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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.