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Title: The globular cluster system in NGC5866: optical observations from HST Advanced Camera for Surveys
Authors: Cantiello, Michele (1,2), Blakeslee, John P. (1), Raimondo, Gabriella (2) ((1) Department of Physics and Astronomy, Washington State University, Pullman, WA; (2)INAF--Osservatorio Astronomico di Teramo, Italy)

 We perform a detailed study of the Globular Cluster (GC) system in the galaxy NGC5866 based on F435W, F555W, and F625W (~ B, V, and R) HST Advanced Camera for Surveys images.
Adopting colour, size and shape selection criteria, the final list of GC candidates comprises 109 objects, with small estimated contamination from background galaxies, and foreground stars.
The colour distribution of the final GC sample has a bimodal form. Adopting colour to metallicity transformations derived from the Teramo--SPoT simple stellar population model, we estimate a metallicity [Fe/H]~ -1.5, and -0.6 dex for the blue and red peaks, respectively. A similar result is found if the empirical colour-metallicity relations derived from Galactic GCs data are used.
The two subpopulations show some of the features commonly observed in the GC system of other galaxies, like a "blue tilt", higher central concentrations of the red subsystem, and larger half--light radii at larger galactocentric distances. However, we do not find evidence of a substantial difference between the average sizes of red and blue clusters.
Our analysis of the GC Luminosity Function indicates a V-band Turn-Over Magnitude V_0^{TOM}=23.46 ±0.06, or M_{V,0}^{TOM}\sim-7.29 ± 0.10 mag, using the distance modulus derived from the average of SBF and the PNLF distances. The absolute Turn-Over Magnitude obtained agrees well with calibrations from literature. The specific frequency is measured to be S_N=1.4 ± 0.3, typical for galaxies of this type.

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This is a unique view of the disk galaxy NGC 5866 tilted nearly edge-on to our line-of-sight. The Hubble space telescope's sharp vision reveals a crisp dust lane dividing the galaxy into two halves. The image highlights the galaxy's structure: a subtle, reddish bulge surrounding a bright nucleus, a blue disk of stars running parallel to the dust lane, and a transparent outer halo.


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Position (2000): R.A. 15h 06m 29s.48 Dec. +55° 45' 47".2

NGC 5866 is a disk galaxy of type "S0". Viewed face on, it would look like a smooth, flat disk with little spiral structure. It remains in the spiral category because of the flatness of the main disk of stars as opposed to the more spherically rotund (or ellipsoidal) class of galaxies called "ellipticals." Such S0 galaxies, with disks like spirals and large bulges like ellipticals, are called 'lenticular' galaxies.
NGC 5866 lies in the Northern constellation Draco, at a distance of 44 million light-years. It has a diameter of roughly 60,000 light-years only two-thirds the diameter of the Milky Way, although its mass is similar to our galaxy. This Hubble image of NGC 5866 is a combination of blue, green and red observations taken with the Advanced Camera for Surveys in February 2006.

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