An Introduction to Galaxies and Cosmology

An Introduction to Galaxies and Cosmology

1.1 Introduction Milky Way (our galaxy - Galaxy) Fig. 1.1 A photograph of one hemisphere of the night sky. (D.di Cicco, Sky Publishing Corp.) 1011 stars 1012 solar mass 30 1 solar mass = 2 x 10 kg flatten disc 100,000 ly the system of stars, gas, dust, and dark matter

1.1 Introduction "Does anything exist beyond the boundary of the Milky Way?" yes! Galaxies Fig. 1.2 An 'external' galaxy. NGC 2997 We think It resembles Our Galaxy.

1.2.1 The structure of the Milky Way Galactic Coordinates l : Galactic longitude b : Galactic latitude Fig 1.3 Galactic Coordinates System.

1.2.1 The structure of the Milky Way Galaxy's main components dark matter halo Galactic disc bulge stellar halo massive l : Galactic latitude b : Galactic longitude Spheroid Let's see each component of the Milky Way!

1.2.1 The structure of the Milky Way Dark matter halo Large and most massive component of the Galaxy. 12 ~10 solar mass never been observed at any wave length. holding the Galaxy together. Its shape is a spheroid. (Oblate spheloid b/a = 0.8) Luminous output are located in the centre of this. dark matter halo The ratio b/a = 0.8 is determined from observation of globular clusters or nearby galaxies (?) spheroid Luminous output

1.2.1 The structure of the Milky Way Galactic disc spiral arm disc Fig 1.4(a) Edge-on view of spiral galaxy. (NGC 891) Fig 1.4(b) Face-on view of spiral galaxy. (NGC 6744) ~ 1011 solar mass (Galactic disc) Spiral arms suggest that galaxies are rotating. The stars and the pattern of spiral arms generally travel at different speeds.

1.2.1 The structure of the Milky Way Bulge and Stellar halo bulge Bulge has ~ 1010 solar mass. Stellar halo has ~ 109 solar mass. Milky Way bulge is elonged,which makes the Milky Way a barred spiral galaxy.

1.2.1 The structure of the Milky Way Galaxy's main components dark matter halo Galactic disc bulge stellar halo massive 12 Spheroid ~10 11 ~10 10 ~10 9 ~10 The Galaxy's major component is the dark matter halo. The Milky Way is a barred spiral galaxy.

1.2.2 The size of the Milky Way parsec Distances in the Galaxy are usually measured in units of parsecs (pc) or kiloparsecs (kpc). 16 1 pc = 3.26 ly = 3.09 x 10 m Earth 1AU 1'' 1 pc Sun

1.2.2 The size of the Milky Way "How big is the galaxy?" The answer depends greatly on which component you measure. dark matter halo luminous component gas disc stellar halo

1.2.2 The size of the Milky Way Measuring the dark matter halo size It is the most difficult to measure. Since its presence is deduced only from its gravitational influence. It can be assesed from its effect on the motions of neighbouring galaxies. The dark matter halo apparently extends at least the Magellanic Clouds which distance are 50 to 60 kpc. Magellanic Clouds as a lower limit on the radius of the dark matter halo Magellanic Clouds 100~120 kpc

1.2.2 The size of the Milky Way Measuring the Galactic disc size It has a gaseous disc that extends out to at least 1.5 times the radius of its stellar disc. Note that the gas extends well beyond the stellar disc, and that the spiral pattern is visible out to the edge of the gas disc. Fig. 1.6 Difference between gaseous and stellar discs in a spiral galaxy NGC 6744. (Ryder et al., 1999)

1.2.2 The size of the Milky Way Measuring the stellar halo and the bulge The stellar halo has no gaseous component, its size is given by the distribution of stars. but difficult, since the density of stars falls off gradually. Now we simply state 20 kpc ~. The bulge size is small, so little relevance to any discussion of the Galaxy size.

1.2.2 The size of the Milky Way Measuring the Galaxy size It is always possible to define the size of the Galaxy as the size of one of the components, but such a definition would be rather arbitary.

