Massive Star Clusters in NonInteracting Galaxies
Dynamical Mass Estimates and the (I)MF
Søren S. Larsen
ESO / ST-ECF, Garching
Tom Richtler, Concepcion  Jean P. Brodie, UCO / Lick  Deidre A. Hunter, Lowell
See also:
Larsen & Richtler, A&A, in press (astro-ph/0407610)
Larsen, Brodie & Hunter, AJ, in press (astro-ph/0407373)
Motivation
• “Massive” (104-106 M) young star clusters have
been found in several nearby galaxies
• Appear similar to old globular clusters in terms of
sizes and masses, but will they really evolve into
bona-fide old (~1 Hubble time) GCs?
• One key question is the (I)MF shape - if deficient in
low-mass stars, clusters might disrupt prematurely
• Is IMF universal, or are there variations? Direct
observations of low-mass stars generally unfeasible
beyond Local Group => dynamical M/L ratios of
YMCs represent one way to constrain IMF
M/L ratios and IMFs for YMCs
M/L ratios from high-dispersion
spectroscopy and HST imaging
Top-heavy IMFs
Mvir ≈ 10 Rhlr vx2 / G
Rhlr = half-light radius
vx = line-of-sight velocity
dispersion
Bottom-heavy IMFs
Degeneracy: IMF slope / lower
mass limit
Mengel et al. 2002, A&A 383, 137
SSP models from Leitherer et al.
1999 (Starburst99; 0.1 - 100 M)
Caveats
• Hard to find good targets (spatial resolution, bright
enough for high-dispersion spectroscopy, isolated,
uniform background)
• Youngest clusters relaxed?
• Mass segregation (primordial or dynamical)
• Macroturbulence in red supergiants ~ 10 km/s =>
dominates over velocity dispersions for masses <
~105 M
• Statistical fluctuations (~20 RSGs in 105 M
cluster at 107 years)
Our Observations
• 7 YMCs in 4 nearby (3-6 Mpc) galaxies: NGC
4214 (irr), NGC 4449 (irr), NGC 5236 (sp) and
NGC 6946 (sp)
• Masses > 105 M, ages 15 Myrs - 800 Myrs (from
broad-band colours)
• HST imaging: cluster profiles well resolved (1
WFPC2 pixel ~ 1.5 pc at 3 Mpc)
• VLT/UVES and Keck/HIRES/NIRSPEC echelle
spectroscopy => velocity dispersions through crosscorrelation analysis (Tonry & Davis 1979)
Target galaxies
NGC 6946
NGC 4449
NGC 4214
NGC 5236
Cluster sizes: EFF Model Fits
F555W
Residuals
Sizes determined with baolab/ishape software (Larsen 1999).
Convolves TinyTim PSF with Elson, Fall & Freeman (EFF)
models of the form
P(r) ~ [1-(r/rc)2]-
Velocity dispersions
Keck/HIRES spectra
Cross-Correlation Functions (CCFs)
Velocity dispersion: vx2 = TC 2 - TT2
where TC and TT are the dispersions of the
cluster-template and template-template CCFs
(Tonry & Davis 1979)
Notes: 1) No individual strong lines are required for
this technique to work. 2) Intrinsic broadening of
lines (macroturbulence etc.) “cancels out”.
Cluster Properties
Rhlr
[pc]
Vx
[km/s]
Log(age)
[yr]
Mvir
[105 M]
0
[M pc-3]
N4214-10
4.33 ± 0.14 5.1 ± 1.0
8.3 ± 0.1
2.6 ± 1.0
(2.5±1.0)103
N4214-13
3.01 ± 0.26 14.8 ± 1.0
8.3 ± 0.1
14.8 ± 2.4
(1.9±0.6)105
N4449-27
3.72 ± 0.32 5.0 ± 1.0
8.9 ± 0.3
2.1 ± 0.9
(1.9±0.8)103
N4449-47
5.24 ± 0.76 6.2 ± 1.0
8.5 ± 0.1
4.6 ± 1.6
(6.8±2.4)103
N5236-502
7.6 ± 1.1
5.5 ± 1.0
8.0 ± 0.1
5.2 ± 0.8
(2.8±1.0) 103
N5236-805
2.8 ± 0.4
8.1 ± 1.0
7.1 ± 0.2
4.2 ± 0.7
(1.6±1.1) 104
N6946-1447
10.2 ± 1.6
8.8 ± 1.0
7.05 ±0.1
17.6 ±5
(2.3±0.8) 104
M/L ratios and the IMF
UVES data (M83)
HIRES data (Dwarfs)
NIRSPEC (N6946)
All 7 clusters consistent with
Kroupa-type or Salpeter
(Mmin=0.1 M) IMF.
Solid black curve:
Bruzual+Charlot SSP models.
Others: Basic SSP models
based on Padua isochrones
No top-heavy IMFs
(Models for Z=0.008)
Summary
• Clusters with masses in the range 104 M - 106
M can form in disks of “normal” spirals and in
dwarf galaxies, in addition to starbursts and
mergers
• The clusters analyzed here have M/L ratios
consistent with “normal” IMFs (usual disclaimers
apply..)
• Such objects may provide direct insight into
processes related to the formation of globular
clusters in the early Universe