Imaging of star clusters in unperturbed spiral galaxies with the Advanced Camera for Surveys

Mora, Marcelo D.; Larsen, Soeren S.; Kissler-Patig, M.

doi:10.1051/0004-6361:20065852

Cited by 24 publications

(46 citation statements)

References 39 publications

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“…Taking T L (U) as a proxy for the cluster formation efficiency, these data thus suggested an increase in the cluster formation efficiency with Σ SFR . It is worth noting here that the Σ SFR values were derived by normalizing Mora et al (2009) and references therein; (a) Schlegel et al (1998); (1) Bresolin et al (2009); (2) Chandar et al (2010b); (3) Walsh & Roy (1997); Larsen et al (2007); (4) Mora et al (2007) ; (5) and Zaritsky et al (1994) at r = 3 Kpc. Chandar et al (2010b) for the galaxies NGC 5236 and NGC 7793, where they found similar values to Larsen & Richtler (2000).…”

Section: Dataset Overviewmentioning

confidence: 99%

“…Fall 2004). Mora et al (2007Mora et al ( , 2009) studied the cluster system for the same set of galaxies used in this work, based on the same HST images. They present detailed estimates of the sizes, ages, and masses for the clusters detected.…”

Section: Dataset Overviewmentioning

confidence: 99%

“…no size cut used to select the clusters; (2.) FWHM ishape ≥ 0.2, same size cut used by Mora et al (2007Mora et al ( , 2009; and (3.) FWHM ishape ≥ 0.7, the size cut used in this paper.…”

Section: Star Clustersmentioning

confidence: 99%

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The star cluster – field star connection in nearby spiral galaxies

Silva-Villa

Larsen

2011

A&A

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109

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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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Section: Dataset Overviewmentioning

confidence: 99%

Section: Dataset Overviewmentioning

confidence: 99%

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The star cluster – field star connection in nearby spiral galaxies

Silva-Villa

Larsen

2011

A&A

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109

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“…The aperture corrections were performed in the same way as in Mora et al (2007): we created 6 artificial source types with different FWHMs: 0.2, 0.5, 0.9, 1.2, 1.5, and 1.8 pixels. On each of them, we performed the aperture correction from 6 pixels up to a nominal 1.…”

Section: Photometrymentioning

confidence: 99%

“…This was performed using the ISHAPE task in BAOLAB (Larsen 1999): each source was modelled using a King (1962) profile with a concentration parameter c = 30, convolved with the instrument PSF (as derived from images of the globular cluster 47 Tuc, see Mora et al (2007) for details). For each object the output of ISHAPE included the physical FWHM, chi-square of the fitting, flux, signal to noise, and a residual image.…”

Section: Size Measurementsmentioning

confidence: 99%

Imaging of star clusters in unperturbed spiral galaxies with the Advanced Camera for Surveys

Mora¹,

Larsen²,

Kissler-Patig³

et al. 2009

A&A

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Aims. Our goal is to investigate the formation of star clusters in relatively unperturbed environments. To do this, we studied the five nearby spiral galaxies: NGC 45, NGC 1313, NGC 4395, NGC 5236, and NGC 7793. Methods. We obtained images of the galaxies and their star cluster systems in UBVI using the Advanced Camera for Surveys (ACS) and the Wide Field Planetary Camera 2 (WFPC2) on board the Hubble Space Telescope. From a comparison of the broad-band colours with simple stellar population (SSP) models, we derived individual properties for each galaxy for the clusters such as masses, ages, and sizes, as well as global star cluster system properties such as the age distribution, luminosity function, and disruption time for clusters. Results. We identified about 600 star cluster candidates in the five galaxies, typically spanning ages from 3.9 Myr up to 1 Gyr and masses from 10 2 M up to 10 5 M . We used the cluster age distribution to reconstruct the recent star formation history of each galaxy and observed significant variations from galaxy to galaxy. We went on to derive the luminosity function of the young star clusters and found slopes around α ∼ −2 (similar to the ones found in previous studies) and the brightest star cluster magnitudes consistent with a random sampling of the luminosity function without involving an upper luminosity cut off. Finally, the sample includes only a handful of old globular clusters in each galaxy from which we derive low globular cluster specific frequencies.

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Star cluster formation and star formation: the role of environment and star-formation efficiencies

Fritze

2009

Astrophys Space Sci

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Analyzing global starburst properties in various kinds of starburst and post-starburst galaxies and relating them to the properties of the star cluster populations they form, I explore the conditions for the formation of massive, compact, long-lived star clusters. The aim is to find out whether the relative amount of star formation that goes into star cluster formation as opposed to field star formation, and into the formation of massive long-lived clusters in particular, is universal or scales with star formation rate, burst strength, star formation efficiency, galaxy or gas mass, and whether or not there are special conditions or some threshold for the formation of star clusters that merit to be called globular clusters a few gigayears later.Keywords star formation; star cluster formation; star formation efficiencies; environment MotivationStar Cluster (SC) formation is a major or even dominant mode of all star formation (SF) and occurs in very different environments. This immediately raises the question whether young star clusters (YSCs) forming in different environments are similar or different.SCs are not only interesting in their own right, but bear considerable power as tracers of SF in their parent galaxies. YSCs trace the spatial distribution of SF and its recent history within a galaxy, old globular clusters (GCs) trace violent SF phases in their parent galaxyall the way back to the very onset of SF, i.e. over a Hubble time. But SCs also fade and dissolve. In actively star forming galaxies, the youngest SCs may still be embedded in their natal dust clouds while part of Uta FritzeUniversity of Hertfordshire the older SCs are already gone, dissolved and/or faded below detection. It is therefore important to take these processes into account when comparing SC populations in different galaxies. They depend on the initial properties of individual SCs, their masses, radii, abundances, stellar IMF, and of the SC population, i.e. the luminosity function, the mass function, distribution of radii, ages, etc.That SC formation is an important mode of SF in starbursts was shown on the example of the Tadpole and Mice interacting galaxies. A pixel-by-pixel analysis of ACS data (BV RI) with GALEV evolutionary synthesis models showed that ∼ 70 % of the blue light is emitted by YSCs as opposed to only ∼ 30 % coming from field stars. We estimated that more than 35 % of all SF went into the formation of YSCs, not only in the main bodies of these two galaxies, but all along their extended tidal tails . Clearly, this analysis needs to be extended to different types of galaxies, starburst/non-starburst, dwarf/normal, gasrich/gas-poor, interacting/non-interacting, in various stages of the interaction, etc. to explore the systematics. There are indications from Meurer et al. (1995)'s work that the contribution from YSCs to the UV light of a galaxy increases with increasing UV surface brightness, which itself is a measure of SF intensity. A burning question that we are currently exploring is whether the amount of SF th...

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Imaging of star clusters in unperturbed spiral galaxies with the Advanced Camera for Surveys

Cited by 24 publications

References 39 publications

The star cluster – field star connection in nearby spiral galaxies

The star cluster – field star connection in nearby spiral galaxies

Imaging of star clusters in unperturbed spiral galaxies with the Advanced Camera for Surveys

Star cluster formation and star formation: the role of environment and star-formation efficiencies

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