An Extended Star Cluster at the Outer Edge of the Spiral Galaxy M 33
ABSTRACT We report the discovery of an extended globular-like star cluster, M 33-EC1, at the outer edge of the spiral galaxy M 33. The distance to the cluster is 890 kpc, and it lies at a projected distance of 12.5 kpc from the center of M 33. Old age (7 Gyr) and low metallicity ([M/H] –1.4) are estimated on the basis of isochrone fits. Color-magnitude diagrams of stars, located in the cluster's area, and photometric and structural parameters of the cluster are presented. The cluster's luminosity (MV = –6.6) and half-light radius (r h = 20.3 pc) are comparable to those of the extended globular clusters, discovered in more luminous Local Group galaxies, the Milky Way and M 31. Extended globular clusters are suspected to be remnants of accreted dwarf galaxies, and the finding of such a cluster in the late-type dwarf spiral galaxy M 33 would imply a complex merging history in the past.
- SourceAvailable from: Paul Goudfrooij[show abstract] [hide abstract]
ABSTRACT: We present the results of medium-resolution spectroscopy of 28 globular clusters (GCs) in six nearby galaxies of different luminosities and morphological types, situated in M33 (15 objects), M31 (three), IC10 (four), UGCA86 (four), Holmberg IX (one) and DDO71 (one) obtained at the Special Astrophysical Observatory 6-m telescope. Measurements of Lick absorption line indices and comparison with Simple Stellar Population models enabled us to obtain their spectroscopic ages, metallicities and α-element to Fe abundance ratios. We found that all old- and intermediate-age GCs in our sample have low metallicities [Z/H]≲ -0. 8 dex. Metal-rich clusters are young and are preferentially found in galaxies more massive than ∼ 109 M⊙. The least massive dwarfs of our sample, DDO71 and Holmberg IX, host one massive intermediate-age and one massive young metal-poor GC, respectively. [α/Fe] abundance ratios tend to be enhanced but closer to solar values for dwarf galaxies compared to GCs in more massive galaxies. We analyse the age–metallicity relation for GCs in our sample and others from the literature, and find that (1) there is a general trend for GCs in low surface brightness dwarf galaxies to be more metal-poor at a given age than GCs in more massive galaxies; (2) the GC metallicity spread is wider for more massive galaxies and (3) intermediate-age GCs in early-type dwarf galaxies are more metal-rich at any given age than those in irregular galaxies of similar luminosity.Monthly Notices of the Royal Astronomical Society 06/2010; 405(2):839 - 856. · 5.52 Impact Factor
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ABSTRACT: We use a combined imaging and spectroscopic survey of the nearby central cluster galaxy, M87, to assemble a sample of 34 confirmed ultra compact dwarfs (UCDs) with half-light radii of >~ 10 pc measured from Hubble Space Telescope images. This doubles the existing sample in M87, making it the largest such sample for any galaxy, while extending the detection of UCDs to unprecedentedly low luminosities (MV = -9). With this expanded sample, we find no correlation between size and luminosity, in contrast to previous suggestions, and no general correlation between size and galactocentric distance. We explore the relationships between UCDs, less luminous extended clusters (including faint fuzzies), globular clusters (GCs), as well as early-type galaxies and their nuclei, assembling an extensive new catalog of sizes and luminosities for stellar systems. Most of the M87 UCDs follow a tight color-magnitude relation, offset from the metal-poor GCs. This, along with kinematical differences, demonstrates that most UCDs are a distinct population from normal GCs, and not simply a continuation to larger sizes and higher luminosities. The UCD color-magnitude trend couples closely with that for Virgo dwarf elliptical nuclei. We conclude that the M87 UCDs are predominantly stripped nuclei. The brightest and reddest UCDs may be the remnant nuclei of more massive galaxies while a subset of the faintest UCDs may be tidally limited and related to more compact star clusters. In the broader context of galaxy assembly, blue UCDs may trace halo build-up by accretion of low-mass satellites, while red UCDs may be markers of metal-rich bulge formation in larger galaxies.The Astronomical Journal 09/2011; 142. · 4.97 Impact Factor
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ABSTRACT: We report the discovery of an extended globular cluster in a halo field in Centaurus A (NGC 5128), situated $\sim 38\kpc$ from the centre of that galaxy, imaged with the Advanced Camera for Surveys on board the Hubble Space Telescope. At the distance of the galaxy, the half-light radius of the cluster is r_h ~ 17pc, placing it among the largest globular clusters known. The faint absolute magnitude of the star cluster, M_(V,o)=-5.2, and its large size render this object somewhat different from the population of extended globular clusters previously reported, making it the first firm detection in the outskirts of a giant galaxy of an analogue of the faint, diffuse globular clusters present in the outer halo of the Milky Way. The colour-magnitude diagram of the cluster, covering approximately the brightest four magnitudes of the red giant branch, is consistent with an ancient, i.e., older than ~8 Gyr, intermediate-metallicity, i.e., [M/H] ~-1.0 dex, stellar population. We also report the detection of a second, even fainter cluster candidate which would have r_h ~ 9pc, and M_(V,o)=-3.4 if it is at the distance of NGC 5128. The properties of the extended globular cluster and the diffuse stellar populations in its close vicinity suggest that they are part of a low mass accretion in the outer regions of NGC 5128. Comment: 9 pages, MNRAS, in pressMonthly Notices of the Royal Astronomical Society 02/2010; · 5.52 Impact Factor
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.