Analysing observed star cluster SEDs with evolutionary synthesis models: systematic uncertainties

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 rate density (Σ SFR ) within the range probed here (Σ SFR ∼ 10 −3 −10 −2 M yr −1 kpc −2 ). The Γ values do, however, appear to correlate with the specific U-band luminosity (T L (U), the fraction of total light coming from clusters compared to the total U-band light of the galaxy).

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“…Anders et al (2004) show that it is necessary to have at least four-band photometry to be able to break degeneracies (e.g. age-metallicity).…”

Section: Aperture Correctionsmentioning

confidence: 99%

The star cluster – field star connection in nearby spiral galaxies

Silva-Villa

Larsen

2011

A&A

109

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“…For NGC 3310 the authors report that the near-infrared colours of a cluster are a good indicator of metallicity, while optical colours do a better job at discerning ages (see also Anders et al 2004).…”

Section: The Effect Of Metallicitymentioning

confidence: 99%

The star cluster population of M 51

Gieles¹,

Bastian²,

Lamers³

2005

A&A

Self Cite

196

277

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Abstract. We use archival Hubble Space Telescope observations of broad-band images from the ultraviolet (F255W-filter) through the near infrared (NICMOS F160W-filter) to study the star cluster population of the interacting spiral galaxy M 51. We obtain age, mass, extinction, and effective radius estimates for 1152 star clusters in a region of ∼7.3 × 8.1 kpc centered on the nucleus and extending into the outer spiral arms. In this paper we present the data set and exploit it to determine the age distribution and relationships among the fundamental parameters (i.e. age, mass, effective radius). We show the critical dependence of the age distribution on the sample selection, and confirm that using a constant mass cut-off, above which the sample is complete for the entire age range of interest, is essential. In particular, in this sample we are complete only for masses above 5 × 10 4 M for the last 1 Gyr. Using this dataset we find: i) that the cluster formation rate seems to have had a large increase ∼50−70 Myr ago, which is coincident with the suggested second passage of its companion, NGC 5195; ii) a large number of extremely young (<10 Myr) star clusters, which we interpret as a population of unbound clusters of which a large majority will disrupt within the next ∼10 Myr; and iii) that the distribution of cluster sizes can be well approximated by a power-law with exponent, −η = −2.2 ± 0.2, which is very similar to that of Galactic globular clusters, indicating that cluster disruption is largely independent of cluster radius. In addition, we have used this dataset to search for correlations among the derived parameters. In particular, we do not find any strong trends between the age and mass, mass and effective radius, nor between the galactocentric distance and effective radius. There is, however, a strong correlation between the age of a cluster and its extinction, with younger clusters being more heavily reddened than older clusters.

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“…For instance, it is now usually admitted that the current cluster mass function decreases with mass as a power law with an index close to −2 (Zhang & Fall 1999;Bik et al 2003;Boutloukos & Lamers 2003) and the debate on the cluster survival rate also rests on distributions obtained using continuous models (Vesperini 1998;Fall & Zhang 2001;Lada & Lada 2003;Rafelski & Zaritsky 2005). The continuous approach has been coupled with statistical data analysis, for instance to provide the impression that including near-IR photometry (K band) solves the age-metallicity degeneracy for clusters (Goudfrooij et al 2001;A&A 521, A22 (2010) Puzia et al 2002;Anders et al 2004;Bridžius et al 2008). Still in the context of continuous population synthesis, followed by González have developed a Bayesian analysis of the integrated spectra of star clusters.…”

Section: Introductionmentioning

confidence: 99%

Accounting for stochastic fluctuations when analysing the integrated light of star clusters

Fouesneau¹,

Lançon²

2010

A&A

109

132

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Context. Star clusters are studied widely both as benchmarks for stellar evolution models and in their own right. Cluster age distributions and mass distributions within galaxies are probes of star formation histories and of cluster formation and disruption processes. The vast majority of clusters in the Universe are small, and it is well known that the integrated fluxes and colours of all but the most massive ones have broad probability distributions, owing to small numbers of bright stars. Aims. This paper goes beyond describing predicted probability distributions to present results of the analysis of cluster energy distributions in an explicitly stochastic context. Methods. The method developed is Bayesian. It provides posterior probability distributions in the age-mass-extinction space, using multi-wavelength photometric observations and a large collection of Monte-Carlo simulations of clusters of finite stellar masses. The main priors are the assumed intrinsic distributions of current mass and current age for clusters in a galaxy. Both UBVI and UBVIK data sets are considered, and the study conducted in this paper is restricted to the solar metallicity. Results. We first use the collection of simulations to reassess and explain errors arising from the use of standard analysis methods, which are based on continuous population synthesis models: systematic errors on ages and random errors on masses are large, while systematic errors on masses tend to be smaller. The age-mass distributions obtained after analysis of a synthetic sample are very similar to those found for real galaxies in the literature. The Bayesian approach, on the other hand, is very successful in recovering the input ages and masses over ages ranging between 20 Myr and 1.5 Gyr, with only limited systematics that we explain. Conclusions. Considering stochasticity is important, more important for instance than the choice of adding or removing near-IR data in many cases. We found no immediately obvious reason to reject priors inspired by previous (standard) analyses of cluster populations in galaxies, i.e., cluster distributions that scale with mass as M −2 and are uniform on a logarithmic age scale.

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Analysing observed star cluster SEDs with evolutionary synthesis models: systematic uncertainties

Cited by 151 publications

References 27 publications

The star cluster – field star connection in nearby spiral galaxies

The star cluster – field star connection in nearby spiral galaxies

The star cluster population of M 51

Accounting for stochastic fluctuations when analysing the integrated light of star clusters

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