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Mon. Not. R. Astron. Soc. 000, 1–9 (2010) Printed 1 March 2010 (MN LATEX style file v2.2)
Galaxy Zoo: Bars in Disk Galaxies∗
Karen L. Masters1, Robert C. Nichol1, Ben Hoyle1,2, Chris Lintott3,
Steven Bamford4, Edward M. Edmondson1, Lucy Fortson5, William C. Keel6,
Kevin Schawinski7, Arfon Smith3, Daniel Thomas1
1Institute for Cosmology and Gravitation, University of Portsmouth, Dennis Sciama Building, Burnaby Road, Portsmouth, PO1 3FX, UK
2Institute for Sciences of the Cosmos (ICCUB), University of Barcelona, Marti i Franques 1, Barcelona, 08024 Spain
3Oxford Astrophysics, Department of Physics, University of Oxford, Denys Wilkinson Building, Keble Road, Oxford, OX1 3RH, UK
4Centre for Astronomy & Particle Theory, University of Nottingham, University Park, Nottingham, NG7 2RD, UK
5Astronomy Department, Adler Planetarium and Astronomy Museum, 1300 Lake Shore Drive, Chicago, IL 60605, USA
6Department of Physics & Astronomy, 206 Gallalee Hall, 514 University Blvd., University of Alabama, Tuscaloosa, AL 35487-0234, USA
7Einstein Fellow/Yale Center for Astronomy and Astrophysics, Yale University, P.O. Box 208121, New Haven, CT 06520, USA
∗This publication has been made possible by the participation of more than 200,000 volunteers in the Galaxy Zoo project.
Their contributions are individually acknowledged at http://www.galaxyzoo.org/Volunteers.aspx.
E-mail: karen.masters@port.ac.uk
Submitted to MNRAS 8th February 2010
ABSTRACT
We present first results from Galaxy Zoo 2, the second phase of the highly suc-
cessful Galaxy Zoo project (www.galaxyzoo.org). Using a volume–limited sample of
13665 disk galaxies (0.01 < z < 0.06 and Mr < −19.38), we study the fraction of
galaxies with bars as a function of global galaxy properties like colour, luminosity and
bulge prominence. Overall, 29.4 ± 0.5% of galaxies in our sample have a bar, in ex-
cellent agreement with previous visually–classified samples of galaxies (although this
overall fraction is lower than measured by automated bar–finding methods). We see
a clear increase in the bar fraction with redder (g − r) colours, decreased luminosity
and in galaxies with more prominent bulges, to the extent that over half of the red,
bulge–dominated, disk galaxies in our sample possess a bar. We see evidence for a
colour bi-modality for our sample of disk galaxies, with a “red sequence” that is both
bulge and bar–dominated, and a “blue cloud” which has little, or no, evidence for a
(classical) bulge or bar. These results are consistent with similar trends for barred
galaxies at higher redshift in the COSMOS survey, and with early studies using the
RC3. We discuss these results in the context of internal (secular) galaxy evolution
scenarios and the possible links to the formation of classical bulges (which have a de
Vaucouleurs profile) and pseudo–bulges (with exponential profiles) in disk galaxies.
Key words: galaxies: spiral - galaxies:structure - galaxies:bulges - galaxies: photom-
etry - galaxies: evolution - surveys
1 INTRODUCTION
Bars are common in disk galaxies, and are thought to
have an important impact on the evolution of galax-
ies through their ability to transfer angular momen-
tum in both the baryonic and dark matter components
of the galaxy (Combes & Sanders 1981; Weinberg 1985;
Debattista & Sellwood 2000; Berentzen et al. 2006). Bars
are efficient at driving gas inwards, perhaps sparking cen-
tral star formation (e.g. Jogee et al. 2005; Sheth et al.
2005), and thus help to grow a central bulge (e.g.
Kormendy & Kennicutt 2004; Debattista et al. 2006). Bars
may also feed a central black hole (see Jogee 2006 for a
review), but so far no correlation has been found between
AGN activity and bar fraction (Hao et al. 2009).
Early visual inspection of spiral galaxies in catalogues
like the RC3 (de Vaucouleurs et al. 1991) gave an optical
bar fraction of fbar ∼ 0.25−0.3, rising to 60% if weaker bars
or oval distortions were included. More recent work on the
bar fraction of disk galaxies has relied on automated meth-
c© 2010 RAS

2 K.L. Masters et al.
ods of detecting bars in galaxies including elliptical isophote
fitting or the Fourier decomposition of CCD images. Such
automated studies find optical bar fractions of ∼50% for
nearby disk galaxies (Barazza et al. 2008; Aguerri et al.
