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

The Astronomical Journal (Impact Factor: 4.05). 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.
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    ABSTRACT: Context. An automatic tool to derive structural parameters of semi-resolved star clusters located in crowded stellar fields in nearby galaxies is needed for homogeneous processing of archival frames. Aims. We have developed a program that automatically derives the structural parameters of star clusters and estimates errors by accounting for individual stars and variable sky background. Methods. Models of observed frames consist of the cluster's surface brightness distribution, convolved with a point spread function; the stars, represented by the same point spread function; and a smoothly variable sky background. The cluster's model is fitted within a large radius by using the Levenberg-Marquardt and Markov chain Monte Carlo algorithms to derive structural parameters, the flux of the cluster, and individual fluxes of all well-resolved stars. Results. FitClust, a program to derive structural parameters of semi-resolved clusters in crowded stellar fields, was developed and is available for free use. The program was tested on simulated cluster frames, and was used to measure clusters of the M31 galaxy in Subaru Suprime-Cam frames. Conclusions. Accounting for bright resolved stars and variable sky background significantly improves the accuracy of derived structural parameters of star clusters. However, their uncertainty remains dominated by the stochastic noise of unresolved stars.
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