An Extended Star Cluster at the Outer Edge of the Spiral Galaxy M 33

The Astronomical Journal (Impact Factor: 4.97). 03/2008; 135(4):1482. DOI: 10.1088/0004-6256/135/4/1482
Source: arXiv

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.

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    ABSTRACT: Star clusters provide a unique and powerful tool for studying the star formation histories of galaxies. In particular, the ages and metallicities of star clusters bear the imprint of the galaxy formation process. M33 is the only nearby late-type spiral galaxy and provides a notable connection between the cluster populations of earlier-type spirals, and the numerous nearby later-type dwarf galaxies. I have carried out a comprehensive study of the M33 star cluster system, including deep photometry as well as high signal-to-noise spectroscopy. I have undertaken a photometric survey for extended sources in a 1deg x 1deg area centered on M33 using the MegaCam camera on the 3.6m Canada-France-Hawaii Telescope. This study mitigates the incompleteness present in the current catalogs of star clusters in M33, especially in the outskirts of this galaxy. I will discuss here the photometric properties of the sample, including color-color diagrams of 599 new candidate stellar clusters, and 204 confirmed clusters. Analysis of the radial density distribution suggests that the cluster system of M33 has suffered from significant depletion, possibly due to interactions with M31. Additionally, I will present the morphological properties of 161 star clusters in M33 using ACS/HST images. I found that the position angles of the M33 clusters show a bimodality with a strong peak perpendicular to the position angle of the galaxy. This evidence supports tidal forces as the reason for cluster elongation. Finally, I will present high-precision velocity measures of a variety of M33 star clusters, based on observations from the 10.4m Gran Telescopio Canarias and 3.6m William Herschel Telescope.
    The Formation and Evolution of M33 as Revealed by Its Star Clusters: , Springer Theses. ISBN 978-1-4614-7326-8. Springer Science+Business Media New York, 2013. 01/2012;
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    ABSTRACT: We use direct N-body simulations to investigate the evolution of star clusters with large size-scales with the particular goal of understanding the so-called extended clusters observed in various Local Group galaxies, including M31 and NGC6822. The N-body models incorporate a stellar mass function, stellar evolution and the tidal field of a host galaxy. We find that extended clusters can arise naturally within a weak tidal field, provided that the tidal radius is filled at the start of the evolution. Differences in the initial tidal filling factor can produce marked differences in the subsequent evolution of clusters and the size-scales that would be observed. These differences are more marked than any produced by internal evolution processes linked to the properties of cluster binary stars or the action of an intermediate-mass black hole, based on models performed in this work and previous work to date. Models evolved in a stronger tidal field show that extended clusters cannot form and evolve within the inner regions of a galaxy, such as M31. Instead, our results support the suggestion that many extended clusters found in large galaxies were accreted as members of dwarf galaxies that were subsequently disrupted. Our results also enhance the recent suggestion that star clusters evolve to a common sequence in terms of their size and mass.
    Monthly Notices of the Royal Astronomical Society 01/2010; 408:2353-2363. · 5.52 Impact Factor
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    ABSTRACT: Aims: We present Subaru/Suprime-Cam deep V and I imaging of seven fields in the outer regions of M 33. Our aim is to search for stellar structures corresponding to extended Hi clouds found in a recent 21-cm survey of the galaxy. Three fields probe a large Hi complex to the southeastern (SE) side of the galaxy. An additional three fields cover the northwestern (NW) side of the galaxy along the Hi warp. A final target field was chosen further north, at a projected distance of approximately 25 kpc, to study part of the large stellar plume recently discovered around M 33. Methods: We analyse the stellar population at R > 10 kpc by means of V, I colour magnitude diagrams reaching the red clump. We constrain the age and metallicity of the different stellar populations, search for density enhancements that correspond to the Hi features, and investigate the radial surface distribution of the stars. Results: We find evolved stellar populations in all fields out to 120'(~30 kpc), while a diffuse population of young stars (~200 Myr) is detected out to a galactocentric radius of 15 kpc. The mean metallicity in the southern fields remains approximately constant at [M/H] = -0.7 beyond the edge of the optical disc, from 40'out to 80'. Along the northern fields probing the outer Hi disc, we also find a metallicity of [M/H] = -0.7 between 35'and 70'from the centre, which decreases to [M/H] = -1.0 at larger angular radii out to 120'. In the northernmost field, outside the disc extent, the stellar population of the large stellar plume possibly related to a M 33-M 31 interaction is on average more metal-poor ([M/H] = -1.3) and older (≳6 Gyr). Conclusions: An exponential disc with a large scale-length (~7 kpc) fits well the average distribution of stars detected in both the SE and NW regions from a galactocentric distance of 11 kpc out to 30 kpc. The stellar disc extends beyond the Hi disc. The stellar distribution at large radii is disturbed and, although there is no clear correlation between the stellar substructures and the location of the Hi clouds, this gives evidence of tidal interaction or accretion events. Based on data collected at Subaru Telescope, which is operated by the National Astronomical Observatory of Japan.Photometry is only available in electronic form at at the CDS via anonymous ftp to ( or via
    Astronomy and Astrophysics 01/2011; 533. · 5.08 Impact Factor

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