S. S. Larsen

The structure and environment of young stellar clusters in spiral galaxies

A search for stellar clusters has been carried out in 18 nearby spiral galaxies, using archive images from the Wide Field Planetary Camera 2 on board the Hubble Space Telescope. All of the galaxies have previously been imaged from the ground in UBVI. A catalogue of structural parameters, photometry and comments based on visual inspection of the clusters is compiled and used to investigate correlations between cluster structure, environment, age and mass. Least-squares fits to the data suggest correlations between both the full-width at half-maximum (FWHM) and half-light radius (Reff )o f the clusters and their masses (M) at about the 3σ level. Although both relations show a large scatter, the fits have substantially shallower slopes than for a constant-density relation (size ∝ M 1/3 ). However, many of the youngest clusters have extended halos which make the Reff determinations uncertain. There is no evidence for galaxy-to-galaxy variations in the mean cluster sizes. In particular, the mean sizes do not appear to depend on the host galaxy star formation rate surface density. Many of the youngest objects (age < 10 7 years) are located in strongly crowded regions, and about 1/3-1/2 of them are double or multiple sources. The HST images are also used to check the nature of cluster candidates identified in a previous ground-based survey. The contamination rate in the ground-based sample is generally less than about 20%, but some cluster identifications remain ambiguous because of crowding even with HST imaging, especially for the youngest objects.

The mass function of young star clusters in spiral galaxies

Aims. The initial cluster mass function (ICMF) in spiral discs is constrained and compared with data for old globular clusters and young clusters in starbursts. Methods. 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, Mc. The calculated relations are com- pared with observations of the brightest clusters in spiral galaxies. Schechter functions are also fitted directly to observed mass functions (MFs). Results. A single Schechter ICMF with an Mc of a few times 10 5 Mcan 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 Mc = (2.1 ± 0.4) × 10 5 M� .T he MFs in cluster-poor and cluster-rich spirals are statistically indistinguishable. An Mc = 2.1 × 10 5 MSchechter 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 M > 10 5 Mwill form about once every 10 7 years, while an M > 10 6 Mcluster will form only once every 50 Gyr. Luminosity functions (LFs) of model cluster populations drawn from an Mc = 2.1 × 10 5 MSchechter ICMF generally agree with LFs observed in spiral galaxies. Conclusions. The ICMF in present-day spiral discs can be modelled as a Schechter function with Mc ≈ 2 × 10 5 M� .H owever, the presence of significant numbers of M > 10 6 M� (and even M > 10 7 M� ) clusters in some starburst galaxies makes it unlikely that the Mc value derived for spirals is universal. In high-pressure environments, such as those created by complex gas kinematics and feedback in mergers, Mc can shift to higher masses than in quiescent discs.

Implications for the formation of star clusters from extragalactic star formation rates

Observations indicate that young massive star clusters in spiral and dwarf galaxies follow a relation between luminosity of the brightest young cluster and the star formation rate (SFR) of the host galaxy, in the sense that higher SFRs lead to the formation of brighter clusters. Assuming that the empirical relation between maximum cluster luminosity and SFR reflects an underlying similar relation between maximum cluster mass (M ecl,max ) and SFR, we compare the resulting SFR(M ecl,max ) relation with different theoretical models. The empirical correlation is found to suggest that individual star clusters form on a free-fall time-scale with their pre-cluster molecular-cloud-core radii typically being a few parsecs independent of mass. The cloud cores contract by factors of 5-10 while building up the embedded cluster. A theoretical SFR(M ecl,max ) relation in very good agreement with the empirical correlation is obtained if the CMF of a young population has a Salpeter exponent of β 2.35 and if this cluster population forms within a characteristic time-scale of a 1-10 Myr. This short time-scale can be understood if the interstellar medium is pressurized, thus precipitating rapid local fragmentation and collapse on a galactic scale. Such triggered star formation on a galactic scale is observed to occur in interacting galaxies. With a global SFR of 3-5 M ○. yr -1 , the Milky Way appears to lie on the empirical SFR(M ecl,max ) relation, given the recent detections of very young clusters with masses near 10 5 M ○. in the Galactic disc. The observed properties of the stellar population of very massive young clusters suggests that there may exist a fundamental maximum cluster mass, 10 6 < M ecl,max */M ○. < 10 7 .

