Characterizing Barred Galaxies in the Abell 901/902 Supercluster
ABSTRACT In dense clusters, higher densities at early epochs as well as physical processes, such as ram pressure stripping and tidal interactions become important, and can have direct consequences for the evolution of bars and their host disks. To study bars and disks as a function of environment, we are using the STAGES ACS HST survey of the Abell 901/902 supercluster (z~0.165), along with earlier field studies based the SDSS and the Ohio State University Bright Spiral Galaxy Survey (OSUBSGS). We explore the limitations of traditional methods for characterizing the bar fraction, and in particular highlight uncertainties in disk galaxy selection in cluster environments. We present an alternative approach for exploring the proportion of bars, and investigate the properties of bars as a function of host galaxy color, Sersic index, stellar mass, star formation rate (SFR), specific SFR, and morphology.
arXiv:0802.3910v1 [astro-ph] 26 Feb 2008
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Characterizing Barred Galaxies in the Abell 901/902
Supercluster from STAGES
I. Marinova,1S. Jogee,1D. Bacon,2M. Balogh,3M. Barden,4
F. D. Barazza,5E. F. Bell,6A. B¨ ohm,7J. A. R. Caldwell,1M. E. Gray,8
B. H¨ außler,8C. Heymans,9,10K. Jahnke,6E. van Kampen,4S.
Koposov,6K. Lane,8D. H. McIntosh,11K. Meisenheimer,6C. Y.
Peng,12,13H.-W. Rix,6S. F. S´ anchez,14A. Taylor,15L. Wisotzki,7C.
Wolf,16and X. Zheng17
1The University of Texas, Department of Astronomy, Austin and Fort
Davis, Texas, USA
2Institute of Cosmology and Gravitation, University of Portsmouth,
3Department of Physics and Astronomy, University Of Waterloo,
4Institute for Astro- and Particle Physics, University of Innsbruck,
5EPFL, Sauverny, Switzerland
6Max-Planck-Institut f¨ ur Astronomie, Heidelberg, Germany
7Astrophysikalisches Insitut Potsdam, Potsdam, Germany
8School of Physics and Astronomy, The University of Nottingham,
9Department of Physics and Astronomy, University of British
Columbia, Vancouver, Canada
10Institut d’Astrophysique de Paris, Paris, France
11Department of Astronomy, University of Massachusetts, Amherst,
12NRC Herzberg Institute of Astrophysics, Victoria, Canada
13Space Telescope Science Institute, Baltimore, MD, USA
14Centro Hispano Aleman de Calar Alto, Almeria, Spain
15The Scottish Universities Physics Alliance (SUPA), Institute for
Astronomy, University of Edinburgh, Edinburgh, UK
16Department of Astrophysics, University of Oxford, Oxford, UK
17Purple Mountain Observatory, National Astronomical Observatories,
Chinese Academy of Sciences, Nanjing, China
ical processes, such as ram pressure stripping and tidal interactions become
important, and can have direct consequences for the evolution of bars and their
host disks. To study bars and disks as a function of environment, we are using
the STAGES ACS HST survey of the Abell 901/902 supercluster (z ∼ 0.165),
along with earlier field studies based the SDSS and the Ohio State University
Bright Spiral Galaxy Survey (OSUBSGS). We explore the limitations of tradi-
tional methods for characterizing the bar fraction, and in particular highlight
uncertainties in disk galaxy selection in cluster environments. We present an
alternative approach for exploring the proportion of bars, and investigate the
In dense clusters, higher densities at early epochs as well as phys-
2 Marinova et al.
properties of bars as a function of host galaxy color, S´ ersic index, stellar mass,
star formation rate (SFR), specific SFR, and morphology.
The most important internal driver of disk galaxy evolution are stellar bars,
because they efficiently redistribute angular momentum between the disk and
dark matter halo (e.g., Combes & Sanders 1981; Weinberg 1985; Athanassoula
2002). To put bars in a cosmological context, we must determine what effects
environment has on bar and galaxy evolution. The bar fraction and properties
in clusters relative to that found in field galaxies depend on several factors, such
as the epoch of bar formation, the higher densities in clusters at early times
leading to earlier collapse of dark matter halos, and the relative importance
of processes such as ram pressure stripping, galaxy tidal interactions, mergers,
and galaxy harassment. For example, tidal interactions can induce a bar in a
dynamically cold disk, but they may also heat the disk, making it less unstable to
bar formation. Previous studies have found opposing results for the bar fraction
in isolated galaxies and those that are perturbed or in clusters (van den Bergh
2002; Varela et al. 2004). We use our large supercluster sample of galaxies
(∼2000 over MV-15.5 to -24.0) from the STAGES survey of the Abell 901/902
supercluster (z ∼ 0.165) to investigate the fraction and properties of bars and
their host disks in a dense environment.
2.STAGES Data and Sample
To study galaxies in a dense cluster environment, we use the Space Telescope
A901/902 Galaxy Evolution Survey (STAGES; Gray et al., in preparation).
The Abell 901/902 supercluster (z ∼ 0.165, number of galaxies per unit area
N=250Mpc−2) consists of three clusters: A901a, A901b, and A902 with an
average core separation of ∼ 1Mpc. The A901 clusters show irregular X-ray
morphologies, suggesting that they are not yet relaxed (Ebeling et al. 1996).
The STAGES survey includes high resolution HST ACS F606W images (PSF
∼ 0.1′′, corresponding to ∼ 300pc at z ∼ 0.1651) of the Abell 901/902 superclus-
ter, along with spectrophotometric redshifts of accuracy δz/(1 + z) ∼ 0.02 down
to RVega= 24 from the COMBO-17 survey (Wolf et al 2004). Multi-wavelength
coverage is available for this field from GALEX, Spitzer, and XMM-Newton.
