THE PANCHROMATIC HUBBLE ANDROMEDA TREASURY. V. AGES AND MASSES OF THE YEAR 1 STELLAR CLUSTERS (original) (raw)
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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.
The Astrophysical Journal, 2010
The newly installed Wide Field Camera 3 (WFC3) on the Hubble Space Telescope has been used to obtain multi-band images of the nearby spiral galaxy M83. These new observations are the deepest and highest resolution images ever taken of a grand-design spiral, particularly in the near ultraviolet, and allow us to better differentiate compact star clusters from individual stars and to measure the luminosities of even faint clusters in the U band. We find that the luminosity function for clusters outside of the very crowded starburst nucleus can be approximated by a power law, dN/dL ∝ L α , with α = −2.04 ± 0.08, down to M V ≈ −5.5. We test the sensitivity of the luminosity function to different selection techniques, filters, binning, and aperture correction determinations, and find that none of these contribute significantly to uncertainties in α. We estimate ages and masses for the clusters by comparing their measured UBVI,Hα colors with predictions from single stellar population models. The age distribution of the clusters can be approximated by a power-law, dN/dτ ∝ τ γ , with γ = −0.9 ± 0.2, for M > ∼ few × 10 3 M ⊙ and τ < ∼ 4 × 10 8 yr. This indicates that clusters are disrupted quickly, with ≈ 80-90% disrupted each decade in age over this time. The mass function of clusters over the same M-τ range is a power law, dN/dM ∝ M β , with β = −1.94 ± 0.16, and does not have bends or show curvature at either high or low masses. Therefore, we do not find evidence for a physical upper mass limit, M C , or for the earlier disruption of lower mass clusters when compared with higher mass clusters, i.e., mass-dependent disruption. We briefly discuss these implications for the formation and disruption of the clusters.
The Astrophysical Journal, 2016
We use the Panchromatic Hubble Andromeda Treasury (PHAT) survey dataset to perform spatially resolved measurements of star cluster formation efficiency (Γ), the fraction of stellar mass formed in long-lived star clusters. We use robust star formation history and cluster parameter constraints, obtained through color-magnitude diagram analysis of resolved stellar populations, to study Andromeda's cluster and field populations over the last ∼300 Myr. We measure Γ of 4-8% for young, 10-100 Myr old populations in M31. We find that cluster formation efficiency varies systematically across the M31 disk, consistent with variations in mid-plane pressure. These Γ measurements expand the range of well-studied galactic environments, providing precise constraints in an Hi-dominated, low intensity star formation environment. Spatially resolved results from M31 are broadly consistent with previous trends observed on galaxy-integrated scales, where Γ increases with increasing star formation rate surface density (Σ SFR). However, we can explain observed scatter in the relation and attain better agreement between observations and theoretical models if we account for environmental variations in gas depletion time (τ dep) when modeling Γ, accounting for the qualitative shift in star formation behavior when transitioning from a H 2-dominated to a Hi-dominated interstellar medium. We also demonstrate that Γ measurements in high Σ SFR starburst systems are well-explained by τ dep-dependent fiducial Γ models.
The Astronomical Journal, 2011
VDB0-B195D is a massive, blue star cluster in M31. It was observed as part of the Beijing-Arizona-Taiwan-Connecticut (BATC) Multicolor Sky Survey using 15 intermediate-band filters covering a wavelength range of 3000-10,000Å. Based on aperture photometry, we obtain its spectral-energy distribution (SED) as defined by the 15 BATC filters. We apply previously established relations between the BATC intermediate-band and the Johnson-Cousins U BV RI broad-band systems to convert our BATC photometry to the standard system. A detailed comparison shows that our newly derived V RI magnitudes are fully consistent with previous results, while our new B magnitude agrees to within 2σ. In addition, we determine the cluster's age and mass by comparing its SED (from 3000 to 20,000Å, comprising photometric data in the 15 BATC intermediate bands, optical broad-band BV RI, and 2MASS near-infrared JHK s data) with theoretical stellar population synthesis models, resulting in age and mass determinations of 60.0 ± 8.0 Myr and (1.1 − 1.6) × 10 5 M ⊙ , respectively. This age and mass confirms previous suggestions that VDB0-B195D is a young massive cluster in M31.