The luminosity function of young star clusters: implications for the maximum mass and luminosity of clusters

We introduce a method to relate a possible truncation of the star cluster mass function at the high mass end to the shape of the cluster luminosity function (LF). We compare the observed LFs of five galaxies containing young star clusters with synthetic cluster population models with varying initial conditions. The LF of the SMC, the LMC and NGC 5236 are characterized by a power-law behavior N dL ∝ L−α dL, with a mean exponent of α = 2.0 ± 0.2. This can be explained by a cluster population formed with a constant cluster formation rate, in which the maximum cluster mass per logarithmic age bin is determined by the size-of-sample effect and therefore increases with log (age/yr). The LFs of NGC 6946 and M51 are better described by a double power-law distribution or a Schechter function. When a cluster population has a mass function that is truncated below the limit given by the size-of-sample effect, the total LF shows a bend at the magnitude of the maximum mass, with the age of the oldest cluster in the population, typically a few Gyr due to disruption. For NGC 6946 and M51 this suggests a maximum mass of Mmax = 0.5−1 × 106 M , although the bend is only a 1–2 σ detection. Faint-ward of the bend the LF has the same slope as the underlying initial cluster mass function and bright-ward of the bend it is steeper. This behavior can be well explained by our population model. We compare our results with the only other galaxy for which a bend in the LF has been observed, the “Antennae” galaxies (NGC 4038/4039). There the bend occurs brighter than in NGC 6946 and M51, corresponding to a maximum cluster mass of Mmax = 1.3−2.5 × 106 M . Hence, if the maximum cluster mass has a physical limit, then it can vary between different galaxies. The fact that we only observe this bend in the LF in the “Antennae” galaxies, NGC 6946 and M51 is because there are enough clusters available to reach the limit. In other galaxies there might be a physical limit as well, but the number of clusters formed or observed is so low, that the LF is not sampled up to the luminosity of the bend. The LF can then be approximated with a single power-law distribution, with an index similar to the initial mass function index.

Stellar clusters in M83: formation, evolution, disruption and the influence of the environment

We study the stellar cluster population in two adjacent fields in the nearby, face-on spiral galaxy M83 using multiwavelength Wide Field Camera 3/Hubble Space Telescope imaging. After automatic det ...

Constraining Globular Cluster Formation Through Studies of Young Massive Clusters: I. A lack of ongoing star formation within young clusters

We present a survey of 130 Galactic and extragalactic young massive clusters (YMCs, 10 4 < M/M⊙ < 10 8 , 10 < t/Myr < 1000) with integrated spectroscopy or resolved stellar photometry (40 presented here and 90 from the literature) and use the sample to search for evidence of ongoing star-formation within the clusters. Such episodes of secondary (or continuous) star-formation are predicted by models that attempt to explain the observed chemical and photometric anomalies observed in globular clusters as being due to the formation of a second stellar population within an existing first population. Additionally, studies that have claimed extended star-formation histories within LMC/SMC intermediate age clusters (1-2 Gyr), also imply that many young massive clusters should show ongoing star-formation. Based on visual inspection of the spectra and/or the colour-magnitude diagrams, we do not find evidence for ongoing star-formation within any of the clusters, and use this to place constraints on the above models. Models of continuous star-formation within clusters, lasting for hundreds of Myr, are ruled out at high significance (unless stellar IMF variations are invoked). Models for the (nearly instantaneous) formation of a secondary population within an existing first generation are not favoured, but are not formally discounted due to the finite sampling of age/mass-space.

ACS imaging of star clusters in M 51 : I. Identification and radius distribution

Context. Size measurements of young star clusters are valuable tools to put constraints on the formation and early dynamical evolution of star clusters. Aims. We use HST/ACS observations of the spiral galaxy M51 in F435W, F555W and F814W to select a large sample of star clusters with accurate effective radius measurements in an area covering the complete disc ofM51.We present the dataset and study the radius distribution and relations between radius, colour, arm/interarm region, galactocentric distance, mass and age. Methods. We select a sample of 7698 (F435W), 6846 (F555W) and 5024 (F814W) slightly resolved clusters and derive their effective radii (Reff) by fitting the spatial profiles with analytical models convolved with the point spread function. The radii of 1284 clusters are studied in detail. Results. We find cluster radii between 0.5 and ∼10 pc, and one exceptionally large cluster candidate with Reff = 21.6 pc. The median Reff is 2.1 pc. We find 70 clusters in our sample which have colours consistent with being old GC candidates and we find 6 new “faint fuzzy” clusters in, or projected onto, the disc of M51. The radius distribution can not be fitted with a power law similar to the one for star-forming clouds. We find an increase in Reff with colour as well as a higher fraction of clusters with B−V >∼ 0.05 in the interarm regions. We find a correlation between Reff and galactocentric distance (RG) of the formReff ∝ R0.12±0.02 G , which is considerably weaker than the observed correlation for old Milky Way GCs. We find weak relations between cluster luminosity and radius: Reff ∝ L0.15±0.02 for the interarm regions and Reff ∝ L−0.11±0.01 for the spiral arm regions, but we do not observe a correlation between cluster mass and radius. Conclusions. The observed radius distribution indicates that shortly after the formation of the clusters from a fractal gas, the radii of the clusters have changed in a non-uniform way. We find tentative evidence suggesting that clusters in spiral arms are more compact.

