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