Age Distribution Of Clusters Research Articles

Imaging of star clusters in unperturbed spiral galaxies with the Advanced Camera for Surveys

Aims. Our goal is to investigate the formation of star clusters in relatively unperturbed environments. To do this, we studied the five nearby spiral galaxies: NGC 45, NGC 1313, NGC 4395, NGC 5236, and NGC 7793. Methods. We obtained images of the galaxies and their star cluster systems in using the Advanced Camera for Surveys (ACS) and the Wide Field Planetary Camera 2 (WFPC2) on board the Hubble Space Telescope. From a comparison of the broad-band colours with simple stellar population (SSP) models, we derived individual properties for each galaxy for the clusters such as masses, ages, and sizes, as well as global star cluster system properties such as the age distribution, luminosity function, and disruption time for clusters. Results. We identified about 600 star cluster candidates in the five galaxies, typically spanning ages from 3.9 Myr up to 1 Gyr and masses from 10 [square] 2 up to 10 [square] 5 M . We used the cluster age distribution to reconstruct the recent star formation history of each galaxy and observed significant variations from galaxy to galaxy. We went on to derive the luminosity function of the young star clusters and found slopes around [alpha] ~ -2 (similar to the ones found in previous studies) and the brightest star cluster magnitudes consistent with a random sampling of the luminosity function without involving an upper luminosity cut off. Finally, the sample includes only a handful of old globular clusters in each galaxy from which we derive low globular cluster specific frequencies.

Infrared spectroscopy and analysis of brown dwarf and planetary mass objects in the Orion nebula cluster

We present near-infrared long-slit and multislit spectra of low-mass brown dwarf candidates in the Orion nebula cluster. The long-slit data were observed in the H and K bands using NIRI on the Gemini-North Telescope. The multi-object spectroscopic observations were made using IRIS2 on the Anglo-Australian Telescope at H band. We develop a spectral typing scheme based on optically calibrated, near-infrared spectra of young sources in the Taurus and IC 348 star-forming regions with spectral types M3.0 to M9.5. We apply our spectral typing scheme to 52 sources, including previously published UKIRT and GNIRS spectra. 40 objects show strong water absorption with spectral types of M3 to >M9.5. The latest type objects are provisionally classified as early L types. We plot our sources on Hertzsprung-Russell diagrams overlaid with theoretical pre-main-sequence isochrones. The majority of our objects lie close to or above the 1-Myr isochrone, leading to an average cluster age that is <1 Myr. We find 38 sources lie at or below the hydrogen-burning limit (0.075 M ⊙ ). 10 sources potentially have masses below the deuterium-burning limit (0.012 M ⊙ ). We use a Monte Carlo approach to model the observed luminosity function with a variety of cluster age and mass distributions. The lowest x 2 values are produced by an age distribution centred at 1 Myr, with a mass function that declines at substellar masses according to an M α power law in the range a = 0.3-0.6. We find that truncating the mass function at 0.012 M ⊙ produces luminosity functions that are starved of the faintest magnitudes, even when using bimodal age populations that contain 10-Myr-old sources. The results of these Monte Carlo simulations therefore support the existence of a planetary mass population in the ONC.