Constraints on mass loss and self-enrichment scenarios for the globular clusters of the Fornax dSph

Recently, high-dispersion spectroscopy has demonstrated conclusively that four of the five globular clusters (GCs) in the Fornax dwarf spheroidal galaxy are very metal-poor with [Fe/H]<−2. The remaining cluster, Fornax 4, has [Fe/H] =−1.4. This is in stark contrast to the field star metallicity distribution which sh ows a broad peak around [Fe/H]≈ −1 with only a few percent of the stars having [Fe/H]<−2. If we only consider stars and clusters with [Fe/H]<−2 we thus find an extremely high GC specific frequency, SN ≈ 400, implying by far the highest ratio of GCs to field stars kno wn anywhere. We estimate that about 1/5‐1/4 of all stars in the Fornax dSph with [Fe/H]<−2 belong to the four most metal-poor GCs. These GCs could, therefore, at most have been a factor of 4‐5 more massive initially. Yet, the Fornax GCs appear to share the same anomalous chemical abundance patterns known from Milky Way GCs, commonly attributed to the presence of multiple stellar generations within the clusters. The extreme ratio of metal-poor GC- versus field stars in the Fornax dSph is di ffi cult to reconcile with scenarios for self-enrichment and ea rly evolution of GCs in which a large fraction (90%‐95%) of the first-generation sta rs have been lost. It also suggests that the GCs may not have formed as part of a larger population of now disrupted clusters with an initial power-law mass distribution. The Fornax dSph may be a rosetta stone for constraining theories of the formation, self-enr ichment and early dynamical evolution of star clusters.

The star cluster - field star connection in nearby spiral galaxies. II. Field star and cluster formation histories and their relation

Context. Recent studies have started to cast doubt on the assumption that most stars are formed in clusters. Observational studies of field stars and star cluster systems in nearby galaxies can lead to better constraints on the fraction of stars forming in clusters. Ultimately this may lead to a better understanding of star formation in galaxies, and galaxy evolution in general. Aims. We aim to constrain the amount of star formation happening in long-lived clusters for four galaxies through the homogeneous, simultaneous study of field stars and star clusters. Methods. Using HST/ACS and HST/WFPC2 images of the galaxies NGC 45, NGC 1313, NGC 5236, and NGC 7793, we estimate star formation histories by means of the synthetic CMD method. Masses and ages of star clusters are estimated using simple stellar population model fitting. Comparing observed and modeled luminosity functions, we estimate cluster formation rates. By randomly sampling the stellar initial mass function (SIMF), we construct artificial star clusters and quantify how stochastic effects influence cluster detection, integrated colors, and age estimates. Results. Star formation rates appear to be constant over the past 10 7 −10 8 years within the fields covered by our observations. The number of clusters identified per galaxy varies, with a few detected massive clusters (M ≥ 10 5 M� ) and a few older than 1 Gyr. Among our sample of galaxies, NGC 5236 and NGC 1313 show high star and cluster formation rates, while NGC 7793 and NGC 45 show lower values. We find that stochastic sampling of the SIMF has a strong impact on the estimation of ages, colors, and completeness for clusters with masses ≤10 3 −10 4 M� , while the effect is less pronounced for high masses. Stochasticity also makes size measurements highly uncertain at young ages (τ 10 8 yr), making it difficult to distinguish between clusters and stars based on sizes. Conclusions. The ratio of star formation happening in clusters (Γ) compared to the global star formation appears to vary for different galaxies. We find similar values to previous studies (Γ ≈ 2%‐10%), but we find no obvious relation between Γ and the star formation ‐ ‐ ‐ ‐

HST observations of star clusters in NGC 1023: Evidence for three cluster populations?

Using Hubble Space Telescope images we have carried out a study of cluster populations in the nearby S0 galaxy NGC 1023. In two WFPC2 pointings we have identified 221 cluster candidates. The small distance (~9 Mpc) combined with deep F555W and F814W images allows us to reach about 2 mag below the expected turnover of the globular cluster luminosity function. NGC 1023 appears to contain at least three identifiable cluster populations: the brighter clusters show a clearly bimodal color distribution with peaks at (V-I)0 = 0.92 and at (V-I)0 = 1.15, and in addition there are a number of fainter, more extended objects with predominantly red colors. Among the brighter clusters, we find that the blue clusters have somewhat larger sizes than the red ones with mean effective radii of Re ~ 2 and Re ~ 1.7 pc, respectively. These clusters have luminosity functions (LFs) and sizes consistent with what is observed for globular clusters in other galaxies. Fitting Gaussians to the LFs of the blue and red compact clusters, we find turnover magnitudes of MTO(blue) = -7.58 and MTO(red) = -7.37 in V and dispersions of σV(blue) = 1.12 and σV(red) = 0.97. The fainter, more extended clusters have effective radii up to Re ~ 10–15 pc, and their LF appears to rise at least down to MV ~ -6, few of them being brighter than MV = -7. We suggest that these fainter objects may have a formation history distinct from that of the brighter GCs.