We analyse multiwavelength Hubble Space Telescope (HST) observations of a large number of star clusters in the nearby (post-)starburst dwarf galaxy NGC 1569. Their spectral energy distributions ...(SEDs) cover at least the wavelength range from U to I in equivalent HST filters, in most cases supplemented by near-infrared data. Using our most up-to-date evolutionary synthesis models of the Göttingen galev code we determine ages, metallicities, extinction values and masses for each individual cluster robustly and independently. We confirm the youth of most of these objects. The majority were formed in a very intense starburst starting around 25 Myr ago. While there are two prominent ‘super star clusters’ present in this galaxy, with masses of (5–16) ×105 M⊙, almost all remaining clusters are significantly less massive than an average Milky Way globular cluster, and are generally consistent with open cluster-type objects. We determine the cluster mass function from individual cluster masses derived by scaling the model SEDs of known mass to the observed cluster SEDs for each individual cluster. We find signs of a change in the cluster mass function as the burst proceeds, which we attribute to the special conditions of star cluster formation in this starburst dwarf galaxy environment.
We use deep Hubble Space Telescope photometry of the rich, young (∼20– to 45–Myr old) star cluster NGC 1818 in the Large Magellanic Cloud to derive its stellar mass function (MF) down to ∼0.15 M⊙. ...This represents the deepest robust MF thus far obtained for a stellar system in an extragalactic, low–metallicity (Fe/H≃−0.4 dex) environment. Combining our results with the published MF for masses above 1.0 M⊙, we obtain a complete present–day MF. This is a good representation of the cluster's initial MF (IMF), particularly at low masses, because our observations are centred on the cluster's uncrowded half–mass radius. Therefore, stellar and dynamical evolution of the cluster will not have affected the low–mass stars significantly. The NGC 1818 IMF is well described by both a lognormal and a broken power–law distribution with slopes of Γ= 0.46 ± 0.10 and Γ≃−1.35 (Salpeter–like) for masses in the range from 0.15 to 0.8 M⊙ and greater than 0.8 M⊙, respectively. Within the uncertainties, the NGC 1818 IMF is fully consistent with both the Kroupa solar neighbourhood and the Chabrier lognormal mass distributions.
Context. Warps occurring in galactic discs have been studied extensively in HI and in the optical, but rarely in the near-infrared (NIR) bands that trace the older stellar populations. Aims. We ...provide NIR data of nearby edge-on galaxies, combined with optical observations, for direct comparison of the properties of galactic warps as a function of wavelength, and calculate warp curves for each galaxy and obtain the characteristic warp parameters. We discuss these properties as possible constraints to the different mechanisms that have been proposed for the development and persistence of galactic warps. Methods. We observed 20 galaxies that were selected from a statistically complete diameter-limited subsample of edge-on disc galaxies. We used the Cerro Tololo Infrared Imager (CIRIM) at the CTIO 1.5 m Ritchey-Chretien telescope to acquire the NIR data. We used the 1.54 m Danish and 0.92 m Dutch telescopes at the European Southern Observatory's La Silla site for our optical observations. Results. Our results show that 13 of our 20 sample galaxies are warped, with the warp more pronounced in the optical than at NIR wavelengths. In the remaining seven galaxies, no warp is apparent within the limitations of our automated detection method. The transition between the unperturbed inner disc and the outer, warped region is rather abrupt. S0 galaxies exhibit very small or no warps. The magnetic model remains one of a number of interesting formation scenarios.
The large majority of extragalactic star cluster studies performed to date essentially use multicolour photometry, combined with theoretical stellar synthesis models, to derive ages, masses, ...extinction estimates and metallicities. M31 offers a unique laboratory for studies of globular cluster (GC) systems. In this paper, we obtain new age estimates for 91 M31 GCs, based on improved photometric data, updated theoretical stellar synthesis models and sophisticated new fitting methods. In particular, we used photometric measurements from the Two Micron All Sky Survey (2MASS), which, in combination with optical photometry, can partially break the well-known age-metallicity degeneracy operating at ages in excess of a few Gyr. We show robustly that previous age determinations based on photometric data were affected significantly by this age-metallicity degeneracy. Except for one cluster, the ages of our other sample GCs are all older than 1 Gyr. Their age distribution shows populations of young- and intermediate-age GCs, peaking at ∼3 and 8 Gyr, respectively, as well as the 'usual' complement of well-known old GCs, i.e. GCs of similar age as the majority of the Galactic GCs. Our results also show that although there is significant scatter in metallicity at any age, there is a notable lack of young metal-poor and old metal-rich GCs, which might be indicative of an underlying age-metallicity relationship among the M31 GC population.
