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Syllabus for 2GLSS, Galaxies and Large Scale Structures. Dr. P. H

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1 Syllabus for 2GLSS, Galaxies and Large Scale Structures. Dr. P. H
Syllabus for 2GLSS, Galaxies and Large Scale Structures Dr. P.H. Regan, 29BC04, x Spring Semester Books 1) Discovering Astronomy, Robins, Jefferys and Shawl, Wiley, (RJS) 2) An Introduction to Modern Astrophysics, Carroll and Osterlie, Wiley (CO) 3) Introductory Astronomy, Haliday, Wiley (HAL) 4) Active Galactic Nuclei, Robson, Wiley, (ROB) 5) Large-Scale Structures in the Universe, Fairall, Wiley (FAI) 2GLSS P.H. Regan

2 2GLSS Course Outline Perspectives Galaxies Large Scale Structure
Size scales Nuclei and atoms interstellar medium Standard Model Galaxies Milky Way Galaxy types, spirals, ellipticals... Colliding Galaxies Large Scale Structure Hubble’s Law Recognition of large scale structures Active Galaxies Active Galactic Nuclei Gamma-ray bursters 2GLSS P.H. Regan

3 Is the Universe Infinite ?
Olber’s Paradox (RJS p534, CO p1222) Q. Why is the sky dark at night ? If the universe was infinitely large and old, you would see a star in your line of sight in all directions, the night sky should be bright! This is evidently NOT the case, ‘Olber’s Paradox’. Olber’s ‘solution’, space not transparent BUT this wouldn’t matter as any interstellar dust would be heated to the same temp. as stellar surface and thus glow the same colour . Also proposed was that the recession velocity moved the light out of visible wavelengths (‘redshifted’), BUT the shift is not large enough. A. Light has a finite speed (c=3x108 ms-1) and the light from the furthest stars has not reached the earth yet (solution proposed by Lord Kelvin and Edgar Allen Poe!) Thus the observable universe is finite in size (and age). 2GLSS P.H. Regan

4 Size Scales Nuclei approx 10-15 m, stellar fuel.
Atoms approx 10-10m, cosmic probes People approx 1m, cosmic observers Stars approx 104m->1012m cosmic furnaces Galaxies approx 1019m->1021m (this course) Units of Distance (RJS p320, CO p64) 1 astronomical unit (AU) = 1.5x1011m (= earth - sun distance) 1 Parsec =3.1x1016m (=1 parallax second) d (pc) = 1 Au / p (in seconds of arc) 1 light year (ly) = 9.5x1015m (=distance light travelled in 1 year) Ro = 7.0x108m (= solar radius) RO = 8.0(5)kpc (= sun -> centre of galaxy) 2GLSS P.H. Regan

5 Nuclei: Stellar Fuel. (CO p349) Elements up to Fe (Z=26) can be formed by nuclear fusion, which keeps the star ‘burning’. H (Z=1), burns to He (Z=2), which burns to Carbon (Z=6). Then Carbon, Oxygen (Z=8) and Silicon (Z=14) burning are all allowed in large (heavy, hot and dense) stars. For Z>26 fusion is no longer energetically favourable, but heavier elements can be formed by neutron capture followed by b- decay. (via the ‘slow’ (s) or ‘rapid’-neutron processes). 2GLSS P.H. Regan

6 See
2GLSS P.H. Regan

7 From Wiescher, Regan and Aprahamian, Physics World, February 2002, p33
2GLSS P.H. Regan

8 Elemental Abundances (CO p526, RJS p109, 307)
Note x-rays from solar photosphere show evidence of elements up to U (Z=92) in the sun. Large stars live for only around years, very short compared to universal time scales. After this, large stars collapse and expel much of their reaction products into space for future star formation. 2GLSS P.H. Regan

9 From 2002 RIA summer school, see
Pm (Z=61) Tc( Z=43) 2GLSS P.H. Regan

10 Atoms, Cosmic Probes. 2GLSS P.H. Regan

11 Emission and Absorption Lines
Star light (black body) Interstellar medium ( diffuse gas and dust) Absorption lines (observation) Emission lines Most (approx 70%) of the Inter-Stellar Medium (ISM) is made up of hydrogen, in either atomic or molecular form. For Hydrogen ATOMS the principal lines come from the Lyman, Balmer and Paschen Series 2GLSS P.H. Regan

12 A hot, dense gas (or solid object) produces a continuous spectrum with no dark spectral lines. (Black-body spectrum) A hot, diffuse gas produces bright emission lines when an electron makes a transition from a higher excited state to a lower one. The wavelength of the emitted photon can be calculated from the energy difference between the initial and final levels. A cool diffuse gas in front of a black-body source produces dark, absorption lines when an electron is raised from a low-excitation energy orbit to a higher one. 2GLSS P.H. Regan

13 Fe absorption spectrum
Fe emission spectrum Fe absorption spectrum H absorption H emission See 2GLSS P.H. Regan

14 Where are our body parts made ? (% of human body by weight)
See 2GLSS P.H. Regan

15 Wiescher, Regan and Aprahamian, Physics World (2001) p33
Note that the elements Tc (Z=43) and Pm (Z=61) have no stable isotopes. Longest lived isotopes are 145Pm (T1/2=18 yrs) and 98Tc (4.2x106yrs). Observation of the atomic spectra of these elements in extra-solar stellar atmospheres is proof of on- going elemental synthesis. Wiescher, Regan and Aprahamian, Physics World (2001) p33 2GLSS P.H. Regan

