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2GLSS Course Outline

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2GLSS Course Outline

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  1. Syllabus for 2GLSS, Galaxies and Large Scale Structures.Dr. P.H. Regan, 29BC04, x6783 p.regan@surrey.ac.uk Spring SemesterBooks1) 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 • 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 http://www.orau.gov/ria 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 106-7 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, seehttp://www.orau.gov/ria Pm (Z=61) Tc( Z=43) 2GLSS P.H. Regan

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

  11. Interstellar medium ( diffuse gas and dust) Star light (black body) Absorption lines (observation) Emission lines (observation) Emission and Absorption 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 emission spectrum Fe absorption spectrum H absorption Hemission See http::/jersey.uoregon.edu/elements/Elements.html 2GLSS P.H. Regan

  14. Where are our body parts made ? (% of human body by weight) See http://www.orau.gov/ria 2GLSS P.H. Regan

  15. 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 http://www.orau.gov/ria 2GLSS P.H. Regan

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

  18. (CO p920) Metallicity 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. Energy Ionisation e- 0 e- proton n=3 proton n=2 1st excited state of HI (spins parallel). Ground state of HI (spins anti-parallel). -13.6 eV n=1 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). Nb. Zeeman effect gives B-field measurement 2GLSS P.H. Regan

  21. 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. Taken from RJS p369 Hydrogen Molecules (H2) (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 http://fuse.pha.jhu.edu/Figures/data 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. 10-90k 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 ~ 102-7 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 ~100-300cm-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 0.05-1 parsec, T~100-200K and densities of 107-109 /cm3. 1000s of GMC are known in our galaxy, mostly in the spiral arms (see later) 2GLSS P.H. Regan

  26. Bok Globules 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. IC-2948 Inside this emission nebula are groups of dark, opaque clouds, i.e., Bok globules. See http://www.aao.gov.au/images/general 2GLSS P.H. Regan

  27. Some good web pages for galaxy informantion and figs. • users.erols.com/arendt/Galaxy/mw.html • www.whfreeman.com/universe6e • antwrp.gsfc.nasa.gov/apod/archivepix.html • adc.gsf.nasa.gov/mw/mmw_sci.html-maps • cdsweb.ustrasbg.fr/astroweb/survey.html • skyandtelescope.com • www.eso.org/outreach/press-rel/pr-2002/pr-17-02.html • zebu.uoregon.edu/~soper/MilkyWay/sahpley.html 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. Scattered (blue) light Original light from source Transmitted (red) light Interstellar dust cloud. 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. 2GLSS P.H. Regan

  31. (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 http://adc.gsfc.nasa.gov/mw/mmw_images.html 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. 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 timeand the universe is older than originally thought? i.e, ‘anti-gravity’ ? Possible evidence for so-called dark energy. (see Burrows, Nature 403 (2000) p727-733). From SPATIUM, no7. May 2001, p11 2GLSS P.H. Regan

  35. See http://physicsweb.org/article/news/2/11/3 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 1011-12 solar masses. • 10% of mass attributed to 200-400 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.(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. http://www.seds.org/messier/m/m002.html 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. http://www.astrophotographer.com/Globular_plot.html See also great pictures and info at the following. www.ipac.caltech.edu/2mass/gallery/Images_galaxies.html www.astr.ua.edu/choosepic.html www.dir.yahoo/Science/Astronomy/Pictures 2GLSS P.H. Regan

  40. http://aether.lbl.gov/www/projects/cobe/cobe_pics.html 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. http://users.erols.com/arendt/Galaxy/mw.html 2GLSS P.H. Regan

  41. halo sun nucleus disk bulge ~15kpc Details of the Structure of the Milky Way (see CO p927ff, RJS p439, p457) 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. 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. star To north galactic pole (NGP) rotation b galactic centre l sun 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) andl (Galactic longitude) are relative to the motion of the sun. • NGP has co-ordinates of b=90o. By convention • Galactic centre has (almost) l=0o and b=0o . 2GLSS P.H. Regan

  45. Q P star rotation z R q galactic centre sun Cylindrical Co-ordinate System (CO p941) • Cylindrical co-ordinate have centre of galaxy at • origin. 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. 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: Motion of Sun in Galaxy (see CO p945) 2GLSS P.H. Regan

  48. vr vq star r observer Measuring Velocities (CO 107, p126-127) 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. Source moving at velocity, u q u q 2nd signal to observer 1st signal to observer For light however, there is no speed of sound, as there is medium involved and the expression must be obtained using special relativity. 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

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