2009), which is consistent with near infra-red (NIR) studies
that find a majority (60%) of disk galaxies appear to have
a bar (Marinova & Jogee 2007). These differences are prob-
ably due to a combination of selection effects, wavelength-
dependences, differences in the strength of the bar, and small
samples sizes. For such reasons, especially to provide a larger
sample of barred galaxies, we started a new phase of the
successful Galaxy Zoo project1 by asking the public to pro-
vide more detailed visual classifications of galaxies seen in
the Sloan Digital Sky Survey (SDSS). This new project is
known as “Galaxy Zoo 2” (GZ2) throughout this paper. The
project and data set will be fully described in a future paper
(Lintott et al. in prep.).
In this article, we present the first results on the bar
properties of 13665 visually classified GZ2 disk galaxies.
This sample is nearly an order of magnitude larger than
previous studies using SDSS data (Barazza et al. 2008;
Aguerri et al. 2009), which facilitates a detailed statistical
study of the fraction of barred disk galaxies as a function of
other galaxy properties like global optical colour, luminos-
ity, and estimates of the bulge size, or prominence. Where
appropriate, we assume a standard cosmological model of
Ωm = 0.3, ΩΛ = 0.7 and H0 = 70 km s−1 Mpc−1and all
photometric quantities are taken from SDSS.
2 IDENTIFICATION OF BARS AND SAMPLE
SELECTION
The first phase of Galaxy Zoo2 (known as GZ1 hereafter)
has now finished collecting data and is described in detail
in Lintott et al. (2008). In this second phase (GZ2), users
are asked to provide more detailed classification of galaxies
than in the original GZ1 (see Figure 1). Specifically, after
identifying a galaxy as possessing “features or a disk” (see
top question in Figure 1), the users are then asked different
questions (depending on their prior answers) as they navi-
gate to the bottom of the decision tree presented in Figure
1. For example, if the user identified a disk from the first
question in the tree, they are then asked if the galaxy could
be an edge-on disk, and if the answer to that question is
“no”, then they are further asked to identify if the galaxy
has a bar or not. This identification process is based on the
SDSS gri composite images as it was in GZ1.
We are still collecting data for the GZ2 project, and
these first results are based upon data collected up to July
2009. Only galaxies for which at least ten answers to the bar
question (the question beginning “Is there a sign of a bar
feature....” in Figure 1) have been included in the sample,
which gives a total superset of 66,864 galaxies in which the
median number of classifications is 20. For the rest of this
paper, we classify a galaxy as being “barred” if the number
of users identifying them as having a bar is equal to, or
larger than, the number identifying them as not having a
1 www.galaxyzoo.org
2 http://zoo1.galaxyzoo.org
bar, i.e., a majority of users voted they saw a bar. A fraction
of the sample has also been visually inspected by us and this
choice seems to make sense (see Figure 2). However, as has
been discussed extensively for GZ1 data (e.g. Lintott et al.
2008; Bamford et al. 2009), there are many ways to go from
“clicks to classifications”. This simple choice means that no
galaxy is left unclassified, and gives equal weight to all users.
Alternative threshold classifications were explored, and were
found to make no qualitative difference to the results. Future
studies will explore this issue further.
We select from the superset of 66,864 galaxies a volume–
limited subsample of GZ2 galaxies with 0.01 < z < 0.06
and Mr < −19.38, where Mr is the SDSS Petrosian r-
band magnitude k-corrected to z=0 and with the standard
Galactic dust extinction correction (Schlegel, Finkbeiner &
Davis 1998). A correction for dust extinction internal to the
galaxy, as described in Masters et al. (2010a), is also ap-
plied (this corrects to for the inclination dependence and is
zero for face-on disks). Furthermore, we limit our sample to
log(a/b) < 0.3 (i ∼ 60◦) as identifying bars in highly in-
clined disk galaxies is challenging. This is a comparable cut
in inclination to previous studies of bars in spiral galaxies
(e.g. Sheth et al. 2008; Barazza et al. 2008; Aguerri et al.
2009). These constraints provide a final sample of 13665 disk
galaxies used throughout this paper (with median number of
22 answers to the GZ2 “bar” question). Examples of barred
and non–barred galaxies in our sample are shown in Figure
2, over the range of redshifts included in this study. We have
cross-matched this sample with the GZ1 (SDSS DR6) sam-
ple discussed in Bamford et al. (2009). The distribution of
GZ1 spiral likelihood, psp for the disk galaxies in this sample
peaks at psp = 1 with a median value of psp = 0.9, and a
long low tail to small spiral likelihoods. This cross-match in-
dicates that what we will call “disk galaxies” in GZ2 should
be interpreted as being mostly classic spiral galaxies, but
also consisting of S0 galaxies in which a “disk or features”
(see the top question of Figure 1) was discernable to GZ2
users.