We present optical HST STIS observations made with two slits crossing four of the optically brightest starburst clumps near the nucleus of M82. These provide Ha kinematics, extinction, electron ...density, and emission measures. From the radial velocity curves derived from both slits we confirm the presence of a stellar bar. We derive a new model for the orientation of the bar and disk with respect to the main starburst clumps and the cluster M82-A1. We propose that clump A has formed within the bar region as a result of gas interactions between the bar orbits, whereas region C lies at the edge of the bar and regions D and E are located farther out from the nucleus but heavily obscured. We derive extremely high interstellar densities of 500 900 cm unk, corresponding to ISM pressures of P/k approximately (0.5-1.0) x 10 super(7) cm super(-3) K, and discuss the implications of the measured gas properties on the production and evolution of the galactic wind. Despite varying pressures, the ionization parameter is uniform down to parsec scales, and we discuss why this might be so. Where the signal-to-noise ratios of our spectra are high enough, we identify multiple emission-line components. Through detailed Gaussian line fitting, we identify a ubiquitous broad (200-300 km s super(-1)) underlying component to the bright Ha line and discuss the physical mechanlsm(s) that could be responsible for such widths. We conclude that evaporation and/or ablation of material from interstellar gas clouds caused by the impact of high-energy photons and fast flowing cluster winds produce a highly turbulent layer on the surface of the clouds from which the emission arises.
We review the complications involved in the conversion of stellar luminosities into masses and apply a range of mass-to-luminosity relations to our Hubble Space Telescope observations of the young ...Large Magellanic Cloud (LMC) star clusters NGC 1805 and 1818. Both the radial dependence of the mass function (MF) and the dependence of the cluster core radii on mass indicate clear mass segregation in both clusters at radii of r≲20–30 arcsec, for masses in excess of ∼1.6–2.5 M⊙. This result does not depend on the mass range used to fit the slopes or the metallicity assumed. It is clear that the cluster MFs, at any radius, are not simple power laws. The global and the annular MFs near the core radii appear to be characterized by similar slopes in the mass range (−0.15 ≤ log m/M⊙ ≤ 0.85), and the MFs beyond r≳30 arcsec have significantly steeper slopes. We estimate that while the NGC 1818 cluster core is between ∼5 and ∼30 crossing times old, the core of NGC 1805 is likely to be ≲3–4 crossing times old. However, since strong mass segregation is observed out to ∼6Rcore and ∼3Rcore in NGC 1805 and 1818, respectively, it is most likely that significant primordial mass segregation was present in both clusters, particularly in NGC 1805.
The large majority of extragalactic star cluster studies performed to date have essentially used two- or three-passband aperture photometry, combined with theoretical stellar population synthesis ...models, to obtain age, mass and extinction estimates, and sometimes also metallicities. The accuracy to which this can be performed depends on the choice of (broad-band) passband combination and, crucially, also on the actual wavelengths and the wavelength range covered by the observations. Understanding the inherent systematic uncertainties (the main aim of this paper) is of the utmost importance for a well-balanced interpretation of the properties of extragalactic star cluster systems. We simultaneously obtain ages, metallicities and extinction values for ∼300 clusters in the nearby starburst galaxy NGC 3310, based on archival Hubble Space Telescope observations from the ultraviolet (UV) to the near-infrared (NIR). We show that for ages 6 ≲ log(age yr−1) ≲ 9, and if one can only obtain partial coverage of the spectral energy distribution (SED), an optical passband combination of at least four filters including both blue and red passbands results in the most representative age distribution, as compared with the better constrained ages obtained from the full UV–NIR SED coverage. We find that while blue-selected passband combinations lead to age distributions that are slightly biased towards lower ages due to the well-known age–metallicity degeneracy, red-dominated passband combinations should be avoided. NGC 3310 underwent a (possibly extended) global burst of cluster formation ∼3 × 107 yr ago. This coincides closely with the last tidal interaction or merger with a low-metallicity galaxy that probably induced the formation of the large fraction of clusters with (significantly) subsolar metallicities. The logarithmic slope of the V-band cluster luminosity function, for clusters in the range 17.7 ≲ F606W ≲ 20.2 mag, is αF606W≃−1.8 ± 0.4. The observed cluster system has a median mass of 〈 log (m/ M⊙) 〉≃ 5.25 ± 0.1, obtained from scaling the appropriate model SEDs for known masses to the observed cluster SEDs.
