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...

We present the classification of 197 point sources observed with the Infrared Spectrograph in the SAGE-Spec Legacy programme on the Spitzer Space Telescope. We introduce a decisiontree method of object classification based on infrared spectral features, continuum and spectral energy distribution shape, bolometric luminosity, cluster membership and variability information, which is used to classify the SAGE-Spec sample of point sources. The decision tree has a broad application to mid-infrared spectroscopic surveys, where supporting photometry and variability information are available. We use these classifications to make deductions about the stellar populations of the Large Magellanic Cloud and the success of photometric classification methods. We find 90 asymptotic giant branch (AGB) stars, 29 young stellar objects, 23 post-AGB objects, 19 red supergiants, eight stellar photospheres, seven background galaxies, seven planetary nebulae, two H II regions and 12 other objects, seven of which remain unclassified.

We use the Advanced Camera for Surveys on the Hubble Space Telescope to study the rich population of young massive star clusters in the main body of NGC 3256, a merging pair of galaxies with a high star formation rate (SFR) and SFR per unit area (Σ SFR ). These clusters have luminosity and mass functions that follow power laws, dN/dL ∝ L α with α = −2.23 ± 0.07, and dN/dM ∝ M β with β = −1.86 ± 0.34 for τ < 10 Myr clusters, similar to those found in more quiescent galaxies. The age distribution can be described by dN/dτ ∝ τ γ , with γ ≈ −0.67 ± 0.08 for clusters younger than about a few hundred million years, with no obvious dependence on cluster mass. This is consistent with a picture where ∼ 80% of the clusters are disrupted each decade in time. We investigate the claim that galaxies with high Σ SFR form clusters more efficiently than quiescent systems by determining the fraction of stars in bound clusters (Γ) and the CMF/SFR statistic (CMF is the cluster mass function) for NGC 3256 and comparing the results with those for other galaxies. We find that the CMF/SFR statistic for NGC 3256 agrees well with that found for galaxies with Σ SFR and SFRs that are lower by 1 − 3 orders of magnitude, but that estimates for Γ are only robust when the same sets of assumptions are applied. Currently, Γ values available in the literature have used different sets of assumptions, making it more difficult to compare the results between galaxies.

The SAGE-Spec Spitzer Legacy program is a spectroscopic follow-up to the SAGE-LMC photometric survey of the Large Magellanic Cloud carried out with the Spitzer Space Telescope. We present an overview of SAGE-Spec and some of its first results. The SAGE-Spec program aims to study the life cycle of gas and dust in the Large Magellanic Cloud, and to provide information essential to the classification of the point sources observed in the earlier SAGE-LMC photometric survey. We acquired 224.6 hours of observations using the InfraRed Spectrograph and the SED mode of the Multiband Imaging Photometer for Spitzer. The SAGE-Spec data, along with archival Spitzer spectroscopy of objects in the Large Magellanic Cloud, are reduced and delivered to the community. We discuss the observing strategy, the specific data reduction pipelines applied and the dissemination of data products to the scientific community. Initial science results include the first detection of an extragalactic "21 µm" feature towards an evolved star and elucidation of the nature of disks around RV Tauri stars in the Large Magellanic Cloud. Towards some young stars, ice features are observed in absorption. We also serendipitously observed a background quasar, at a redshift of z ≈ 0.14, which appears to be host-less. 31 The SAGE-LMC point source catalog can be accessed on http://irsa.ipac.caltech.edu/applications/Gator/ sl/ll KDM 4554 127 22429952 05h30m46.74s −67d16m56.92s MC J053046.81−671657.2 sl NGC 2004 Robb B45 Cluster 128 22430208 05h30m48.40s −67d16m45.88s MC J053048.42−671645.8 sl/ll NGC 2004 Wes 18−13 Cluster 129 22430464 05h30m52.25s −67d17m34.22s MC J053052.28−671734.4 sl/ll NGC 2004 Wes 6−14 Cluster 130 22430720 05h31m28.43s −70d10m27.65s MC J053128.44−701027.1 sl/ll 131 22430976 05h31m51.01s −69d11m46.56s MC J053150.98−691146.4 sl/ll MACHO 82.8405.15 132 22431232 05h31m58.96s −72d44m36.35s MC J053158.92−724436.0 sl KDM 4665 133 22431488 05h32m06.64s −70d10m25.34s MC J053206.70−701024.8 sl Cluster 134 22431744 05h32m18.66s −67d31m46.16s MC J053218.64−673145.9 sl Cluster 135 22432000 05h32m19.31s −67d31m20.34s MC J053219.33−673120.5 sl/ll NGC 2011 SAGE IRS 1 Cluster 136 22432256 05h32m26.52s −73d10m06.99s MC J053226.51−731006.8 sl KDM 4718 137 24318208 05h32m39.71s −69d30m49.25s MC J053239.68−693049.4 sl/ll RP 774 138 22432768 05h32m53.35s −66d07m27.17s MC J053253.36−660727.8 sl/ll 139 22433024 05h32m54.98s −67d36m47.10s MC J053254.99−673647.2 sl KDM 4774 140 24314112 05h33m06.86s −70d30m34.22s MC J053306.86−703033.9 sl/ll MSX LMC 736 141 22433280 05h33m18.61s −66d00m39.91s MC J053318.58−660040.2 sl/ll 142 22433536 05h33m43.18s −70d59m21.16s MC J053343.27−705921.1 sl Cluster 143 22433792 05h33m44.00s −70d59m01.14s MC J053343.98−705901.9 sl Cluster 144 22434048 05h33m46.97s −68d36m44.08s MC J053346.97−683644.2 sl/ll LHα 120−N 151 145 22434304 05h34m41.46s −69d26m30.74s MC J053441.40−692630.6 sl/ll 146 22434560 05h34m44.20s −67d37m50.82s MC J053444.17−673750.1 sl/ll SHP LMC 256 147 22434816 05h35m19.01s −67d02m19.50s MC J053518.91−670219.5 sl/ll HV ...

We study the stellar content in the tidal tails of three nearby merging galaxies, NGC 520, NGC 2623, and NGC 3256, using BVI imaging taken with the Advanced Camera for Surveys on board the Hubble Space Telescope. The tidal tails in all three systems contain compact and fairly massive young star clusters, embedded in a sea of diffuse, unresolved stellar light. We compare the measured colors and luminosities with predictions from population synthesis models to estimate cluster ages and find that clusters began forming in tidal tails during or shortly after the formation of the tails themselves. We find a lack of very young clusters (≤ 10 Myr old), implying that eventually star formation shuts off in the tails as the gas is used up or dispersed. There are a few clusters in each tail with estimated ages that are older than the modeled tails themselves, suggesting that these may have been stripped out from the original galaxy disks. The luminosity function of the tail clusters can be described by a single power-law, dN/dL ∝ L α , with −2.6 < α < −2.0. We find a stellar age gradient across some of the tidal tails, which we interpret as a superposition of 1) newly formed stars and clusters along the dense center of the tail and 2) a sea of broadly distributed, older stellar material ejected from the progenitor galaxies.