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Distant galaxies and quasars Prof Andy Lawrence Astronomy 1G 2011-12 The ages of things • • • • Rock and Roll Science Civilisation Early Hominids 50 years 400 years 5000 years 5 million years • • • • • Pleiades 60 million years Life on Earth 3.8 billion years Earth 4.5 billion years Globular Clusters 12 billion years Universe 14 billion years* * see later lectures Were things different in the past ? Astronomy 1G 2011-12 Light-travel time • Sun D=1AU → 8.3 minutes • Alpha Centauri D= 1.338 pc → 4.365 years – 1 "light year" = c x 3600 x 24 x 365.25 = 9.461x1015m – 1 pc = 3.086e16m so 1 pc = 3.262 lt yr • M31 D=778 kpc → 2.5 million years • Virgo cluster D=18 Mpc → 60 million years – redshift z=0.004 • Coma cluster D=99 Mpc → 300 million years – redshift z=0.024 • Quasar 3c273 D=658 Mpc → 2.1 billion years – redshift z=0.158 – 15% of the age of the Universe !!! Observations of distant objects allows us to study the history of the Universe Observing the early universe Naively, half the age of the Universe would correspond to D=2145 Mpc, and with V=cz=H0D to z=0.52 However, the expansion of the universe has not been constant.. see later Sparke and Gallagher CUP 2007 "Lookback time" vs redshift shown to left for various cosmological models. Details not important... confirms that halfway back is roughly z=0.5 and observations at z>3 show Universe when it was only 10-20% of its current age. Geometrical vs Physical effects • in the past, the universe : – was smaller – was maybe expanding at different rate – was more curved; Euclidean formulae don't work • so flux vs distance, number counts, angular size etc all more subtle • put aside until cosmology section • concentrate on physical effects • did galaxies look different ? High redshift galaxies • In the past galaxies were on average bluer : smaller : messier • This has only become clear in recent years with Hubble Space Telescope deep images • It is consistent with the idea that galaxies have formed from the merger of sub-units Hubble Ultra Deep Field shows large numbers of faint galaxies which are on average smaller and look more peculiar than current day galaxies 2003 NASA/ESA/STScI Mugshots of individual distant galaxies From random piece of sky Selected as distant radio sources Best, Longair, and Roettgering, MNRAS 292, 758 (1997) Comparison of galaxies at different distances Motorway pile-up ! Rich clusters of galaxies As usual for astronomical objects, note that small ones are common and big ones are rare Rosati, Borgani, and Norman 2002 AnnRev 40 539 No per unit volume • Massive, rich clusters of galaxies were probably rarer in the past • This result has been controversial over the last twenty years .. but latest studies seem to confirm it is correct • The best way to find massive clusters is by X-ray surveys... they stand out very clearly X-ray luminosity of cluster Recent cluster results Number per unit volume Vikhlinin et al 2009 ApJ 692 1033 Not much difference in smaller clusters When universe was half its current age, massive clusters were rarer by a factor of several Mass of cluster Evolution of quasars • Quasars (the most luminous Active Galactic Nuclei, or AGN) have evolved dramatically over cosmic time • Because they are easy to see to large distances, this (unlike galaxy evolution) has been clearly known for several decades • In the past they were both much more numerous and substantially more luminous The most luminous quasars were several hundred times more common at z=2 than they are today adapted from Shaver et al. 1996 Nature 384, 449 This is actually much more striking if plotted in linear number density against linear time since the Big Bang... The first third of the history of the Universe was the age of quasars ... we now see only a faint leftover History of star formation By adding up starlight from blue stars / IR from deep samples, we can now estimate the total rate of star formation in the Universe (regardless which galaxies it is taking place in ... This, like the quasar evolution, shows a dramatic peak at earlier times ... but somewhat later than the quasars The Inter Galactic Medium (IGM) Bill Keel website http://www.astr.ua.edu/keel/agn/forest.html Some absorption lines seen in quasars are due to small Hydrogen gas clouds between us and the quasar High redshift quasars show hugely more of these ... this is known as the "Lyα forest" The increase happens both because of the longer sight line and because the IGM evolves