An assessment of broad-band optical colours as age indicators for star clusters (original) (raw)
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Properties of young star cluster systems: the age signature from near-infrared integrated colours
Astronomy & Astrophysics, 2013
A recent JHK s study of several grand-design spiral galaxies, including NGC 2997, shows a bimodal distribution of their system of star clusters and star forming complexes in colour-magnitude and colour-colour diagrams. In a comparison with stellar population models including gas, the (J − H) vs (H − K s ) diagram reveals that embedded clusters, still immersed in their parental clouds of gas and dust, generally have a redder (H − K s ) colour than older clusters, whose gas and dust have already been ejected. This bimodal behaviour is also evident in the colour-magnitude diagram M K vs (J − K s ), where the brightest clusters split into two sequences separating younger from older clusters. In addition, the reddening-free index Q d =(H − K s ) − 0.884 (J − H) has been shown to correlate with age for the young clusters and thus provided an effective way to differentiate the embedded clusters from the older ones. Aims. We aim to study the behaviour of these photometric indices for star cluster systems in the Local Group. In particular, we investigate the effectiveness of the Q d index in sorting out clusters of different ages at their early evolutionary stages. In addition, the whole set of homogeneous measurements will serve as a template for analyses of the populations belonging to distant galaxies that are unresolved clusters or complexes. Methods. Surface photometry was carried out for 2MASS images of populous clusters younger than ∼ 100 Myr whose ages were available. The integrated magnitude and colours were measured to a limiting radius and combined to generate the photometric diagrams. Some clusters, particularly the embedded ones, were studied for the first time using this method. Results. The integrated magnitudes and colours extracted from the surface photometry of the most populous clusters/complexes in the Local Group shows the expected bimodal distribution in the colour-colour and colour-magnitude diagrams. In particular, we confirm the index Q d as a powerful tool for distinguishing clusters younger than about 7 Myr from older clusters.
The Astrophysical Journal, 2012
The apparent age and mass of a stellar cluster can be strongly affected by stochastic sampling of the stellar initial mass function, when inferred from the integrated color of low mass clusters ( 10 4 M ). We use simulated star clusters to show that these effects are minimized when the brightest, rapidly evolving stars in a cluster can be resolved, and the light of the fainter, more numerous unresolved stars can be analyzed separately. When comparing the light from the less luminous cluster members to models of unresolved light, more accurate age estimates can be obtained than when analyzing the integrated light from the entire cluster under the assumption that the initial mass function is fully populated. We show the success of this technique first using simulated clusters, and then with a stellar cluster in M31. This method represents one way of accounting for the discrete, stochastic sampling of the stellar initial mass function in less massive clusters and can be leveraged in studies of clusters throughout the Local Group and other nearby galaxies.
Predicted colours for simple stellar populations
Astronomy and Astrophysics Supplement Series, 2000
This paper presents theoretical integrated colours of old stellar populations as computed adopting an homogeneous set of stellar models, covering all the major evolutionary phases of globular cluster stars. We show that adopting Reimers parameterization of mass loss rates, the choice η = 0.4 gives synthetic CM diagrams of simple stellar populations in agreement with the typical dependence of globular cluster CM diagrams on metallicity. We present theoretical U − B, B − V , V − R and V − I integrated colours for cluster metallicity in the range Z = 0.0001 to 0.02 and for age between 8 and 15 Gyr. The stochastic occurrence of luminous post-AGB stars is briefly discussed. We find that for an age t ∼ 15 Gyr the predicted integrated colours appear in good agreement with available data for both galactic and LMC old globulars. We discuss the uncertainties of integrated colours due to statistical fluctuations in the number of luminous stars, giving for each colour the expected uncertainty as a function of the cluster integrated V-magnitude. Comparison with Kurth et al. (1999) discloses that the still existing differences in the evolutionary results appear of minor relevance as far as integrated cluster colours is concerned. Finally, we show that reasonable uncertainties either in the cluster age or in the efficiency of mass loss have marginal effects on the predicted colours.
Monthly Notices of the Royal Astronomical Society, 2008
We present integrated JHK s 2MASS photometry and a compilation of integratedlight optical photoelectric measurements for 84 star clusters in the Magellanic Clouds. These clusters range in age from ≈ 200 Myr to > 10 Gyr, and have [Fe/H] values from −2.2 to −0.1 dex. We find a spread in the intrinsic colours of clusters with similar ages and metallicities, at least some of which is due to stochastic fluctuations in the number of bright stars residing in low-mass clusters. We use 54 clusters with the most reliable age and metallicity estimates as test particles to evaluate the performance of four widely used SSP models in the optical/NIR colour-colour space. All models reproduce the reddening-corrected colours of the old ( 10 Gyr) globular clusters quite well, but model performance varies at younger ages. In order to account for the effects of stochastic fluctuations in individual clusters, we provide composite B − V , B − J, V − J, V − K s and J − K s colours for Magellanic Cloud clusters in several different age intervals. The accumulated mass for most composite clusters are higher than that needed to keep luminosity variations due to stochastic fluctuations below the 10% level. The colours of the composite clusters are clearly distinct in optical-NIR colour-colour space for the following intervals of age: > 10 Gyr, 2 − 9 Gyr, 1 − 2 Gyr, and 200 Myr−1 Gyr. This suggests that a combination of optical plus NIR colours can be used to differentiate clusters of different age and metallicity.
Integrated photometric characteristics of galactic open star clusters
Astronomy & Astrophysics, 2002
Integrated UBV RI photometric parameters of 140 galactic open clusters have been computed. Integrated I(V − R)0 and I(V − I)0 colours as well as integrated parameters for 71 star clusters have been obtained for the first time. These, in combination with published data, altogether 352 objects, are used to study the integrated photometric characteristics of the galactic open clusters. The I(MV ) values range from −9.0 to −1.0 mag corresponding to a range in total mass of the star clusters from ∼25 to 4×10 4 M . The integrated colours have a relatively narrow range, e.g., I(B − V )0 varies from −0.4 to 1.2 mag.