Context. Like other starburst galaxies, M 82 hosts compact, massive (>5 $\times$ 105 $M_{\odot}$) young star clusters that are interesting both in their own right and as benchmarks for population ...synthesis models. Aims. In addition to assessing or reassessing the properties of some of the brightest near-IR sources in M 82, this paper addresses the following questions. Can population synthesis models at $\lambda/\delta \lambda \simeq$ 750 adequately reproduce the near-IR spectral features and the energy distribution of these clusters between 0.8 and 2.4 μm? How do the derived cluster properties compare with previous results from optical studies? Methods. We analyse the spectra of 5 massive clusters in M 82, using data acquired with the spectrograph SpeX on the InfraRed Telescope Facility (NASA/IRTF) and a new population synthesis tool with a highly improved near-IR extension, based on a recent collection of empirical and theoretical spectra of red supergiant stars. Results. We obtain excellent fits across the near-IR with models at quasi-solar metallicity and a solar neighbourhood extinction law. Spectroscopy breaks a strong degeneracy between age and extinction in the near-IR colours in the red supergiant-dominated phase of evolution. The estimated near-IR ages cluster between 9 and 30 Myr, i.e. the ages at which the molecular bands due to luminous red supergiants are strongest in the current models. They do not always agree with optical spectroscopic ages. Adding optical data sometimes leads to the rejection of the solar neighbourhood extinction law. This is not surprising considering small-scale structure around the clusters, but it has no significant effect on the near-IR based spectroscopic ages. Conclusions. The observed IR-bright clusters are part of the most recent episode of extended star formation in M 82. The near-IR study of clusters that are too faint for optical observation adds important elements to the age distribution of massive clusters in dusty starbursts. Further joint optical and near-IR spectroscopic studies will provide strong constraints on the uncertain physics of massive stars on which population synthesis models rest.
The diagnostic age versus mass-to-light ratio diagram is often used in attempts to constrain the shape of the stellar initial mass function (IMF) and the potential longevity of extragalactic young to ...intermediate-age massive star clusters. Here, we explore its potential for Galactic open clusters. On the basis of a small, homogenised cluster sample, we provide useful constraints on the presence of significant binary fractions. Using the massive young Galactic cluster Westerlund 1 as a key example, we caution that stochasticity in the IMF introduces significant additional uncertainties. We conclude that, for an open cluster to survive for any significant length of time, and in the absence of substantial external perturbations, it is necessary but not sufficient to be located close to or (in the presence of a significant binary population) somewhat below the predicted photometric evolutionary sequences for “normal” simple stellar populations, although such a location may be dominated by a remaining “bound” cluster core and thus not adequately reflect the overall cluster dynamics.
We obtain new age and mass estimates for the star clusters in the fossil starburst region B of M82, based on improved fitting methods. Our new age estimates confirm the peak in the age histogram ...attributed to the last tidal encounter with M81; we find a peak formation epoch at slightly older ages than previously published, log(tpeak yr−1) = 9.04, with a Gaussian σ of Δ log(twidth) = 0.273. The actual duration of the burst of cluster formation may have been shorter because uncertainties in the age determinations may have broadened the peak. Our improved mass estimates confirm that the (initial) masses of the M82 B clusters with V≤ 22.5 mag are mostly in the range 104–106 M⊙, with a median mass of Mcl= 1.08 × 105 M⊙. The formation history of the observed clusters shows a steady decrease towards older ages. This indicates that cluster disruption has removed a large fraction of the older clusters. Adopting the expression for the cluster disruption time-scale of tdis(M) =t4dis(M/104 M⊙)γ with γ≃ 0.62 (Paper I), we find that the ratios between the real cluster formation rates in the pre-burst phase log(t yr−1) ≥ 9.4, the burst phase 8.4 < log(t yr−1) < 9.4 and the post-burst phase log(t yr−1) ≤ 8.4 are approximately . The formation rate during the burst may have been higher if the actual duration of the burst was shorter than adopted. The mass distribution of the clusters formed during the burst shows a turnover at log (Mcl/ M⊙) ≃ 5.3 that is not caused by selection effects. This distribution can be explained by cluster formation with an initial power-law mass function of slope α= 2 up to a maximum cluster mass of Mmax= 3 × 106 M⊙ and cluster disruption with a normalization time-scale tdis4/tburst= (3.0 ± 0.3) × 10−2. For a burst age of 1 × 109 yr, we find that the disruption time-scale of a cluster of 104 M⊙ is tdis4∼ 3 × 107 yr, with an uncertainty of approximately a factor of 2. This is the shortest disruption time-scale known in any (disc region of a) galaxy.
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