The mass function of young star clusters in spiral galaxies

<i>Aims. <i/> The initial cluster mass function (ICMF) in spiral discs is constrained and compared with data for old globular clusters and young clusters in starbursts. <i>Methods. <i/> For a given absolute magnitude, the cluster age distribution depends on the ICMF. Here, the behaviour of the median age-magnitude relation is analysed in detail for Schechter ICMFs with various cut-off masses, <i>M<i/><sub>c<sub/>. The calculated relations are compared with observations of the brightest clusters in spiral galaxies. Schechter functions are also fitted directly to observed mass functions (MFs).<i>Results. <i/>A single Schechter ICMF with an <i>M<i/><sub>c<sub/> of a few times 10<sup>5<sup/> can reproduce the observed ages and luminosities of the brightest (and 5th brightest) clusters in the spirals if disruption of optically visible clusters is dominated by relatively slow secular evolution. A Schechter function fit to the combined cluster MF for all spirals in the sample yields <i>M<i/><sub>c<sub/> = (2.1<i>±<i/>0.4)10<sup>5<sup/> . The MFs in cluster-poor and cluster-rich spirals are statistically indistinguishable. An <i>M<i/><sub>c<sub/> = 2.110<sup>5<sup/> Schechter function also fits the MF of young clusters in the Large Magellanic Cloud. If the same ICMF applies in the Milky Way, a bound cluster with <i>M<i/> > 10<sup>5<sup/> will form about once every 10<sup>7<sup/> years, while an <i>M<i/> > 10<sup>6<sup/> cluster will form only once every 50 Gyr. Luminosity functions (LFs) of model cluster populations drawn from an <i>M<i/><sub>c<sub/> = 2.110<sup>5<sup/> Schechter ICMF generally agree with LFs observed in spiral galaxies. <i>Conclusions. <i/> The ICMF in present-day spiral discs can be modelled as a Schechter function with <i>M<i/><sub>c<sub/> <i>≈<i/> 210<sup>5<sup/> . However, the presence of significant numbers of <i>M<i/> > 10<sup>6<sup/> (and even <i>M<i/> > 10<sup>7<sup/> ) clusters in some starburst galaxies makes it unlikely that the <i>M<i/><sub>c<sub/> value derived for spirals is universal. In high-pressure environments, such as those created by complex gas kinematics and feedback in mergers, <i>M<i/><sub>c<sub/> can shift to higher masses than in quiescent discs.

Star cluster evolution in the Magellanic Clouds revisited

AbstractThe evolution of star clusters in the Magellanic Clouds has been the subject of significant recent controversy, particularly regarding the importance and length of the earliest, largely mass-independent disruption phase (referred to as “infant mortality”). Here, we take a fresh approach to the problem, using a large, independent, and homogeneous data set of UBVR imaging observations, from which we obtain the cluster age and mass distributions in both the Large and Small Magellanic Clouds (LMC, SMC) in a self-consistent manner. We conclude that the (optically selected) SMC star cluster population has undergone at most ~30% (1σ) infant mortality between the age range from about 3–10 Myr, to that of approximately 40–160 Myr. We rule out a 90% cluster mortality rate per decade of age (for the full age range up to 109 yr) at a &gt; 6σ level. Using a simple approach, we derive a “characteristic” cluster disruption time-scale for the cluster population in the LMC that implies that we are observing the initial cluster mass function (CMF). Preliminary results suggest that the LMC cluster population may be affected by &lt;10% infant mortality.

An alternative method to study star cluster disruption

Many embedded star clusters do not evolve into long-lived bound clusters. The most popular explanation for this infant mortality of young clusters is the expulsion of natal gas by stellar winds and supernovae, which leaves up to 90% of them unbound. A cluster disruption model has recently been proposed in which this mass- independent disruption of clusters proceeds for another Gyr after gas expulsion. In this scenario, the survival chances of massive clusters are much smaller than in the traditional mass-dependent disruption models. The most common way to study cluster disruption is to use the cluster age distribution, which, however, can be heavily affected by incompleteness. To avoid this, we introduce a new method, based on size-of-sample effects, namely the relation between the most massive cluster, M_max, and the age range sampled. Assuming that clusters are sampled from a power-law initial mass function, with index -2 and that the cluster formation rate is constant, M_max scales with the age range sampled, such that the slope in a log(M_max) vs. log(age) plot is equal to unity. This slope decreases if mass-independent disruption is included. For 90% mass-independent cluster disruption per age dex, the predicted slope is zero. For the solar neighbourhood, SMC, LMC, M33, and M83, based on ages and masses taken from the literature, we find slopes consistent with the expected size-of-sample correlations for the first 100 Myr, hence ruling out the 90% mass-independent cluster disruption scenario. For M51, however, the increase of log(M_max) with log(age) is slightly shallower and for the Antennae galaxies it is flat. This simple method shows that the formation and/or disruption of clusters in the Antennae must have been very different from that of the other galaxies studied here, so it should not be taken as a representative case.