16 Proposed Creation Sites of Elements.
(taken from 2GLSS P.H. Regan

17 S and R process peaks due to nuclear shell effects
2GLSS P.H. Regan

18 Metallicity (CO p920) The iron (Fe, Z=26) content can be used as a good indicator of the age of the star. Newer stars have a higher iron content than their predecessors (more generations of reactions). The metallicity is defined as the iron-to-hydrogen (Fe:H) ratio in the atmosphere of a star compared to the solar value. This is given by the expression [Fe/H] = log10 (NFe/NH) - log10(NFe/NH)o where log10(NFe/NH)o corresponds to the solar value. This stars with metallicities identical to the sun’s have [Fe/H]=0. Typical values range from -4.5 for old, metal-poor stars, to +1 for young, metal-rich ones. Note that some astronomers believe Type 1a supernovas may distort the local values of the [Fe/H] ratio for different regions of the inter-stellar medium (ISM) and thus prefer the [O/H] ratio instead as an measure of stellar age. 2GLSS P.H. Regan

19 Forbidden Lines, ‘Metastable states, ‘isomers’
CO p404, Certain transitions from excited atomic and molecular states are hindered in their decay, usually by some quantum mechanical decay selection rule. This can give rise to very long lifetimes for such states. For such atomic states to exist in interstellar gas etc. the density must be very small since (as on earth) random atomic collisions would de-excite this state. Examples of such states at the l=21cm decay in the neutral hydrogen atom (H-I) and the green glow associated with some nebular arising from emissions in doubly ionised oxygen (O-III). 2GLSS P.H. Regan

20 (spins anti-parallel). 1st excited state of HI (spins parallel).
Hydrogen (see RJS p445) HI = hydrogen atoms. Existence is often seen by radiotelescopes via the observation of the electron-proton ‘spin-flip’ transition from hyperfine splitting. This has a wavelength of 21cm (n =1420 MHz, E =5.9x10-6eV). proton e- Ground state of HI (spins anti-parallel). 1st excited state of HI (spins parallel). -13.6 eV Energy Ionisation n=1 n=2 n=3 Nb. Zeeman effect gives B-field measurement 2GLSS P.H. Regan

21 This discovery was important because
Hendrik Van de Hulst’s prediction of the observation of the 21 cm line allowed the study of cold, neutral hydrogen in the cosmos. If H atoms collide with neighbouring atoms, the atom can be raised from its anti-parallel spins ground state to the excited, parallel-spin config. This is a low-energy, excited metastable state (~107 years due to non-conservation of spin, 1s->1s, but photon has intrinsic spin 1). Due to the low density, the atom can remain in this state for a long time before decaying back to the ground state via the 21 cm emission. This discovery was important because It’s a feature of H (most abundant element) It occurs only in low density regions It indicates the presence of neutral ( i.e.non-ionised and non-molecular) HI atoms It is not easily absorbed by interstellar gas. This means that 21cm radiation emission fromalmost anywhere in the galaxy can be measured on earth via radiotelescopes. 2GLSS P.H. Regan

22 Hydrogen Molecules (H2)
Taken from RJS p369 (RJS p445) If the interstellar hydrogen is close to a hot star, the H2 molecules can be ionised and form a region of H-II. Interstellar lines often show different component with slightly different wavelengths. This is caused by Doppler shifts which depend on the relative velocity of the specific cloud. Dust absorption means that emissions in the visible region are not useful in determining the overall structure of the galaxy. 2GLSS P.H. Regan

23 From
Far Ultraviolet Spectroscopic Explorer (FUSE) data on AGN which interstellar absorption from gas in milky way, including H2 molecules. 2GLSS P.H. Regan

24 The Interstellar Medium (ISM)
Importance of radio-astronomy in seeing further due to less scattering/absorption in interstellar dust. Interstellar dust is heated to approx k by the stars in the galaxy, which then radiated in the far infra-red region (30-300mm). Stars radiate strongly in the near infrared region (1-10 mm). Very little stellar ‘extinction’ of light (as which occurs for the visible region) occurs in this range. See e.g., COBE spectra. Spin-flip transition in hydrogen gives rise to a 21cm radiowave emission. Thus, radiotelescope surveys allow the distribution if hydrogen across the plane of the milky way and its Doppler shift allows us to determine the speed at which the (H) gas in the galaxy rotates. 2GLSS P.H. Regan

25 Molecular Gas Clouds (CO p446)
Typical temperatures of around 20K (c.f. ISM typical temp ~ 100K). Density in such clouds ~ atoms /cm3 (c.f. sea level earth atmosphere ~3x1019 /cm3). Gas is mostly molecular hydrogen, H2…(ISM gas is mostly H-II, i.e., ionised H2 mols). Note that the H2 molecule does NOT emit the 21cm line. Thus hard to identify in visible region (use rotational decays)…also need to use tracers with known relative abundances, such as CO, CH, OH and C3H2. Giant Molecular Clouds (GMC) These are very large collections of dust and gas with T~20K with typical densities of ~ cm-3. They can stretch for distances of 50 parsecs and contain up to 106 solar masses. Inside the GMCs are more hot and dense cores with dimensions of parsec, T~ K and densities of /cm s of GMC are known in our galaxy, mostly in the spiral arms (see later) 2GLSS P.H. Regan

26 Bok Globules (RJS p375, CO p446)
Names after Bart Bok, these are dark (cold, dense) regions which are often seen projected against bright nebulae. Bok globules are small, dense clouds, with large visual extinctions (al~10) and low temperatures (~10K). They are relatively dense (~104cm-3) and have masses between 1 and 1000 solar masses, with sizes of around 1 parsec. About one quarter of these have young, proto-stars inside. Bok Globules IC-2948 Inside this emission nebula are groups of dark, opaque clouds, i.e., Bok globules. See 2GLSS P.H. Regan