THE HIGH-MASS STELLAR INITIAL MASS FUNCTION IN M31 CLUSTERS
The Astrophysical Journal, 2015
We have undertaken the largest systematic study of the high-mass stellar initial mass function (IMF) to date using the optical color-magnitude diagrams (CMDs) of 85 resolved, young (4 Myr < t < 25 Myr), intermediate mass star clusters (10 3 -10 4 M ), observed as part of the Panchromatic Hubble Andromeda Treasury (PHAT) program. We fit each cluster's CMD to measure its mass function (MF) slope for stars 2 M . By modeling the ensemble of clusters, we find the distribution of MF slopes is best described by Γ = +1.45 +0.03 −0.06 with a very small intrinsic scatter. This model allows the MF slope to depend on cluster mass, size, and age, but the data imply no significant dependencies within this regime of cluster properties. The lack of an age dependence suggests that the MF slope has not significantly evolved over the first ∼ 25 Myr, and provides direct observational evidence that the measured MF represents the IMF. Taken together, this analysis -based on an unprecedented large sample of young clusters, homogeneously constructed CMDs, well-defined selection criteria, and consistent principled modeling -implies that the high-mass IMF slope in M31 clusters is universal. The IMF has a slope (Γ = +1.45 +0.03 −0.06 ) that is slightly steeper than the canonical Kroupa (+1.30) and Salpeter (+1.35) values, with no drastic outliers in this sample of nearly 100 clusters. Using our inference model on select Milky Way (MW) and LMC high-mass IMF studies from the literature, we find Γ MW ∼ +1.15 ± 0.1 and Γ LMC ∼ +1.3 ± 0.1, both with intrinsic scatter of ∼0.3-0.4 dex. Thus, while the high-mass IMF in the Local Group may be universal, systematics in literature IMF studies preclude any definitive conclusions; homogenous investigations of the high-mass IMF in the local universe are needed to overcome this limitation. Consequently, the present study represents the most robust measurement of the high-mass IMF slope to date. To facilitate practical use over the full stellar mass spectrum, we have grafted the M31 high-mass IMF slope onto widely used sub-solar mass Kroupa and Chabrier IMFs. The increased steepness in the M31 high-mass IMF slope implies that commonly used UV-and Hα-based star formation rates should be increased by a factor of ∼1.3-1.5 and the number of stars with masses > 8 M are ∼ 25% fewer than expected for a Salpeter/Kroupa IMF.
Hubble Space Telescope Observations of the Oldest Star Clusters in the LMC
1999
We present V, V-I color-magnitude diagrams (CMDs) for three old star clusters in the Large Magellanic Cloud (LMC): NGC 1466, NGC 2257 and Hodge 11. Our data extend about 3 magnitudes below the main-sequence turnoff, allowing us to determine accurate relative ages and the blue straggler frequencies. Based on a differential comparison of the CMDs, any age difference between the three LMC clusters is less than 1.5 Gyr. Comparing their CMDs to those of M 92 and M 3, the LMC clusters, unless their published metallicities are significantly in error, are the same age as the old Galactic globulars. The similar ages to Galactic globulars are shown to be consistent with hierarchial clustering models of galaxy formation. The blue straggler frequencies are also similar to those of Galactic globular clusters. We derive a true distance modulus to the LMC of (m-M)=18.46 +/- 0.09 (assuming (m-M)=14.61 for M 92) using these three LMC clusters.
An HST/WFPC2 survey of bright young clusters in M31
Astronomy and Astrophysics, 2009
Aims. We present the main results of an imaging survey of possible young massive clusters (YMC) in M31 performed with the Wide Field and Planetary Camera 2 (WFPC2) on the Hubble Space Telescope (HST), with the aim of estimating their age and their mass. We obtained shallow (to B∼ 25) photometry of individual stars in 19 clusters (of the 20 targets of the survey). We present the images and color magnitude diagrams (CMDs) of all of our targets. Methods. Point spread function fitting photometry of individual stars was obtained for all the WFPC2 images of the target clusters, and the completeness of the final samples was estimated using extensive sets of artificial stars experiments. The reddening, age, and metallicity of the clusters were estimated by comparing the observed CMDs and luminosity functions (LFs) with theoretical models. Stellar masses were estimated by comparison with theoretical models in the log(Age) vs. absolute integrated magnitude plane, using ages estimated from our CMDs and integrated J, H, K magnitudes from 2MASS-6X. Results. Nineteen of the twenty surveyed candidates were confirmed to be real star clusters, while one turned out to be a bright star. Three of the clusters were found not to be good YMC candidates from newly available integrated spectroscopy and were in fact found to be old from their CMD. Of the remaining sixteen clusters, fourteen have ages between 25 Myr and 280 Myr, two have older ages than 500 Myr (lower limits). By including ten other YMC with HST photometry from the literature, we assembled a sample of 25 clusters younger than 1 Gyr, with mass ranging from 0.6 × 10 4 M ⊙ to 6 × 10 4 M ⊙ , with an average of ∼ 3 × 10 4 M ⊙ . Our estimates of ages and masses well agree with recent independent studies based on integrated spectra. Conclusions. The clusters considered here are confirmed to have masses significantly higher than Galactic open clusters (OC) in the same age range. Our analysis indicates that YMCs are relatively common in all the largest star-forming galaxies of the Local Group, while the lack of known YMC older than 20 Myr in the Milky Way may stem from selection effects.