The stellar mass range of this volume-limited GZ2 sam-
ple of 13665 disk galaxies is approximately 109 < M <
1011M (using the stellar mass estimates from Baldry et al.
2006) with a colour and luminosity dependence such that the
dimmest, bluest objects in our sample have stellar masses of
109 < M < 1010M, while the reddest, most luminous,
galaxies have higher stellar masses (red galaxies in our sam-
ple are only complete to stellar masses of 1010M).
3 RESULTS
First, we compute the overall mean bar fraction of our
sample which is 29.4 ± 0.5% (we find 4020 barred disk
galaxies in our sample of 13665 GZ2 galaxies). This value
is in excellent agreement with the fraction of 25-30% of
galaxies having strong bars found by visual inspection of
classic optical galaxy samples (e.g. the RC3 and UGC
de Vaucouleurs et al. 1991; Nilson 1973) and also with
Marinova et al. (2009) who found a bar fraction of ∼30%
for disk galaxies in a dense cluster at z ∼ 0.165 (in the
STAGES survey). However, it is lower than recent bar frac-
tions quoted for nearby disk galaxies using automated ellipse
fitting techniques to find bars, e.g., Barazza et al. (2008)
c© 2010 RAS, MNRAS 000, 1–9

Galaxy Zoo: Bars in Disk Galaxies 3
Smooth Star or
Artifact
Features
or disk
Is the galaxy simply smooth and rounded, with no sign of a disk?
How rounded is is?
Completely
round
In
between
Cigar
shaped
Is there anything odd?
Yes No
Is the odd feature a ring, or is the
galaxy disturbed or irregular?
Ring Lens or
arc
Disturbed
Other Merger Irregular
Dust lane
Could this be a disk viewed edge-on?
Yes No
Does the galaxy have a bulge at its
centre? If so what shape?
Rounded Boxy No bulge
Is there a sign of a bar feature
through the centre of the galaxy?
Bar No bar
Is there any sign of a spiral
arm pattern?
Spiral
No
Spiral
How tightly wound do the
spiral arms appear?
Tight Medium Loose
How many spiral arms are there?
1 2 3
More
than 4
Can’t tell 4
How prominent is the central
bulge, compared to the rest of
the galaxy?
No bulge Just
noticeable
Obvious Dominant
Figure 1. We present a schematic diagram of the decision tree for GZ2 classifications. We provide the questions asked of the user for
each SDSS galaxy image (starting with the top question first). For each question, we provide the possible answers they are allowed.
Depending on their answers, the user can navigate down different branches of the tree.
Figure 2. (Top row) Examples of GZ2 classified barred disk galaxies. (Bottom row) Examples of GZ2 classified disk galaxies with no
bar. The galaxies on the left are at z ' 0.02, the galaxies in the middle at z ' 0.04 and the galaxies on the right are at z ' 0.06, thus
spanning the full redshift range of the volume–limited sample used herein (see Section 2). The images are taken from the SDSS (gri
composite) and are one arcminute squared in size. (These images differ to those presented to users for classification, which are scaled
using the Petrosian radius of the galaxy.)
c© 2010 RAS, MNRAS 000, 1–9

4 K.L. Masters et al.
find a bar fraction of 50 ± 2%, while Aguerri et al. (2009)
find 45%. This difference could depend on the strength of
bars included in these analyses as it has been known for
sometime that the bar fraction in the RC3 catalogue in-
creases to ∼ 60% if weak or ovally distorted systems are
included (e.g. Sheth et al. 2008). We return to this issue in
Section 3.1 but it appears our GZ2 bars are fully consistent
with the classic optically–identified strong bars.
We see no trend of bar fraction with redshift in our
sample, and only a mild trend with inclinations (for i <
60◦) such that bars are slightly less likely to be identified as
the galaxies become more inclined. As the inclinations are
random with respect to other galaxy properties, we do not
expect this trend to have a significant effect on our results.
3.1 Bar Fraction with Colour
In Figure 3, we present the bar fraction of our GZ2 volume–
limited sample as a function of the (g−r) global, k-corrected
colour of the galaxy3. We find a significant trend for redder
GZ2 disk galaxies to have larger bar fractions, e.g., over
half of the reddest galaxies in our sample possess a bar.
This is consistent with the recent results of Masters et al.
