arXiv:0802.0501v1 [astro-ph] 4 Feb 2008
AN EXTENDED STAR CLUSTER AT THE OUTER EDGE
OF THE SPIRAL GALAXY M331
Rima Stonkut˙ e2, Vladas Vanseviˇ cius2, Nobuo Arimoto3,4,
Takashi Hasegawa5, Donatas Narbutis2, Naoyuki Tamura6,
Pascale Jablonka7, Kouji Ohta8, and Yoshihiko Yamada3
We report a discovery of an extended globular-like star cluster, M33-EC1,
at the outer edge of the spiral galaxy M33.
890kpc, and it lies at a 12.5kpc projected distance from the center of M33.
Old age (?7Gyr) and low metallicity ([M/H]?-1.4) are estimated on the ba-
sis of isochrone fits. Color-magnitude diagrams of stars, located in the cluster’s
area, photometric and structural parameters of the cluster are presented. Clus-
ter’s luminosity (MV=-6.6) and half-light radius (rh= 20.3pc) are comparable
to those of the extended globular clusters, discovered in more luminous Local
Group galaxies, the Milky Way and M31. Extended globular clusters are sus-
pected to be remnants of accreted dwarf galaxies, and the finding of such a cluster
in the late-type dwarf spiral galaxy M33 would imply a complex merging history
in the past.
The distance to the cluster is
Subject headings: galaxies: individual (M33) — galaxies: star clusters
1Based on data collected at Subaru Telescope, which is operated by the National Astronomical Observa-
tory of Japan
2Institute of Physics, Savanori¸ u 231, Vilnius LT-02300, Lithuania
3National Astronomical Observatory of Japan, Mitaka, Tokyo 181-8588, Japan
4Department of Astronomy, Graduate University of Advanced Studies, Mitaka, Tokyo 181-8588, Japan
5Gunma Astronomical Observatory, Agatuma, Gunma 377-0702, Japan
6Subaru Telescope, National Astronomical Observatory of Japan, 650 North A’ohoku Place, Hilo, HI
7Universit´ e de Gen` eve, Laboratoire d’Astrophysique de l’Ecole Polytechnique F´ ed´ erale de Lausanne
(EPFL), Observatoire, CH-1290 Sauverny, Switzerland
8Department of Astronomy, Kyoto University, Kyoto 606-8502, Japan
– 2 –
Resolved stellar diagnostics has been extensively applied for investigation of merging
history of galaxies. In this context extended stellar systems have been recently known to
be informative. Firstly, some of the extended stellar systems in the Milky Way (MW), e.g.,
M54 and ωCen, are suggested to be remnants of accreted dwarf galaxies, which might be
responsible for the thick disk and halo formation. Such systems have produced large-scale
stellar streams in the MW, thus they are useful to highlight various substructures of the host
galaxies and to reveal their merging history. Secondly, while the key physical processes that
discriminate extended star clusters and low surface brightness dwarf spheroidals (dSphs) are
poorly understood, their distinction is rather ambiguous.
Searches for extended stellar systems discovered at least a dozen of low surface bright-
ness dSphs in the vicinity of MW (Sakamoto & Hasegawa 2006; Belokurov et al.
Irwin et al. 2007) and M31 (Martin et al. 2006). Recently Huxor et al. (2005) and Mackey et al.
(2006) discovered four extended luminous star clusters in the vicinity of M31. Star clusters
of this type are also found in the spirals M51 and M81 (Chandar et al. 2004), and in the
giant elliptical galaxy NGC5128 (G´ omez et al. 2006). It is important to stress, however,
that all extended star clusters found so far belong to massive luminous galaxies. Therefore,
any piece of evidence on extended stellar systems in smaller galaxies would play an important
role in disclosing the merging history of galaxies on all scales.
M33 is a unique late-type (Scd) dwarf spiral galaxy in the Local Group, resolvable
by ground-based observations, and it is claimed (Ferguson et al. 2006) to possess an unper-
turbed stellar disk without any remarkable sign of thick disk and halo. However, Chandar et al.