27 Some good web pages for galaxy informantion and figs. 2GLSS P.H. Regan

28 ISM, Composition (RJS p444)
Interstellar Dust Early studies of UV radiation showed spectral features at 220 nm wavelength, corresponding to known transitions from graphite (carbon). Infra-red astronomy then showed that some stars were surrounded by dust shells which heat up and subsequently re-radiate in the infra-red region. Although these spectra are generally continuous, for some stars, an extra continuous peak was superimposed on the usual black body spectrum at wavelengths of approx. 10,000 nm. This was consistent with significant amounts of silicates (e.g. quartz SiO2) in the dust cloud. It has been suggested that C and Si grains are formed in the carbon-rich atmospheres of giant pulsating stars. At expansion, the outer layers of such stars cool and the carbon atoms can stick together to make ‘grains’. When this region heats up again, the increased radiation pressure from the star pushes these out of the star’s atmosphere and into space. 2GLSS P.H. Regan

29 Interstellar Gas (RJS p366ff,CO p447)
The ISM also contains large amounts of gas, which is mostly made up of hydrogen. Depending on the density and temperature of the regions where this hydrogen is, it can exists either in its natural atomic form (HI), its stable molecular form (H2) or the ionised form of the molecule (HII). In addition to hydrogen, a number of other molecules have been observed in the ISM, such as CO (carbon monoxide), SO2 (sulphur dioxide), OH (hydroxyl), H2O (water), NH3 (Ammonia), H2CO (formaldehyde), H2S (hydrogen sulphide) CH3CH2OH (ethyl alcohol!), HC11N (organic?). These molecules are identified by their emissions which occur in the UV, visible and IR regimes. Usually decays from excited states are by photon with wavelengths in the mm region corresponding from the decay from a rotational state to one with a slightly slower rotation. Such wavelengths can penetrate the earth’s atmosphere and be observed. 2GLSS P.H. Regan

30 Interstellar Reddening.
(CO p265) Dust particles scatter short wavelengths (blue) more effectively than long (red) ones. (Sky is blue, setting sun is red!) This effect can give rise to an interstellar reddening effect of stars which are partially obscured by dust clouds. This scattered light also tends to be (partially) polarised. Dust clouds can obscure the view of stars (and galaxies), and the centre of the Milky Way. The amount of interstellar extinction depends on the wavelength, l, and the path length, s. Original light from source Interstellar dust cloud. Transmitted (red) light Scattered (blue) light 2GLSS P.H. Regan

31 Absorption includes the scattering of light and
(See CO p265, p438) Absorption includes the scattering of light and true absorption from e.g., electrons being to higher energy states in atoms and molecules. It is wavelength dependent. If dIl is the change in intensity through distance ds, then Opacity is the cross-section for absorbing photons of wavelength, l, per gm of stellar material. 2GLSS P.H. Regan

32 Spectral Maps of the Galaxy
Ref 2GLSS P.H. Regan

33 Standard Model of the Universe.
Big Bang ~2x1010 years ago, created an expanding universe, now 2x1010 ly radius (constant expans.) Primordial Abundances: ~80% Hydrogen, ~20% Helium, trace amounts of Li and Be. After ~105 years, regions condense, gravitational energies leads to heating, nuclear reactions (proton-proton chain), stars form. H burns to He (p-p chain). For heavy stars, nuclear fusion reactions can burn to form elements all the way up to Iron (Fe, Z=26). Small stars eject planetary nebula which releases some material into space, but most kept in core (to form white dwarf). Large stars (>10Mo) have life cycles of ~107 years followed by cataclysmic supernova. Most of their material is expelled into the ISM. New stars form in the ISM which are metals-rich ( i.e., higher metalicities). 2GLSS P.H. Regan

34 Galaxy formation…. a problem
Galaxy formation….a problem. An early period of rapid expansion is need to give sufficient inhomogeneity. Inflation theory. Recent distant (> 5 billion ly) supernova type 1A (good ‘standard candles’) red-shifts imply that expansion rate is increasing with time and the universe is older than originally thought? i.e, ‘anti-gravity’ ? Possible evidence for so-called dark energy. (see Burrows, Nature 403 (2000) p ). From SPATIUM, no7. May 2001, p11 2GLSS P.H. Regan

35 See
Type 1a supernova (see HOL p246) are excellent standard candles for distance measurements using Hubble’s law (see later). See Perlmutter et al., Nature, 391 (1997) p51 2GLSS P.H. Regan

36 Our Galaxy, The Milky Way (CO, chapter 22 RJS chapter 20)
Sun located 8(1)kpc from galactic centre. Orbital period 2.2x108 years, orbital speed of 790,000 km/h. 50kpc disk of 600pc thickness and a central bulge of approx. 3kpc thick. Total galactic mass of approx solar masses. 10% of mass attributed to million stars, gas and dust with other possible 90% ‘dark matter’. Supermassive black hole in centre with mass of approx. 3x106 solar masses. 2GLSS P.H. Regan

37 Distribution of Globular Clusters
Distribution of Globular Clusters. (RJS p436) (1917) Before effects of dust were known, Harlow Shapley studied the distribution of globular clusters in space. He calculated their distances using (variable) standard candles known as RR-Lyrae stars located within these clusters. Shapley found that these clusters were further from the sun that thought and thus the galaxy must be larger than previously believed. Shapley reasoned that these large globular clusters were such large components of the galaxy that they would be unlikely to be distributed to one side. He thus proposed that the centre of the globular cluster distribution coincided with the centre of the galaxy, and thus that the sun was actually quite far from the centre. (Modern value is between 25-30Kly). 2GLSS P.H. Regan

