Galaxies: Our Galaxy: the Milky Way

. The Structure of the Milky Way Galactic Plane Galactic Center The actual structure of our Milky Way is very hard to determine because: 1) We are inside 2) Distance measurements are difficult 3) Our view towards the center is obscured by gas and dust

Structure of the Milky Way (MW) So what can we do to explore the MW ?? So what can we do to explore the MW ?? a) space craft? No b) select bright objects that can be seen throughout the MW c) observe in different wavelengths d) trace velocities of all visible objects

Structure of Milky Way: a) space craft a) How long would it take to get good “outside view” of our Galaxy a) How long would it take to get good “outside view” of our Galaxy (travel at speed of light) ? (travel at speed of light) ? i) 2 months i) 2 months ii) 1 year ii) 1 year iii) 500 years iii) 500 years iv) years iv) years v) 5 million years ?? v) 5 million years ??

Answer: iv) 30,000 years The Sun is about 8.5 kpc = 8,500 pc ≈ 30,000 light years from the Galactic center. Sun Galactic Center => No spacecraft will ever travel a significant distance through or even out of the Milky Way

Structure of Milky Way: b) bright objects b) What are bright objects?  A type stars ?  Brown dwarfs ?  White dwarfs ?  O type stars ?

Structure of Milky Way: b) bright objects Answer: O- and B-stars ! Answer: O- and B-stars ! Remember: O and B stars are the most massive, most luminous stars Look for very young clusters or associations: O/B- Associations !

Structure of Milky Way: b) bright objects optically bright objects O/B Associations Distances to O/B Associations determined using Cepheid Variables O/B Associations trace out 3 spiral arms near the Sun. Sagittarius arm Orion-Cygnus arm Perseus arm Sun

Structure of Milky Way: b) bright objects Globular Clusters Dense clusters of 50,000 – a million stars Approx. 200 globular clusters in our Milky Way Old (11 billion years), lower-main-sequence stars Globular Cluster M80

Structure of Milky Way: b) bright objects Globular cluster distribution: Globular cluster distribution: we are not in the centre of our Galaxy we are not in the centre of our Galaxy

Structure of Milky Way: c) different wavelengths Galaxy (optical): Galaxy (optical): absorption by gas and dust absorption by gas and dust Galaxy (near-infrared): Galaxy (near-infrared): emission from warm dust

Structure of Milky Way: c) different wavelengths Galaxy more transparent at longer (than optical) wavelengths…. Galaxy more transparent at longer (than optical) wavelengths…. most transitions in hydrogen atom at “short” wavelengths, but … most transitions in hydrogen atom at “short” wavelengths, but … coupling magnetic moments electron and proton in neutral hydrogen hydrogen: coupling magnetic moments electron and proton in neutral hydrogen hydrogen: 21cm radio emission 21cm radio emission

Structure of the Milky Way Neutral hydrogen creates Neutral hydrogen creates radio emission ( = 21cm): radio emission ( = 21cm): coupling between magnetic coupling between magnetic moments of proton and electron… moments of proton and electron… “21-cm radiation” can be “21-cm radiation” can be used to trace the used to trace the distribution of neutral distribution of neutral hydrogen in the Galaxy hydrogen in the Galaxy

Structure of the Milky Way Animation 75,000 light years Disk Nuclear Bulge Halo Sun Globular Clusters Open Clusters, O/B Associations

Structure of Milky Way Stellar Populations: Stellar Populations: …heavier elements are formed in various burning stages of stars… burning stages of stars…  Question: 1) Old stars should be more metal-rich… 2) Young stars should be more metal-rich… 3) They should be the same… how does the metal content of young and old stars differ?

Structure of Milky Way Of course: young stars are “metal”-rich Of course: young stars are “metal”-rich Stellar Populations Stellar Populations Population I: Young stars: metal rich; located in spiral arms and disk Population II: Old stars: metal poor; located in the halo (globular clusters) and nuclear bulge

Dynamics in the Milky Way (I) Population I (disk stars) Population II (halo stars)

Dynamics in the Milky Way (II) Differential Rotation Sun orbits around Galactic center with 220 km/s 1 orbit takes approx. 240 million years.

