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  • 1.
    Adamo, Angela
    et al.
    Stockholm University, Faculty of Science, Department of Astronomy. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
    Hollyhead, Katherine
    Stockholm University, Faculty of Science, Department of Astronomy. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
    Messa, M.
    Ryon, J. E.
    Bajaj, V.
    Runnholm, Axel
    Stockholm University, Faculty of Science, Department of Astronomy. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
    Aalto, S.
    Calzetti, D.
    Gallagher, J. S.
    Hayes, Matthew J.
    Stockholm University, Faculty of Science, Department of Astronomy. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
    Kruijssen, J. M. D.
    König, S.
    Larsen, S. S.
    Melinder, Jens
    Stockholm University, Faculty of Science, Department of Astronomy. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
    Sabbi, E.
    Smith, L. J.
    Östlin, Göran
    Stockholm University, Faculty of Science, Department of Astronomy. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
    Star cluster formation in the most extreme environments: insights from the HiPEEC survey2020In: Monthly notices of the Royal Astronomical Society, ISSN 0035-8711, E-ISSN 1365-2966, Vol. 499, no 3, p. 3267-3294Article in journal (Refereed)
    Abstract [en]

    We present the Hubble imaging Probe of Extreme Environments and Clusters (HiPEEC) survey. We fit HST NUV to NIR broad-band and H alpha fluxes to derive star cluster ages, masses, and extinctions and determine the star formation rate (SFR) of six merging galaxies. These systems arc excellent laboratories to trace cluster formation under extreme gas physical conditions, rare in the local Universe, but typical for star-forming galaxies at cosmic noon. We detect clusters with ages of 1-500 Myr and masses that exceed 10(7) M-circle dot. The recent cluster formation history and their distribution within the host galaxies suggest that systems such as NGC 34, NGC 1614, and NGC 4194 are close to their final coalescing phase, while NGC 3256, NGC 3690, and NGC 6052 are at an earlier/intermediate stage. A Bayesian analysis of the cluster mass function in the age interval 1-100 Myr provides strong evidence in four of the six galaxies that an exponentially truncated power law better describes the observed mass distributions. For two galaxies, the fits arc inconclusive due to low number statistics. We determine power-law slopes beta similar to -1.5 to -2.0 and truncation masses, M-c, between 10(6) and a few times 10(7) M-circle dot, among the highest values reported in the literature. Advanced mergers have higher M-c than early/intermediate merger stage galaxies, suggesting rapid changes in the dense gas conditions during the merger. We compare the total stellar mass in clusters to the SFR of the galaxy, finding that these systems are among the most efficient environments to form star clusters in the local Universe.

  • 2.
    Hollyhead, Katherine
    et al.
    Stockholm University, Faculty of Science, Department of Astronomy. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
    Lardo, C.
    Kacharov, N.
    Bastian, N.
    Hilker, M.
    Rejkuba, M.
    Koch, A.
    Grebel, E. K.
    Georgiev, I.
    Kron 3: a fourth intermediate age cluster in the SMC with evidence of multiple populations2018In: Monthly notices of the Royal Astronomical Society, ISSN 0035-8711, E-ISSN 1365-2966, Vol. 476, no 1, p. 114-121Article in journal (Refereed)
    Abstract [en]

    We present the results of a spectroscopic study of the intermediate age (approximate to 6.5 Gyr) massive cluster Kron 3 in the Small Magellanic Cloud. We measure CN and CH band strengths (at similar or equal to 3839 and 4300 angstrom, respectively) using VLT FORS2 spectra of 16 cluster members and find a sub-population of five stars enriched in nitrogen. We conclude that this is evidence for multiple populations in Kron 3, the fourth intermediate age cluster, after Lindsay 1, NGC 416 and NGC 339 (ages 6-8 Gyr), to display this phenomenon originally thought to be a unique characteristic of old globular clusters. At approximate to 6.5 Gyr this is one of the youngest clusters with multiple populations, indicating that the mechanism responsible for their onset must operate until a redshift of at least 0.75, much later than the peak of globular cluster formation at redshift similar to 3.

  • 3.
    Hollyhead, Katherine
    et al.
    Stockholm University, Faculty of Science, Department of Astronomy. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
    Martocchia, S.
    Lardo, C.
    Bastian, N.
    Kacharov, N.
    Niederhofer, F.
    Cabrera-Ziri, I.
    Dalessandro, E.
    Mucciarelli, A.
    Salaris, M.
    Usher, C.
    Spectroscopic detection of multiple populations in the similar to 2 Gyr old cluster Hodge 6 in the LMC2019In: Monthly notices of the Royal Astronomical Society, ISSN 0035-8711, E-ISSN 1365-2966, Vol. 484, no 4, p. 4718-4725Article in journal (Refereed)
    Abstract [en]

    We report the spectroscopic discovery of abundance spreads (i.e. multiple populations) in the similar to 2 Gyr old cluster in the LMC, Hodge 6. We use low-resolution VLT FORS2 spectra of 15 member stars in the cluster to measure their CN and CH band strengths at similar or equal to 3883 and 4300 angstrom, respectively, as well as [C/Fe] and [N/Fe] abundances. We find a subpopulation of two stars that are enriched in nitrogen, and we conclude that this subpopulation is evidence of multiple populations in Hodge 6. This is the second similar to 2 Gyr old cluster (the first being NGC 1978 in the LMC) to show multiple populations and the first spectroscopic detection of MPs in a cluster of this age. This result is interesting as it hints at a possible relationship between the disappearance of extended main sequence turn-offs in clusters younger than similar to 2 Gyr and the onset of multiple populations at similar to 2 Gyr, which should be explored further.