38 M2 cluster (‘Messier object’).
M2 cluster (‘Messier object’). First found by Maraldi (1746). Messier (1760) catalogued it as nebula but without individual stars. Hershel (~1780) resolved individual stars within the cluster. Approx. 150,000 stars and diameter of 140 ly 2GLSS P.H. Regan

39 See also great pictures and info at the following.
See also great pictures and info at the following. 2GLSS P.H. Regan

40 Milky Way from combined images at
Milky Way from combined images at near-infrared wavelengths of 1.2, 2.2 and 3.4 microns from the COBE satellite. Note thin disk and central bulge. Redder regions are due to light absorption from dust. Artist’s impression of above view of Milky Way from computer simulation. See web page given below. 2GLSS P.H. Regan

41 Details of the Structure of the Milky Way (see CO p927ff, RJS p439, p457)
disk sun ~15kpc halo bulge nucleus Thin disk is metal-rich, [Fe/H]~0, star formation, lots of young blue stars. Around 325 parsec region of sun. Thick disk, metal-poor [Fe/H]~-0.5, older stars. Disk thickness increases towards the inner regions of galaxy.Gas/dust in disk absorbs visible light, but 21cm H-I line ok. Thus, radiotelescopes can map velocity and distribution of H-I gas. Galactic bulge. Spheroidal region near centre of galaxy. Only certain w.lengths observed due to dust. Disc meets the galactic bulge at a radius of approx. 1kpc. Vertical scale in bulge is around 400 parsecs (along the bulge’s minor axis). Major to minor axis ratio for the bulge is thought to be a ~0.6. Wide range in metallicity, in the bulge with -1 < [Fe/H] <+1, average ~ +0.3, i.e., average Fe to H ratio is about 2 x solar value. i.e., younger stars dominate here. Bulge mass is ~ 1010Mo. 2GLSS P.H. Regan

42 The surface brightness of the bulge, I, in units of Lopc-2 is given by the r1/4 law, also known as de Vaucloulers profile (1948). Confirmed by COBE results. Re is the reference radius, Ie is the surface brightness at re. Formally, re is defined as the radius at which one half of the bulge’s light is emitted. Baade’s window is a gap in the dust clouds in the bulge which allows the observation of RR-Lyrae stars (standard candles) beyong the galactic centre. Appears 3.9o below and within 550 parsecs of the galactic centre. Galactic B-field: Disk field estimated to be approx 0.4nT (~10-5 times solar B-field). Deduced from Zeeman-splitting effect on the two states in H-I resposible for the 21cm line). Also from polarisation of scattered light. 2GLSS P.H. Regan

43 The Stellar Halo is the region around the disk and bulge
The Stellar Halo is the region around the disk and bulge. It is made up of a few hundred globular clusters and many high-velocity (‘field’) stars. The globular clusters consist of old, metal-poor stars, with the oldest clusters ([Fe/H] <-0.8) scattered throughout the halo. Appear to be approx. 1.6x1010years old ( i.e. as old as the universe, surely not possible..see RJS p930 and chaps. 25 and 27) (CO p930) Two distinct regions of metallicity for globular clusters, Generally, more metal-poor (older) in spherical distribution around galactic centre with more metal-rich (younger) in galactic plane. 2GLSS P.H. Regan

44 Dynamics of the MilkyWay
(CO p935, RJS p448) Motion of the Sun Note that the celestial equator ( i.e. the plane through the earth’s equator) is at 63o to the galactic equator ( i.e., the plane through the galaxy’s disk). Definition of galactic co-ordinates, b (Galactic latitude) and l (Galactic longitude) are relative to the motion of the sun. To north galactic pole (NGP) centre star rotation l b sun NGP has co-ordinates of b=90o. By convention Galactic centre has (almost) l=0o and b=0o . 2GLSS P.H. Regan

45 Cylindrical Co-ordinate System (CO p941)
Cylindrical co-ordinate have centre of galaxy at origin. galactic centre star rotation sun q R z P Q In this co-ordinate system, the corresponding velocity components are given by, 2GLSS P.H. Regan

46 Local Standard of Rest (LSR) CO p942)
To investigate the motion of the sun and other local stars, we must first define the Local Standard of Rest (LSR). This is defined as a point which is instantaneously centred on the sun and moving in a perfect circular orbits about the galactic centre. Thus, by definition, the velocity components about the LSR must be The velocity of star relative to the LSR is known as the peculiar velocity and is given by 2GLSS P.H. Regan

47 Motion of Sun in Galaxy (see CO p945)
The LSR has Q0~220 km/s and a rotational period of about 230 million years. To a good approximation, there is no motion relative to the LSR in the R and z directions, but there is a significant Q effect (see CO p943). The sun’s peculiar velocity (relative to the LSR) is called the solar motion and has values of: 2GLSS P.H. Regan

48 Measuring Velocities (CO 107, p126-127) Proper motion. vr vq star
observer star Proper motion. Change over time of stellar co-ordinates. Note that need to know distance (only useful for nearby stars). Doppler Shift Christian Doppler (1842) found that as the sound waves moved through air, the observed w.length, lobs, is compressed in the forward direction and expanded in the backward direction (compared to a stationary observer) by the expression, 2GLSS P.H. Regan

49 For light however, there is no speed of sound,
as there is medium involved and the expression must be obtained using special relativity. q Source moving at velocity, u 2nd signal to observer 1st signal u If Dtrest is the time difference between the emission of light crests and Dtobs is the difference in time between their arrival at the observer then 2GLSS P.H. Regan