From Star Complexes to the Field: Open Cluster Families

The currently accepted paradigm for star formation assumes that field stars are born in clusters. These are not formed in isolation but in stellar complexes born out of giant molecular clouds. In the Galactic disk, molecular clouds have distinctive orbits, which, as they disappear after star formation is complete, may seed the Galactic disk with families of young clusters. These families gradually disperse to become individual clusters and, eventually, field populations. We investigate the existence of dynamical families of open clusters in the solar neighborhood using both age- and volume-limited samples from WEBDA in the framework of scan statistics. Our analysis indicates that a significant number of known young clusters organize in groups when age, spatial distribution, and kinematics are taken into account simultaneously. We find compelling statistical evidence for the presence of at least five dynamical families of young open clusters in the Milky Way disk associated to the underlying spiral structure. The young cluster population seems to be dominated by families of 10-20 objects; they are short-lived and the likely progenitors of classical moving groups, and stellar streams. Available observational data suggests that 50%-80% of newly formed open clusters dissolve within 20 Myr of formation to become field population. The overall age distribution of open clusters shows a steep decline, -->dN/dτ τβ, with -->β = − 3.6 ± 0.5 for clusters younger than 100 Myr, although it could be dependent on the local conditions as it ranges from -->β = − 1.0 ± 0.2 in the direction of Puppis to -->β = − 2.8 ± 0.5 in Norma. Due to the high rate of destruction among young clusters, any cluster-related coherent substructure must be younger than about 30 Myr unless it is the result of dynamically induced corotation resonances within the Galactic disk or minor mergers. The characteristic timescale for stars to become part of the field stellar populations is 10-20 Myr.

Star cluster ‘infant mortality’ in the Small Magellanic Cloud (Redivivus)

The early evolution of star clusters in the Small Magellanic Cloud (SMC) has been the subject of significant recent controversy, particularly regarding the importance and length of the earliest, largely mass-independent disruption phase (referred to as ‘infant mortality’). Here, we take a fresh approach to the problem, using an independent, homogeneous data set of UBVR imaging observations, from which we obtain the SMC's cluster age and mass distributions in a self-consistent manner. We conclude that the (optically selected) SMC star cluster population has undergone at most ∼30 per cent (1σ) infant mortality between the age range from about (3–10) Myr, to that of approximately (40–160) Myr. We rule out a 90 per cent cluster mortality rate per decade of age (for the full age range up to 109 yr) at a >6σ level. We independently affirm this scenario based on the age distribution of the SMC cluster sample.

The poorly constrained cluster disruption time-scale in the Large Magellanic Cloud

We use Monte Carlo simulations, combined with homogeneously determined age and mass distributions, based on multiwavelength photometry, to constrain the cluster formation history and the rate of bound cluster disruption in the Large Magellanic Cloud (LMC) star cluster system. We evolve synthetic star cluster systems formed with a power-law initial cluster mass function (ICMF) of spectral index a = -2 assuming different cluster disruption time-scales. For each of these cluster disruption time-scales, we derive the corresponding cluster formation rate (CFR) required to reproduce the observed cluster age distribution. We then compare, in a 'Poissonian' Χ 2 sense, model mass distributions and model two-dimensional distributions in log(mass) versus log(age) space of the detected surviving clusters to the observations. Because of the bright detection limit (M lim V ≃ -4.7 mag) above which the observed cluster sample is complete, one cannot constrain the characteristic cluster disruption time-scale for a 104 M ⊙ cluster, t dis 4 [where the disruption time-scale depends on cluster mass as t dis = t dis 4 (M cl /10 4 M ⊙ )ν, with γ ≃ 0.62], to better than a lower limit, t dis 4 > 1 Gyr. We conclude that the CFR has been increasing steadily from 0.3 clusters Myr -1 5 Gyr ago to a present rate of (20-30) clusters Myr -1 for clusters spanning a mass range of ∼100-10 7 M ⊙ . For older ages, the derived CFR depends sensitively on our assumption of the underlying CMF shape. If we assume a universal Gaussian ICMF, then the CFR has increased steadily over a Hubble time from ∼1 cluster Gyr -1 15 Gyr ago to its present value. On the other hand, if the ICMF has always been a power law with a slope close to a = -2, the CFR exhibits a minimum some 5 Gyr ago, which we tentatively identify with the well-known age gap in the LMC's cluster age distribution.