(2010b) who found that passive red spiral galaxies had a
high fraction of bars; these passive spirals are the extreme
red population in our GZ2 sample. Interestingly, the trend
seen in Figure 3 does not appear to be monotonic, given the
Poisson error bars, and we see a slight increase in the bar
fraction for the bluest objects (compared to intermediate
colours of (g − r) ∼ 0.5).
Furthermore, one could argue that the trend seen in
Figure 3 is consistent with a difference in bar fraction which
is correlated with the well-established colour bi-modality re-
lationship of galaxies (illustrated in the lower panel of Fig-
ure 3). Disks in the “blue cloud” (e.g., with (g − r) < 0.6)
have a constant bar fraction (with colour) of ' 20%, while
disk galaxies in the “red sequence” (e.g., (g − r) > 0.7)
have a clear increase in bar fraction with colour. The split
between “normal” and “red” passive spiral galaxies dis-
cussed by Masters et al. (2010b) falls approximately at
the dip of the colour distribution of our GZ2 disk galax-
ies (the colour cut applied by Masters et al. (2010b) was
(g− r) = 0.63− 0.02(Mr +20) while the median magnitude
of our GZ2 sample used here is Mr = −20.9, but note that
Masters et al. (2010b) also removed spiral galaxies with any
sign of a bulge, something which has not been done here).
We should consider if the trend of bar fraction with
colour could be an artifact of the visual identification of bars
in the SDSS composite gri images. It has been seen that
bar fraction increases when moving from the optical into
the NIR (e.g. Eskridge et al. 2000; Marinova & Jogee 2007),
with more bars being revealed in the NIR (e.g. Keel et al.
1996; Mulchaey et al. 1997). Therefore, it is possible that
the bluer GZ2 disks are able to hide stellar bars under the
on–going star formation which would dominate the SDSS
g-band, while in the redder disks, the light would be domi-
nated by the i-band, and thus led to an increased bar frac-
3 Standard Galactic extinction corrections are also applied, but
no correction for internal extinction – this last corrections would
be at most 0.04 mag because of the inclination limit log(a/b) < 0.3
Figure 3. (Top panel) The bar fraction as a function of global
galaxy (g−r) colour. The dashed line shows the median bar frac-
tion for the entire volume–limited sample of GZ2 disk galaxies.
Poisson error bars are shown. (Lower panel) The distribution of
(g − r) colours for GZ2 disk galaxies used in this study.
tion. Contrary to this interpretation, we cite the findings of
Sheth et al. (2008) who show for a sample of 139 local SDSS
galaxies, that bar fraction (both from visual inspection and
ellipse fitting) is constant over the SDSS griz passbands,
and only drops significantly in the u-band. Furthermore, we
highlight the fact that the bar fraction for our bluest GZ2
disk galaxies ((g − r) < 0.5) is greater than at interme-
diate colours (as discussed above) and is probably consis-
tent with a constant bar fraction for all colours bluer than
(g−r) < 0.6. These results argue against a significant colour
bias in identifying bars in our composite gri images but fur-
ther studies are necessary to discover any subtle biases with
colour.
3.1.1 Bar Fraction, Colour and Luminosity
The trend observed in Figure 3, and in particular the upturn
in bar fraction at the bluest colours, suggests that colour
may not be the only variable of importance for the bar frac-
tion. We therefore split the sample into four subsamples of
absolute magnitude and show the results in Figure 4 (we
avoid using stellar mass estimates which introduce complete-
ness effects dependent on colour). This figure shows that at
a fixed (g−r) colour, there is a residual trend of bar fraction
with luminosity such that bars are more common in lower
luminosity disk galaxies. However, the trend with luminos-
ity is still sub–dominant compared to the correlation of bar
fraction with colour.
Figure 4 highlights a maximum bar fraction of ∼ 70%
c© 2010 RAS, MNRAS 000, 1–9

Galaxy Zoo: Bars in Disk Galaxies 5
Figure 4. Similar to Figure 3, but with the sample split into
four bins of absolute magnitude: Mr > −20 (black solid), −20 <
Mr < −21 (blue dotted), −21 < Mr < −22 (green dashed) and
Mr < −22 (red dot-dashed).. Bins are only plotted if they have
at least 10 galaxies.
(for our optical GZ2 sample) for low luminosity disk galaxies
with colours in the range of 0.7 < (g − r) < 0.8, and a
drop in bar fraction for the most massive, red disks in this
sample. Interestingly the overall trend is for an increase in
bar fraction with galaxy luminosity, but this is obviously
driven by the trend of bar fraction with colour and the fact
that more luminous disk galaxies tend to be redder.
