Properties and spatial distribution of galaxy superclusters
Date
2017-01-16
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Abstract
Käesoleva töö eesmärgiks on kirjeldada suuremastaabilist galaktikate paiknemist universumis, uurides selleks suuri galaktikate ja galaktikaparvede kogumeid, mida nimetatakse superparvedeks. Superparved on esimeses lähenduses ulatuslikud, valdavalt ebasümmeetrilised, märgatava galaktikate ületihedusega alad, mis võivad sisaldada kümneid kuni sadu galaktiparvi ning tuhandeid üksikuid või väikestes gruppides olevaid galaktikaid. Koos neid ümbritsevate hõredate piirkondade – tühikutega – moodustavad nad iseloomuliku suuremastaabilise struktuuri, mida selle välimuse tõttu “kosmiliseks võrguks” kutsutakse. Superparvede suurus, kuju ja ruumiline jaotus võimaldavad hinnata erinevaid kosmoloogilisi sktruktuuri tekke mudeleid. Lisaks peaksid nad massiivsete aine kontsentratsioonidena jätma jälgi kosmilisse mikrolainelisse taustkiirgusse ja mõjutama neis sisalduvate galaktikate omadusi. Universumi suuremastaabilise struktuuriga seotud uurimustöid on Tartu Observatooriumis läbi viidud juba mitme aastakümne jooksul.
Antud töös on superparved leitud galaktate silutud heledustihedusväljade, mis on arvutatud kahe erineva vaatlusliku ja ühe simuleeritud galaktikavalimi, põhjal. Vaatluslikke valimeid korrigeeriti sealjuures punanihkemoonutuste ja teiste, kaugusest sõltuvate nn. selektsiooniefektide, kõrvaldamiseks. Superparved ise on defineeritud tihedusvälja piirkondadena, mille sees on tihedus kõrgem teatud läviväärtusest. Iga galaktikavalimi kohta on koostatud kaks superparvede kataloogi: üks konstantsel tihedusnivool ja teine kasutades algoritmi, mis leiab igale objektile individuaalse piirtiheduse.
Superparvede kataloogides esitatakse järgmised superparvede omadused: asukoht, galaktikate ja galaktikaparvede arv, ruumala, diameeter, koguheledus ning morfoloogilised kujuparameetrid. Superparvede omaduste jaotused osutavad, et valitud meetod ja selle parameetrid on olnud põhjendatud. Omaduste sõltuvused kaugusest üldjuhul väikesed, mis näitab, et vaatluslikke piiranguid on õnnestunud edukalt kompenseerida. Superparvede morfoloogia näitab, et superparved on valdavalt filamentaarse kujuga, kusjuures nende väljavenitatus suureneb koos objektide koguheledusega. Superparvede ruumilises jaotuses võib omakorda märgata suure tihedusega ja rikaste objektide teatava regulaarsusega grupeerumist.
Publitseeritud katalooge ja superparvede leidmise meetodit on kasutatud mitmetes uurimustöödes nii Tartu Observatoorumis kui ka mujal.
Astronomy is a science that can offer plenty of unforgettable imagery, and the large-scale distribution of galaxies is no exception. Among the first features the viewer's eye is likely to be drawn to, are large concentrations of galaxies – galaxy superclusters, contrasting to the seemingly empty regions beside them. Superclusters can extend from tens to over hundred megaparsecs, they contain from hundreds to thousands of galaxies, and many galaxy groups and clusters. Unlike galaxy clusters, superclusters are clearly unrelaxed systems, not gravitationally bound as crossing times exceed the age of the universe, and show little to no radial symmetry. Superclusters, as part of the large-scale structure, are sensitive to the initial power spectrum and the following evolution. They are massive enough to leave an imprint on the cosmic microwave background radiation. Superclusters can also provide an unique environment for their constituent galaxies and galaxy clusters. Large-scale galaxy structures have been studied at the Tartu Observatory already for several decades. In this study we used two different observational and one simulated galaxy samples to create several catalogues of structures that, we think, correspond to what are generally considered galaxy superclusters. Superclusters were delineated as continuous over-dense regions in galaxy luminosity density fields. When calculating density fields several corrections were applied to remove small-scale redshift distortions and distance-dependent selection effects. Resulting catalogues of objects display robust statistical properties, showing that flux-limited galaxy samples can be used to create nearly volume-limited catalogues of superstructures. Generally, large superclusters can be regarded as massive, often branching filamentary structures, that are mainly characterised by their length. Smaller superclusters, on the other hand, can display a variety of shapes. Spatial distribution of superclusters shows large-scale variations, with high-density concentrations often found in semi-regularly spaced groups. Future studies are needed to quantify the relations between superclusters and finer details of the galaxy distribution. Supercluster catalogues from this thesis have already been used in numerous other studies.
Astronomy is a science that can offer plenty of unforgettable imagery, and the large-scale distribution of galaxies is no exception. Among the first features the viewer's eye is likely to be drawn to, are large concentrations of galaxies – galaxy superclusters, contrasting to the seemingly empty regions beside them. Superclusters can extend from tens to over hundred megaparsecs, they contain from hundreds to thousands of galaxies, and many galaxy groups and clusters. Unlike galaxy clusters, superclusters are clearly unrelaxed systems, not gravitationally bound as crossing times exceed the age of the universe, and show little to no radial symmetry. Superclusters, as part of the large-scale structure, are sensitive to the initial power spectrum and the following evolution. They are massive enough to leave an imprint on the cosmic microwave background radiation. Superclusters can also provide an unique environment for their constituent galaxies and galaxy clusters. Large-scale galaxy structures have been studied at the Tartu Observatory already for several decades. In this study we used two different observational and one simulated galaxy samples to create several catalogues of structures that, we think, correspond to what are generally considered galaxy superclusters. Superclusters were delineated as continuous over-dense regions in galaxy luminosity density fields. When calculating density fields several corrections were applied to remove small-scale redshift distortions and distance-dependent selection effects. Resulting catalogues of objects display robust statistical properties, showing that flux-limited galaxy samples can be used to create nearly volume-limited catalogues of superstructures. Generally, large superclusters can be regarded as massive, often branching filamentary structures, that are mainly characterised by their length. Smaller superclusters, on the other hand, can display a variety of shapes. Spatial distribution of superclusters shows large-scale variations, with high-density concentrations often found in semi-regularly spaced groups. Future studies are needed to quantify the relations between superclusters and finer details of the galaxy distribution. Supercluster catalogues from this thesis have already been used in numerous other studies.
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Keywords
galaktikad, galaktikaparved, galaxies, galaxy clusters