  • 4. Martocchia, S.
    et al.
    Cabrera-Ziri, I.
    Lardo, C.
    Dalessandro, E.
    Bastian, N.
    Kozhurina-Platais, V.
    Usher, C.
    Niederhofer, F.
    Cordero, M.
    Geisler, D.
    Hollyhead, Katherine
    Stockholm University, Faculty of Science, Department of Astronomy. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
    Kacharov, N.
    Larsen, S.
    Li, C.
    Mackey, D.
    Hilker, M.
    Mucciarelli, A.
    Platais, I.
    Salaris, M.
    Age as a major factor in the onset of multiple populations in stellar clusters2018In: Monthly notices of the Royal Astronomical Society, ISSN 0035-8711, E-ISSN 1365-2966, Vol. 473, no 2, p. 2688-2700Article in journal (Refereed)
    Abstract [en]

    It is now well established that globular clusters (GCs) exhibit star-to-star light-element abundance variations (known as multiple populations, MPs). Such chemical anomalies have been found in (nearly) all the ancient GCs (more than 10 Gyr old) of our Galaxy and its close companions, but so far no model for the origin of MPs is able to reproduce all the relevant observations. To gain new insights into this phenomenon, we have undertaken a photometric Hubble Space Telescope survey to study clusters with masses comparable to that of old GCs, where MPs have been identified, but with significantly younger ages. Nine clusters in the Magellanic Clouds with ages between similar to 1.5 and 11 Gyr have been targeted in this survey. We confirm the presence of MPs in all clusters older than 6 Gyr and we add NGC 1978 to the group of clusters for which MPs have been identified. With an age of similar to 2 Gyr, NGC 1978 is the youngest cluster known to host chemical abundance spreads found to date. We do not detect evident star-to-star variations for slightly younger massive clusters (similar to 1.7 Gyr), thus pointing towards an unexpected age dependence for the onset of MPs. This discovery suggests that the formation of MPs is not restricted to the early Universe and that GCs and young massive clusters share common formation and evolutionary processes.

  • 5. Martocchia, S.
    et al.
    Niederhofer, F.
    Dalessandro, E.
    Bastian, N.
    Kacharov, N.
    Usher, C.
    Cabrera-Ziri, I.
    Lardo, C.
    Cassisi, S.
    Geisler, D.
    Hilker, M.
    Hollyhead, Katherine
    Stockholm University, Faculty of Science, Department of Astronomy. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
    Kozhurina-Platais, V.
    Larsen, S.
    Mackey, D.
    Mucciarelli, A.
    Platais, I.
    Salaris, M.
    The search for multiple populations in Magellanic Cloud clusters - IV. Coeval multiple stellar populations in the young star cluster NGC 19782018In: Monthly notices of the Royal Astronomical Society, ISSN 0035-8711, E-ISSN 1365-2966, Vol. 477, no 4, p. 4696-4705Article in journal (Refereed)
    Abstract [en]

    We have recently shown that the similar to 2 Gyr old Large Magellanic Cloud star cluster NGC 1978 hosts multiple populations in terms of star-to-star abundance variations in [N/Fe]. These can be seen as a splitting or spread in the subgiant and red giant branches (SGB and RGB) when certain photometric filter combinations are used. Because of its relative youth, NGC 1978 can be used to place stringent limits on whether multiple bursts of star formation have taken place within the cluster, as predicted by some models for the origin of multiple populations. We carry out two distinct analyses to test whether multiple star formation epochs have occurred within NGC 1978. First, we use ultraviolet colour-magnitude diagrams (CMDs) to select stars from the first and second population along the SGB, and then compare their positions in optical CMDs, where the morphology is dominantly controlled by age as opposed to multiple population effects. We find that the two populations are indistinguishable, with age differences of 1 +/- 20 Myr between them. This is in tension with predictions from the asymptotic giant branch scenario for the origin of multiple populations. Second, we estimate the broadness of the main-sequence turn-off (MSTO) of NGC 1978 and we report that it is consistent with the observational errors. We find an upper limit of similar to 65 Myr on the age spread in the MSTO of NGC 1978. This finding is in conflict with the age spread scenario as origin of the extended MSTO in intermediate-age clusters, while it fully supports predictions from the stellar rotation model.

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