The Astrophysical Journal, 2009
We present integrated photometry and color-magnitude diagrams for 161 star clusters in M33, of which 115 were previously uncataloged, using the Advanced Camera For Surveys Wide Field Channel onboard the Hubble Space Telescope. The integrated V-band magnitudes of these clusters range from M V ∼-9 to as faint as M V ∼-4, extending the depth of the existing M33 cluster catalogs by ∼1 mag. Comparisons of theoretical isochrones to the color-magnitude diagrams using the Padova models yield ages for 148 of these star clusters. The ages range from Log (t)∼7.0 to Log (t)∼9.0. Our color-magnitude diagrams are not sensitive to clusters older than ∼1 Gyr. We find that the variation of the clusters' integrated colors and absolute magnitudes with age is consistent with the predictions of simple stellar population models. These same models suggest that the masses of the clusters in our sample range from 5 × 10 3 to 5 × 10 4 M ⊙ .
The Astrophysical Journal, 2013
In the past decade, the notion that globular clusters (GCs) are composed of coeval stars with homogeneous initial chemical compositions has been challenged by growing evidence that they host an intricate stellar population mix, likely indicative of a complex history of star formation and chemical enrichment. Several models have been proposed to explain the existence of multiple stellar populations in GCs, but no single model provides a fully satisfactory match to existing data. Correlations between chemistry and global parameters such as cluster mass or luminosity are fundamental clues to the physics of GC formation. In this Letter, we present an analysis of the mean abundances of Fe, Mg, C, N, and Ca for 72 old GCs from the Andromeda galaxy. We show for the first time that there is a correlation between the masses of GCs and the mean stellar abundances of nitrogen, spanning almost two decades in mass. This result sheds new light on the formation of GCs, providing important constraints on their internal chemical evolution and mass loss history.
Spectroscopic constraints on the form of the stellar cluster mass function
Astronomy & Astrophysics, 2012
This contribution addresses the question of whether the initial cluster mass function (ICMF) has a fundamental limit (or truncation) at high masses. The shape of the ICMF at high masses can be studied using the most massive young (<10 Myr) clusters, however this has proven difficult due to low-number statistics. In this contribution we use an alternative method based on the luminosities of the brightest clusters, combined with their ages. The advantages are that more clusters can be used and that the ICMF leaves a distinct pattern on the global relation between the cluster luminosity and median age within a population. If a truncation is present, a generic prediction (nearly independent of the cluster disruption law adopted) is that the median age of bright clusters should be younger than that of fainter clusters. In the case of an non-truncated ICMF, the median age should be independent of cluster luminosity. Here, we present optical spectroscopy of twelve young stellar clusters in the face-on spiral galaxy NGC 2997. The spectra are used to estimate the age of each cluster, and the brightness of the clusters is taken from the literature. The observations are compared with the model expectations of Larsen (2009) for various ICMF forms and both mass dependent and mass independent cluster disruption. While there exists some degeneracy between the truncation mass and the amount of mass independent disruption, the observations favour a truncated ICMF. For low or modest amounts of mass independent disruption, a truncation mass of 5 − 6 × 10 5 M ⊙ is estimated, consistent with previous determinations. Additionally, we investigate possible truncations in the ICMF in the spiral galaxy M83, the interacting Antennae galaxies, and the collection of spiral and dwarf galaxies present in Larsen (2009) based on photometric catalogues taken from the literature, and find that all catalogues are consistent with having a truncation in the cluster mass functions. However for the case of the Antennae, we find a truncation mass of a few ×10 6 M ⊙ , suggesting a dependence on the environment, as has been previously suggested.