50 Remembering that the frequencies of the light
from the source (nrest) and and the observed frequency (nobs) are given by The Relativistic Doppler Shift is then given by Note v =vel. n = freq For RADIAL MOTION ONLY 2GLSS P.H. Regan

51 For objects moving away from the observer,
there is a shift down in frequency ( i.e. up in wavelength) towards to RED end of the spectrum ( i.e. lobs>lrest and vr>0). For movement towards the direction of the observer, the shift is to the BLUE (i.e. lobs<lrest and vr<0) Since most astronomical objects are moving away from the earth, a redshift parameter, z, is used to describe the change in observed wavelength (and thus the radial velocity), where 2GLSS P.H. Regan

52 Galaxy Rotation (CO p955-8, RJS p477)
Can use the circular orbits of stars and Newton’s laws for objects far from the galaxy’s interior, equating the centripetal and grav. forces This predicted density dependence (~1/r2) is much slower than deduced from the star counts beyond the centre of the galaxy. The density of luminous (i.e., visible) stars in the stellar halo appears to fall off as (~1/r3.5)….. This has been put forward as an argument that the majority of mass in the galaxy is actually in the form of Dark Matter. In a spherically symmetric system, the condition for mass conservation is given by (r=density) 2GLSS P.H. Regan

53 Measuring Galactic Rotation Curves
// // // The rotation curves for galaxies are the measured velocities (via the Doppler shift) of H-I and carbon mononxide (CO) gases. The z-value observed in these lines can be used to calculate the radial velocity of the gas. Note that CO emits spectral lines which are easier detect through the region of the bright centre of a galaxy (unlike the visible H lines). Note, max Doppler shift (solid line above) is when motion is directly towards or away from observer. 2GLSS P.H. Regan

54 This can be used to estimate mass of galaxies.
CO p960) (RJS p477) From Kepler’s 3rd law as modified by Newton, for a star travelling about a galaxy with a Rotational Period, P in years, Mstar and Mgalaxy are in solar masses and R is the distance to the galaxy’s centre in AU, then This can be used to estimate mass of galaxies. 2GLSS P.H. Regan

55 specific spectral line (2) measure z(v) for each
(1) measure Dl for specific spectral line (2) measure z(v) for each point along galaxy disk (3) Subtract vrecession from galactic centre. (4) residual is rotation curve 2GLSS P.H. Regan

56 Measuring the Doppler at each end of the galaxy
gives the radial velocity as a function of distance from the galaxy centre This allows the rotation curve to be measured (Radial velocity, vr as a function of distance from galaxy centre, r) 2GLSS P.H. Regan

57 Examples of galaxy rotation curves, see Y. Sofue, PASJ 49 (1997) p17
2GLSS P.H. Regan

58 The Galaxy (cont). 21cm radiowaves from neutral H gas can penetrate the ISM. Large hydrogen clouds can be found in the spiral arms of galaxies (see e.g., M31). Looking across the galactic plane, one can observe a series of emission peaks, all slightly shifted with respect to each other. This can be explained by each spiral arm of the Milky Way moving with its own specific rotational velocity (and Doppler shift). It is thus possible to produce maps of the number and orientation of arms in the Milky Way. Other molecules (as well as H2) such as OH, CH and CO can also be detected via radio frequency emissions. Giant Molecular Clouds (GMC), which represent stellar formation sites. CO which emits radiation at l=2.6mm and regions of this molecule suggest that our galaxy has four major spiral arms (Perscus, Sagitarius, Centaurus and Cygnus). The Sun is situated on a smaller branched arm spiral known as Orion. 2GLSS P.H. Regan

59 From Astronomy by Zeilikk (John Wiley and Sons)
Note, artists impression…not data! 2GLSS P.H. Regan

60 There is optical evidence for the spiral structure of the Milky Way, using so-called SPIRAL TRACERS (see RJS p203). These include the presences of young, (Population I) blue (O and B-type) stars, high-metallicity stars. Hot, ionised hydrogen (H-II) regions are also associated with stellar formation and might be expected to be found in the spiral arms. Other spiral tracers include (i) Emission Nebula illuminated by hot O and B stars, (ii) Giant Molecular Clouds (GMCs) and (iii) Supernova Remnants 2GLSS P.H. Regan

61 Andromeda galaxy (M31) at various wavelengths
See 2GLSS P.H. Regan

62 Rotation Curve for the Milky Way (dark matter)
Optical and radioastronomy seem to indicate that all stars in the galaxy travel at the approx. the same rotational speed. Thus stars closer to the centre complete their orbits faster than the ones at the edge. This results in spiral shapes. But as we know the approx. age of the galaxy and the rate at which the sun rotates around the galactic centre, the Milky Way should be more tightly wound than it is. Failure of Kepler’s third law…more matter in middle maybe ? Dark matter ? Why don’t the arms wind up ? Supernovae may stretch new groups of stars into arms. Also spiral density waves and self-propogating star formation, the ‘ice-skater’ model. Freeman’s Law (see CO p999, all spirals have ~equal surface brightnesses) 2GLSS P.H. Regan

63 Supermassive Black Hole ? (CO p970, HOL p263)
Our galaxy is probably a ‘barred-spiral’. The strongest infra-red emission line is found in a region at the centre of the galaxy called Sagittarius A (SgrA), which is also a strong source of synchrotron radiation (I.e. a point radio-source, Sgr A*. From around 1995 onwards, infra-red (2.2mm) enabled the velocities of ~90 stars close to to SgrA* to be measured. Implication from Newtonian mechanics, radio-source and x-ray flares is a supermassive black hole with approx 2.6x106 solar masses at centre of Milky Way (see Ghez et al,. Nature 407 (2000) p349, see also Distance of galactic centre is approx 26,000 ly from earth 2GLSS P.H. Regan