STAR CLUSTERS IN THE NEARBY LATE-TYPE GALAXY NGC 1311

Ultraviolet, optical and near-infrared images of the nearby (D 5.5 Mpc) SBm galaxy NGC 1311, obtained with the Hubble Space Telescope, reveal a small population of 13 candidate star clusters. We identify candidate star clusters based on a combination of their luminosity, extent, and spectral energy distribution. The masses of the cluster candidates range from ~103 M☉ up to ~105 M☉, and show a strong positive trend of larger mass with increasing with cluster age. Such a trend follows from the fading and dissolution of old, low-mass clusters, and the lack of any young super-star clusters of the sort often formed in strong starbursts. The cluster age distribution is consistent with a bursting mode of cluster formation, with active episodes of age ~10 Myr, ~100 Myr, and Gyr. The ranges of age and mass we probe are consistent with those of the star clusters found in quiescent Local Group dwarf galaxies.

Simple stellar population models as probed by the Large Magellanic Cloud star cluster ESO 121-SC03

The presence of blue straggler stars (BSs) in star clusters has proven a challenge to conventional simple stellar population (SSP) models. Conventional SSP models are based on the evolution theory of single stars. Meanwhile, the typical locations of BSs in the colour-magnitude diagram of a cluster are brighter and bluer than the main-sequence turn-off point. Such loci cannot be predicted by single-star evolution theory. However, stars with such properties contribute significantly to the integrated light of the cluster. In this paper, we reconstruct the integrated properties of the Large Magellanic Cloud cluster European Southern Observatory (ESO) 121-SC03, the only cluster populating the well-known age gap in the cluster age distribution, based on a detailed exploration of the individual cluster stars, and with particular emphasis on the cluster's BSs. We find that the integrated light properties of ESO 121-SC03 are dramatically modified by its BS component. The integrated spectral energy distribution (ISED) flux level is significantly enhanced towards shorter wavelengths, and all broad-band colours become bluer. When fitting the fully integrated ISED of this cluster based on conventional SSP models, the best-fitting values of age and metallicity are significantly underestimated compared to the true cluster parameters. The age underestimate is similar to 40 per cent if we only include the BSs within the cluster's half-light radius and similar to 60 per cent if all BSs are included. The corresponding underestimates of the cluster's metallicity are similar to 30 and similar to 60 per cent, respectively. The populous star clusters in the Magellanic Clouds are ideal objects to explore the potential importance of BSs for the integrated light properties of more distant unresolved star clusters in a statistically robust manner, since they cover a large range in age and metallicity.

On the Interpretation of the Age Distribution of Star Clusters in the Small Magellanic Cloud

We re-analyze the age distribution (dN/dt) of star clusters in the Small Magellanic Cloud (SMC) using age determinations based on the Magellanic Cloud Photometric Survey. For ages younger than 3x10^9 yr the dN/dt distribution can be approximated by a power-law distribution, dN/dt propto t^-beta, with -beta=-0.70+/-0.05 or -beta=-0.84+/-0.04, depending on the model used to derive the ages. Predictions for a cluster population without dissolution limited by a V-band detection result in a power-law dN/dt distribution with an index of ~-0.7. This is because the limiting cluster mass increases with age, due to evolutionary fading of clusters, reducing the number of observed clusters at old ages. When a mass cut well above the limiting cluster mass is applied, the dN/dt distribution is flat up to 1 Gyr. We conclude that cluster dissolution is of small importance in shaping the dN/dt distribution and incompleteness causes dN/dt to decline. The reason that no (mass independent) infant mortality of star clusters in the first ~10-20 Myr is found is explained by a detection bias towards clusters without nebular emission, i.e. cluster that have survived the infant mortality phase. The reason we find no evidence for tidal (mass dependent) cluster dissolution in the first Gyr is explained by the weak tidal field of the SMC. Our results are in sharp contrast to the interpretation of Chandar et al. (2006), who interpret the declining dN/dt distribution as rapid cluster dissolution. This is due to their erroneous assumption that the sample is limited by cluster mass, rather than luminosity.