1.2.3 The major constituents of the Milky Way The structural component of the Milky Way dark matter halo disc bulge stellar halo Their constituents dark matter star gas dust Now, we look these constituents more closely.

1.2.3 The major constituents of the Milky Way Dark matter Baryon - Proton and Neutron. ( etc.) "Is dark matter a baryon?" Some may be baryonic matter. BUT, there is strong evidence indicating that much of the dark matter is non baryonic. The nature of non-baryonic dark matter is still a mystery.

1.2.3 The major constituents of the Milky Way Stars 1011 solar mass (If sun is typical.) 11 10 stars in the Galaxy. Stars can differ from one another in their mass, their age, and their chemical composition. Spectral Class O High B A F G temperature K M Low

1.2.3 The major constituents of the Milky Way Gas Most of the Milky Way's gas and dust lies in the disc. and is found within a vertical distance of 150 pc of the Galactic plane. Hydrogen - 70% Helium - 28% The other elements - 2% In various forms molecular hydrogen ionized hydrogen atomic hydrogen These are collectively reffered to as metals.

1.2.3 The major constituents of the Milky Way Dust - consists of tiny lumps of solid compounds of carbon, oxygen, silicon and other metals. The size of dust particles is comparable to the wavelength of light. 10-7 ~ 10-6 m particularly effective at scattering light, as well as absorbing it.

1.2.3 The major constituents of the Milky Way Zone of obscuration very difficult The presence of dust in the disc of the Milky Way severely limits our ability to make optical observation in certain directions. impossible Galaxy Map Dust in our Galaxy prevents us from seeing the light from other galaxies in directions close to the Galactic plane Fig. 1.7

1.2.3 The major constituents of the Milky Way Interstellar medium (ISM) - gas and dust that occupies the space between stars. Almost half of the ISM is contained cool dense gas. Molecular cloud - 1% of volume molecular cloud H II region - a few per cent of mass and volume The Orion Nebula One of the best known region 6 pc in diameter containing gas~100 solar mass Much of the remaining volume Intercloud medium. may be hot, or warm Fig. 1.8 The Orion Nebula (M42) (NASA)

1.2.3 The major constituents of the Milky Way The ISM is intimately associated with star formation and stellar evolution. We have now completed our survey of the main Galactic components and their constituents. dark matter, stars, gas, and dusts. dark matter halo, disc, stellar halo and bulge.

1.2.4 The stellar populations of the Milky Way star mass age chemical composition Does any differences in these parameters exist between the stars in each of the different Galactic components?

1.2.4 The stellar populations of the Milky Way Globular clusters 104~106 members compact dense very old stars spherical 50pc 2/3 in the stellar halo 1/3 in the stellar disc Fig1.9 Globular cluster 47 Tuc. They account for only about 1% of all stars in the stellar halo.

1.2.4 The stellar populations of the Milky Way The distribution of globular clusters They are distributed approximately spherically about the centre of the Galaxy.

1.2.4 The stellar populations of the Milky Way Baade's discovery He noted difference in colour between the stars in the disc and spheroid of M31. disc stars spheroid stars BLUE RED Population I (Pop. I) Population II (Pop. II) Fig1.11 Walter Baade Figure M 31

1.2.4 The stellar populations of the Milky Way metallicity Z= the mass of elements heavier than helium in the object the mass of all elements in the object The metallicity of the sun is Z = 0.02.

1.2.4 The stellar populations of the Milky Way Population I Baade associated with the disc includes very young stars a few million years ~ 1010yr Z = 0.01 to 0.04 move in circular orbits confined to the Galactic disc

1.2.4 The stellar populations of the Milky Way Population II occupy the spheroid 9 (12 to 15) x 10 yr Little or no interstellar gas is still associated with Pop. II stars only low-mass stars still shine Pop. I circular orbits Pop. II eccentric orbits Fig. 1.12 A face on view of the Galaxy showing sample orbits for Pop. I and Pop. II stars.