(2002) revealed an old cluster population, which has a velocity distribution that they at-
tributed to the thick-disk/halo component. Warp of the M33 gaseous disk has already
been known from HI observation (Corbelli et al. 1989), and a stellar stream was suggested
recently from spectroscopy of individual stars (McConnachie et al. 2006). Any further ev-
idence on the M33 perturbation and accretion events is indispensable to disclose the real
formation history of the galaxy. Since stellar systems of accretion origin are often found far
from the hosts’ central part, wide and deep searches for such objects are crucial.
We report a discovery of an extended star cluster, M33-EC1, in the M33 photometric
survey (P.I. N.Arimoto) frames obtained on Subaru Telescope (Fig.1). The cluster is located
at R.A.=01h32m58.s5, Decl.=29◦52′03′′(J2000.0), lying far south from the M33 center at a
projected galactocentric distance of 48.′4. Previous M33 cluster studies did not reveal any
clusters of a comparably large size (Chandar et al. 1999, 2001). An extensive catalogue of
M33 star clusters recently compiled by Sarajedini & Mancone (2007) does not include this
– 3 –
In section 2 we present details of observations and data reduction. In section 3 the
derived cluster parameters and resolved stellar photometry results are given. In section 4 we
briefly discuss the impact of our finding in the context of galaxy formation.
2.Observations and Data Reductions
Photometric data of the discovered star cluster, M33-EC1, were obtained during the
course of the M33 wide field photometric survey performed on Subaru Telescope, equipped
with Prime Focus Camera (Suprime-Cam; Miyazaki et al.
Cam mosaic (5×2 CCD chips; pixel size of 0.′′2) covers a field of 34′×27′, and a magnitude of
V ∼ 25mis reached in 60s. Broad-band images: V -band (exposures 5×90s; seeing ∼1.′′0), R-
band (5×90s; ∼0.′′6), and I-band (5×200s; ∼0.′′8) were acquired during photometric nights.
For standard reduction procedures we used the software package (Yagi et al. 2002) dedi-
cated to the Suprime-Cam data. We employed the DAOPHOT (Stetson 1987) program
set implemented in the IRAF software package (Tody 1993) for crowded-field stellar PSF
(point spread function) photometry and integrated aperture photometry of the cluster. The
PSF stellar photometry on 5 individual exposures in each passband was performed.
2002). Single shot Suprime-
Instrumental magnitudes were transformed to the standard photometric system by re-
ferring to the published M33 photometric catalogue (Massey et al. 2006). In total 220
stars spanning the I-band magnitude range from 19mto 21mand wide color ranges (R − I
from -0.15 to 1.3; V − I from -0.25 to 2.5) were selected as local standards. R.M.S. errors
of the transformation equations for V − I and R − I colors, and the I-band are less than
0.m035 which, taking into account the number of employed stars, assures accurate calibration.
Considering the intrinsic calibration accuracy of the standard stars (Massey et al. 2006),
we estimate the accuracy of our photometric data to be of ∼0.m015 at I = 22m. We used
a bilinear R − I color transformation equation due to a significant difference between the
transmission curve of the Suprime-Cam R-band interference filter and that of the standard
Cousins R-band filter.
The star cluster M33-EC1 is located far beyond the M33 galaxy’s disk, therefore, it
is reasonable to assume that its colors are contaminated only by the MW’s foreground
extinction. Photometric data were de-reddened using the E(B − V ) = 0.06 value, derived
at the cluster’s position from the extinction maps (Schlegel et al. 1998), as follows AV =
3.1 · E(B − V ), AI= 0.11, E(R − I) = 0.045.
– 4 –
The color-magnitude diagram (CMD) of a region of 20′′radius, centered on the star
cluster M33-EC1, is dominated by red giant branch (RGB) stars, see Fig.2. Reduction
and photometry procedures enable us to recognize and remove obvious bright non-stellar
objects (star/galaxy separation was performed by eye referring to PSF fitting parameters –
sharpness and χ2), however, faint unresolved background galaxies can still be present in this
In order to resolve well-known age-metallicity degeneracy of the RGB position in CMD,
inherent to old populations, it is helpful to introduce faint RGB and horizontal branch stars
into the isochrone fitting procedure, see e.g., Martin et al. (2006). The global shape of
our CMD resembles the CMD plotted in Fig.7 from Martin et al. (2006), implying the
presence of a very old population with a prominent horizontal branch. However, the limiting
magnitude of our observations is too shallow for reliable morphology study of the lower
part of the CMD. Therefore, to estimate the intrinsic RGB width over the entire magnitude
range, and to derive radial and magnitude dependence of data completeness, we performed
an artificial star test (AST) on R- & I-band images. The AST results quantify in detail
the photometry errors, confusion limits and data completeness, making the isochrone fitting
procedure more robust and better constrained.
