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Register a new userThe Luminosity, Mass, and Age Distributions of Compact Star Clusters in M83 Based on HST/WFC3 Observations
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Rupali Chandar
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The newly installed Wide Field Camera 3 (WFC3) on the Hubble Space Telescope has been used to obtain multi-band images of the nearby spiral galaxy M83. These new observations are the deepest and highest resolution images ever taken of a grand-design spiral, particularly in the near ultraviolet, and allow us to better differentiate compact star clusters from individual stars and to measure the luminosities of even faint clusters in the U band. We find that the luminosity function for clusters outside of the very crowded starburst nucleus can be approximated by a power law, dN/dL propto L^{alpha}, with alpha = -2.04 +/- 0.08, down to M_V ~ -5.5. We test the sensitivity of the luminosity function to different selection techniques, filters, binning, and aperture correction determinations, and find that none of these contribute significantly to uncertainties in alpha. We estimate ages and masses for the clusters by comparing their measured UBVI,Halpha colors with predictions from single stellar population models. The age distribution of the clusters can be approximated by a power-law, dN/dt propto t^{gamma}, with gamma=-0.9 +/- 0.2, for M > few x 10^3 Msun and t < 4x10^8 yr. This indicates that clusters are disrupted quickly, with ~80-90% disrupted each decade in age over this time. The mass function of clusters over the same M-t range is a power law, dN/dM propto M^{beta}, with beta=-1.94 +/- 0.16, and does not have bends or show curvature at either high or low masses. Therefore, we do not find evidence for a physical upper mass limit, M_C, or for the earlier disruption of lower mass clusters when compared with higher mass clusters, i.e. mass-dependent disruption. We briefly discuss these implications for the formation and disruption of the clusters.
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We study star clusters in two fields in the nearby spiral galaxy M83 using broad and narrow band optical imaging taken with the WFC3 on-board HST. We present results based on several different catalogs of star clusters in an inner and outer field, and conclude that different methods of selection do not strongly impact the results, particularly for clusters older than $approx$10 Myr. The age distributions can be described by a power law, $dN/dtau proptotau^{gamma}$, with $gammaapprox -$0.84$pm$0.12 in the inner field, and $gammaapprox -$0.48$pm$0.12 in the outer field for $taugtrsim$10 Myr. We bracket the difference, $Delta gamma$, between the two fields to be in the range 0.18$-$0.36, based on estimates of the relative star formation histories. The mass functions can also be described by a power law, $dN/dMpropto M^{beta}$, with $betaapprox -$1.98$pm$0.14 and $betaapprox $2.34$pm$0.26 in the inner and outer fields, respectively. We conclude that the shapes of the mass and age distributions of the clusters in the two fields are similar, as predicted by the quasi-universal model. Any differences between the two fields are at the $approx$2$-$3$sigma (approx$1$-$2$sigma)$ level for the age (mass) distributions. Therefore any dependence of these distributions on the local environment is probably weak. We compare the shapes of the distributions with those predicted by two popular cluster disruption models, and find that both show evidence that the clusters are disrupted at a rate that is approximately independent of their mass, but that the observational results do not support the earlier disruption of lower mass clusters relative to their higher mass counterparts.
We investigate the loss of low-mass stars in two of the faintest globular clusters known, AM 4 and Palomar 13 (Pal 13), using HST/WFC3 F606W and F814W photometry. To determine the physical properties of each cluster --- age, mass, metallicity, extinction, present day mass function (MF) --- we use the maximum likelihood color-magnitude diagram (CMD) fitting program MATCH and the Dartmouth, Padova and BaSTI stellar evolution models. For AM 4, the Dartmouth models provide the best match to the CMD and yield an age of >13 Gyr, metallicity log Z/Z_solar = -1.68 +/- 0.08, a distance modulus (m-M)_V = 17.47 +/- 0.03 and reddening A_V = 0.19 +/- 0.02. For Pal 13 the Dartmouth models give an age of 13.4 +/- 0.5 Gyr, log Z/Z_solar = -1.55 +/- 0.06, (m-M)_V = 17.17 +/- 0.02 and A_V = 0.43 +/- 0.01. We find that the systematic uncertainties due to choice in assumed stellar model greatly exceed the random uncertainties, highlighting the importance of using multiple stellar models when analyzing stellar populations. Assuming a single-sloped power law MF, we find that AM 4 and Pal 13 have spectral indices alpha = +0.68 +/- 0.34 and alpha = -1.67 +/- 0.25 (where a Salpeter MF has alpha = +1.35), respectively. Comparing our derived slopes with literature measurements of cluster integrated magnitude (M_V) and MF slope indicates that AM 4 is an outlier. Its MF slope is substantially steeper than clusters of comparable luminosity, while Pal 13 has a MF in line with the general trend. We discuss both primordial and dynamical origins for the unusual MF slope of AM 4 and tentatively favor the dynamical scenario. However, MF slopes of more low luminosity clusters are needed to verify this hypothesis.