Dynamics in the Milky Way Question: What determines the velocity with which the sun is moving around the Galactic centre? Question: What determines the velocity with which the sun is moving around the Galactic centre?  Mass of the sun?  Rotational period of the spiral arm pattern?  Mass inside the orbit of the sun ?  Angular momentum of the Milky Way ?

Dynamics in the Milky Way Answer: Answer: Newton’s Laws tell us that it is the mass inside the radius of the sun that determines its velocity Newton’s Laws tell us that it is the mass inside the radius of the sun that determines its velocity The more mass there is inside the orbit, the faster the sun has to orbit around the Galactic center (argument similar to Kepler’s III. law)… V= 220 km/s M inside ~ M sol R= 8.5 kpc

Dynamics in the Milky Way Forms of rotation Forms of rotation   rigid rotation   differential rotation…

Dynamics of the Milky Way Use the 21-cm-radiation of neutral hydrogen to determine the rotation curve (“velocity as a function of radius”) of our Galaxy Use the 21-cm-radiation of neutral hydrogen to determine the rotation curve (“velocity as a function of radius”) of our Galaxy observed observed expected (if expected (if mass concentrated mass concentrated in centre) in centre)

Dynamics in the Milky Way explanation for the observed rotation curve: explanation for the observed rotation curve: There must be more mass than is visible !!! “ DARK MATTER “ (DM) “ DARK MATTER “ (DM) - 90 % of the matter in the - 90 % of the matter in the Galaxy is “invisible” Galaxy is “invisible” - only 10 % in stars - only 10 % in stars

Dynamics in the Galaxy What could dark matter be made of? What could dark matter be made of? i) dim stars, massive planets, black holes? i) dim stars, massive planets, black holes? (= massive compact halo objects= MACHOS ) (= massive compact halo objects= MACHOS ) experiments: only small fraction of experiments: only small fraction of DM are MACHOS DM are MACHOS ii) A new kind of particle ? (=weakly interacting massive particle= WIMP ) (=weakly interacting massive particle= WIMP ) maybe, but none such particle has maybe, but none such particle has been detected yet… been detected yet…

The centre of our Galaxy Wide-angle optical view of the GC region Galactic center Our view (in visible light) towards the Galactic center (GC) is heavily obscured by gas and dust: Only 1 out of optical photons makes its way from the GC towards Earth!

Radio View of the Galactic Center Many supernova remnants; shells and filaments Sgr A Arc Sgr A*: The Center of our Galaxy supermassive black hole2.6 million solar masses Galactic Center contains a supermassive black hole of approx. 2.6 million solar masses. Sgr A

Centre of our Galaxy motion of stars close to Galactic centre motion of stars close to Galactic centre (observed !) (observed !) everything consistent with a black everything consistent with a black hole of 2.6 million solar masses hole of 2.6 million solar masses

Other Galaxies types galaxies types galaxies   spiral galaxies   barred spiral galaxies

Other Galaxies elliptical elliptical galaxies galaxies irregular irregular galaxies galaxies

Other galaxies...some more beautiful galaxies…...some more beautiful galaxies… Sombrero galaxy Blackeye galaxy Andromeda galaxy

Other galaxies Stars rarely collide… Stars rarely collide… But galaxies do… But galaxies do… (observations !) (observations !)

Other Galaxies galaxy mergers galaxy mergers (computer simulation !)

Other galaxies Do other galaxies also contain supermassive black holes ? YES ! Do other galaxies also contain supermassive black holes ? YES ! Similar to accretion disk-jet connection in young stellar objects

Going to larger scales: Many galaxies are in clusters (“group”= small cluster)Many galaxies are in clusters (“group”= small cluster) Local group: Local group: - small cluster containing the milky way - small cluster containing the milky way - about 20 galaxies within a cubic parsec - about 20 galaxies within a cubic parsec - 3 spirals: Milky Way, Andromeda (“M31”), - 3 spirals: Milky Way, Andromeda (“M31”), M33 M irregulars: MW-satellites Small and Large - 4 irregulars: MW-satellites Small and Large Magellanic Clouds Magellanic Clouds - several dwarf ellipticals - several dwarf ellipticals Virgo Cluster: - nearest rich clusterVirgo Cluster: - nearest rich cluster - about 2500 galaxies - about 2500 galaxies - distance about 15 Mpc - distance about 15 Mpc

Virgo Cluster