3.2 Bar Fraction and Bulge Prominence
As part of GZ2, users were also asked to identify the size, or
prominence, of the bulges in disk galaxies (excluding edge-
on disks), and were given the four options of “no bulge”,
“just noticeable”, “obvious” and “dominant” (see Figure 1
for details). Similar to our treatment of the bar question,
we uniquely place all of our GZ2 disk galaxies into one of
these four categories based on majority voting. We find that
most galaxies are placed into the middle two categories, with
only a small number of disks having dominant or no bulge
classification. In Masters et al. (2010a), the use of the SDSS
parameter fracdeV4 was used as a proxy for bulge size in
GZ1 spirals. In bright spirals, fracdeV is dominated by the
inner light profile and should be increased in the presence
of a large bulge component.
In Figure 5, we show the fracdeV distribution of GZ2
4 The fraction of the best fit light profile which comes from a de
Vaucouleurs fit as opposed to an exponential fit
Figure 5. The distribution of values of fracdeV for GZ2 disk
galaxies classified by their visual bulge size into “no bulge” (blue),
“just noticeable” (green) and “obvious” (orange). We have not
plotted “dominant” bulges as they are few in this category and
a vast majority have fracdeV= 1. Histograms for barred galaxies
are shown by the dashed lines, unbarred by the solid lines.
disk galaxies separated into the four GZ2 bulge categories.
We find that most GZ2 disks with low values of fracdeV are
categorised as having “just noticeable” bulges by the GZ2
users (the green line in Figure 5), while most GZ2 disk galax-
ies with large values of fracdeV are categorised as having an
“obvious” bulge (orange line). While the number of galaxies
in the “no bulge” (blue line) and “dominant” bulge cate-
gories are small, there is still a clear trend with fracdeV in
the expected direction. These results are reassuring as they
demonstrate that both fracdeV and the GZ2 bulge classifi-
cations are meaningful and do provide a measure of the bulge
prominence in disk galaxies. We further split the sample into
barred and non-barred disks using the GZ2 classifications to
check that the presence of a bar does not have a significant
effect on fracdeV. The resulting histograms (dashed and
solid lines in Figure 5) are identical thus proving there is no
impact on fracdeV from the presence of a bar.
In Figure 6, we show the fraction of GZ2 disk galaxies
in our sample as a function of fracdeV. We have chosen to
use this SDSS measured quantity, instead of the GZ2 bulge
classification, to follow the work of Masters et al. (2010a)
and because it is a continuous variable. Figure 6 shows a
clear monotonic increase of bar fraction with fracdeV, i.e,.
half of the bulge–dominated disk galaxies in our sample have
bars.
As is well known, and recently shown for GZ1 spirals by
Masters et al. 2010a, early–type spiral galaxies with large
bulges tend to be redder than late–type spirals, so the ob-
served trend in Figure 6 could be due to a correlation be-
tween colour and bulge size. To explore this, we split the
trend of bar fraction with colour (seen in Figure 3) into four
broad disk galaxy types of
• no bulge present with fracdeV < 0.1,
• small bulge with 0.1 <fracdeV < 0.5,
• large bulge with 0.5 <fracdeV < 0.9, and
• dominant bulge of fracdeV > 0.9.
The results of this division are shown in Figure 7, which
c© 2010 RAS, MNRAS 000, 1–9

6 K.L. Masters et al.
Figure 6. (Top panel) The bar fraction as a function of fracdeV.
The dashed line shows the median bar fraction for the entire
volume limited sample of GZ2 disks. Poisson error bars are shown.
(Lower panel) The distribution of fracdeV values of GZ2 disks
used in this study. Most galaxies have either fracdeV = 0 or
fracdeV = 1.
shows that late-type disk galaxies (with low fracdeV) have
a low bar fraction (except the very reddest as also seen in
Masters et al. 2010b) while early-type disks have a high bar
fraction. This shows that the bar fraction correlation with
colour is primarily driven by bulge prominence.
At this point, it is important to recognise the differ-
ent types of bulges that could be present in disk galaxies
(Athanassoula 2005). In particular, there is a distinction in
the literature between classical bulges and pseudo-bulges.