A Catalog of Star Cluster Candidates in M33

We present a new catalog of star cluster candidates in the nearby spiral galaxy M33. It is based on eight existing catalogs wherein we have cross-referenced identifications and endeavored to resolve inconsistencies between them. Our catalog contains 451 candidates of which 255 are confirmed clusters based on HST and high resolution ground-based imaging. The catalog contains precise cluster positions (RA and Dec), magnitudes and colors in the UBVRIJHKs filters, metallicities, radial velocities, masses and ages, where available, and galactocentric distances for each cluster. The color distribution of the M33 clusters appears to be similar to those in the Large Magellanic Cloud with major peaks at (B-V)o~0.15, and (B-V)o~0.65. The intrinsic colors are correlated with cluster ages, which range from 10^{7.5} to 10^{10.3} years. The age distribution of the star clusters supports the notion of rapid cluster disruption with a slope of alpha=-1.09 +/- 0.07 in the dN_{cluster}/dt ~ t^{alpha} relation. In addition, comparison to theoretical single stellar population models suggests the presence of an age-metallicity relation among these clusters with younger clusters being more metal-rich. Analysis of the radial distribution of the clusters yields some evidence that younger clusters (age <~ 1 Gyr) may be more concentrated toward the center of M33 than older ones. A similar comparison with the radial profile of the M33 field stars shows the clusters to be more centrally concentrated at the greater than 99.9% confidence level. Possible reasons for this are presented and discussed; however, the overwhelming conclusion seems to be that a more complete and thorough cluster search is needed covering at least 4 square degrees centered on M33.

Relations between the Luminosity, Mass, and Age Distributions of Young Star Clusters

We derive and interpret some relations between the luminosity, mass, and age distributions of star clusters, denoted here by (L), ψ(M), and χ(τ), respectively. Of these, (L) is the easiest to determine observationally, whereas ψ(M) and χ(τ) are more informative about formation and disruption processes. For a population of young clusters, with a relatively wide range of ages, (L) depends on both ψ(M) and χ(τ) and thus cannot serve as a proxy for ψ(M) in general. We demonstrate this explicitly by four illustrative examples with specific forms for either ψ(M) or χ(τ). In the special case in which ψ(M) is a power law and is independent of χ(τ), however, (L) is also a power law with the same exponent as ψ(M). We conclude that this accounts for the observed similarity between (L) and ψ(M) for the young clusters in the Antennae galaxies. This result reinforces our picture in which clusters form with ψ(M) ∝ M-2 and are then disrupted rapidly at a rate roughly independent of their masses. The most likely disruptive process in this first stage is the removal of interstellar matter by the energy and momentum input from young stars (by photoionization, winds, jets, and supernovae). The few clusters that avoid this infant mortality are eventually disrupted in a second stage by the evaporation of stars driven by two-body relaxation, a process with a strong dependence on mass. We suspect this picture may apply to many, if not all, populations of star clusters, but this needs to be verified observationally by determinations of ψ(M) and χ(τ) in more galaxies.

Star Cluster Population of the Interacting Galaxy System M51

We present a star cluster population study in the interacting galaxy system M51 based on HST ACS BV I mosaic images taken by the Hubble Heritage Team to commemorate the HST's 15th anniversary. We have found and classified star clusters in M51 using SExtractor and visual inspection. We have derived the photometry, size, and age of the clusters. It is found that the companion SB0 galaxy NGC 5195 harbors about 50 faint fuzzy clusters and that the age distribution of star clusters appears to be correlated with the epochs of dynamical events in M51 system.

