M82-F: a doomed super star cluster?

By Linda J. Smith, J. Gallagher III. 2001

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We present high dispersion echelle spectroscopy of the very luminous, young super star cluster (SSC) ‘F’ in M82, obtained with the 4.2-m William Herschel Telescope (WHT), for the purpose of deriving its dynamical mass and assessing whether it will survive to become an old globular cluster. In addition to the stellar lines, the spectrum contains complex Na I absorption and broad emission lines from the ionized gas. We measure a stellar velocity dispersion of 13.4±0.7 kms −1 , a projected half-light radius of 2.8 ± 0.3 pc from archival HST/WFPC2 images, and derive a dynamical mass of 1.2±0.1×10 6 M⊙, demonstrating that M82-F is a very massive, compact cluster. We determine that the current luminosity-to-mass ratio (LV /M)⊙ for M82-F is 45 ± 13. Comparison with spectral synthesis models shows that (LV /M)⊙ is a factor of � 5 higher than that predicted for a standard Kroupa (2001) initial mass function (IMF) at the well-determined age for M82-F of 60 ± 20 Myr. This high value of (LV /M)⊙ indicates a deficit of low mass stars in M82-F; the current mass function (MF) evidently is ‘top-heavy’. We find that a lower mass cutoff of 2–3M⊙ is required to match the observations for a MF with a slope � = 2.3. Since the cluster apparently lacks longlived low mass stars, it will not become an old globular cluster, but probably will dissolve at an age of 62 Gyr. We also derive up-dated luminosity-to-mass ratios for the younger SSCs NGC 1569A and NGC 1705-1. For the first object, the observations are consistent with a slightly steeper MF (� = 2.5) whereas for NGC 1705-1, the observed ratio requires the MF to be truncated near 2 M⊙ for a slope of � = 2.3. We discuss the implications of our findings in the context of large scale IMF variations; with the present data the top-heavy MF could reflect a local mass segregation effect during the birth of the cluster. M82-F likely formed in a dense molecular cloud; however, its high radial velocity with respect to the centre of M82 (� 175 km s −1 ) suggests it is on an eccentric orbit and now far from its birthplace, so the environment of its formation is unknown.

Subfields: Astronomy and Astrophysics, Instrumentation

View Paper: https://www.semanticscholar.org/paper/7e3088733d74bcaab94af49862beaa20a44653fc