The former appear to resemble an elliptical galaxy within a
disk, and are most likely formed by merger events, while
the latter are disk-like bulges, which seem to be formed
by the re-distribution of material within the disk. These
pseudo-bulges are they are called should have almost expo-
nential profiles (Kormendy & Kennicutt 2004), while classi-
cal bulges would follow a de Vaucouleur profile. Therefore,
our use of fracdeV as a proxy for bulge size is likely to be
most effective in selecting classical bulges. In the bottom
panel of Figure 7, we see that most of our GZ2 disk galaxies
with large fracdeV values have red (g − r) colours. This is
consistent with the work of Drory & Fisher (2007) who show
that classical bulges tend to live in galaxies close to, or on
the red sequence, while spirals with pseudo-bulges remain in
the blue cloud. Therefore, we can interpret our trend of in-
creased bar fraction with fracdeV as probably being caused
by the fact that disk galaxies with a classical bulge have a
higher fraction of bars.
We can attempt to study the difference between classi-
Figure 7. Similar to Figure 3 but split into four bins of fracdeV:
fracdeV < 0.1 (no bulge; black), 0.1 <fracdeV < 0.5 (small bulge;
blue), 0.5 <fracdeV < 0.9 (large bulge; green) and fracdeV > 0.9
(dominant bulge; red). . Bins are only plotted if they have at least
10 galaxies.
cal and pseudo–bulges by using a combination of fracdeV
and the GZ2 bulge classifications. In fact, in Figure 5, we see
a relatively large fraction of our disk galaxies have a “just
noticeable” bulge (green lines), according to our GZ2 users,
but possess small values of fracdeV. If we restrict our sam-
ple to fracdeV < 0.1, we find that 84% of such galaxies are
classified as having “just noticeable” bulges and could be
candidates for pseudo–bulges, i.e., visible bulges that might
not have a de Vaucouleurs profile. The bar fraction in this
subset is only 11 ± 1%. Alternatively, we can consider only
GZ2 disk galaxies with large values of fracdeV (> 0.9) which
most likely host a classical bulge. In this subset, we find a
bar fraction of 56± 5% for “just noticeable” bulges. We dis-
cuss the interpretation of these fractions below.
4 DISCUSSION
4.1 Comparison with Previous Work
We observe a significant increase in the bar fraction of disk
galaxies as the galaxies become redder and have more promi-
nent bulges. This is a similar trend of bar fraction that has
been seen in the COSMOS sample of higher redshift galax-
ies (z ∼ 0.2–0.8) by both Sheth et al. (2008, in 2157 spiral
galaxies) and Cameron et al. (2010, in 3187 disk galaxies).
Both these studies find more bars in redder disk galaxies at
intermediate stellar masses of 1010.5 < M < 1011M, which
is similar to the high end of the mass range explored here.
We note that Cameron et al. (2010) finds this trend changes
c© 2010 RAS, MNRAS 000, 1–9

Galaxy Zoo: Bars in Disk Galaxies 7
at masses> 1011M, but there are almost no such galaxies
in our volume–limited sample.
Qualitatively similar trends of bar fraction with Hubble
type were also observed using data on local galaxies from the
RC3 (de Vaucouleurs et al. 1991). Both Odewahn (1996)
and Elmegreen et al. (2004) observe that the (strong) bar
fraction (i.e. spiral types SB) decreases from around 60% in
S0/a to 30% in Sc, after which it increases again towards
very late type disks. Interestingly both studies also show
that the weak (or mixed type, SAB) bar fraction is much
flatter with Hubble type and shows the opposite trend.
While we find a similar overall bar fraction to the
STAGES study of barred galaxies in a dense cluster at
z ∼ 0.2 (Marinova et al. 2009) our findings on the trends of
bar fraction with other properties differ substantially from
that study. They observed that bar fraction (in ∼ 800 galax-
ies found from ellipse fitting to B-band images) rises in
brighter galaxies and those which have no significant bulge
component and that bar fraction had no dependence on disk
galaxy colour. While we do see an increase in bar fraction
for brighter spirals (from 26± 1% for those with Mr > −20,
to 37 ± 2% for those with Mr < −22), we argue that this
is driven by the strong colour dependence of the bar frac-
tion and at a fixed colour we see little dependence of bar
fraction on luminosity (Figure 4) - the trend we find is also
much smaller than seen by Marinova et al. (2009). We also
observe a dependence on bulge prominence exactly opposite
to the sense seen Marinova et al. (2009).
Similarly, we agree with the overall bar fraction of 25%
found for 945 galaxies by Barazza et al. (2009) across both
field and clusters environments at z ∼0.4–0.8 (observed in
rest frame B–V). However we again find opposite trends of
bar fraction with bulge-prominence (they find more bars in
bluer disk-dominated galaxies).