Connection between the Age Distributions of Star Clusters and Field Stars: A First Application to the Small Magellanic Cloud

We present the age distributions for star clusters and individual stars in the Small Magellanic Cloud (SMC) based on data from the Magellanic Clouds Photometric Survey by Zaritsky and collaborators. The age distribution of the SMC clusters shows a steep decline, dN_{cluster}/dt \propto t^{-0.85\pm0.15}, over the period 10^7 < t <10^9 yr. This decline is essentially identical to that observed previously for more massive clusters in the merging Antennae galaxies, and also for lower-mass embedded clusters in the solar neighborhood. The SMC cluster age distribution therefore provides additional evidence for the rapid disruption of star clusters (``infant mortality''). These disrupted clusters deliver their stars to the general field population, implying that the field star age distribution, dN_{fld star}/dt, should have an inverse relation to dN_{cluster}/dt if most stars form initially in clusters. We make specific predictions for dN_{fldstar}/dt based on our cluster disruption models, and compare them with current data available for stars in the SMC. While these data do not extend to sufficiently young ages for a definitive test, they are consistent with a scenario wherein most SMC stars formed in clusters. Future analyses of dN_{fldstar}/dt that extend down to ages of approximately few million years are needed to verify the age relationship between stars residing in clusters and in the field.

Fundamental parameters of six neglected old open clusters

In this paper, we present the first BV I CCD photometry of six overlooked old open clusters (Berkeley 44, NGC 6827, Berkeley 52, Berkeley 56, Skiff 1 and Berkeley 5) and derive estimates of their fundamental parameters by using isochrones from the Padova library. We found that all the clusters are older than the Hyades, with ages ranging from 0.8 Gyr (NGC 6827 and Berkeley 5) to 4.0 Gyr (Berkeley 56). The latter is one of the old open clusters with the largest heliocentric distance. In the field of Skiff 1, we recognize a faint blue main sequence identical to the one found in the background of open clusters in the second and third Galactic quadrant, and routinely attributed to the Canis Major accretion event. We use the synthetic colour–magnitude diagram method and a Galactic model to show that this population can be easily interpreted as a thick disc and halo population towards Skiff 1. We finally revise the age distribution of the old open clusters, showing that the previously suggested peak at 5 Gyr looses importance as additional old clusters are discovered.

Age distribution of young clusters and field stars in the Small Magellanic Cloud

Aims. In this paper we discuss the cluster and field star formation in the central part of the Small Magellanic Cloud. The main goal is to study the correlation between young objects and their interstellar environment. Methods. The ages of about 164 associations and 311 clusters younger than 1 Gyr are determined using isochrone fitting. The spatial distribution of the clusters is compared with the HI maps, with the HI velocity dispersion field, with the location of the CO clouds and with the distribution of young field stars. Results. The cluster age distribution supports the idea that clusters formed in the last 1 Gyr of the SMC history in a roughly continuous way with periods of enhancements. The two super-shells 37A and 304A detected in the HI distribution are clearly visible in the age distribution of the clusters: an enhancement in the cluster formation rate has taken place from the epoch of the shell formation. A tight correlation between young clusters and the HI intensity is found. The degree of correlation is decreasing with the age of the clusters. Clusters older than 300 Myr are located away from the HI peaks. Clusters and associations younger than 10 Myr are related to the CO clouds in the SW region of the SMC disk. A positive correlation between the location of the young clusters and the velocity dispersion field of the atomic gas is derived only for the shell 304A, suggesting that the cloud-cloud collision is probably not the most important mechanism of cluster formation. Evidence of gravitational triggered episode due to the most recent close interaction between SMC and LMC is found both in the cluster and field star distribution.

Extremely α‐Enriched Globular Clusters in Early‐Type Galaxies: A Step toward the Dawn of Stellar Populations?

