Dynamics and Mass Estimation of Massive Star Clusters in Non-Interacting Galaxies

1. Massive Star Clusters in Non-Interacting 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)

2. 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

3. M/L ratios and IMFs for YMCs M/L ratios from high-dispersion spectroscopy and HST imaging Mvir ≈ 10 Rhlr vx2 / G Rhlr = half-light radius vx = line-of-sight velocity dispersion Degeneracy: IMF slope / lower mass limit Top-heavy IMFs Bottom-heavy IMFs Mengel et al. 2002, A&A 383, 137 SSP models from Leitherer et al. 1999 (Starburst99; 0.1 - 100 M)

4. 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)

5. 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)

6. Target galaxies NGC 6946 NGC 4449 NGC 4214 NGC 5236

7. 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]-

8. Cross-Correlation Functions (CCFs) Velocity dispersion: vx2 = TC2 - 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”. Velocity dispersions Keck/HIRES spectra

9. Cluster Properties

10. 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)

11. 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