The source of these discrepancies is unclear. Disk galax-
ies were identified visually in both studies so should be sim-
ilar to our GZ disks. We do use quite different bar finding
techniques (visual versus ellipse fitting), so perhaps this indi-
cates a difference in the trends for strong (visual) and weaker
bars (as hinted at by Odewahn 1996 and Elmegreen et al.
2004). More interestingly it could be pointing to a difference
between disk galaxies in high density regions and those else-
where. Barazza et al. (2009) explored differences between
bar fractions in the field and clusters finding hints that high
density regions are favourable locations for bars, however
this could only be done for a subset of the sample (N = 241),
making the results of limited statistical significance. These
possibilities will be explored in future work exploting the
huge increase in bar classifications available to us in GZ2.
Our findings are also in contrast to the results of
Barazza et al. (2008) and Aguerri et al. (2009) who both
find larger bar fractions in bluer disk galaxies than presented
herein (using a local samples of ∼ 2000 “disk” or spiral
galaxies from the SDSS). The source of this disagreement
again is unclear, but one possible explanation is the differ-
ences in the sample selections used and the colour ranges
probed. For example, both use automated techniques to
identify a disk/spiral sample of galaxies based on concentra-
tion and velocity dispersion (Aguerri et al. 2009) or colour
(Barazza et al. 2008, this study also considered Sersic fits,
but the final results were for a colour–selected sample of
“spiral” galaxies). If we restrict our analysis to the range of
colours explored by Barazza et al. (2008), then much of the
trend we see in Figure 3 is missed and we would have ac-
tually witnessed a mild decrease in bar fraction towards the
redder spirals, fully consistent with their findings. In more
detail, we mimic the Barazza et al. (2008) selection by using
their U-V colour cut (from Bell et al. (2004)) with colour
transformations from Smith et al. (2002), plus i < 60◦, and
0.01 < z < 0.03, Mg < −18.5. For this matched GZ2 sam-
ple, we find a flat bar fraction of 25%, with no obvious trend
with (g−r) colour, except for a slight upturn for the reddest
objects in this matched sample (at (g − r) ∼ 0.6).
There still however remains a difference in our overall
conclusions with Barazza et al. (2008) and Aguerri et al.
(2009) and, in particular, the total bar fraction we find
is lower than either of these studies. It has been argued
that bars in early-type spiral galaxies tend to be longer
(and thus stronger) than those seen in late–type spirals
(Athanassoula 2003), so it may be that the remaining dif-
ferences are due to our sensitivity to these longer, stronger
bars, while Barazza et al. (2008) and Aguerri et al. (2009)
may detect weaker bars using their ellipse–fitting techniques.
Further studies directly comparing the bars identified from
ellipse–fitting methods and the GZ2 identifications will be
needed to understand the main reason for this difference.
4.2 The Impact of Bars on Disk Galaxies
Given the trends we have observed, we now focus on the in-
terpretations we can make for the effect of bars on the sec-
ular and dynamical evolution of disk galaxies. We observe a
significant increase in bar fraction as disk galaxies become
redder and have larger (classical) bulges; over half of red,
bulge–dominated disk galaxies have a bar. Similar trends
were observed for intermediate stellar mass disk galaxies in
COSMOS (Sheth et al. 2008; Cameron et al. 2010) which
have been interpreted as signaling an earlier formation time
for such galaxies, compared to disk galaxies with smaller
bulges. Our observations at low redshift are harder to inter-
pret in this view, as our GZ2 galaxies should have formed
a long time ago and therefore, the bar formation timescales
are negligible compared to the age of the galaxy.
Instead, our observations would suggest a more impor-
tant link between the presence of a classical bulge (with a
de Vaucouleurs profile) and the existence of a bar instabil-
ity. This is contrary to expectations as classical bulges must
have formed during a fast, dissipative process (most likely re-
lated to galaxy mergers), which would have likely disrupted
any bar. Furthermore, bar instabilities are often invoked as
a way to form pseudo-bulges (with a exponential profile) by
moving material around in the disk of a spiral galaxy (see
Kormendy & Kennicutt (2004) for a comprehensive review
of this subject). In Section 3.2, we argue that most of the
bulges selected using our fracdeV methodology are proba-
bly classical bulges, yet we see an increase in bar fraction
with bulge prominence. Moreover, all the pseudo–bulges in
our sample are likely to be in blue disk galaxies with low
bar fractions, e.g., in Section 3.2, we use the combination of
fracdeV and the GZ2 bulge classifications to identify poten-
tial pseudo–bulges and find that only 11 ± 1% have a bar,
far below the average across our sample. So, if the bulges in
these galaxies have grown by internal secular evolution (e.g.
re-distribution of mass and star formation), this suggests
c© 2010 RAS, MNRAS 000, 1–9

8 K.L. Masters et al.
that methods of angular momentum transfer other than bar
instabilities (e.g. ovally distorted disks, or the spiral arm
structure itself) are more important, or that the process of
growing the pseudo–bulges leads to the bar being destroyed;
even though Shen & Sellwood (2004) argue that it is un-
likely for bulges to be formed from the destruction of bars,
and both Shen & Sellwood (2004) and Athanassoula et al.
