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The sensitivity and angular resolution of photometric surveys executed by the Hubble Space Telescope (HST) enable studies of individual star clusters in galaxies out to a few tens of megaparsecs. The fitting of spectral energy distributions (SEDs) of star clusters is essential for measuring their physical properties and studying their evolution. We report on the use of the publicly available Code Investigating GALaxy Emission (CIGALE) SED fitting package to derive ages, stellar masses, and reddenings for star clusters identified in the Physics at High Angular resolution in Nearby GalaxieS–HST (PHANGS–HST) survey. Using samples of star clusters in the galaxy NGC 3351, we present results of benchmark analyses performed to validate the code and a comparison to SED fitting results from the Legacy Extragalactic Ultraviolet Survey. We consider procedures for the PHANGS–HST SED fitting pipeline, e.g. the choice of single stellar population models, the treatment of nebular emission and dust, and the use of fluxes versus magnitudes for the SED fitting. We report on the properties of clusters in NGC 3351 and find, on average, the clusters residing in the inner star-forming ring of NGC 3351 are young (

The redshift range z = 4–6 marks a transition phase between primordial and mature galaxy formation in which galaxies considerably increase their stellar mass, metallicity, and dust content. The study of galaxies in this redshift range is therefore important to understanding early galaxy formation and the fate of galaxies at later times. Here, we investigate the burstiness of the recent star formation history (SFH) of 221z ~ 4.5 main-sequence galaxies at log(M/M⊙) > 9.7 by comparing their ultra-violet (UV) continuum, Hα luminosity, and Hα equivalent-width (EW). The Hα properties are derived from the Spitzer [3.6 μm]−[4.5 μm] broadband color, thereby properly taking into account model and photometric uncertainties. We find a significant scatter between Hα- and UV-derived luminosities and star formation rates (SFRs). About half of the galaxies show a significant excess in Hα compared to expectations from a constant smooth SFH. We also find a tentative anticorrelation between Hα EW and stellar mass, ranging from 1000 Å at log(M/M⊙) < 10 to below 100 Å at log(M/M⊙) > 11. Consulting models suggests that most z ~ 4.5 galaxies had a burst of star formation within the last 50 Myr, increasing their SFRs by a factor of >5. The most massive galaxies on the other hand might decrease their SFRs and may be transitioning to a quiescent stage by z = 4. We identify differential dust attenuation (f) between stars and nebular regions as the main contributor to the uncertainty. With local galaxies selected by increasing Hα EW (reaching values similar to high-z galaxies), we predict that f approaches unity at z > 4, consistent with the extrapolation of measurements out to z = 2.

The redshift range z=4-6 marks a transition phase between primordial and mature galaxy formation in which galaxies considerably increase their stellar mass, metallicity, and dust content. The study of galaxies in this redshift range is therefore important to understand early galaxy formation and the fate of galaxies at later times. Here, we investigate the burstiness of the recent star-formation history (SFH) of 221 $z\sim4.5$ main-sequence galaxies at log(M) > 9.7 by comparing their ultra-violet (UV) continuum, H$��$ luminosity, and H$��$ equivalent-width (EW). The H$��$ properties are derived from the Spitzer [3.6$��$m]-[4.5$��$m] broad-band color, thereby properly taking into account model and photometric uncertainties. We find a significant scatter between H$��$ and UV-derived luminosities and star-formation rates (SFRs). About half of the galaxies show a significant excess in H$��$ compared to expectations from a constant smooth SFH. We also find a tentative anti-correlation between H$��$ EW and stellar mass, ranging from 1000$��$ at log(M) < 10 to below 100$��$ at log(M) > 11. Consulting models suggests that most $z\sim4.5$ galaxies had a burst of star-formation within the last 50 Myrs, increasing their SFRs by a factor of > 5. The most massive galaxies on the other hand might decrease their SFRs, and may be transitioning to a quiescent stage by z=4. We identify differential dust attenuation (f) between stars and nebular regions as the main contributor to the uncertainty. With local galaxies selected by increasing H$��$ EW (reaching values similar to high-z galaxies), we predict that f approaches unity at $z>4$ consistent with the extrapolation of measurements out to z=2., 31 pages (including appendix), 21 figures, accepted for publication in ApJ

Galaxy interactions/mergers have been shown to dominate the population of IR luminous galaxies (log(LIR)>11.6Lsun) in the local Universe (z 11.5 Lsun, >50% of the objects show evident features of strongly interacting/merger systems, where this percentage is similar to the studies of local IR-luminous galaxies. The fraction of interacting/merger systems also systematically increases with the deviation from the SFR-M relation, supporting the view that galaxies fall above the MS are more dominated by mergers than the MS galaxies. Meanwhile, we find that ~18% of massive IR-luminous MS galaxies are classified as interacting systems, where this population may not evolve through the evolutionary track predicted by a simple gas exhaustion model., 14 pages, 11 figures, ApJ accepted