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Home , Density wave theory, NGC 2787, Triangulum Galaxy, Virgo Supercluster

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Galaxies and Cosmology

• Milky Way means – Line of dust visible in the sky – Our own

• 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

• W. Herschel at the end of the 18th century: Milky Way is disk-like, 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 ()

• Disk – Young stars and open clusters and associations

• Halo – Globular clusters – Old stars

• Corona – Hot gas – Possibly old stars

• halo https://commons.wikimedia.org/w/index.php?curid=52696960 Milky Way –

• With the help of the Spitzer-satellite, the distribution of 30 million stars at the center of the Milky Way was studied

•

• 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 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

• Stars with different ages in different regions – Study of the – Old, metal-poor stars located at the central bulge and halo – Young, metal-rich stars located at the spiral arms Other

● In early 1900s it was debated whether the “Andromeda ” 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, used the established period- 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 ” 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

● Our local group of galaxies – Milky Way – – Galaxy – Lots of smaller dwarf galaxies and satellite galaxies, e.g. Large and and Andromeda’s satellites M32 and M110 – Probably many undiscovered dwarf galaxies, especially in the plane of Milky Way

● Part of the “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 (M33) will merge with Milky Way / Andromeda further in the future

• Fate of the – 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 in late 1700s ● Messier was interested in , and made a list of similar objects which are not comets

● NGC () – 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 –

• 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

• (ISM) – Non-existent – no

• 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

• 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

• – Extremely bright galaxy nuclei with strong jets

• – with jet directed directly towards us 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)

• – Size megaparsecs, tens of galaxy groups and clusters – Local Group part of , 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 –

• 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 : 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 temperature of 3 K – Redshifted, originally about 3000 K

• Very homogeneous – only small perturbations of 10-5 K, which are the seeds of the structures we see today

• Studied by COBE, WMAP, Planck -satellites The Cosmic Microwave Background measured by Planck Hubble Deep Field – deepest optical image of the Universe Shape of the Universe

● The large scale geometry of the Univserse can be – Closed (Ω0 > 1) – Open (Ω0 < 1) – Flat (Ω0 = 1)

● Depends on the density

parameter of the Universe (Ω0) – According to the General Theory of Relativity mass and energy cause space- time to curve – If the critical density is exceeded, the Universe is closed, if not, the Universe is open – Measurements indicate that the density is very close to the critical density – Universe is flat History of the universe

• t = 0 ???

• t < 10-43s: Planck Epoch; current models don’t work; quantum gravity important?

• t = 10-30s: cosmic inflation – Universe expanded faster than speed of light

• t = 10-8 – 10-4s: protons and neutrons form – Neutrons not stable, start to decrease with a mean lifetime of about 15 min

• t = 10-4 – 1s: electrons form

• t = 100s: neutrons getting bound in helium cores

• t = 300 000 years: disconnection of matter and radiation – Electrons combine into nuclei forming atoms -> electrically neutral gas -> transparent universe – Radiation can move freely -> Cosmic Microwave Background History of the universe

• The first darkening of the universe – t ≈ 300 000 – 200 000 000 years

• t ≈ 200-500 Myr: first stars light up – Second brightening of the universe – Heavier elements are produced – At the same times galaxies are forming inside dark matter structures

• t = 8 – 9 Gyr: universe at its brightest

• t = 9 Gyr: accelerating expansion starts

• t = about 9 Gyr: Solar System formation

• t = 13.8 Gyr: NOW

Future of the universe

• Now: t=0

– t+1012 y: Local galaxy group has merged and no other galaxies are visible – t+1014 y: star formation will cease – t+1027 y: galaxies disband, mass concentrates on black holes – t+1034 y: protons decay? – t+10100 y: black holes dominate, until they decay, and only redshifted radiation is left

• Heat death Other universes?

• How many universes can there be? – Informal coffee table conversation: 0, 1 or infinite – 0: universes don’t exist, we think that we exist – 1: only this –∞(infinite): many universes..

• Universes can be – Hierarchical: universes inside a universe • Maybe every elementary particle IS another universe? – Parallel: another universe in the neighborhood • Maybe every possible outcome (on quantum level) actually happens in SOME universe?