Six reference points on the observed RGB (I,R − I =20.90, 0.72; 21.90, 0.65; 22.90,
0.57; 23.40, 0.53; 23.90, 0.49; 24.40, 0.46) were selected to represent the entire magnitude
range of the cluster’s stellar population. DAOPHOT’s addstar procedure was employed
to add artificial stars to the images. To avoid self-crowding we generated individual AST
images at every reference point. Each AST image contains 400 artificial stars of the same
magnitude distributed on a regular grid (step 3′′) over the region of 60′′×60′′centered on the
cluster. However, only 140 artificial stars fall within the actual cluster radius of 20′′. In order
to increase the number of artificial stars and derive radial data completeness distributions
more reliably, we generated 21 individual images for each passband and every reference
point by shifting the grid around the initial position to 8 and 12 symmetrically distributed
locations around the initial position at the radial distances of ∼0.′′6 and ∼1.′′2, respectively.
Therefore, within the radius of 20′′we used 2940 artificial stars in total at each reference
point on the RGB. The photometry procedure of the AST images was exactly the same as
the one employed for the real star photometry.
To understand the morphology of star distribution in the lower part of CMD we con-
structed artificial star CMD. Radial distribution of the artificial stars at every reference AST
– 5 –
point on the RGB was chosen to represent the observed radial density distribution of the
cluster stars. However, to increase robustness of the artificial star CMD, we used a number
of artificial stars 5 times greater than the number of real stars. The observed cluster stars
over-plotted on the artificial star CMD are shown in Fig.2, panel b).
“Christmas tree-like” artificial star CMD (Fig.2, panel b) implies that CMD of the star
cluster M33-EC1 is composed solely of RGB stars, experiencing very low contamination by
foreground stars and background galaxies. Note, however, the enhanced (in respect to the
artificial stars) density of the faint blue (R−I < 0.25) objects, which could be attributed to
horizontal branch stars of the cluster or faint blue galaxies. Therefore, the straightforward
isochrone fit to the observed stars can be applied down to I = 23m, using only the RGB part
of the isochrones.
We constructed the radial data completeness plot (Fig.3) by counting the recovered
artificial stars in 2′′wide annulus zones centered on the cluster. Stars down to I = 23mare
well recovered even at the very center of the cluster. At this magnitude level we are able to
find and measure more than 70% of the stars at the cluster’s center and more than 95% at
larger radii (Fig.3).
The ∼100% data completeness of the brightest RGB stars is of high importance for the
cluster’s distance determination by fitting the tip of RGB (TRGB). This method is based
on the assumption (valid for [Fe/H]≤-0.7 and ages of ?2Gyr) that the absolute I-band
magnitude of TRGB (MI= 4.05 ± 0.10) is independent of metallicity and age (Lee et al.
1993; Bellazzini et al. 2001). The AST data completeness results imply that the TRGB
method can be applied throughout the radial extent of the star cluster.
A magnitude of the brightest RGB star (it is located within the cluster’s core, however,
in an uncrowded area, and thus measured accurately) is of I = 20.81 ± 0.01. Taking into
account the MW foreground extinction (AI= 0.11), this converts to a distance modulus of
(m − M)0= 24.75±0.10
The distance modulus error is dominated by the systematic error of the TRGB calibration
(±0.10) and by an additional increase of the distance modulus, arising due to a probability,
that the brightest observed star is below the very tip of theoretical RGB, because of a small
total number of RGB stars in the cluster.
0.20and places the star cluster M33-EC1 at a distance of 890±40
It is noteworthy to stress, that we determine the RGB tip of the M33 galaxy’s outer disk
at I = 20.68 ± 0.02, which converts, by applying the MW foreground extinction, AI= 0.08,
and assuming validity of the TRGB method for the case of M33 outer disk’s metallicity, to
∼850kpc. The derived distance of M33 is in agreement with recent M33 galaxy distance
determinations, based on the TRGB method, by Galleti et al.(2004) and Tiede et al.