64 Intense radio-source in centre of Galaxy, Sagittarius A*
radio-frequencies between cm) 2GLSS P.H. Regan

65 False colour image of x-ray flare from
Baganoff et al.,f rom the CHANDRA X-ray observatory, 01_releases/press_09050/flare.html False colour image of x-ray flare from supermassive black hole at centre of galaxy, associated with the compact radio source, Sagittarius A*. It is suggested that the flare is evidence for local matter falling into the black hole, which fuels energetic activity in the centre of the Milky Way. 2GLSS P.H. Regan

66 Galaxies in General: A Brief History
Messier ( ) recorded 103 fuzzy objects (nebulae) in the Messier catalogue Dreyer ( ) published the New General Catalogue (NGC) of almost 8,000 nebulae but not known at this point whether they were galactic or extra-galactic phenomena. 1923, Hubble detected Cephied variables in M31 (Andromeda galaxy, NGC224), showed extra-galactic nature. 2GLSS P.H. Regan

67 Morphology of Galaxies: Galaxy Types.
(RJS p468, CO p990, HOL p257) Galaxies are broadly classified into three types by a scheme known as the Hubble Sequence. This divides galaxies into (i)Ellipticals (E), (ii) Spirals (S) & (iii) Irregulars (Ir). The spirals are further subdivided into two sequences, namely (iia) Normal Spirals (S or SA) and (iib) Barred Spirals (SB). There are also a class of galaxies which are transitional between ellipticals and spirals, known as Lenticulars, which can be either ‘normal’ (SO) or barred (SBO). These are arranged by Hubble’s Tuning Fork Diagram. normal spirals ellipticals barred spirals Hubble thought (wrongly!) this was an evolution of type 2GLSS P.H. Regan

68 (RJS p468, CO p997, HOL p258) ELLIPTICAL (E) galaxies are ellipsoidal in shape and contain no spiral arms. They contain little interstellar dust and gas and are found mostly in rich clusters of galaxies. Elliptical galaxies typically appear yellow-red in colour. There are few blue, young, recycling stars (in contrast to spiral galaxies, whose spiral arms are full of young stars and appear quite blue). Supergiant Ellipticals have radii up to 106 pc. M87 elliptical galaxy NGC4881 Giant Elliptical galaxy) 2GLSS P.H. Regan

69 SPIRAL (Sa, Sb, etc.) galaxies consist of a large, dominant central bulge and tightly wound spiral arms. Star formation in the bulge region is thought to have taken place long ago and thus light from this region is blue-deficient (since massive, blue stars are relatively short lived), but rather reddish (from red-giants). This contrasts with the spiral arms, where new, blue, stars are formed. Spirals have (i) a flat disk, which usually contains a lot of interstellar matter and (ii) hot, massive, young (blue. O and B type) star clusters, which are usually arranged in spiral patterns structures. Some spirals have a large ‘bar’ through their central region and are known as ‘barred spirals’. (The milky way is thought to be a barred-spiral galaxy). Hubble subdivided spiral galaxies into Sa, Sab, Sb, Sbc, Sc and Sba, Sbab, SBb, SBbc and SBc. Those galaxies with the largest bulge-to-disk luminosity ratios, the most tightly wound spiral arms and the smoothest distribution of stars are classified as Sa or Sba. Conversely, Sc and SBc galaxies have loosely wound arms etc. 2GLSS P.H. Regan

70 closest galaxy ~ 2million ly
Andromeda galaxy (M31) closest galaxy ~ 2million ly Whirlpool Galaxy (M51) companion galaxy (NGC 5195) 2GLSS P.H. Regan

71 The ‘Sombrero’sprial galaxy (M104),
clearly showing the dust in the disk, the central bulge and globular clusters within the stellar halo. See 2GLSS P.H. Regan

72 NGC7479 barred spiral galaxy
NGC7479 barred spiral galaxy NGC3992, M109 barred spiral galaxy 2GLSS P.H. Regan

73 NGC 4546 (SB0) Lenticular galaxy from
LENTICULAR (SO, SBO) galaxies consist of flat disk with a central bulge but do not show any spiral arms (unlike spiral galaxies). Depending on the orientation relative to the earth, such galaxies can appear circular (if seen face on), elliptical, or thin and elongated. NGC 4546 (SB0) Lenticular galaxy from NGC 3384 (SO) lenticular galaxy 2GLSS P.H. Regan

74 IRREGULAR (Irr) galaxies have a chaotic
appearance which does not resemble either spiral or elliptical galaxies. Many of the smallest galaxies are irregular (including the Magellanic clouds which accompany the Milky Way). M82 Irregular galaxy 2GLSS P.H. Regan

75 Size of Galaxies (CO p998) By mapping contours of constant brightness,
(ISOPHOTES), one can infer the matter radius of galaxies. BUT, galaxies do not have have sharp edges, thus radii are not well defined. The effective radius (re ) , is defined as the radius within which half of the galaxies light is emitted. For the bulges of spirals and large ellipticals, this looks like (from equn, on page 42) where m( r) is the surface brightness in units of magnitude/arcsec2 and me is the surface brightness at the effective radius, re. (Note, that galaxial disks are usually modelled with an exponential , rather than r1/4 decay). 2GLSS P.H. Regan