(2005) show that large central concentrations of stellar mass
are required before bars become unstable.
We finish by returning to the suggestion that we see two
populations of disk galaxies. Both Figure 3 and 7 suggest a
split between disk galaxies on the “red sequence”, which
have large classical bulges (maybe formed during merger
processes), and disk galaxies in the “blue cloud” with either
no bulge or a pseudo–bulge. The red sequence population
show little change in their bar fraction with luminosity and
colour and overall have a high fraction of bars, approaching
50%. The blue cloud population also show little trend with
colour with low bar fractions of 10-20%.
5 SUMMARY
We present here an analysis of the bar fraction of 13665 disk
galaxies selected from the new Galaxy Zoo 2 dataset. This
sample is volume–limited, with z < 0.06 and Mr < −19.38,
and overall we find that 29.4 ± 0.4% of these galaxies have
a bar. We split this sample as a function of global colour
and luminosity, as well as the prominence of the bulge, and
find that redder disk galaxies, with larger bulges have a
high fraction of bars (up to 50%). At a fixed colour, bar
fraction is seen to decrease slightly with luminosity. These
results are consistent with previous visual studies of spiral
galaxies (from the RC3 catalogue) but the trends appear
to be in contrast with results in the local universe from re-
cent automated bar–finding methods (Barazza et al. 2008;
Aguerri et al. 2009). However, if we restrict our GZ2 sam-
ple to the same colour range as these automated searches,
we find more consistent trends with colour but still disagree
on the overall fraction of galaxies with bars. This difference
requires further investigation but is probably due to the au-
tomated searches being more sensitive to weaker bars than
the GZ2 users.
We discuss the implication of our results for different
scenarios of disk and bulge formation. It would appear that
our results suggest a strong link between the presence of
a bar and the existence of a large classical bulge, i.e., a
bulge with a de Vaucouleurs profile likely formed via merg-
ers. Likewise, we see that pseudo-bulges (best modeled with
an exponential profile) are most likely found in blue, disk
galaxies with no bar. This is contrary to expectations which
suggest pseudo–bulges are built via the re-distribution of
stellar mass (or induced star-formation) driven by a bar in-
stability. Furthermore, we observe a colour bimodiality in
our GZ2 disk galaxies with a “red sequence” hosting large
classical bulges and possessing a bar fraction up to 50%,
while the majority of disk galaxies are in the “blue cloud”
which have either no bulge (or a pseudo–bulge) and possess
low bar fractions of 10-20%.
This paper provides the first results from the GZ2
project on bars in disk galaxies. In the future, we will explore
the dependence of bar fraction on stellar mass and environ-
ment. We will also report on a satellite Galaxy Zoo project,
which invited the GZ2 users to measure the length, strength
and orientation of bars (detected in the GZ2 sample) via an
interactive Google Maps interface, as well as identify the
links between the bar and spiral structure. Such data will
allow us to extend this work to studies of the correlation of
the bar lengths (and bar colours) with global galaxy prop-
erties.
ACKNOWLEDGEMENTS. This publication has
been made possible by the participation of more
than 200,000 volunteers in the Galaxy Zoo project.
Their contributions are individually acknowledged at
http://www.galaxyzoo.org/Volunteers.aspx. KLM ac-
knowledges funding from the Peter and Patricia Gruber
Foundation as the 2008 Peter and Patricia Gruber Founda-
tion International Astronomical Union Fellow, and from the
University of Portsmouth and SEPnet (www.sepnet.ac.uk).
BH thanks Google for funding during this project and
RCN acknowledge financial support from STFC. Galaxy
Zoo 2 was developed with the help of a grant from The
Leverhulme Trust. CJL acknowledges support from an
STFC Science in Society fellowship. Funding for the SDSS
and SDSS-II has been provided by the Alfred P. Sloan
Foundation, the Participating Institutions, the National
Science Foundation, the U.S. Department of Energy,
the National Aeronautics and Space Administration, the
Japanese Monbukagakusho, the Max Planck Society, and
the Higher Education Funding Council for England. The
SDSS Web Site is http://www.sdss.org/.
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