Star cluster catalogues for the LEGUS dwarf galaxies

Abstract

We present the star cluster catalogues for 17 dwarf and irregular galaxies in the HST Treasury Program 'Legacy ExtraGalactic UV Survey' (LEGUS). Cluster identification and photometry in this sub-sample are similar to that of the entire LEGUS sample, but special methods were developed to provide robust catalogues with accurate fluxes due to low cluster statistics. The colours and ages are largely consistent for two widely used aperture corrections, but a significant fraction of the clusters are more compact than the average training cluster. However, the ensemble luminosity, mass, and age distributions are consistent suggesting that the systematics between the two methods are less than the random errors. When compared with the clusters from previous dwarf galaxy samples, we find that the LEGUS catalogues are more complete and provide more accurate total fluxes. Combining all clusters into a composite dwarf galaxy, we find that the luminosity and mass functions can be described by a power law with the canonical index of −2 independent of age and global SFR binning. The age distribution declines as a power law, with an index of ≈− 0.80 ± 0.15, independent of cluster mass and global SFR binning. This decline of clusters is dominated by cluster disruption since the combined star formation histories and integrated-light SFRs are both approximately constant over the last few hundred Myr. Finally, we find little evidence for an upper-mass cut-off (<2σ ) in the composite cluster mass function, and can rule out a truncation mass below ≈10 4.5 M but cannot rule out the existence of a truncation at higher masses.

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

“…On the other hand, Mok et al (2019) applied a uniform maximum-likelihood fitting procedure to a sample of young clusters in eight nearby galaxies, including M83, M51, and the Antennae, and found that the majority of the galaxies do not show evidence for a physical cutoff, but are consistent with the expectations of a statistical cutoff. Whitmore et al (2020) reached a similar conclusion about physical cutoffs for clusters in NGC 4449, as did Cook et al (2019) for a composite of 17 dwarf galaxies studied as part of the LEGUS project. For GMC populations, evidence of a cutoff M * at the > 3σ level has been claimed for some galaxies (e.g., M33 (Rosolowsky et al 2007), NGC 4256 (Utomo et al 2015)), with weak (2 − 3σ) evidence (e.g., M51 (Colombo et al 2014), NGC 300 (Faesi et al 2018), NGC 6946 (Wu et al 2017)), or no evidence found in others (e.g.…”

“…First, we determine an average aperture correction of 0.834 mag from a number of relatively isolated clusters. Second, we fitted a function to the measured aperture correction and concentration index (C, the difference between aperture magnitudes in 0.5 and 3 pixels; see for example Cook et al (2019)) for synthetic clusters. We find that the mass functions are quite similar for both methods of determining aperture corrections.…”

LEGUS and Hα-LEGUS Observations of Star Clusters in NGC 4449: Improved Ages and the Fraction of Light in Clusters as a Function of Age

“…An additional 121 cluster candidates were identified and added to the sample, resulting in a total of 594 category 1+2+3 cluster candidates in the final catalog. This will be called the H α -LEGUS catalog, and is somewhat different from the LEGUS catalog (used in Cook et al 2019 for example), as described below. Figure 3 shows three examples of objects that were added (the white circles) in Region 23, along with several original category 1 and 2 objects for comparison.…”

“…Figure 3 shows three examples of objects that were added (the white circles) in Region 23, along with several original category 1 and 2 objects for comparison. The sample, including the added clusters, was also vetted by Dave Cook as part of the Cook et al (2019) study. He retained 94 % of the added cluster candidates.…”

Star clusters in the dwarf irregular galaxy Leo A

“…The finding of such low-mass (∼300 M ) and young-to intermediate-age (∼20-100 Myr) star clusters in the low stellar mass (3.3 · 10 6 M ) and the extremely low metallicity (12 + log(O/H) = 7.4) dwarf irregular galaxy Leo A (in which CO emission has not been detected so far) could help constrain star formation scenarios in the early Universe. The problems of lowmass star clusters that reside in similar environments have been addressed only recently: an extensive study of star clusters in the LEGUS dwarf galaxies has been published by Cook et al (2019). The Leo A galaxy is at the lowest mass and at the lowest metallicity limits of the LEGUS dwarf galaxies.…”