– 6 –
(2004) – 855kpc and 867kpc, respectively. However, the detached eclipsing binary method
gives a significantly longer distance of 964kpc (Bonanos et al. 2006), while a cepheid based
distance is shorter – 802kpc (Lee et al. 2002). Therefore, further in this paper we will use
the distance modulus of 24.75, which places M33-EC1 at a projected distance of 12.5kpc
from the M33 center.
To estimate the cluster’s age and metallicity we compared the shape and slope of the
observed RGB with the isochrones of Girardi et al. (2002) and VandenBerg et al. (2006).
In the case of Girardi et al. (2002) isochrones, we achieved the best fit for the interpolated
isochrone of the age of 13Gyr and metallicity of [M/H]=-1.2 (Fig.2, panel a; isochrone of
the age of 14Gyr and metallicity of [M/H]=-1.3 is over-plotted). However, the isochrone
of lower metallicity, [M/H]=-1.4, and age of ∼18Gyr, as well as the isochrone of higher
metallicity, [M/H]=-1.0, and age of ∼2.5Gyr, can also be fitted reasonably well. Therefore,
additional information is needed in order to break the age (2.5–18 Gyr) and metallicity
([M/H]=-1.4 – -1.0) degeneracy.
We obtained more constrained fits by employing VandenBerg et al. (2006) isochrones
(2–18Gyr), which are available on the finer metallicity grid for three alpha element abun-
dance ratios ([α/Fe]=0.0, 0.3, 0.6). We achieved good, although degenerate, fits for the
ages of >7Gyr and metallicity of [M/H]<-1.4 independent on alpha element abundance,
see Fig.4. The isochrones spanning a narrow age and metallicity range are over-plotted on
CMDs for the illustrative purpose. Assuming the reasonably old cluster’s age of 13Gyr,
we derived metallicity of [M/H]=-1.6. Note, that for the same age (13Gyr) metallicity de-
rived from the Girardi et al. (2002) isochrones is higher by 0.4dex. The derived metallicity
[M/H]?-1.4 is in good agreement with the recent spectroscopic metallicity determination of
the M33 halo stars in a nearby field to the M33-EC1 location (McConnachie et al. 2006).
3.2.Integrated photometry and structural parameters
The M33-EC1 age of >7Gyr, estimated from the isochrone fitting, implies that it should
possess a globular cluster-like surface number density profile, which is traditionally fitted by
the King model (King 1962):
ρ(r) = ρ0· [(1 + (r/rc)2)−1/2− (1 + (rt/rc)2)−1/2]2,
where ρ0– central surface number density, rc– core radius, and rt– tidal radius are profile
fitting parameters. However, due to a small number of bright stars and an incompleteness
of the stellar photometry catalogue at fainter magnitudes (see Fig.3), we decided to fit the
– 7 –
King model to the surface brightness, rather than to the surface number density profile.
This assumption is reasonable for the surface brightness profiles, constructed from aperture
photometry, which even at large radial distances sample the cluster’s stellar population
On the other hand, large M33-EC1 extent and relatively low luminosity (mass), as
well as long (∼12.5kpc) projected distance from the host galaxy’s center, can lead to an
assumption, that the cluster is dynamically young and possesses a surface brightness profile,
which could be reproduced by the empirical EFF (Elson et al. 1987) profile derived for young
(<300Myr) Large Magellanic Cloud clusters. We employed the EFF model in differential
form representing surface brightness profile
µ(r) = µ0· (1 + (r/re)2)−n,
and in integral form representing integrated luminosity profile
Σ(r) = Σ0· r2
e/(n − 1) · [1 − (1 + (r/re)2)1−n],
where µ0– central surface brightness, Σ0 – central luminosity, re – scale-length, and n –
Determination of an accurate center of the well resolved cluster is a sensitive procedure
in constructing the surface brightness profile. In the central part of M33-EC1 luminous stars
are distributed slightly asymmetrically (see Fig.1), therefore, the location of the center was
derived by fitting the luminosity weighted and spatially smoothed surface brightness profile
in the area of 20′′×20′′. M33-EC1 exhibits perfectly round isophotes at large radii, suggesting
that the mass distribution is spherical in general – hence we used circular apertures for the
construction of surface brightness and integrated luminosity profiles, by integrating in 0.′′4
wide annuli up to the radius of 20′′.