76 The Tully-Fisher Relation (CO p1001, RJS p476)
It is reasonable to assume that the greater the mass in a given galaxy, the larger its luminosity. It then follows, the greater the mass, the larger the gravitational force at each point in the galaxy would be. Since it is the size of this gravitation force which determines the rotation velocity of the galaxy (see equn. P52), there will be a relationship between the rotational velocity and luminosity of galaxies. The rotational velocity of galaxy can be deduced from the Doppler broadening of the 21 cm line. This will be both red and blue shifted. A value for the absolute luminosity for the galaxy can be estimated and compared with the observed brightness. The inverse square law of light can be used to make an estimate of galaxy’s distance. The relationship between the luminosity and rotational velocity of galaxies is known as the Tully-Fisher relation (1977). 2GLSS P.H. Regan

77 Tully-Fisher Relations at large (0. 2<z<1
Tully-Fisher Relations at large (0.2<z<1.1) redshift see The exact form of the Tully-Fisher relation depends on the galaxial mass distribution, and thus the type of galaxy. But to first order, the relationship between the absolute magnitude (M), the luminosity (L) and the maximum rotational velocity, (Vmax) can be given by c=constant 2GLSS P.H. Regan

78 Stellar Distributions and Metallicities
H-I gas in our galaxy is distributed in spiral arms, connected to the bulge. Most young (blue) stars are in these arms, i.e., these are regions of on-going star formation. Since stars form from gas clouds, and massive (blue) stars form and die relatively quickly, one expects (all) massive, main sequence stars to be found in the spiral arms. Older stars are red in colour (further down main sequence, red giants..) These are found in the disk, not in the spiral arms. Red stars can are also found in the bulge and halo. Observations indicate that the metallicity of a galaxy correlates with its absolute magnitude and thus luminosity. This implies that chemical enrichment is more efficient in luminous, massive galaxies. Possibly due to them having more massive stars in which subsequently go to type II supernova ? 2GLSS P.H. Regan

79 Metallicity-Magnitude Relation (CO p1006)
From Pagel and Edwards, Annu. Rev. Astron. Astrophysics 19 (1981) 2GLSS P.H. Regan

80 Galaxy Clusters and Colliding Galaxies (CO p1053-10073, p1119ff)
Most galaxies belong to clusters, which can contain 1000s of individual galaxies. Groups of galaxies usually have less than 50 members, with clusters containing between 50 and several thousand individual galaxies. Galaxy clusters are classified as either regular (spherical and centrally condensed) or irregular. The Milky Way belongs to the LOCAL GROUP, which consists of approx. 30 galaxies, including the Andromeda Galaxy (M31), M32, M33, M110 and the Small and Large Magellanic Clouds (LMC, SMC). The Milky Way and Andromeda galaxies are by far the largest and most dominant members of the local group. There are also around 20 small groups of galaxies within approx 14Mpc of the local group, including the Maffei-1, the South Polar or ‘Sculptor’, the M81 and the M83 groups. Out group of galaxies is in gravitational interaction with these galaxy groups. 2GLSS P.H. Regan

81 The M81 Group is one of the closest galaxy groups
visible radio M81 group M82 M81 NGC 3077 2 dwarf galaxies NGC3077 The M81 Group is one of the closest galaxy groups to our own (~ 12Mly). The group has two large galaxies, M81 (‘Bodes galaxy’) and M82 plus NGC 3077 which all have a mutual gravitational interaction. Note the two dwarf galaxies close to M81 and the common gaseous envelope which are clearly apparent on on the radiowave image. radio 2GLSS P.H. Regan

82 It is thought that a few 10s of million years ago
a ‘close encounter’ occurred between M81 and M82, with the result that M82 was dramatically deformed by the gravitational attraction of the larger and more massive M81. This encounter also left traces in the spiral pattern of M81, firstly making the overall pattern more pronounced. Note that the M81 and M82 are still relatively close at around 150kly apart. 2GLSS P.H. Regan

83 Gravitational attraction between nearby
galaxies means they can interact and even collide with each other. Colliding galaxies does not imply colliding stars (size between individual stars too large). There can be dynamical friction between galaxies due to the gravity (see CO p1055). This dynamical friction force (‘drag’), fd, can be estimated assuming fd ~ (r / v2) where v is the relative speed of the colliding galaxies and r is the density of the colliding material. Energy will also be converted from potential and self-energy into kinetic energy of individually scattered stars. Kinetic energy from the galaxies’ motions of can be transferred into internal KE of the gravitationally interacting galaxies. Gravitationally bound galaxies willeventually merge (e.g. Milky Way and LMC & SMC, also some evidence double nucleus in Andromeda, M31 galaxy). The mergers and tidal interactions ‘tidal stripping’ between colliding galaxies may be responsible for matter transfer and creating new active star-forming regions. 2GLSS P.H. Regan

84 CO p1066 2GLSS P.H. Regan

85 2GLSS P.H. Regan

86 Calculating Galaxy Collisions (CO p1088)
Realistic ‘N-body’ computer simulations can be carried out using Newton’s gravitational equations. Such calculations suggest that many large elliptical galaxies are actually the result of mergers between two spiral galaxies. It is thus thought that over time, spiral galaxies are destroyed by mergers, resulting in the creation of very large elliptical galaxies. Evidence for this includes Computer based N-body simulations The observation that elliptical galaxies appear to dominate in the more dense, centres of galaxy clusters Observations also suggest that the more distant (and thus younger) galaxies appear to have (had?) a greater fraction of spiral galaxies compared to ellipticals. 2GLSS P.H. Regan

