Deriving physical parameters of unresolved star clusters. II. The degeneracies of age, mass, extinction, and metallicity (original) (raw)
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Deriving physical parameters of unresolved star clusters. I. Age, mass, and extinction degeneracies
2013
Context. Stochasticity and physical parameter degeneracy problems complicate the derivation of the parameters (age, mass, and extinction) of unresolved star clusters when using broad-band photometry. Aims. We develop a method to simulate stochasticity and degeneracies, and to investigate their influence on the accuracy of derived physical parameters. Then we apply it to star cluster samples of M31 and M33 galaxies. Methods. Age, mass and extinction of observed star clusters are derived by comparing their broad-band UBVRI integrated magnitudes to the magnitudes of a large grid of star cluster models with fixed metallicity Z=0.008. Masses of stars for a cluster model are randomly sampled from the initial mass function. Models of star clusters from the model grid, which have all of their magnitudes located within 3 observational errors from the magnitudes of the observed cluster, are selected for the computation of its age, mass, and extinction. Results. In the case of the M31 galaxy, ...
Deriving physical parameters of unresolved star clusters
Astronomy & Astrophysics, 2013
Context. This study is the fifth of a series that investigates the degeneracy and stochasticity problems present in the determination of physical parameters such as age, mass, extinction, and metallicity of partially resolved or unresolved star cluster populations in external galaxies when using HST broad-band photometry. Aims. In this work we aim to derive parameters of star clusters using models with fixed and free metallicity based on the HST WFC3+ACS photometric system. The method is applied to derive parameters of a subsample of 1363 star clusters in the Andromeda galaxy observed with the HST. Methods. Following Paper III, we derive the star cluster parameters using a large grid of stochastic models that are compared to the six observed integrated broad-band WFC3+ACS magnitudes of star clusters. Results. We show that the age, mass, and extinction of the M 31 star clusters, derived assuming fixed solar metallicity, are in agreement with previous studies. We also demonstrate the ability of the WFC3+ACS photometric system to derive metallicity of star clusters older than ∼1 Gyr. We show that the metallicity derived using broad-band photometry of 36 massive M 31 star clusters is in good agreement with the metallicity derived using spectroscopy.
Deriving physical parameters of unresolved star clusters III. Application to M31 PHAT clusters
2015
This study is the third of a series that investigates the degeneracy and stochasticity problems present in the determination of physical parameters such as age, mass, extinction, and metallicity of partially resolved or unresolved star cluster populations situated in external galaxies when using broad-band photometry. This work tests the derivation of parameters of artificial star clusters using models with fixed and free metallicity for the WFC3+ACS photometric system. Then the method is applied to derive parameters of a sample of 203 star clusters in the Andromeda galaxy observed with the HST. Following Papers I \& II, the star cluster parameters are derived using a large grid of stochastic models that are compared to the observed cluster broad-band integrated WFC3+ACS magnitudes. We derive the age, mass, and extinction of the sample of M31 star clusters with one fixed metallicity in agreement with previous studies. Using artificial tests we demonstrate the ability of the WFC3+ACS...
Stochasticity effects on derivation of physical parameters of unresolvedstar clusters
Memorie della Società Astronomica Italiana
We developed a method for a fast modeling of broad-band UBVRI integrated magnitudes of unresolved star clusters and used it to derive their physical parameters (age, mass, and extinction). The method was applied on M33 galaxy cluster sample and consistency of ages and masses derived from unresolved observations with the values derived from resolved stellar photometry was demonstrated. We found that interstellar extinction causes minor age-extinction degeneracy for the studied sample due to a narrow extinction range in M33.
Astronomy & Astrophysics, 2021
Context. This study is the sixth of a series that investigates degeneracy and stochasticity problems present in the determination of the age, mass, extinction, and metallicity of partially resolved or unresolved star clusters in external galaxies when using Hubble Space Telescope broadband photometry. In the fifth publication, it was noticed that inconsistencies in cluster colour indices, which arise due to projected foreground and background stars on the apertures, enhance age-metallicity-extinction degeneracies. Aims. In this work we aim to present new aperture photometry results for a sample of star clusters from the M 31 Panchromatic Hubble Andromeda Treasury (PHAT) survey. Methods. We employed two methods of aperture photometry. The first method is ordinary aperture photometry to measure total cluster fluxes. The second method was introduced to avoid the brightest foreground and background stars that project onto large apertures. This method employs smaller apertures, adapted t...
