Star Cluster Formation and Destruction in the Merging Galaxy NGC 3256

Whitmore

et al. 2017

ApJ

Self Cite

153

We estimate the fraction of stars that form in compact clusters (bound and unbound), Γ F , in a diverse sample of eight star-forming galaxies, including two irregulars, two dwarf starbursts, two spirals, and two mergers. The average value for our sample is Γ F ≈ 24 ± 9%. We also calculate the fraction of stars in clusters that survive to ages between τ 1 and τ 2 , denoted by Γ S (τ 1 , τ 2 ), and find Γ S (10, 100) = 4.6 ± 2.5% and Γ S (100, 400) = 2.4 ± 1.1%, significantly lower than Γ F for the same galaxies. We do not find any systematic trends in Γ F or Γ S with the star formation rate (SFR), the SFR per unit area (Σ SF R ), or the surface density of molecular gas (Σ H 2 ) within the host galaxy. Our results are consistent with those found previously from the CMF/SFR statistic (where CMF is the cluster mass function), and with the quasi-universal model in which clusters in different galaxies form and disrupt in similar ways. Our results, however, contradict many previous claims that the fraction of stars in bound clusters increases strongly with Σ SF R and Σ H 2 . We find that the previously reported trends are largely driven by comparisons that mixed Γ F ≈ Γ S (0, 10) and Γ S (10, 100), where Γ S (0, 10) was systematically used for galaxies with higher Σ SF R and Σ H 2 , and Γ S (10, 100) for galaxies with lower Σ SF R and Σ H 2 .We showed recently that the mass functions of young clusters (with ages τ < 10 7 yr), when divided by the star formation rate (SFR), are also similar among different galaxies (Chandar et al. 2015, hereafter CFW15;Mulia et al. 2016). In our sample of 8 galaxies, the amplitude of the cluster mass function (CMF) and the SFR vary by factors ∼ 10 3 , while their ratio (CMF/SFR) varies by less than a factor of two. Moreover, we find no significant correlations between the CMF/SFR statistic and the other properties of the galaxies. These results mean that the rates of star and cluster formation are essentially proportional to each other-another sign of quasi-universality.The CMF/SFR statistic is closely related to another important quantity Γ, the fraction of stars that form in compact clusters. Indeed, CMF/SFR and Γ are proportional to each other (as we show in Section 2). Γ has figured prominently in several recent studies of cluster populations (e.g., Bastian 2008;Goddard et al. 2010;Kruijssen 2012, Adamo et al. 2015Johnson et al. 2016). It is usually defined as the fraction of stars that form in gravitationally bound clusters, i.e., those with negative total energy (kinetic plus potential). However, since in practice the binding energies of clusters are never measured or even estimated, Γ, despite its putative definition, must be regarded as the fraction of stars that form in all compact clusters, both bound and unbound. The most striking claim about Γ from recent studies is that it increases systematically with Σ SF R and Σ H 2 , the mean surface densities of SFR and molecular gas in galaxies. Thus, there is a stark discrepancy between our findings for CMF/SFR, which shows no depende...

Section: Introductionsupporting

confidence: 71%

mentioning

confidence: 99%

mentioning

confidence: 99%

See 1 more Smart Citation

The Fraction of Stars That Form in Clusters in Different Galaxies

Whitmore

et al. 2017

ApJ

Self Cite

153

“…While it is possible that the cluster mass functions for the most distant galaxies in our sample may suffer from biases related to crowding, previous works have found the shapes of these distributions to be fairly stable out to the distances of ≈ 40 Mpc. Randriamanakoto et al (2013) and Mulia et al (2016) tested the impact of 'distance bias' by degrading images of galaxies to simulate distances similar to the two most distant galaxies used here, and found the resulting power-law index of the mass function does not differ by more than ≈ 0.2.…”

Section: Previous Catalogs and Mass Functionsmentioning

confidence: 99%

Mass Functions of Giant Molecular Clouds and Young Star Clusters in Six Nearby Galaxies

Mok

2020

ApJ

We compare the mass functions of young star clusters (ages ≤ 10 Myr) and giant molecular clouds (GMCs) in six galaxies that cover a large range in mass, metallicity, and star formation rate (LMC, M83, M51, NGC 3627, the Antennae, and NGC 3256). We perform maximum-likelihood fits of the Schechter function, ψ(M ) = dN/dM ∝ M β exp(−M/M * ), to both populations. We find that most of the GMC and cluster mass functions in our sample are consistent with a pure power-law distribution (M * → ∞). M51 is the only galaxy that shows some evidence for an upper cutoff (M * ) in both populations. Therefore, physical upper mass cutoffs in populations of both GMCs and clusters may be the exception rather than the rule. When we perform power-law fits, we find a range of indices β PL = −2.3 ± 0.3 for our GMC sample and β PL = −2.0 ± 0.3 for the cluster sample. This result, that β Clusters ≈ β GMC ≈ −2, is consistent with theoretical predictions for cluster formation and suggests that the star-formation efficiency is largely independent of mass in the GMCs.

“…We re-examine the mass functions of the cluster population in eight galaxies from our previous studies (Chandar et al 2015(Chandar et al , 2017. The selection and photometry of clusters is based on U BV IHα images taken with the Hubble Space Telescope (HST ) for NGC 4214 (Chandar et al 2017), NGC 4449 (Rangelov et al 2011), M83 (Whitmore et al in prep), M51 , the Antennae , and NGC 3256 (Mulia et al 2016), and U BV R images taken with the Michigan Curtis Schmidt telescope for the LMC and SMC (Hunter et al 2003). Clusters were selected to be compact, but no attempt was made to distinguish bound from unbound clusters based on their appearance.…”

Section: Cluster Mass Functionsmentioning

confidence: 99%

Constraints on Upper Cutoffs in the Mass Functions of Young Star Clusters

Mok

2019

ApJ

We test claims that the power-law mass functions of young star clusters (ages < ∼ few× 10 8 yr) have physical upper cutoffs at M * ∼ 10 5 M . Specifically, we perform maximumlikelihood fits of the Schechter function, ψ(M ) = dN/dM ∝ M β exp(−M/M * ), to the observed cluster masses in eight well-studied galaxies (LMC, SMC, NGC 4214, NGC 4449, M83, M51, Antennae, and NGC 3256). In most cases, we find that a wide range of cutoff mass is permitted (10 5 M M * < ∞). We find a weak detection at M * ∼ 10 5 M in one case (M51) and strong evidence against this value in two cases. However, when we include realistic errors in cluster masses in our analysis, the constraints on M * become weaker and there are no significant detections (even for M51). Our data are generally consistent with much larger cutoffs, at M * ∼ few×10 6 M . This is the predicted cutoff from dynamical models in which old globular clusters and young clusters observed today formed by similar physical processes with similar initial mass functions.