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/ ) Larsen, Brodie & Hunter, AJ, in press (astro-ph/ )

Motivation “Massive” ( 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, 137 SSP models from Leitherer et al (Starburst99; M  ) M/L ratios from high-dispersion spectroscopy and HST imaging M vir ≈ 10 R hlr v x 2 / G R hlr = half-light radius v x = 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 < ~10 5 M  Statistical fluctuations (~20 RSGs in 10 5 M  cluster at 10 7 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 > 10 5 M , ages 15 Myrs 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 galaxies NGC 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/r c ) 2 ] -  F555W Residuals

Velocity dispersions Keck/HIRES spectra Velocity dispersion: v x 2 =  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 R hlr [pc] V x [km/s] Log(age) [yr] M vir [10 5 M  ]  0 [M  pc -3 ] N ± ± ± ± 1.0 (2.5±1.0)  10 3 N ± ± ± ± 2.4 (1.9±0.6)  10 5 N ± ± ± ± 0.9 (1.9±0.8)  10 3 N ± ± ± ± 1.6 (6.8±2.4)  10 3 N ± ± ± ± 0.8 (2.8±1.0)  10 3 N ± ± ± ± 0.7 (1.6±1.1)  10 4 N ± ± ± ±5 (2.3±0.8)  10 4

M/L ratios and the IMF  UVES data (M83)  HIRES data (Dwarfs)  NIRSPEC (N6946) All 7 clusters consistent with Kroupa-type or Salpeter (M min =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 10 4 M  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

Velocity dispersions - MK type

M/L: Reddening effects