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Register a new userStar cluster versus field star formation in the nucleus of the prototype starburst galaxy M82
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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.
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Spectroscopic, photometric and dynamical data of the inner 3 kpc part of the starburst galaxy M82 are analyzed in order to investigate the star formation history of the stellar disk. The long-slit spectra along the major axis are dominated by Balmer absorption lines in the region outside the nuclear starburst all the way up to ~3.5 scalelengths (mu_B=22 mag/arcsec**2). Single Stellar Population (SSP) spectra of age 0.4-1.0 Gyr match well the observed spectra in the 1-3 kpc zone, with a mean age of the stellar population marginally higher in the outer parts. The mass in these populations, along with that in the gas component, make up for the inferred dynamical mass in the same annular zone for a Kroupa initial mass function, with a low mass cut-off m_l=0.4 Msun. The observed ratio of the abundances of alpha elements with respect to Fe, is also consistent with the idea that almost all the stars in M82 disk formed in a burst of short duration (0.3 Gyr) around 0.8 Gyr ago. We find that the optical/near infrared colors and their gradients in the disk are determined by the reddening with visual extinction exceeding 1 mag even in the outer parts of the disk, where there is apparently no current star formation. The disk-wide starburst activity was most likely triggered by the interaction of M82 with its massive neighbor M81 around 1~Gyr ago. The properties of the disk of M82 very much resemble the properties of the disks of luminous compact blue galaxies seen at 0.2-1.0 redshift.
(Abridged) Interacting galaxies are well-known for their high star formation rates and rich star cluster populations, but the rapidly changing tidal field can also efficiently destroy clusters. We use numerical simulations of merging disc galaxies to investigate which mechanism dominates. The simulations include a model for the formation and dynamical disruption of the entire star cluster population. We find that the dynamical heating of clusters by tidal shocks is about an order of magnitude higher in interacting galaxies than in isolated galaxies. This is driven by the increased gas density, and is sufficient to destroy star clusters at a higher rate than new clusters are formed: the total number of clusters in the merger remnant is 2-50% of the amount in the progenitor discs, with low-mass clusters being disrupted preferentially. By adopting observationally motivated selection criteria, we find that the observed surplus of star clusters in nearby merging galaxies is caused by the bias to detect young, massive clusters. We provide a general expression for the survival fraction of clusters, which increases with the gas depletion time-scale. Due to the preferential disruption of low-mass clusters, the mass distribution of the surviving star clusters in a merger remnant develops a peak at a mass of about 10^3 Msun, which evolves to higher masses at a rate of 0.3-0.4 dex per Gyr. The peak mass initially depends weakly on the galactocentric radius, but this correlation disappears as the system ages. We discuss the similarities between the cluster populations of the simulated merger remnants and (young) globular cluster systems. Our results suggest that the combination of cluster formation and destruction should be widespread in the dense star-forming environments at high redshifts, which could provide a natural origin to present-day globular cluster systems.
We have used new, deep, visible and near infrared observations of the compact starburst cluster in the giant HII region NGC 3603 and its surroundings with the WFC3 on HST and HAWK-I on the VLT to study in detail the physical properties of its intermediate mass (~ 1 - 3 M_sun) stellar population. We show that after correction for differential extinction and actively accreting stars, and the study of field star contamination, strong evidence remains for a continuous spread in the ages of pre-main sequence stars in the range ~ 2 to ~ 30 Myr within the temporal resolution available. Existing differences among presently available theoretical models account for the largest possible variation in shape of the measured age histograms within these limits. We also find that this isochronal age spread in the near infrared and visible Colour-Magnitude Diagrams cannot be reproduced by any other presently known source of astrophysical or instrumental scatter that could mimic the luminosity spread seen in our observations except, possibly, episodic accretion. The measured age spread and the stellar spatial distribution in the cluster are consistent with the hypothesis that star formation started at least 20-30 Myrs ago progressing slowly but continuously up to at least a few million years ago. All the stars in the considered mass range are distributed in a flattened oblate spheroidal pattern with the major axis oriented in an approximate South-East - North-West direction, and with the length of the equatorial axis decreasing with increasing age. This asymmetry is most likely due to the fact that star formation occurred along a filament of gas and dust in the natal molecular cloud oriented locally in this direction.
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 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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