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We aim to identify and characterize binary systems containing red supergiant (RSG) stars in the Small Magellanic Cloud (SMC) using a newly available ultraviolet (UV) point source catalogue obtained using the Ultraviolet Imaging Telescope (UVIT) on board AstroSat. We select a sample of 560 SMC RSGs based on photometric and spectroscopic observations at optical wavelengths and cross-match this with the far-UV point source catalogue using the UVIT F172M filter, finding 88 matches down to m F 172 M  = 20.3 ABmag, which we interpret as hot companions to the RSGs. Stellar parameters (luminosities, effective temperatures, and masses) for both components in all 88 binary systems are determined and we find mass distributions in the ranges 6.1 < M / M ⊙ < 22.3 for RSGs and 3.7 < M / M ⊙ < 15.6 for their companions. The most massive RSG binary system in the SMC has a combined mass of 32 ± 4 M⊙, with a mass ratio (q) of 0.92. By simulating observing biases, we find an intrinsic multiplicity fraction of |$18.8\, \pm \, 1.5\, {{\ \rm per\ cent}}$| for mass ratios in the range 0.3 < q < 1.0 and orbital periods approximately in the range |$3 \lt \log P [\rm days] \lt 8$|⁠. By comparing our results with those of a similar mass on the main sequence, we determine the fraction of single stars to be ∼20  per cent and argue that the orbital period distribution declines rapidly beyond log  P ∼ 3.5. We study the mass-ratio distribution of RSG binary systems and find that a uniform distribution best describes the data below 14 M⊙. Above 15 M⊙, we find a lack of high mass ratio systems. [ABSTRACT FROM AUTHOR]

The analysis of star cluster ages in tandem with the morphology of their H  ii regions can provide insight into the processes that clear a cluster's natal gas, as well as the accuracy of cluster ages and dust reddening derived from Spectral Energy Distribution (SED) fitting. We classify 3757 star clusters in 16 nearby galaxies according to their H α morphology (concentrated, partially exposed, no emission), using Hubble Space Telescope (HST) imaging from the Legacy ExtraGalactic Ultraviolet Survey (LEGUS). We find: (1) The mean SED ages of clusters with concentrated (1–2 Myr) and partially exposed H  ii region morphologies (2–3 Myr) indicate a relatively early onset of gas clearing and a short (1–2 Myr) clearing time-scale. (2) The reddening of clusters can be overestimated due to the presence of red supergiants, which is a result of stochastic sampling of the IMF in low mass clusters. (3) The age-reddening degeneracy impacts the results of the SED fitting – out of 1408 clusters with M * ≥ 5000 M⊙, we find that at least 46 (3 per cent) have SED ages which appear significantly underestimated or overestimated based on H α and their environment, while the total percentage of poor age estimates is expected to be several times larger. (4) Lastly, we examine the dependence of the morphological classifications on spatial resolution. At HST resolution, our conclusions are robust to the distance range spanned by the sample (3–10 Mpc). However, analysis of ground-based H α images shows that compact and partially exposed morphologies frequently cannot be distinguished from each other. [ABSTRACT FROM AUTHOR]

A key task of observational extragalactic astronomy is to determine where – within galaxies of diverse masses and morphologies – stellar mass growth occurs, how it depends on galaxy properties and what processes regulate star formation. Using spectroscopic indices derived from the stellar continuum at ∼4000 Å, we determine the spatially resolved star formation histories of 980 000 spaxels in 2404 galaxies in the SDSS-IV MaNGA IFU survey. We examine the spatial distribution of star forming, quiescent, green valley, starburst, and post-starburst spaxels as a function of stellar mass and morphology to see where and in what types of galaxy star formation is occurring. The spatial distribution of star formation is dependent primarily on stellar mass, with a noticeable change in the distribution at M * > 1010 $$\, \rm {M_{_\odot}}$$. Galaxies above this mass have an increasing fraction of regions that are forming stars with increasing radius, whereas lower mass galaxies have a constant fraction of star-forming regions with radius. Our findings support a picture of inside–out growth and quenching at high masses. We find that morphology (measured via concentration) correlates with the fraction of star-forming spaxels, but not with their radial distribution. We find (post-)starburst regions are more common outside of the galaxy centre, are preferentially found in asymmetric galaxies, and have lower gas-phase metallicity than other regions, consistent with interactions triggering starbursts and driving low-metallicity gas into regions at <1.5 R e. [ABSTRACT FROM AUTHOR]

Recently acquired WFC3 UV (F275W and F336W) imaging mosaics under the Legacy Extragalactic UV Survey (LEGUS), combined with archival ACS data of M51, are used to study the young star cluster (YSC) population of this interacting system. Our newly extracted source catalogue contains 2834 cluster candidates, morphologically classified to be compact and uniform in colour, for which ages, masses and extinction are derived. In this first work we study the main properties of the YSC population of the whole galaxy, considering a mass-limited sample. Both luminosity and mass functions follow a power-law shape with slope -2, but at high luminosities and masses a dearth of sources is observed. The analysis of the mass function suggests that it is best fitted by a Schechter function with slope -2 and a truncation mass at 1.00 ± 0.12 × 105 M☉. Through Monte Carlo simulations, we confirm this result and link the shape of the luminosity function to the presence of a truncation in the mass function. A mass limited age function analysis, between 10 and 200 Myr, suggests that the cluster population is undergoing only moderate disruption. We observe little variation in the shape of the mass function at masses above 1 × 104 M☉ over this age range. The fraction of star formation happening in the form of bound clusters in M51 is ~20 per cent in the age range 10-100 Myr and little variation is observed over the whole range from 1 to 200 Myr. [ABSTRACT FROM AUTHOR]

We examine the HI-based star formation efficiency (SFEHI), the ratio of star formation rate to the atomic hydrogen (HI) mass, in the context of a constant stability star-forming disc model. Our observations of HI-selected galaxies show SFEHI to be fairly constant (log SFEHI = -9.65 yr-1 with a dispersion of 0.3 dex) across ∼5 orders of magnitude in stellar masses. We present a model to account for this result, whose main principle is that the gas within galaxies forms a uniform stability disc and that stars form within the molecular gas in this disc. We test two versions of the model differing in the prescription that determines the molecular gas fraction, based on either the hydrostatic pressure or the stellar surface density of the disc. For high-mass galaxies such as the Milky Way, we find that either prescription predicts SFEHI similar to the observations. However, the hydrostatic pressure prescription is a more accurate SFEHI predictor for low-mass galaxies. Our model is the first model that links the uniform SFEHI observed in galaxies at low redshifts to star-forming discs with constant marginal stability. While the rotational amplitude Vmax is the primary driver of disc structure in our model, we find that the specific angular momentum of the galaxy may play a role in explaining a weak correlation between SFEHI and effective surface brightness of the disc. [ABSTRACT FROM AUTHOR]