Monte Carlo simulations of metal-poor star clusters
Astronomy and Astrophysics, 2006
Context. Metal-poor globular clusters (GCs) can provide a probe of the earliest epoch of star formation in the Universe, being the oldest stellar systems observable. In addition, young and intermediate-age low-metallicity GCs are present in external galaxies. Nevertheless, inferring their evolutionary status by using integrated properties may suffer from large intrinsic uncertainty caused by the discrete nature of stars in stellar systems, especially in the case of faint objects. Aims. In this paper, we evaluate the intrinsic uncertainty (due to statistical effects) affecting the integrated colours and mass-to-light ratios as a function of the cluster integrated visual magnitude (M tot V), which represents a quantity directly measured. We investigate the case of metal-poor single-burst stellar populations with age from a few million years to a likely upper value for the Galactic globular cluster ages (∼15 Gyr). Methods. Our approach is based on Monte Carlo techniques for randomly generating stars distributed according to the cluster's mass function. Results. Integrated colours and mass-to-light ratios in different photometric bands are checked to be in good agreement with the observational values of low-metallicity Galactic clusters; the effect of different assumptions on the Horizontal Branch (HB) morphology is shown to be not relevant, at least for the photometric bands explored here. We present integrated colours and mass-to-light ratios as a function of age for different assumptions on the cluster total V magnitude. We find that the intrinsic uncertainty cannot be neglected. In particular, in models with M tot V = −4 the broad-band colours show an intrinsic uncertainty so high as to prevent precise age evaluation of the cluster. The effects of different assumptions on the initial mass function and on the minimum mass for which carbon burning is ignited on both integrated colours and mass-to-light ratios are also analyzed. Finally, the present predictions are compared with recent results available in the literature, showing in some cases non-negligible differences.
The accuracy of integrated star clusters parameters. The effects of stochasticity
2015
Star clusters are important tools to probe the star formation mechanisms and star formation history in their host galaxy. The traditional way to derive the physical parameters of star clusters using integrated broad-band photometry makes use of the Simple Stellar Population models (SSP), which can be considered as oversimplified. Indeed, these models consider that the way stellar masses are created in star clusters is by continuously populating the Initial Mass Function (IMF), which is an idealized view.
Astronomy & Astrophysics, 2004
We present homogeneous scales of ages and metallicities for star clusters from very young objects, through intermediate-age ones up to the oldest known clusters. All the selected clusters have integrated spectra in the visible range, as well as reliable determinations of their ages and metallicities. From these spectra equivalent widths (EWs) of K Ca II, G band (CH) and Mg I metallic, and Hδ, Hγ and Hβ Balmer lines have been measured homogeneously. The analysis of these EWs shows that the EW sums of the metallic and Balmer H lines, separately, are good indicators of cluster age for objects younger than 10 Gyr, and that the former is also sensitive to cluster metallicity for ages greater than 10 Gyr. We propose an iterative procedure for estimating cluster ages by employing two new diagnostic diagrams and age calibrations based on the above EW sums. For clusters older than 10 Gyr, we also provide a calibration to derive their overall metal contents.
Monthly Notices of the Royal Astronomical Society
The Magellanic Clouds are the most massive and closest satellite galaxies of the Milky Way (MW), with stars covering ages from a few Myr up to 13 Gyr. This makes them important for validating integrated light methods to study stellar populations and star formation processes, which can be applied to more distant galaxies. We characterized a set of stellar clusters in the Small Magellanic Cloud (SMC), using the Southern Photometric Local Universe Survey. This is the first age (metallicity) determination for 11 (65) clusters of this sample. Through its seven narrow bands, centred on important spectral features, and five broad bands, we can retrieve detailed information about stellar populations. We obtained ages and metallicities for all stellar clusters using the Bayesian spectral energy distribution fitting code bagpipes. With a sample of clusters in the colour range −0.20 < r − z < +0.35, for which our determined parameters are most reliable, we modeled the age–metallicity rel...