We present Wide Field Camera 3 images taken with the Hubble Space Telescope within a single field in the southern grand design star-forming galaxy M83. Based on their size, morphology and photometry in continuum-subtracted H$alpha$, [SII], H$beta$, [OIII] and [OII] filters, we have identified 60 supernova remnant candidates, as well as a handful of young ejecta-dominated candidates. A catalog of these remnants, their sizes and, where possible their H$alpha$ fluxes are given. Radiative ages and pre-shock densities are derived from those SNR which have good photometry. The ages lie in the range $2.62 < log(tau_{rm rad}/{rm yr}) < 5.0$, and the pre-shock densities at the blast wave range over $0.56 < n_0/{rm cm^{-3}} < 1680$. Two populations of SNR have been discovered. These divide into a nuclear and spiral arm group and an inter-arm population. We infer an arm to inter-arm density contrast of 4. The surface flux in diffuse X-rays is correlated with the inferred pre-shock density, indicating that the warm interstellar medium is pressurised by the hot X-ray plasma. We also find that the interstellar medium in the nuclear region of M83 is characterized by a very high porosity and pressure and infer a SNR rate of one per 70-150 yr for the nuclear ($R<300 $pc) region. On the basis of the number of SNR detected and their radiative ages, we infer that the lower mass of Type II SNe in M83 is $M_{rm min} = 16^{+7}_ {-5}$ M$_{odot}$. Finally we give evidence for the likely detection of the remnant of the historical supernova, SN1968L.
We use new WFC3 observations of the nearby grand design spiral galaxy M83 to develop two independent methods for estimating the ages of young star clusters. The first method uses the physical extent and morphology of Halpha emission to estimate the ages of clusters younger than tau ~10 Myr. It is based on the simple premise that the gas in very young (tau < few Myr) clusters is largely coincident with the cluster stars, is in a small, ring-like structure surrounding the stars in slightly older clusters (e.g., tau ~5 Myr), and is in a larger ring-like bubble for still older clusters (i.e., ~5-10 Myr). The second method is based on an observed relation between pixel-to-pixel flux variations within clusters and their ages. This method relies on the fact that the brightest individual stars in a cluster are most prominent at ages around 10 Myr, and fall below the detection limit (i.e., M_V < -3.5) for ages older than about 100 Myr. These two methods are the basis for a new morphological classification system which can be used to estimate the ages of star clusters based on their appearance. We compare previous age estimates of clusters in M83 determined from fitting UBVI Halpha measurements using predictions from stellar evolutionary models with our new morphological categories and find good agreement at the ~95% level. The scatter within categories is ~0.1 dex in log tau for young clusters (<10 Myr) and ~0.5 dex for older (>10 Myr) clusters. A by-product of this study is the identification of 22 single-star HII regions in M83, with central stars having ages ~4 Myr.
The circumnuclear starburst of M83 (NGC 5236), the nearest such example (4.6 Mpc), constitutes an ideal site for studying the massive star IMF at high metallicity (12+log[O/H]=9.1$pm$0.2, Bresolin & Kennicutt 2002). We analyzed archival HST/STIS FUV imaging and spectroscopy of 13 circumnuclear star clusters in M83. We compared the observed spectra with two types of single stellar population (SSP) models, semi-empirical models, which are based on an empirical library of Galactic O and B stars observed with IUE (Robert et al. 1993), and theoretical models, which are based on a new theoretical UV library of hot massive stars described in Leitherer et al. (2010) and computed with WM-Basic (Pauldrach et al. 2001). The models were generated with Starburst99 (Leitherer & Chen 2009). We derived the reddenings, the ages, and the masses of the clusters from model fits to the FUV spectroscopy, as well as from optical HST/WFC3 photometry.
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