Universe Now Galaxies and Cosmology Milky Way • Milky Way means – Line of dust visible in the sky – Our own galaxy • Milky Way in other languages – Linnutee (Estonian) – Tejút (Hungarian) – Linnunrata (Finnish) – Vintergatan (Swedish) – MilchStraße (German) – Via Lactea (Latin) – Voie Lactée (French) Milky Way • Galilei concluded in the 17th century that the Milky Way consists of countless stars • W. Herschel at the end of the 18th century: Milky Way is disk-like, Sun is close to the center • H. Shapley in the 1920s, observed spatial distribution of the globular clusters. Result: the center of the Milky Way is 30000 ly away. Structure of the Milky Way • Central bulge, bar with a lot of stars – Milky Way central regions (black hole) • Disk – Young stars and open star clusters and associations • Halo – Globular clusters – Old stars • Corona – Hot gas – Possibly old stars • Dark matter halo https://commons.wikimedia.org/w/index.php?curid=52696960 Milky Way – Spiral galaxy • With the help of the Spitzer-satellite, the distribution of 30 million stars at the center of the Milky Way was studied • Barred spiral galaxy • The size of the bar is 27 000 ly • Contains 100-400 billion stars The Milky Way - Spirals • Differential rotation – Lifespan of fixed spiral arms is 100 million years -> spiral arms must renew • 1960s: density wave theory – Density wave circulates around the Milky Way • When collides with ISM forms condensations – spiral arms • Individual stars move in and out of the spiral arms as the galaxy rotates The Milky Way - Center • The Sun’s distance 8.5 kpc (28000 ly) – Orbital period around the center is 240 million years • Has done 20 rounds – Orbital velocity 254 km/s 11 • The mass of the Milky Way 2∙10 M⊙, disk size 100-200 000 ly and thickness 1 kpc (stars) and 200 pc (ISM) • Radio source in the center – Sgr A* – Mass 4 million M⊙ and radius 12 million km – Super-massive black hole G2 – star inside a dust cloud? https://www.youtube.com/watch?v=A2jcVusR54E Formation and evolution of the Milky Way • Formed 13.6±0.8 billion years ago • Shrinking into a disk took a few billion years – The visible galaxy formed inside a dark matter halo • Stars with different ages in different regions – Study of the metallicity – Old, metal-poor stars located at the central bulge and halo – Young, metal-rich stars located at the spiral arms Other galaxies ● In early 1900s it was debated whether the “Andromeda Nebula” was outside our galaxy – Novae seen in Andromeda were much fainter than in other parts of the sky: is Andromeda further away than previously thought? ● In 1925, Edwin Hubble used the established period-luminosity relation of Cepheid variables in Andromeda, and proved that it is an entire galaxy outside our own ● In 1943 Walter Baade discovered that there are two types of Cepheids – Hubble’s estimate for Andromeda’s distance was too short – Distance 2.5 million ly (780 kpc) – Since all longer distances were calibrated with the Cepheid measurements in Andromeda, the size of the whole Universe “doubled in one night” ● At first galaxies were thought to be “island universes” with little interaction – In reality, galaxies interact much with each other (such as galaxy mergers) – Distances between galaxies are much shorter relative to their size, than distances between stars Local Group ● Our local group of galaxies – Milky Way – Andromeda Galaxy – Triangulum Galaxy – Lots of smaller dwarf galaxies and satellite galaxies, e.g. Large and Small Magellanic Cloud and Andromeda’s satellites M32 and M110 – Probably many undiscovered dwarf galaxies, especially in the plane of Milky Way ● Part of the Virgo Supercluster “End” of the Milky Way • Andromeda and Milky Way collide with each other • Collision in 4-5 billion years – Andromeda approaches with a velocity of 110 km/s • Eventually the galaxies will merge into one galaxy – It is also possible that the Triangulum galaxy (M33) will merge with Milky Way / Andromeda further in the future • Fate of the Solar System – Merge happens during the lifespan of the Sun – Migrates outward in the new galaxy – May even be completely ejected from the galaxy Simulation of the Antennae galaxy mergers. Credit: Natalia Lahén Galaxy catalogs ● Messier objects (M) – List of 110 deep-sky objects listed by Charles Messier in late 1700s ● Messier was interested in comets, and made a list of similar objects which are not comets ● NGC (New General Catalogue) – List of 7840 galaxies, star clusters and nebulae ● UGC (Uppsala General Catalogue) – List of 12921 galaxies ● PGC (Principal Galaxies Catalogue) – List of 73197 galaxies Galaxies - classification • Edwin Hubble in 1926 – Hubble sequence • Main types – Elliptical galaxies – Lenticular glaxies – Spiral galaxies • Divided into two sub groups • Other – Irregular galaxies Elliptical galaxies - E • Observed as elliptical concentration of stars – No large scale rotation • Different ellipticity – Spherical (E0) – elliptical (E7) • Other elliptical galaxies – Giant ellipses – cD-galaxies – largest existing galaxies – Dwarf ellipses – dE-galaxies • Interstellar medium (ISM) – Non-existent – no star formation • No uniform rotation – stars in random orbits around the center Lenticular galaxies • A flat disk that consists of stars – Rotation in the plane of the disk • No spiral structure • Not much interstellar matter • Do not mix up lenticular galaxies with gravitational lenses! NGC 2787 Spiral galaxies - S • Milky Way –like structure (=SB) – Rotation in the plane of the disk • More interstellar matter and star formation than in ellipticals • Three sub classes: a, b and c (Sa, Sb and Sc) • Properties change when moving from a to c – Central bulge gets smaller – Spirals open up – Spiral arms more blurry and scattered – The amount of ISM increases • Another main group: barred spirals (SB) – Sub groups similar to normal spiral galaxies Spiral galaxies: Sa, Sb and Sc • NGC 300: Sa • M 81: Sb • M 99: Sc Galaxy evolution • The study of galaxy types in near space and at 6 billion ly distance Portion % Near space 6 billion ly E 3 4 S0 15 13 S 72 31 Irr 10 52 • Have irregular galaxies turned into spiral galaxies? Irregular galaxies • Two main classes: Irr I and Irr II (irr = irregular) • Irr I – Some scattered spiral structures • Irr II – Completely irregular Irregular galaxies LMC SMC Active galaxies • Some of the galaxies have abnormal activity • Concentrated on the galaxy center – Active Galaxy Nuclei (AGN) – material falling to the supermassive black hole • Large brightness – Brightest known objects in the Universe • Not a distinct class of galaxies, but rather a phase any galaxy can go through Active galaxies – main types • Star burst -galaxies • Seyfert galaxies – Active disk galaxies • Radiogalaxies – Active ellipse galaxies • Quasars – Extremely bright galaxy nuclei with strong jets • Blazars – Quasar with jet directed directly towards us Radio galaxy Cygnus A Quasar PKS 2349 Galaxy AM 0644-741 Galaxy concentrations • Galaxy • Galaxy groups – Contains a few bright galaxies – Milky Way is part of the Local Group • Galaxy clusters – A large cluster of bright galaxies (min. 50) • Superclusters – Size megaparsecs, tens of galaxy groups and clusters – Local Group part of Virgo Supercluster, which in turn is part of Lanikea Supercluster • Huge empty voids between clusters https://commons.wikimedia.org/w/index.php?curid=71065242 470 million years 2,1 billion years 13,7 billion years Distriution of galaxies in the Sloan Digital Sky Survey https://commons.wikimedia.org/w/index.php?curid=13251597 Gravitational lenses • First found in 1979 – Double quasar turned out to be two images of the same quasar • Caused by a galaxy between us and the quasar – Bends light coming from the quasar ● Gravitation bends light according to Einstein’s theory of General Relativity – Galaxy called a gravitational lens • Gravitational lens can be – A single galaxy – Galaxy group • Microlenses (used for example to find new exoplanets) Abell 370 Gravitational lens and microlens Cosmological observations • Galaxies’ homogeneous and isotropic distribution – On large scales the Universe looks similar everywhere • Expansion of the Universe – Cosmic redshift: distances expand -> electromagnetic waves stretch out and become less energetic -> become redder – The farther away, the faster it’s moving away -> cosmic redshift larger (Hubble’s law) ● Allows also distance measurements for far away objects based on the redshift Cosmology - observations • Distance measurements of supernovae: expansion not slowing down – The expansion of the Universe is accelerating – dark energy • Visible matter makes up only a small part of the total matter/energy density of the Universe – Dark matter, dark energy Dark side of the Universe • Dark matter – Measurements of movements and rotation of galaxies shows that there must be more mass than is seen – Probably unknown weakly interacting particle(s) – Galaxies could not have formed in the early Universe without dark matter • Dark energy – Most of the content of the Universe seems to be dark energy – Responsible for the accelerating expansion of the Universe – Cosmological constant (“vacuum energy”)? Dynamical energy field? Not universal, but only a local feature in our parts of the Universe? General relativity is wrong? Cosmic Microwave Background • Oldest light in the Universe – A remnant of the Big Bang – Originates from a 380 000 yr old Universe – recombination – electrons and protons combine to atoms and Universe becomes transparent for photons • Corresponds to a black-body