1.2.4 The stellar populations of the Milky Way Population III Theoretical Population formed unprocessed gas (by the big bang) -9 Z ~ 10

1.2.5 The chemical evolution of the Milky Way Why the different populations have different ranges of metallicity? main sequense star Z = ISM Z at the time the star formed ISM star formation main sequence chemical enrichment materials return to the ISM COSMIC RECYCLING late stage

1.3 The mass of the Milky Way We saw (in 1.2.*) dark matter halo ~ 1012 solar mass Galactic disc ~ 1011 solar mass stellar halo ~ 1010 solar mass interstellar medium ~ 109 solar mass How astronomers know these values? How uncertain? Let's learn about it

1.3 The mass of the Milky Way rotation curve - rigid body rotation r angular speed = const. @ each point

1.3 The mass of the Milky Way rotation curve - differential rotation http://www.intelore.com/solar-system/solar-system-large.jpg Fig. 1.13 (b) Solar system rotation curve each travels at different angular speed differential rotation The mass of the sun domiates solar system.

1.3 The mass of the Milky Way rotation curve - The Milky Way There is no massive central body dominating the Milky Way.

1.3.1 Calculating the mass of a gravitating system v r M m If M m, rotation curve is GM 12 v= r. 2 v r M= G Note that the orbital speed of the less massive body does not depend on its mass.

1.3.1 Calculating the mass of a gravitating system Question 1.6 summary (a) The Circumference of the Earth's orbit around the Sun is ~ 1012m (b) The speed at which the Earth orbits the sun is v= 2 r 1 year ~ 3x104m/s 2 v r (c) From equation M = G, M = 2 x 1030 kg. Can we use this way to calculate the mass of inner part of Galaxy? It is possible to make a simple estimate of the mass, using Newtonian gravitation theory. r = 1.5x1011m M

1.3.1 Calculating the mass of a gravitating system Newtonian gravitational theory r equal r spherically symmetry mass distribution total mass = M M We can calculate the mass within the sphere at the radius r. M(r)

1.3.2 Using rotation curves The flat rotation curve does not imply constant density. real symmetry assumption inadequency Fig. 1.13(b) Galaxy rotation curve Several independent investigations have failed to show any sign of a decline in the rotation curve of the Milky Way out to a radius of 20kpc. dark matter

1.3 Summary We can use the equation v2 r M= G to estimate the mass of inner part at radius r.

1.4 The disc of the Milky Way If we consider the disc alone, we find that there the dark matter has less impact. No more than 30-50% of the disc's mass is due to dark matter. Importance of the disc - two reason most of the visible matter belongs the main site of current star formation in the Milky Way

1.4 The disc of the Milky Way Near infrared Visible stars cool stars Radio Far infrared dust gas Fig. 1.14 All-sky views of the Milky Way at various wave length.

1.4.1 The stellar content of the disc Open clusters typically 2-3 pc across the density of stars is enhanced locally stars formed at the same time thousands of open clusters in the disc relatively short lives ~ no more than 109yr Fig. 1.15 The Pleiades Trapezium Regin in M42.

1.4.1 The stellar content of the disc OB associations have diameter of 100 pc or so. densities not so much greater than their general surroundings unusually high proportion of O- and Bclass stars Many OB associations have a very young open clusters at their center About 70 OB associations are known. Fig. 1.16 A portion of the Milky Way showing the Hyades and Pleiades open clusters and OB associations in Taurus and Perseus.

1.4.1 The stellar content of the disc subpopulations The disc stars are primarilly Pop. I objects. They can divide into a range of subpopulations Pop. I, spiral arm stars Pop. I, thin-disc stars Intermediate Population (thick disc stars)

1.4.1 The stellar content of the disc Pop. I, spiral-arm stars The youngest stars in the galaxy ~ 109 yr Hyades, Pleiades, O and B stars, super giant star, pulsating giant stars (Cepheid), T tauli stars Z = 0.02 ~ 0.04 Associated Object - glowing HII region

1.4.1 The stellar content of the disc Pop. I, thin disc stars Older than spiral arm stars ~ 1 to 10 x 109 yr Z = 0.005 to 0.04 move in circular orbits found within 500 pc from mid-plane of the disc

1.4.1 The stellar content of the disc Intermediate population (thick disc stars) lower metallicities than thin-disc stars Z = 0.002 ~ 0.01 their ages closer to the thin disc max. 10 x 109 yr still basically circular orbits travel to greater distances from the Galactic plane

1.4.2 The gaseous content of the disc The gaseous interstellar medium is intimately associated with stellar evolution. Gas and dust are important constituents of the disc.