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Citations in the corpus

The mass function of young star clusters in spiral galaxies (2008, background): “…but have confirmed that young clusters with masses above 105 M (and, in some cases, 106 M ) are quite common in external starforming galaxies (Ho & Filippenko 1996; Larsen et al. 2001, 2004; Larsen & Richtler 2004; McCrady & Graham 2007; Smith & Gallagher 2001; de Grijs & Parmentier 2007).”
Evidence for the Strong Effect of Gas Removal on the Internal Dynamics of Young Stellar Clusters (2006, background): “A appears to have a standard IMF (Smith & Gallagher 2001) based on dynamical measurements; however, we show that this cluster is probably also out of equilibrium.”
Thermal physics, cloud geometry and the stellar initial mass function (2005, background): “…by Rieke et al. (1993) in the small starburst galaxy M82, and detailed studies of the ‘super star cluster’ M82-F have also suggested such a bottom-light IMF in some clusters in M82 (Smith & Gallagher 2001; McCrady, Graham & Vacca 2005), although this evidence too is indirect and not conclusive.”
Gas expulsion and the destruction of massive young clusters (2006, unknown): “We compare the dynamical (assuming virial equilibrium) and luminous (assuming a standard Kroupa IMF) masses for a large sample of young clusters (limited by the number of clusters for which reliable values for both parameters are available).”
THE DISRUPTION OF GIANT MOLECULAR CLOUDS BY RADIATION PRESSURE & THE EFFICIENCY OF STAR FORMATION IN GALAXIES (2009, background): “The clusters are unresolved in the HST images (which have a resolution of order 15pc at the distance of Arp 220), except possibly their brightest cluster, with a half light radiusrcl ≈ 20pc. Wilson et al. (2006) do not obtain either a velocity dispersion or a half light radius for their clusters, so”
The Nature of Starburst Activity in M82 (2000, methodology): “Dynamical information for three individual young stellar clusters in M82 (Smith & Gallagher 2001; McCrady et al. 2003) provides similar evidence, but clearly more clusters need to be investigated.”
A Hubble Space Telescope Census of Nuclear Star Clusters in Late-Type Spiral Galaxies. II. Cluster Sizes and Structural Parameter Correlations (2003, methodology): “This kind of analysis has been successfully applied to NCs (Böker, van der Marel, & Vacca 1999), as well as young clusters in the disks of nearby galaxies (Smith & Gallagher 2001; Mengel et al. 2002).”
Extragalactic Globular Clusters and Galaxy Formation (2006, background): “…2004; Larsen, Brodie, & Hunter 2004), others may have abnormal IMFs that preclude their long-term survival (e.g., McCrady, Gilbert, & Graham 2003; Smith & Gallagher 2001; Brodie et al. 1998), though important uncertainties in dynamical mass estimates due to mass segregation remain (McCrady,…”
Young Massive Star Clusters (2010, background): “Kroupa 2001) although it has been suggested that the mass functions of some YMCs may be deficient in low-mass stars and/or “top-heavy” (Smith & Gallagher 2001), in the sense that the slope d logN/d logm of the mass function at the high-mass end is shallower (i.e. less negative) than the standard…”
Production of dust by massive stars at high redshift (2011, background): “Evidence for IMF variations towards higher stellar masses alsocomes from observations of the Galactic Center region and Galactic globular clusters (.g., D’Antona and Caloi, 2004; Ballero et al, 2007; Maness et al, 2007; Bartko et al, 2010) and star clusters (e.g., Smith and Gallagher, 2001).”
A Universal Stellar Initial Mass Function? A critical look at variations in extreme environments (2010, methodology): “Smith & Gallagher (2001) measured a LV/Mdyn ratio for M82F, and found it was ∼ 3 times larger than the LV/Mpop ratio predicted for a population with a Kroupa IMF.”
The formation of ultracompact dwarf galaxies (2001, methodology): “These are not to be confused with the commonly-used terminology ‘super-star-cluster’, SSC, which is one young star cluster with a mass typical for globular clusters (e.g. Smith & Gallagher 2001).”
Star cluster formation and evolution in nearby starburst galaxies – II. Initial conditions (2003, background): “In fact, these masses are similar to or exceed the spectroscopically confirmed mass estimates of the so-called ‘super star clusters’ (SSCs) in M82 (M82-F; Smith & Gallagher 2001), and the Antennae galaxies (Mengel et al. 2002).”
The stellar and sub-stellar IMF of simple and composite populations (2011, methodology): “Using spectroscopy of the unresolved M82-F cluster, Smith & Gallagher (2001) derive, via the inferred velocity dispersion, a mass and from the luminosity a mass-to-light ratio that is significantly smaller than the ratio expected from the canonical IMF for a 60 Myr population.”
Disruption time scales of star clusters in different galaxies (2004, background): “Such a lower mass cut-off for the initial mass function is controversial, but is suggested in young star clusters like MGG-11 (McCrady et al. 2003) and MGG-F, both in M82 (Smith and Gallagher 2001) and in several of the young star clusters in the Antennae galaxies (Mengel et al. 2002).”
On the massive stellar population of the super star cluster Westerlund 1 (2005, unknown): “F; Smith & Gallagher 2001).”
Kinematic Masses of Super-Star Clusters in M82 from High-Resolution Near-Infrared Spectroscopy (2003, methodology): “Smith & Gallagher (2001) estimated the age of MGG-F as 60 20 Myr.”