Young Star Clusters and Clumps in the Local Universe

114 Pages·2017·7.84 MB·English

by  Matteo Messa

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.