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Register a new userSuper Star Cluster Velocity Dispersions and Virial Masses in the M82 Nuclear Starburst
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We use high-resolution near-infrared spectroscopy from Keck Observatory to measure the stellar velocity dispersions of 19 super star clusters (SSCs) in the nuclear starburst of M82. The clusters have ages on the order of 10 Myr, which is many times longer than the crossing times implied by their velocity dispersions and radii. We therefore apply the Virial Theorem to derive the kinematic mass for 15 of the SSCs. The SSCs have masses of 2 x 10^5 to 4 x 10^6 solar masses, with a total population mass of 1.4 x 10^7 solar masses. Comparison of the loci of the young M82 SSCs and old Milky Way globular clusters in a plot of radius versus velocity dispersion suggests that the SSCs are a population of potential globular clusters. We present the mass function for the SSCs, and find a power law fit with an index of gamma = -1.91 +/- 0.06. This result is nearly identical to the mass function of young SSCs in the Antennae galaxies.
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We present optical spectroscopy obtained with the Space Telescope Imaging Spectrograph (STIS) of five young massive star clusters in the starburst galaxy M82. A detailed analysis is performed for one cluster `M82-A1 and its immediate environment in the starburst core. From HST archive images, we find that it is elliptical with an effective radius of 3.0+/-0.5 pc and is surrounded by a compact (r=4.5+/-0.5 pc) H II region. We determine the age and reddening of M82-A1 using synthetic spectra from population synthesis models by fitting both the continuum energy distribution and the depth of the Balmer jump. We find an age of 6.4+/-0.5 Myr and a photometric mass estimate of M=7-13 x 10^5 solar masses. We associate its formation with the most recent starburst event 4-6 Myr ago. We find that the oxygen abundance of the H II region surrounding M82-A1 is solar or slightly higher. The H II region has a high pressure P/k = 1-2 x 10^7 cm^-3 K. The diffuse gas in region A has a slightly lower pressure, which together with the broad H alpha emission line width, suggests that both the thermal and turbulent pressures in the M82 starburst core are unusually high. We discuss how this environment has affected the evolution of the cluster wind for M82-A1. We find that the high pressure may have caused the pressure-driven bubble to stall. We also obtain spectroscopic ages for clusters B1-2 and B2-1 in the `fossil starburst region and for the intermediate age clusters F and L. These are consistent with earlier studies and demonstrate that star formation activity, sufficiently intense to produce super star clusters, has been going on in M82 during the past Gyr, perhaps in discrete and localized episodes.
We analyse high-resolution Hubble Space Telescope/Advanced Camera for Surveys imaging of the nuclear starburst region of M82, obtained as part of the Hubble Heritage mosaic made of this galaxy, in four filters (Johnson-Cousins equivalent B, V, and I broad bands, and an Halpha narrow-band filter), as well as subsequently acquired U-band images. We find a complex system of ~150 star clusters in the inner few 100 pc of the galaxy. We do not find any conclusive evidence of a cluster-formation epoch associated with the most recent starburst event, believed to have occurred about 4-6 Myr ago. This apparent evidence of decoupling between cluster and field-star formation is consistent with the view that star cluster formation requires special conditions. However, we strongly caution, and provide compelling evidence, that the `standard simple stellar population analysis method we have used significantly underestimates the true uncertainties in the derived ages due to stochasticity in the stellar initial mass function and the corresponding sampling effects.
Many young extra-galactic clusters have a measured velocity dispersion that is too high for the mass derived from their age and total luminosity, which has led to the suggestion that they are not in virial equilibrium. Most of these clusters are confined to a narrow age range centred around 10 Myr because of observational constraints. At this age the cluster light is dominated by luminous evolved stars, such as red supergiants, with initial masses of ~13-22 Msun for which (primordial) binarity is high. In this study we investigate to what extent the observed excess velocity dispersion is the result of the orbital motions of binaries. We demonstrate that estimates for the dynamical mass of young star clusters, derived from the observed velocity dispersion, exceed the photometric mass by up-to a factor of 10 and are consistent with a constant offset in the square of the velocity dispersion. This can be reproduced by models of virialised star clusters hosting a massive star population of which ~25 is in binaries, with typical mass ratios of ~0.6 and periods of ~1000 days. We conclude that binaries play a pivotal role in deriving the dynamical masses of young (~10 Myr) moderately massive and compact (<1e5 Msun; > 1 pc) star clusters.
Much progress has been made in measuring black hole (BH) masses in (non-active) galactic nuclei using the tight correlation between stellar velocity dispersions (sigma) in galaxies and the mass of their central BH. The use of this correlation in quasars, however, is hampered by the difficulty in measuring sigma in host galaxies that tend to be overpowered by their bright nuclei. We discuss results from a project that focuses on z~0.3 quasars suffering from heavy extinction at shorter wavelengths. This makes it possible to obtain clean spectra of the hosts in the spectral regions of interest, while broad lines (like H-alpha) are still visible at longer wavelengths. We compare BH masses obtained from velocity dispersions to those obtained from the broad line region and thus probe the evolution of this relation and BH growth with redshift and luminosity. Our preliminary results show an offset between the position of our objects and the local relation, in the sense that red quasars have, on average, lower velocity dispersions than local galaxies. We discuss possible biases and systematic errors that may affect our results.
We investigate the Initial Mass Function and mass segregation in super star cluster M82-F with high resolution Keck/NIRSPEC echelle spectroscopy. Cross-correlation with template supergiant spectra provides the velocity dispersion of the cluster, enabling measurement of the kinematic (virial) mass of the cluster when combined with sizes from NICMOS and ACS images. We find a mass of 6.6 +/- 0.9 x 10^5 M_sun based on near-IR light and 7.0 +/- 1.2 x 10^5 M_sun based on optical light. Using PSF-fitting photometry, we derive the clusters light-to-mass ratio in both near-IR and optical light, and compare to population synthesis models. The ratios are inconsistent with a normal stellar initial mass function for the adopted age of 40 to 60 Myr, suggesting a deficiency of low-mass stars within the volume sampled. King model light profile fits to new HST/ACS images of M82-F, in combination with fits to archival near-IR images, indicate mass segregation in the cluster. As a result, the virial mass represents a lower limit on the mass of the cluster.
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