On the coevolution of galaxies and their host clusters
Date
2019-04-15
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Abstract
Galaktikad pole ruumis ühtlaselt jaotunud. Pidevalt mõjuva gravitatsioonilise tõmbe tõttu moodustavad nad erineva tihedusega struktuure: hiiglaslikke, väga väheste galaktikatega tühikuid; tühikuid ääristavaid “seinu”; seinte lõikumisjoont markeerivaid filamente; filamentide ristumiskohtades paiknevaid galaktikaparvi. Galaktikaparved võivad omakorda moodustada superparvi – suurimaid struktuure Universumis. Selles struktuuride võrgustikus toimub pidev areng, mille käigus galaktikagrupid ja väiksemad parved ühinevad suurematega. Omavahel võivad ühineda ka suured galaktikaparved.
Vaatlusandmed näitavad, et erinevate struktuuride galaktiline koostis on erinev. Kõige hõredamates keskkondades domineerivad aktiivse tähetekkega spiraalgalaktikad, kuid suurte parvede siseosasid asustavad valdavalt tähetekketa elliptilised galaktikad. Seni ei ole täielikku selgust, milliste mehhanismide toimel on galaktikate tüüp ning tähetekke kiirus seotud keskkonnatihedusega. Käesolevas väitekirjas otsimegi seoseid galaktikate ja neid ümbritseva keskkonna arenemise vahel. Selleks analüüsime galaktikate tähetekke aktiivsust ühes kokkupõrkefaasis olevas galaktikaparves. Samuti uurime külma, tähetekkeks vajaliku gaasi sisaldust galaktikates, mis paiknevad ühes ebatavaliselt rohke tähetekkega galaktikaparves. Lõpuks võrdleme neid kaht parve suurema parvevalimiga, mille liikmed paiknevad meist ligikaudu samal kaugusel.
Jõuame järeldusele, et sarnase massiga parvede põrkumise tagajärjel tähetekkeaktiivsus kahaneb. Samas võib põrke algfaasis tähetekkeaktiivsus lühiajaliselt ka suureneda. Näeme, et 2,5 miljardit aastat nooremana paistvate galaktikaparvede gaasisisaldus on sarnane praeguse Universumi parvede omaga, ning et aktiivse tähetekkega galaktikate suurem osakaal kaugetes galaktikaparvedes on tõenäoliselt seotud parvedest väljaspool toimuvate protsessidega.
Galaxies, are not uniformly distributed throughout space. Due to the continuous pull of gravity they form structures which range in densities from giant voids, almost completely devoid of galaxies, through sheets which surround the voids, filaments where sheets intersect and clusters which are forming at the crossing of filaments. Clusters of galaxies are often found in conglomerates called super-clusters which are the largest structures in the universe. This network of structures is continuously evolving with groups and smaller clusters of galaxies being accreted onto larger clusters. Sometimes large clusters also collide. It is observed that throughout this network the mixture of galaxies varies. The more rarefied environments are dominated by star forming, spiral galaxies while the interiors of large clusters are dominated by elliptical galaxies, devoid of star formation. The exact physical mechanisms that determines this connection between environment and morphology and activity of galaxies is not yet fully understood. In this thesis we search for a link between the evolution of galaxies and the evolution of the structures within which they are embedded. We analyse the star formation activity in galaxies which are members of a merging cluster of galaxies. We also analyse the cold gas content (fuel for star formation) of galaxies residing in a cluster with unusually high fraction of star forming galaxies. Finally we compare the galaxies found in these two clusters with a sample of other clusters at similar distance from us. We conclude that mergers of clusters of similar mass tend to quench star formation. We find indications that during the initial phase of the merger the star formation can be enhanced briefly. We find the gas content in cluster galaxies observed 2.5 Gyrs ago to be similar to that of local cluster-members, and that the increased fraction of star forming galaxies in distant clusters is likely related to processes working outside these clusters.
Galaxies, are not uniformly distributed throughout space. Due to the continuous pull of gravity they form structures which range in densities from giant voids, almost completely devoid of galaxies, through sheets which surround the voids, filaments where sheets intersect and clusters which are forming at the crossing of filaments. Clusters of galaxies are often found in conglomerates called super-clusters which are the largest structures in the universe. This network of structures is continuously evolving with groups and smaller clusters of galaxies being accreted onto larger clusters. Sometimes large clusters also collide. It is observed that throughout this network the mixture of galaxies varies. The more rarefied environments are dominated by star forming, spiral galaxies while the interiors of large clusters are dominated by elliptical galaxies, devoid of star formation. The exact physical mechanisms that determines this connection between environment and morphology and activity of galaxies is not yet fully understood. In this thesis we search for a link between the evolution of galaxies and the evolution of the structures within which they are embedded. We analyse the star formation activity in galaxies which are members of a merging cluster of galaxies. We also analyse the cold gas content (fuel for star formation) of galaxies residing in a cluster with unusually high fraction of star forming galaxies. Finally we compare the galaxies found in these two clusters with a sample of other clusters at similar distance from us. We conclude that mergers of clusters of similar mass tend to quench star formation. We find indications that during the initial phase of the merger the star formation can be enhanced briefly. We find the gas content in cluster galaxies observed 2.5 Gyrs ago to be similar to that of local cluster-members, and that the increased fraction of star forming galaxies in distant clusters is likely related to processes working outside these clusters.
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Keywords
galaktikad, galaktikaparved