Massive Star Clusters in Non-Interacting Galaxies Dynamical Mass Estimates and the (I)MF
description
Transcript of Massive Star Clusters in Non-Interacting Galaxies Dynamical Mass Estimates and the (I)MF
Massive Star Clusters in Non-Interacting Galaxies
Dynamical Mass Estimates and the (I)MF
Søren S. LarsenESO / 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
Mengel et al. 2002, A&A 383, 137SSP models from Leitherer et al. 1999 (Starburst99; 0.1 - 100 M)
M/L ratios from high-dispersion spectroscopy and HST imaging
Mvir ≈ 10 Rhlr vx2 / G
Rhlr = half-light radiusvx = line-of-sight velocity dispersion
Degeneracy: IMF slope / lower mass limit
Top-heavy IMFs
Bottom-heavy IMFs
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 cross-correlation analysis (Tonry & Davis 1979)
Target galaxiesNGC 6946
NGC 5236
NGC 4449
NGC 4214
Cluster sizes: EFF Model Fits
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]-
F555W
Residuals
Velocity dispersions
Keck/HIRES spectra
Velocity dispersion: vx2 = TC 2 - TT
2
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”.
Cross-Correlation Functions (CCFs)
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 IMFUVES 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