Dark matter maps for the supercluster have been constructed using gravitational
lensing by Gray et al. (2002) and Heymans et al. (2008, MNRAS submitted).
The cluster sample contains 798 bright, MV≤ −18.0, galaxies.
3.Method for Identification and Characterization of Bars
To identify and characterize the properties of bars, we employ the widely used
method of fitting ellipses to the galaxy isophotes out to sky level with the iraf
task ’ELLIPSE’ (e.g., Friedli et al. 1996; Jogee et al. 1999; Knapen et al. 2000;
1We assume a flat cosmology with ΩM = 1 − ΩΛ = 0.3 and H0 =70km s−1Mpc−1.
Characterizing Barred Galaxies in Abell 901/9023
Marinova & Jogee 2007). We generate plots of the surface brightness (SB),
ellipticity (e), and position angle (PA) as a function of radius for each galaxy.
We also plot overlays of the fitted ellipses onto the galaxy image. We use both
the overlays and radial plots to identify bars. A galaxy is identified as barred
if (a) the e profile rises to a global maximum while the PA stays constant and
(b) after the global max, the e drops and the PA changes characterizing the disk
region. To ensure reliable morphological classification, we exclude all galaxies
with outer e > 0.5 (i > 60o). With our PSF of ∼ 0.1′′, (∼ 300pc at z ∼ 0.165),
we cannot reliably detect bars with diameter smaller than ∼ 1.8kpc. However,
such small bars are usually nuclear bars, whereas we focus only on primary
bars, which have diameters greater than 2kpc. When working in the rest-frame
optical, bars heavily obscured by dust and SF will be missed, while such bars
can be detected in the rest-frame NIR (Eskridge et al. 2000, Knapen et al.
2000, Marinova & Jogee 2007). Furthermore, partially obscured bars can be
missed in ellipse-fits because dust and SF along the bar can cause the PA to
vary marginally more than the 10oallowed by the constant PA criterion. In
fact, studies of nearby galaxies suggest that the bar fraction increases by a
factor of ∼ 1.3 in the NIR, compared to the optical (Marinova & Jogee 2007).
As we do not have rest-frame NIR images for Abell 901/902, we resort to a
second method of classifying bars: visual classification. The visual classification
method tends to capture partially obscured bars somewhat better than ellipse
fits, because visual classification takes into account, not only the stellar light
in the bar, but also secondary signatures, such as the shape of dust lanes, the
overall morphology of the disk, and spiral arms.
All bar studies to date carried out in field samples define the bar fraction fbar
as the ratio (number of barred disks/ total number of disks). An accurate deter-
mination of fbartherefore hinges on an accurate way to identify disk galaxies.
In field samples, both locally and at intermediate redshifts, two techniques are
widely used to identify disks: S´ ersic cuts (n <2.5) based on single component fits,
and luminosity-color cuts to isolate blue cloud galaxies from the red sequence.
These methods have limitations even in field samples: the S´ ersic cut can miss
bright disks with prominent bulges (where n >2.5) and the luminosity-color se-
lection misses bright disks with red colors (caused by old stellar populations or
In dense cluster environments, where disk galaxies can be red, and where
the luminosity function is dominated by faint dwarf galaxies, it becomes even
harder to identify disks via either method. We illustrate these uncertainties in
disk selection in the supercluster as follows. The barred galaxies identified in
§ 3, from ellipse fit and visual classification, are plotted on the color-luminosity
(Figure 1a) and S´ ersic -luminosity (Figure 1b) planes. Because bars are disk
signatures, we can use the strongly barred galaxies missed by the two methods
as a lower limit on their failure to select disk galaxies. For bright galaxies, we find
that 46% (45/97) and 40% (39/97) of disks with prominent, visually-identified
bars are missed, respectively, by the blue cloud color-luminosity cut and S´ ersic
4 Marinova et al.
It is clear that the uncertainties in disk selection will cause a large and
dominant error in the optical bar fraction fbarin clusters. We therefore adopt the
following approach in the A901/902 supercluster (1) We define a new quantity
Pbar as the proportion of all galaxies (rather than disk galaxies), which are
barred. Thus, Pbaris not as heavily affected as fbarby the uncertainties in disk
selection. (2) We explore how Pbarand bar properties vary as a function of galaxy
properties, such as color (Figure 1a), stellar mass (Figure 1c,d) , SFR (Figure
1c), specific SFR (Figure 1d), and bulge-to-disk ratio. (3) When comparing the
frequency of bars in clusters to that in the field, we can only use fbar, since no
measurements of Pbarexist in the field. We estimate fbarby selecting disks via
visual classification, rather than from color-luminosity or S´ ersic cuts. Disk are
identified visually based on features such as spiral arms and stellar bars, or the
presence of a bulge+disk from the light distribution.
AST 06-07748, NASA LTSA grant NAG5-13063, as well as HST G0-10395.
I.M. and S.J. acknowledge support from NSF grant
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Characterizing Barred Galaxies in Abell 901/9025
with visually-identified strong bars are marked as triangles. Many of these lie
on the red sequence and would be missed by assuming disks lie only in the
blue cloud. (b) Rest-frame U-V color vs. S´ ersic n plane. Many disks with
prominent bars would be missed by a S´ ersic cut n <2.5. (c) SFR vs. stellar
mass. (d) Specific SFR vs. stellar mass. Values for red sequence galaxies are
(a) Bright galaxies in rest-frame U-V vs. MV plane. Galaxies