (2008) found that galaxy morphology as a function of the nearest neighbor separation. We use the SDSS data to study the properties of barred galaxies. Our goal is to understand how the properties of the galaxies, like as environmental factors, affect the formation of the bar. 2. Morphological Classification (a) Fig. (3) (b) Fig. (1) ~ (3) show distribution of the barred galaxies in 2-D physical parameters spaces. Blue dots are barred galaxies, and grey dots are non – barred galaxies with axial ratio b/a≥0.65. Contours of constant barred galaxy fraction are superposed on each the scatter plot. (a) Sample definitions of our volume – limited SDSS sample in redshift vs. Mr space. (b) Distributions of galaxies in our sample. Red dots are early types, blue dots are late types. We use a large sample of 31,398 galaxies, with Mr < -19.5 and redshift 0.025 < z < 0.05485, drawn from the SDSS Data Release 7. We classify these galaxies into early type (E & S0) and late type (S & Irr) using automated method introduced in Park & Choi (2005). After performing additional visual check, we confirm that our sample consists of 13,808 early type galaxies, 17,510 late type galaxies, and 78 unclassified galaxies. We use the IRAF task ELLIPSE to detect barred galaxies. To find “candidate” barred galaxy, we check the bar signatures in ellipticity radial profiles and position angle profiles obtained by ellipse fitting. We also perform visual inspection on g+r+i combined color images to determine whether a “real” bar is present or not. Finally, we identify 2,201 galaxies have a bar structure. The bar fraction turns to be 12.6%, and it rises to 17.6% ( 1,539 / 8,725 ) when consider only moderately inclined systems with axial ratio b/a≥0.65. Fig. (1) – The bar fraction is higher in redder, brighter galaxies. When color is fixed, it seems barred galaxies have higher star formation activity relative to non-barred galaxies. And the bar fraction also depend on central velocity dispersion (σ ). Fig. (2) – All contours are nearly parallel to x-axis that means local density. It means that the bar fraction has no or weak dependence on the local density when color or luminosity is fixed. Fig. (3) – Rn/rvir,n is the nearest neighbor distance normalized by the virial radius of the neighbor galaxy. It is difficult to find the environmental dependence attributed to the neighbor’s distance and morphology, when color is fixed. We also use the galaxy data associated with Abell gaalxy cluster from Park & Hwang (2008). R/r200,cl means the clustercentric radius normalized to the cluster virial radius. Left picture show that the bar fraction as a function of the R/r200,cl. We find the region where the fraction of barred galaxy abruptly decreases at R/r200,cl ≈3~6. 4. Conclusion These results indicate that the formation of the bar structure is affected by internal factors (u-r color, luminosity, star formation activity, velocity dispersion … ) and by external factors like as cluster centric radius. However, we need a further study to interpret the results physically and to understand the properties of barred galaxies sufficiently.