We compare (� /Fe), metallicity, and age distributions of globular clusters in elliptical, lenticular, and spiral gal- axies, which we derive from Lick line index measurements. We find a large number of globular clusters in elliptical galaxies that reach significantly higher (� /Fe) values (½� /Fe� > 0:5) than any clusters in lenticular and spiral gal- axies. Most of these extremely � -enriched globular clusters are old (t > 8 Gyr), and cover the metallicity range � 1P ½Z/HP0.Acomparisonwithsupernovayieldmodelssuggeststhattheprogenitorgascloudsofthese globular clustersmusthavebeenpredominantlyenrichedbymassivestars(k20 M� ),withlittlecontributionfromlowermass stars. The measured (� /Fe) ratios are also consistent with yields of very massive pair-instability supernovae (� 130Y190 M� ). Both scenarios imply that the chemical enrichment of the progenitor gas was completed on ex- tremely short timescales of the order of a few Myr. Given the lower (� /Fe) average ratios of the diffuse stellar popu- lation in early-type galaxies, our results suggest that these extremely � -enhanced globular clusters could be members of the very first generation of star clusters formed, and that their formation epochs would predate the formation of the majority of stars in giant early-type galaxies. Subject headingg galaxies: evolution — galaxies: formation — galaxies: star clusters — globular clusters: general

ISM gas removal from starburst galaxies and the premature death of star clusters

Recent observational studies of the age distribution of star clusters in nearby merging galaxies and starburst (SB) galaxies indicate a premature death of the young clusters. The fate of an evolving star cluster crucially depends of its gas content. This behaves like a glue that helps to keep the star system gravitationally bound. In SB systems where the rate of supernovae (SNe) explosions is elevated one should expect an efficient heating of the gas and its complete removal which could then favor the rapid dissociation of the evolving star clusters. Based on a contemporaneous study of the dynamical evolution of the interstellar gas in SB environments (Melioli & de Gouveia Dal Pino 2004, A&A, 424, 817) where it has been considered also the presence of dense clouds that may inhibit the heating efficiency of the interstellar gas by the SNe, we have here computed the timescales for gas removal from young clusters embedded in these systems and found that they are consistent with the very short timescales for cluster dissolution which are inferred from the observational studies above. Our results indicate that typical SB proto-clusters should start to disperse after less than 5 Myr. For a given total gas mass content, this result is nearly insensitive to the initial star formation efficiency.

Properties of Resolved Star Clusters in M51

We present a study of compact star clusters in the nearby pair of interacting galaxies NGC 5194/5195 (M51), based on multifilter Hubble Space Telescope (HST) WFPC2 archival images. We have detected ~400 isolated, resolved clusters in five HST WFPC2 fields of the two-galaxy system. Due to our requirement that the clusters be detected based only on their morphology, which results in the selection of relatively isolated objects, we estimate that we are missing the majority (by a factor of 4?6) of clusters younger than ~10?Myr due to the extreme crowding in the spiral arms and star-forming regions. Hence, we focus on the cluster population older than 10?Myr. An age distribution of the detected clusters shows a broad peak between 100 and 500?Myr, which is consistent with the crossing times of the companion galaxy NGC?5195 through the NGC?5194 disk estimated in both single- and multiple-passage dynamical models. We estimate that the peak contains ~2.2?2.5 times more clusters than expected from a constant rate of cluster formation over this time interval. While there is also evidence for individual peaks near 100 and 500?Myr in the cluster age distribution (consistent with the predictions of multiple-passage models), this result requires verification. We estimate the effective radii of our sample clusters and find a median value of ~3?4 pc. In addition, we see correlations of (increasing) cluster size with cluster mass (with a best-fit slope of 0.14 ? 0.03) at the ~4 ? level, and with cluster age (0.06 ? 0.02) at the 3 ? level. Finally, we report for the first time the discovery of faint, extended star clusters in the companion, NGC?5195, an SB0 galaxy. These have red [(V - I) > 1.0] colors and effective radii >7?pc, and are scattered over the disk of NGC?5195. Our results therefore indicate that NGC?5195 is currently the third known barred lenticular galaxy to have formed so-called faint fuzzy star clusters.