The sky background was determined in a circular annulus centered on the cluster and
spanning the radial range from 20′′to 30′′(see Fig.1). The correct sky background sub-
traction is critical for determining a shape of surface brightness profile in the cluster’s outer
region (for detailed discussion see Hill & Zaritsky 2006), therefore, systematic sky back-
ground subtraction errors were evaluated by constructing the profiles with over-subtracted
and under-subtracted sky background. The sky background variation by R.M.S. of the sky
background value, led to insignificant structural parameter changes.
Resultant M33-EC1 radial surface brightness profiles are smooth up to ∼16′′radius,
where a bright RGB star is located. Therefore, we performed the King and EFF model
– 8 –
profile fitting up to the radius of 14′′. The best model fits to the V -band sky background-
subtracted integrated luminosity (top panel) and surface brightness (bottom panel) radial
profiles are presented in Fig.5.
Directly fitted and derived (rh and full-width at half maximum, FWHM) M33-EC1
structural parameters, as well as corresponding standard deviations, are listed in Table1.
Derived parameters were computed basing on transformation equations (6, 7, 9, 10) presented
by Larsen (2006) for the EFF profile
FWHM = 2 · re·
and the King profile
FWHM = 2 · rc·
1 + (rt/rc)2+
rh= 0.547 · rc· (rt/rc)0.486.
For further discussion we choose the conservative lower limit of the cluster’s half-light
radius of rH= 4.′′7, that, at the estimated distance of 890kpc, converts to ∼20.3pc, revealing
the extended M33-EC1 nature. It is also important to note, that the differences between
V -, R-, I-band profile fit parameters are smaller than their standard deviations. Therefore,
in Table1 we give averaged parameters for the three passbands. It is worth noting, that a
change of the fitting radius from 12′′to 19′′does not influence the derived cluster parameters
significantly. Integrated magnitudes and colors derived at the cluster’s center and radii of
(1 – 4)·rHare listed in Table2. We find no significant color gradient over the entire radial
cluster’s extent. V −I color of M33-EC1, taking into account that only foreground extinction
is present at this galactocentric distance, is in the color range of the intermediate and old
age (?5Gyr) M33 clusters (Sarajedini & Mancone 2007).
We report a discovery of an extended globular-like star cluster (eGC) at the outer edge of
the M33 galaxy, M33-EC1. All the previously known clusters in M33 are compact ones with
core radii of rc? 2pc (Chandar et al. 1999, 2001). Therefore, M33-EC1 with rc∼ 25pc is
of a very rare type, and the only such object found in the Subaru Suprime-Cam wide-field
survey frames (∼1.◦1×1.◦7) of the M33 galaxy.
– 9 –
The rh– MV diagram was proven to be a very informative and suggestive tool for a
star cluster study (van den Bergh & Mackey 2004). In Fig.6 we plot this diagram, taking
representative objects from various studies published recently, and mark M33-EC1. The
MW galaxy has ten exceptionally large (rh? 15pc) globular clusters (Harris 1996). How-
ever, only two of them (NGC5053 and NGC2419) are of comparable luminosity or brighter
(MV<-6.5) than M33-EC1. Recently, four clusters of such extreme-type have also been
found in the vicinity of M31 (Huxor et al. 2005; Mackey et al. 2006). Huxor et al. (2005)
pointed out, that eGCs in the MW are fainter than those in the M31 galaxy, due to differing
formation and evolution scenarios of the host galaxies.
By means of luminosity, structural parameters, and metal-poor nature, M33-EC1 is
very similar to NGC5053 (MW eGC) and to four M31 eGCs. Therefore, regardless of
the difference in morphological type, size, and luminosity, in the vicinity of three different
galaxies eGCs of the same type reside. Similar eGCs discovered in the spirals M51 and M81
(Chandar et al. 2004), and in the giant elliptical galaxy NGC5128 (G´ omez et al. 2006)
expand further the variety of eGC’s host galaxies (Fig.6). To our knowledge, M33 is the
smallest spiral galaxy hosting eGC.
