TY - JOUR
T1 - Dynamical mass of a star cluster in M 83: a test of fibre-fed multi-object spectroscopy
AU - Moll, S.L.
AU - Grijs, R.
AU - Anders, P.
AU - Crowther, P.A.
AU - Larsen, S.S.
AU - Smith, L.J.
AU - Portegies Zwart, S.F.
PY - 2008
Y1 - 2008
N2 - Aims. We obtained VLT/FLAMES+UVES high-resolution, fibre-fed spectroscopy of five young massive clusters (YMCs) in M 83
(NGC 5236). This forms the basis of a pilot study testing the feasibility of using fibre-fed spectroscopy to measure the velocity dispersions
of several clusters simultaneously, in order to determine their dynamical masses. In principle, this reduces the telescope time
required to obtain a statistically significant sample of dynamical cluster masses. These can be used to assess the long-term survivability
of YMCs by comparing their dynamical and photometric masses, which are necessary to ascertain the potential evolution of
YMCs into second-generation globular clusters.
Methods. We adopted two methods for determining the velocity dispersion of the star clusters: cross-correlating the cluster spectrum
with the template spectra and minimising a χ2 value between the cluster spectrum and the broadened template spectra. We also
considered both red giant and red supergiant template stars. Cluster 805 in M 83 (following the notation of Larsen) was chosen as a
control to test the reliability of the results obtained by this observational method, through a comparison with the results obtained from
a standard echelle VLT/UVES spectrum obtained by Larsen & Richtler.
Results. We find no dependence of the velocity dispersions measured for a cluster on the choice of red giant versus red supergiant
templates, nor on the method adopted. However, we do find that the standard deviation of the results obtained with only one method
may underestimate the true uncertainty. We measure a velocity dispersion of σlos = 10.2 ± 1.1 kms−1 for cluster 805 from our fibrefed
spectroscopy. This is in excellent agreement with the velocity dispersion of σlos = 10.6 ± 1.4kms−1 determined from the standard
echelle UVES spectrum of cluster 805. Our FLAMES+UVES velocity dispersion measurement gives Mvir = (6.6 ± 1.7) × 105 M ,
consistent with previous results. This value of the virial mass is a factor of ∼3 greater than the cluster’s photometric mass, indicating
a lack of virial equilibrium. However, based on its effective star formation efficiency, the cluster is likely to virialise, and may survive
for a Hubble time, in the absence of external disruptive forces. Unfortunately, our observations of the other M 83 star clusters have
insufficient signal-to-noise ratios to determine robust cluster velocity dispersions.
Conclusions. We find that reliable velocity dispersions can be determined from high-resolution, fibre-fed spectroscopy. The advantages
of observing several clusters simultaneously outweighs the difficulty of accurate galaxy background subtraction, providing that
the targets are chosen to provide sufficient signal-to-noise ratios, and are much brighter than the galaxy background.
AB - Aims. We obtained VLT/FLAMES+UVES high-resolution, fibre-fed spectroscopy of five young massive clusters (YMCs) in M 83
(NGC 5236). This forms the basis of a pilot study testing the feasibility of using fibre-fed spectroscopy to measure the velocity dispersions
of several clusters simultaneously, in order to determine their dynamical masses. In principle, this reduces the telescope time
required to obtain a statistically significant sample of dynamical cluster masses. These can be used to assess the long-term survivability
of YMCs by comparing their dynamical and photometric masses, which are necessary to ascertain the potential evolution of
YMCs into second-generation globular clusters.
Methods. We adopted two methods for determining the velocity dispersion of the star clusters: cross-correlating the cluster spectrum
with the template spectra and minimising a χ2 value between the cluster spectrum and the broadened template spectra. We also
considered both red giant and red supergiant template stars. Cluster 805 in M 83 (following the notation of Larsen) was chosen as a
control to test the reliability of the results obtained by this observational method, through a comparison with the results obtained from
a standard echelle VLT/UVES spectrum obtained by Larsen & Richtler.
Results. We find no dependence of the velocity dispersions measured for a cluster on the choice of red giant versus red supergiant
templates, nor on the method adopted. However, we do find that the standard deviation of the results obtained with only one method
may underestimate the true uncertainty. We measure a velocity dispersion of σlos = 10.2 ± 1.1 kms−1 for cluster 805 from our fibrefed
spectroscopy. This is in excellent agreement with the velocity dispersion of σlos = 10.6 ± 1.4kms−1 determined from the standard
echelle UVES spectrum of cluster 805. Our FLAMES+UVES velocity dispersion measurement gives Mvir = (6.6 ± 1.7) × 105 M ,
consistent with previous results. This value of the virial mass is a factor of ∼3 greater than the cluster’s photometric mass, indicating
a lack of virial equilibrium. However, based on its effective star formation efficiency, the cluster is likely to virialise, and may survive
for a Hubble time, in the absence of external disruptive forces. Unfortunately, our observations of the other M 83 star clusters have
insufficient signal-to-noise ratios to determine robust cluster velocity dispersions.
Conclusions. We find that reliable velocity dispersions can be determined from high-resolution, fibre-fed spectroscopy. The advantages
of observing several clusters simultaneously outweighs the difficulty of accurate galaxy background subtraction, providing that
the targets are chosen to provide sufficient signal-to-noise ratios, and are much brighter than the galaxy background.
M3 - Article
SN - 0004-6361
VL - 490
SP - 125
EP - 133
JO - Astronomy and Astrophysics
JF - Astronomy and Astrophysics
IS - 1
ER -