1.4.2 The gaseous content of the disc The 21cm emission line of atomic hydrogen higher energy level 21cm radio wave 5.9x10-6eV parallel (higher energy) radiation collision lower energy level T > 0.046 K reasonable anti-parallel (lower energy)

1.4.2 The gaseous content of the disc The gas forms a disc about 300 pc thick surrounded by hotter gas in the form of clouds, ~ thousands ~ 300 pc The individual clouds take the form of sheets or filaments of gas. clouds intercloud medium - not occupied by clouds (exists mainly in the form of HI) hotter gas

1.4.2 The gaseous content of the disc The gaseous disc is not completely flat. r = 14 kpc r = 16 kpc declined Galactic centre Fig. 1.18 The Galactic HI density on cylindrical sufaces. Fig. 1.19 A schematic diagram of the warped disc of gas. inner six - same plane outer three - tilted by a few degrees

1.4.2 The gaseous content of the disc Molecular Hydrogen (H2) In the disc, about 50%of the mass of the hydrogen is molecular. HII region Condition the density is high temperature below 100K UV flux - low molecular cloud Fig. horsehead nebula cool dense clouds numelous between 4 and 7 kpc from Galactic centre about 100 different molecules have been detected (CO, CH3CH2OH, etc.)

1.4.2 The gaseous content of the disc Carbon monoxide (CO) as a tracker of molecular gas H2 (, C2, N2,..) symmetric No radiation from rotational energy transition UNDETECTABLE CO rotational transitions can be observed = 1.3mm and 2.6mm An important tracer of cold molecular cloud

1.4.2 The gaseous content of the disc Interstellar dust sublimate If dust is heated too much (T > 2000) Ordinally, dust grains ~ 20K easily able to survive Fig. 1.21 The infrared spectrum of radiation emitted by sources in the Galactic plane.

1.4.3 A cross-section through the disc The vertical distribution of stars n z =n0 e n(z) z h n0 : : : : z h number density distance above or below the mid-plane scale height number density of stars in the mid-plane The scale height of the disc is the distance over which the number density of disc stars decreases to 1/e times its mid-plane value. whole collection, subpopulation, other entities (ISM etc.)

1.4.3 A cross-section through the disc The thin disc G, K, M O, B most stars are formed near the mid-plane h ~ 300 pc h ~ 50 to 60 pc scattered by giant molecular clouds

1.4.3 A cross-section through the disc The thick disc G, K h ~ 1000 to 1300 pc Why the thin disc and the thick disc differ in this way? currently uncertain Does it reflect different origins of the two subpopulations? Did some event occur during the formation of the disc?

1.4.3 A cross-section through the disc The ISM n z =n0 e z h z h z = 0 e is the mass density. ISM h ~ 150 pc Star Formation Rate (SFR) The rate at which stars are formed from the ISM. Measured in solar masses per year in any specified region. SFR n power law

1.4.4 The spiral arms Fig. NGC 2997 Tracing spiral structure Why the spiral arms stand out? concentrations of bright objects associated with recent star fomation strong concentrations of mass

1.4.4 The spiral arms Spiral-arm tracers molecular clouds HII regions open clusters OB associations Fig. 1.23(a) face-on view of the Galaxy mapped out the spiral-arm tracers. Fig. 1.23(b) artists' conceptions of the Milky Way

1.4.4 The spiral arms The winding dilemma Spiral arms are not just transient (short-lived) structures. composed of an unchanging set of stars differential rotation the pattern is soon broken they cannot always be made of the same stars

1.4.4 The spiral arms Density wave theory disc (approximately) smooth, axially symmetric distribution of matter density enhanced regions are naturally developed certain spiral patterns of density enhancement were especially favoured and could become self-perpetuating. spiral density waves Fig. 1.24