We note that recent studies (Sakamoto & Hasegawa 2006; Belokurov et al. 2007; Irwin et al.
2007) do not warrant a simple classification of stellar systems by using their structural pa-
rameters, therefore, structural parameters of M33-EC1 may also be shared with low surface
brightness dwarf galaxies. In this context, the extended nature and very low concentration
(rt/rc∼ 2.5; Table 1) of M33-EC1 suggests that this stellar system could be a low surface
brightness dwarf galaxy. The central surface brightness of µ0,V ∼ 23mag·arcsec−2is both
consistent with lower end of surface brightness of the MW globular clusters (Harris 1996)
and with local dwarf galaxies (Mateo 1998). At present, we have no clear diagnostics to
discriminate between these possibilities. The best way to constrain the origin of M33-EC1
would be to conduct a study of cluster dynamics. The velocity dispersion data, in partic-
ular, would make it possible to determine whether this cluster contains dark matter or not
(Bender et al. 1992), since low surface brightness dSphs are found to exhibit very large
mass-to-light ratio, see e.g. Kleyna et al. (2005), Martin et al. (2007).
The extended nature of M33-EC1 becomes very important when it is considered in
light of merging history of M33. Basing on the assumption that M33 has no (prominent)
thick disk and/or halo (Ferguson et al. 2006), it has long been postulated that very few,
if any, massive accretion events have taken place. It is still controversial issue, however,
whether M33 is a pure stellar disk system or it has a thick disk and/or a halo component.
It is interesting to note, however, that accumulating evidences suggest a complex merging
history of M33. Chandar et al. (2002) reported that there is a wide spread in the age of star
– 10 –
clusters and some old clusters have velocities consistent with the halo component dynamics.
McConnachie et al. (2006) suggested a halo component and a possible stream by means
of spectroscopic study of individual RGB stars. The discovered M33-EC1 cluster may also
give support for the merging history scenario – it could be a stripped dwarf galaxy that
has accreted and merged onto M33 – a scenario suggested for eGCs in M31 (Huxor et al.
2005). We note that the proximity of M33-EC1 relative to the stream (McConnachie et al.
2006) could suggest a physical connection, however, the preliminary metallicity estimates
for both parties differ significantly. We here just mention that the metallicity of globular
clusters in Sagittarius dwarf spheroidal do not necessarily agree with that of parent galaxy
(Bellazzini et al. 2003).
Vanseviˇ cius et al. (2004) discovered an extended halo in the dwarf irregular galaxy
Leo A and suggested that even such a small dwarf galaxy was formed in a much more com-
plex way than believed before, implying hierarchical galaxy formation on all scales. Recent
findings indicate that small late-type disk galaxies, such as M33, could have experienced
merging events. Therefore, the eGC presented in this paper, M33-EC1, together with vari-
ous objects, which are suggested to associate with the M33 halo, are all important targets for
detailed study in order to understand the merging history of not only M33, but of galaxies
on all scales.
We are indebted to Chisato Ikuta for her invaluable help with observations on Subaru
telescope. We are thankful to the anonymous Referee for constructive suggestions and pro-
posed corrections. This work was financially supported in part by a Grant of the Lithuanian
State Science and Studies Foundation, and by a Grant-in-Aid for Scientific Research by the
Japanese Ministry of Education, Culture, Sports, Science and Technology (No. 19540245).
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This preprint was prepared with the AAS LATEX macros v5.2.
– 13 –
Table 1: Structural parameters of the star cluster M33-EC1.
Note. All parameters are given in arc-seconds.
– 14 –
Table 2: Photometric parameters of the star cluster M33-EC1.
rVV − RR − IMV
Note. Distance from the cluster’s center, r, is given in cluster’s half-light radius, rH= 4.′′7,
– 15 –
Fig. 1.— Suprime-Cam R-band image of the star cluster M33-EC1. The circles indicate
radii of 20′′(solid line) and 30′′(dashed line), delineating the cluster and the sky background
determination areas, respectively.