J/ApJ/705/1533 NGC 5128 globular clusters (Chattopadhyay+, 2009)
Study of NGC 5128 globular clusters under multivariate statistical paradigm.
Chattopadhyay A.K., Chattopadhyay T., Davoust E., Mondal S., Sharina M.
<Astrophys. J., 705, 1533-1547 (2009)>
=2009ApJ...705.1533C 2009ApJ...705.1533C
ADC_Keywords: Clusters, globular ; Galaxies, optical ; Models
Keywords: galaxies: general - globular clusters: general - methods: statistical
Abstract:
An objective classification of the globular clusters (GCs) of NGC 5128
has been carried out by using a model-based approach of cluster
analysis. The set of observable parameters includes structural
parameters, spectroscopically determined Lick indices and radial
velocities from the literature. The optimum set of parameters for this
type of analysis is selected through a modified technique of principal
component analysis, which differs from the classical one in the sense
that it takes into consideration the effects of outliers present in
the data. Then a mixture model based approach has been used to
classify the GCs into groups. The efficiency of the techniques used is
tested through the comparison of the misclassification probabilities
with those obtained using the K-means clustering technique. On the
basis of the above classification scheme three coherent groups of GCs
have been found. We propose that the clusters of one group originated
in the original cluster formation event that coincided with the
formation of the elliptical galaxy, and that the clusters of the two
other groups are of external origin, from tidally stripped dwarf
galaxies on random orbits around NGC 5128 for one group, and from an
accreted spiral galaxy for the other.
Description:
In the present study, we have first used a modified technique of
principal component analysis (PCA; Salibian-Barrera et al. 2006, J.
Am. Stat. Assoc., 101, 1198) to search for the optimum set of
parameters which gives the maximum variation for the GCs in NGC 5128.
Then we have classified the GCs on the basis of these significant
parameters using a model-based method of cluster analysis (CA; Qui, D.
& Tamhane, A. C. 2007, J. Stat. Plan. Inference, 137, 3722) which
finds the structure of the optimum groups of GCs instead of choosing
groups in an ad hoc manner on the basis of a single parameter.
Our analysis is based on the sample of GCs of the early-type central
giant elliptical galaxy in the Centaurus group, NGC 5128, whose
structural parameters have been derived by fitting King and Wilson
models to the surface brightness profiles based on HST/ACS imaging in
the F606W bandpass (McLaughlin et al. 2008MNRAS.384..563M 2008MNRAS.384..563M).
File Summary:
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FileName Lrecl Records Explanations
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ReadMe 80 . This file
table1.dat 82 130 List of parameters of the globular clusters of
NGC 5128 used in principal component analysis
(PCA) and cluster analysis (CA)
table2.dat 76 130 List of parameters as well as derived parameters
of the globular clusters of NGC 5128
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See also:
J/AJ/139/1871 : Kinematics of globulars in NGC 5128 (Woodley+, 2010)
J/AJ/140/2101 : NGC 5128 globular clusters photometry (Sinnott+, 2010)
J/ApJ/708/1335 : Spectroscopy of globulars in NGC 5128 (Woodley+, 2010)
J/ApJ/682/199 : Globulars with X-ray sources in NGC 5128 (Woodley+, 2008)
J/MNRAS/386/1443 : 2dF study of globular clusters in NGC 5128 (Beasley+, 2008)
J/AJ/134/494 : NGC 5128 globular clusters UBVRICMT1 phot. + RV (Woodley+,
2007)
J/AJ/132/2187 : MCT1 photometry of NGC 5128 GCs (Harris+, 2006)
