Young Star Clusters and Clumps in the Local Universe PDF
Preview Young Star Clusters and Clumps in the Local Universe
Young Star Clusters and Clumps in the Local Universe The effect of galactic environment on star formation Matteo Messa Academic dissertation for the Degree of Doctor of Philosophy in Astronomy at Stockholm University to be publicly defended on Friday 15 February 2019 at 13.00 in sal FA31, AlbaNova universitetscentrum, Roslagstullsbacken 21. Abstract Stars do not form in isolation, but rather out of a hierarchical structure set by the turbulence of the interstellar medium. At the densest peak of the gas distribution, the star formation process can produce young star clusters (YSCs), which are gravitationally bound systems of stars with mass between ~100 and 106 M and typical size of few parsecs. At larger Sun scales, clusters are themselves arranged into cluster complexes, on scales of hundreds of parsecs and up to kiloparsec scales, which are usually referred to as ‘star-forming clumps’. Observations of local star-forming galaxies show that YSCs form over a wide range of galactic environment. However, it is not yet clear if and how the galactic environment relates to the properties of star clusters. I present the results obtained by studying the YSC population of the nearby spiral galaxy M51. We find that the cluster mass function, dN/dM, can be described by a power-law with a -2 slope and an exponential truncation at 105 M , consistent with what is observed Sun in similar galaxies in the literature. The shape of the mass function is similar when looking at increasing galactocentric distances. We observe significant differences, however, when comparing clusters located in the spiral arm with those the inter-arm environments. On average, more massive clusters are formed in the spiral arms, as also previously found for the YSC progenitors, the giant molecular clouds (GMCs). Finally, we see that clusters are more quickly disrupted in denser environments, as expected if their disruption is mainly caused by tidal interaction with dense gas structures like the GMCs. I have also undertaken the analysis of the interplay between galactic scale properties and larger star forming units, the stellar clumps. The analysis has been conducted in a sample of 14 low-redshift starburst galaxies, the Lyman-Alpha Reference Sample (LARS). The elevated star formation rate densities of such galaxies allow to form clumps with densities comparable to clumps at high-redshift, typically more massive and denser than what is normally observed in the local universe. The clumps in the LARS galaxies contribute to a large fraction to the UV flux of the galaxy itself (in many galaxies > 50%), resulting in galaxies which appear ‘clumpy’. In agreement with formation theories we observe that clumpiness is higher in galaxies with higher SFR surface density and dominated by turbulent gas motion. Keywords: Galaxies, Star formation, Star clusters. Stockholm 2019 http://urn.kb.se/resolve?urn=urn:nbn:se:su:diva-163079 ISBN 978-91-7797-554-0 ISBN 978-91-7797-555-7 Department of Astronomy Stockholm University, 106 91 Stockholm YOUNG STAR CLUSTERS AND CLUMPS IN THE LOCAL UNIVERSE Matteo Messa Young Star Clusters and Clumps in the Local Universe The effect of galactic environment on star formation Matteo Messa ©Matteo Messa, Stockholm University 2019 ISBN print 978-91-7797-554-0 ISBN PDF 978-91-7797-555-7 Cover image: Detail of the spiral galaxy M51, with the location of star clusters highligthed with red boxes. The green boxes correspond to the three zoomed-in clusters. The galaxy three-color composite image was produced using HST observations in B, V and I bands. The coordinates of the cluster population were taken from the LEGUS catalogs, using only clusters more massive than 104 MSun. Both the HST observations and the cluster catalogue are publicy available on the LEGUS webpage: https://archive.stsci.edu/prepds/legus/ Graphical realization by Chiara Bini & Matteo Messa Printed in Sweden by Universitetsservice US-AB, Stockholm 2019 Le cose tutte quante hanno ordine tra loro, e questo è forma che l'universo a Dio fa simigliante. (Dante, Paradiso I) All things whate'er they be Have order among themselves, and this is form, That makes the universe resemble God. (Dante, Paradise I) to Chiara Abstract Stars do not form in isolation, but rather out of a hierarchical structure set by the turbulence of the interstellar medium. At the densest peak of the gas dis- tribution, the star formation process can produce young star clusters (YSCs), whicharegravitationallyboundsystemsofstarswithmassbetween 100and ∼ 106 M and typical size of few parsecs. At larger scales, clusters are them- (cid:12) selves arranged into cluster complexes, on scales of hundreds of parsecs and uptokiloparsecscales,whichareusuallyreferredtoas‘star-formingclumps’. Observationsoflocalstar-forminggalaxiesshowthatYSCsformoverawide range of galactic environment. However, it is not yet clear if and how the galacticenvironmentrelatestothepropertiesofstarclusters. IpresenttheresultsobtainedbystudyingtheYSCpopulationofthenearby spiral galaxy M51. We find that the cluster mass function, dN/dM, can be described by a power-law with a -2 slope and an exponential truncation at 105 M , consistent with what is observed in similar galaxies in the litera- (cid:12) ture. The shape of the mass function is similar when looking at increasing galactocentric distances. We observe significant differences, however, when comparing clusters located in the spiral arm with those the inter-arm environ- ments. Onaverage,moremassiveclustersareformedinthespiralarms,asalso previouslyfoundfortheYSCprogenitors,thegiantmolecularclouds(GMCs). Finallyweseethatclustersaremorequicklydisruptedindenserenvironments, asexpectediftheirdisruptionismainlycausedbytidalinteractionwithdense gasstructuresliketheGMCs. Ihavealsoundertakentheanalysisoftheinterplaybetweengalacticscale properties and larger star forming units, the stellar clumps. The analysis has been conducted in a sample of 14 low-redshift starburst galaxies, the Lyman- Alpha Reference Sample (LARS). The elevated star formation rate densities of such galaxies allow to form clumps with densities comparable to clumps athigh-redshift, typicallymoremassiveanddenserthanwhatisnormallyob- served in the local universe. The clumps in the LARS galaxies contribute to a large fraction to the UV flux of the galaxy itself (in many galaxies >50%), resultingingalaxieswhichappear‘clumpy’. Inagreementwithformationthe- oriesweobservethatclumpinessishigheringalaxieswithhigherSFRsurface densityanddominatedbyturbulentgasmotion.
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