87 CO p1088 2GLSS P.H. Regan

88 In hydrotstatic equilibrium, from the VIRIAL THEOREM (CO p56) the Total (E), Potential (U) and Kinetic (K) energies of a galaxies are related by the expression 2K= -U = -2E For a head-on galaxy collision, we can use the impulse approximation (CO p1061) to assume the encounter between the galaxies occurs so quickly that the individual stars do not have time to move much from their positions. The gravitational potential energy, U, of the galaxy is virtually unaltered by the collision. If a galaxy increases its internal kinetic energy, from K to K+DK as a result of the gravitational ‘work’ each galaxy has done on the other one, the total energy will also increase from K to K+DK, but the potential energy, U remains unchanged (=K). The galaxy would thus no longer be in hydrostatic equilibrium. To regain equilibrium, the extra 2DK of kinetic energy obtained just after collision, must be ‘lost’. One way this can be done is for this excess energy to be converted in to increased (i.e., less negative) gravitational potential energy, by expansion. 2GLSS P.H. Regan

89 Colliding Galaxies The Cartwheel galaxy (approx 500 Mly away), is an example of a ‘ring galaxy’ (see CO p1062) which followed from a nearly head-on collision between galaxies. This figure, from the Hubble Space Telescope, shows the effect of a collision by an intruder galaxy (it is not clear which one of the two objects to the right), which smashed through the core of the host galaxy. Note blue, new stars in the ring. The ring is expanding at approx, 90km/s. 2GLSS P.H. Regan

90 Sleeping Beauty Galaxy(M64)
There is a central region where the stars rotate in a different direction to the dust and gas. This effect is thought to be result of collision between a large and small galaxy which has yet to equilibrate out. 2GLSS P.H. Regan

91 ‘Interacting Galaxies, Arp 273’ distance
Interacting galaxies Arp 273 ‘Interacting Galaxies, Arp 273’ distance approx 200Mly, false colour image from the Anglo-Australian telescope. Note the strong tidal distortion of the larger of the two galaxies and the notable ‘extra’ red nucleus in the smaller, side on galaxy in the botton right of the picture. The lower galaxy has a very active nucleus (see later). 2GLSS P.H. Regan

92 Superclusters A supercluster is a group of galaxy clusters
which appear to be associated (gravitationally) with each other. Unfortunately, this information is not usually known for most galaxy clusters and thus definitions of members of superclusters are made on the basis of how far different galaxy clusters are separated from each other. This definition has some problems, as the distances between galaxies is much more difficult to determine that the distance to them (since we can use the doppler shift to give us more accurate measures of the distance perpendicular to the radial direction, than the radial direction specifically.) 2GLSS P.H. Regan

93 Perseus galaxy cluster, see
The Perseus cluster of galaxies, approx. 300Mly. This is part of the Pisces-Perseus supercluster. Note that the confinement of gas in the large galaxial gravitational field (purple in figure). This extra gravity’ which is sufficient to hold the gas in place, has been proposed as further potential evidence for Dark Matter ROSAT False colour x-ray image Abell 426 2GLSS P.H. Regan

94 Coma cluster, see
The Coma cluster contains >1,000 galaxies, is around 6 Mpc across and lies ~360 Mly away. Centre of galaxy is hot (T~100x106K) gas cloud (see X-ray image) held in by gravity. Coma cluster, see visible X ray 2GLSS P.H. Regan

95 Central portion of the Virgo cluster
The Virgo cluster of galaxies which is over 3Mpc across and consists of more than 2000 individual galaxies lies around 16 Mpc (~60 Mly) away. It represents the centre of the local supercluster which includes the local group. M87 M84 M86 ‘Makarian’s chain’ NGC4371 NGC4429 M89 M90 Central portion of the Virgo cluster 2GLSS P.H. Regan

96 Expansion of the Universe, Hubble’s Law (CO p1110)
Using luminosity of Cephied variable stars, Edwin Hubble plotted the recessional velocity of such standard candles in extra-galactic objects against their induced distance (deduced using the inverse square law of light). The resulting (nearly?) linear relationship is known as Hubble’s Law, which has the form, Where V is the recessional velocity and r is the distance from the earth. H is known as the Hubble constant. 2GLSS P.H. Regan

97 From Hubble’s original paper, published in
Proceedings of the National Academy of Sciences Volume 15 March 15, (1929), number 3. ‘Distances of extra-galactic nebulae depend ultimately on the application of absolute luminosity criteria to involved stars whose types can be recognised. These include….Cepheid variables, novae and blue stars involved in nebulae emission. ……The apparent luminosities of the brightest stars in such nebulae are thus criteria which, although rough and to be applied with caution, furnish reasonable estimates of the distances of extra-galactic systems’. 2GLSS P.H. Regan

98 Value of the Hubble Constant, Scale Factors and Hubble Time
Hubble’s orginal value for H=530kms-1Mpc-1. But subsequent measurements showed that he underestimated the distances involved as what he thought were the brightest stars were actually groups of stars. The value of H appears to have varied over time, thus while it is still a constant of proportionality between V and r, H=H(t). By convention, we the present day value of the Hubble constant is given the symbol, H0 . There is still much debate and measurement on the value of the Hubble constant. Currently, it is standard to define constant using the dimensionless quantity, h, and h has a value between 0.5 and 0.8. 2GLSS P.H. Regan

99 Thus, by measuring redshift, the distance can
Hubble’s law implies a universal expansion. In this context, the recessional velocity (due to the expansion of space) is distinct from the peculiar velocity (which is due to the motion through space). i.e v = Hod + vpec The recessional velocity arises because space itself is expanding, this is the realm of COSMOLOGY. The cosmological redshift needs to take into account the curvature of space-time (beyond the scope of this course). However, it is common practice to use the redshift equation to obtain the effective radial velocity. Thus, by measuring redshift, the distance can be determined. H0 is poorly known, use H0=75(20)kms-1Mpc-1 (h=0.75(0.2)) 2GLSS P.H. Regan

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