J/AJ/128/723 : Washington Photometry in NGC 5128. II (Harris+, 2004)
J/AJ/128/712 : Washington Photometry in NGC 5128 (Harris+, 2004)
J/ApJS/150/367 : Globular clusters in NGC 5128 (Peng+, 2004)
J/A+A/369/812 : UVKs photometry of globular clusters in NGC 5128 (Rejkuba,
2001)
Byte-by-byte Description of file: table1.dat
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Bytes Format Units Label Explanations
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1- 9 A9 --- Name Globular cluster name (G1)
10 A1 --- f_Name [a] discarded in CA (G2)
12 I1 --- Group [1,3]? Group membership found by CA (G3)
14- 17 F4.2 --- c Concentration (c∼log(Rtid/Rc)) (G1)
19- 23 F5.2 mag/arcsec2 muV V surface brightness averaged over
half-light radius (<µv>h) (G1)
25- 29 F5.2 mag/arcsec2 mu0 Extinction-corrected central surface
brightness in the F606W bandpass (G1)
31- 35 F5.2 --- W0 Dimensionless central potential of the
best-fitting model (G1)
37- 42 F6.3 [pc] logRc Log core radius (G1)
44- 47 F4.2 [pc] logRtid Log tidal radius (G1)
49- 54 F6.3 [pc] logrh Log half-light radius (G1)
56- 59 F4.2 [Msun] logM Integrated model mass (logMtot) (G1)
61- 70 E10.6 Msun/pc3 rho0 Central volume density (G1)
72- 76 F5.2 km/s sig.P Predicted line of sight velocity dispersion
at the cluster center (σp,0) (G1)
78- 82 F5.2 10+9yr trh Two-body relaxation time at the model
projected half-mass radius (G1)
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Byte-by-byte Description of file: table2.dat
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Bytes Format Units Label Explanations
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1- 9 A9 --- Name Globular cluster name (G1)
10 A1 --- f_Name [a] discarded in CA (G2)
12 I1 --- Group [1,3]? Group membership found by CA (G3)
14- 18 F5.2 kpc Rgc Galactocentric radius (G1)
20- 24 F5.2 mag T1 Washington T1 magnitude (G1)
26- 30 F5.2 [Sun] [Fe/H] Fe/H abundance (G1)
32- 35 F4.2 mag (C-T1)0 Extinction corrected (C-T1)0 color index (G1)
37- 39 I3 km/s RV ? Weighted mean radial velocity from Woodley
et al. (2007, Cat. J/AJ/134/494)
41- 45 F5.2 [Sun] [Z/H] ? Metallicity
46 A1 --- u_[Z/H] [:] Uncertainty flag on [Z/H] (1)
48- 51 F4.2 [Sun] e_[Z/H] ? [Z/H] uncertainty
53- 57 F5.2 Gyr Age ? Age
58 A1 --- u_Age [:] Uncertainty flag on Age (1)
60- 63 F4.2 Gyr e_Age ? Age uncertainty
65- 69 F5.2 km/s Vrot ? Rotational velocity
71- 76 F6.2 km/s sigma ? Velocity dispersion (σV)
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Note (1): ":" are used for the errors of [Z/H] and ages when the error of
[Z/H]>0.4dex and the error of Age>4Gyr.
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Global notes:
Note (G1): From McLaughlin et al. (2008MNRAS.384..563M 2008MNRAS.384..563M)
Note (G2): the globular clusters markedf "a" have not been considered
during the cluster analysis (CA) as they are outliers.
Note (G3): the cluster analysis (CA) segregated the sample of GCs into
three groups, G1, G2, G3, according to their structural properties.
The different properties of the three groups are presented in Table 4
of the paper. The main difference between the groups lies in the mean
luminosities (T1) of the GCs and their individual central velocity
dispersions (σp,0), which are both indicators of their
individual masses. The GCs of G2 are the most massive, followed by
those of G1, and then G3. Main characteristics for the 3 groups are:
* G1 (47 GCs): T1=19.462±0.081mag ; σp,0=9.984±0.400km/s
* G2 (35 GCs): T1=18.218±0.159mag ; σp,0=20.528±1.130km/s
* G3 (45 GCS): T1=21.156±0.092mag ; σp,0=3.533±0.144km/s
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History:
From electronic version of the journal
(End) Emmanuelle Perret [CDS] 07-Dec-2011