Arxiv preprint arXiv: …, 2009
Using deep two-band imaging from the Hubble Space Telescope, we measure the color-magnitude relations (CMR) of E/S0 galaxies in a set of 9 optically-selected clusters principally from the Red-Sequence Cluster Survey (RCS) at 0.9 < z < 1.23. We find that the mean scatter in the CMR in the observed frame of this set of clusters is 0.049±0.008, as compared to 0.031±0.007 in a similarly imaged and identically analyzed X-ray sample at similar redshifts. Single-burst stellar population models of the CMR scatter suggest that the E/S0 population in these RCS clusters truncated their star-formation at z l ≃ 1.6, some 0.9 Gyrs later than their X-ray E/S0 counterparts which were truncated at z l ≃ 2.1. The notion that this is a manifestation of the differing evolutionary states of the two populations of cluster galaxies is supported by comparison of the fraction of bulge-dominated galaxies found in the two samples which shows that optically-selected clusters contain a smaller fraction of E/S0 galaxies at the their cores.
Predicted HST FOC and broad band colours for young and intermediate simple stellar populations
Astronomy and Astrophysics Supplement Series, 1999
This paper presents theoretical HST and broad band colours from population synthesis models based on an homogeneous set of stellar evolutionary tracks as computed under canonical (no overshooting) assumptions, covering the range of cluster ages from t = 8 Myr to t = 5 Gyr for three different metallicities (Z = 0.02, 0.006, and 0.001). Statistical fluctuations in the cluster population have been investigated, assessing the predicted fluctuations of the various colours as a function of the cluster integrated absolute magnitude. We show that the red leak in HST UV filters deeply affects the predicted fluxes and colours. However, we find that for F152M−F307M≤ 0.5 and for F170M−F278M≤ 0.5 (which means ages lower than 1 Gyr) the HST UV colours can still be used to infer reliable indications on the age of distant clusters. Moreover, one finds that the age calibration of these colours is scarcely affected by the amount of original helium or by the assumed IMF. On this basis, we present a calibration of the HST UV two-colours (F152M−F307M vs F170M−F278M) in terms of cluster ages for the three above quoted metallicities. We suggest the combined use of HST UV colours and IR colours (V−K in particular) to disentangle the metallicity-age effect in integrated colours of young stellar populations (t ≤ 1Gyr).
THE PANCHROMATIC HUBBLE ANDROMEDA TREASURY. V. AGES AND MASSES OF THE YEAR 1 STELLAR CLUSTERS
The Astrophysical Journal, 2014
We present ages and masses for 601 star clusters in M31 from the analysis of the six filter integrated light measurements from near ultraviolet to near infrared wavelengths, made as part of the Panchromatic Hubble Andromeda Treasury (PHAT). We derive the ages and masses using a probabilistic technique, which accounts for the effects of stochastic sampling of the stellar initial mass function. Tests on synthetic data show that this method, in conjunction with the exquisite sensitivity of the PHAT observations and their broad wavelength baseline, provides robust age and mass recovery for clusters ranging from ∼ 10 2 − 2 × 10 6 M ⊙ . We find that the cluster age distribution is consistent with being uniform over the past 100 Myr, which suggests a weak effect of cluster disruption within M31. The age distribution of older (> 100 Myr) clusters fall towards old ages, consistent with a power-law decline of index −1, likely from a combination of fading and disruption of the clusters. We find that the mass distribution of the whole sample can be well-described by a single power-law with a spectral index of −1.9 ± 0.1 over the range of 10 3 − 3 × 10 5 M ⊙ . However, if we subdivide the sample by galactocentric radius, we find that the age distributions remain unchanged. However, the mass spectral index varies significantly, showing best fit values between −2.2 and −1.8, with the shallower slope in the highest star formation intensity regions. We explore the robustness of our study to potential systematics and conclude that the cluster mass function may vary with respect to environment.
2002
We present a photometric investigation of the variation in galaxy colour with environment in 11 X-ray-luminous clusters at 0:07 # z # 0:16 taken from the Las Campanas/AAT Rich Cluster Survey. We study the properties of the galaxy populations in individual clusters, and take advantage of the homogeneity of the sample to combine the clusters together to investigate weaker trends in the composite sample. We find that modal colours of galaxies lying on the colour-magnitude relation in the clusters become bluer by dðB 2 RÞ=dr p ¼ 20:022^0:004 from the cluster core out to a projected radius of r p ¼ 6 Mpc, further out in radius than any previous study. We also examine the variation in modal galaxy colour with local galaxy density, S, for galaxies lying close to the colour-magnitude relation, and find that the median colour shifts bluewards by dðB 2 RÞ=d log 10 ðSÞ ¼ 20:076^0:009 with decreasing local density across three orders of magnitude. We show that the position of the red envelope of galaxies in the colour-magnitude relation does not vary as a function of projected radius or density within the clusters, suggesting that the change in the modal colour results from an increasing fraction of bluer galaxies within the colour-magnitude relation, rather than a change in the colours of the whole population. We show that this shift in the colour-magnitude relations with projected radius and local density is greater than that expected from the changing morphological mix based on the local morphology-density relation. We therefore conclude that we are seeing a real change in the properties of galaxies on the colour-magnitude relation in the outskirts of clusters. The simplest interpretation of this result (and similar constraints in local clusters) is that an increasing fraction of galaxies in the lower density regions at large radii within clusters exhibit signatures of star formation in the recent past, signatures which are not seen in the evolved galaxies in the highest density regions.