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Our Milky Way Galaxy: basics Disk, bulge, halo Stellar populations: Baade’s observations Stellar populations: Current thinking Based on Sarah Bridle’s.

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Presentation on theme: "Our Milky Way Galaxy: basics Disk, bulge, halo Stellar populations: Baade’s observations Stellar populations: Current thinking Based on Sarah Bridle’s."— Presentation transcript:

1 Our Milky Way Galaxy: basics Disk, bulge, halo Stellar populations: Baade’s observations Stellar populations: Current thinking Based on Sarah Bridle’s slides

2 Our Galaxy Contents: –stars (~10 11 ) –gas (~10 10 M o ) –dust (~10 8 M o ) –dark matter (~10 12 M o )

3 Location of galaxy contents Most of the stars are in a disc, with spiral arms Significant concentration of stars in the center –The bulge; is an elongated bar Also: stellar halo (~10 9 stars) and some gas Mass is believed to be mainly in a spheroidal halo, dominated by dark matter –Extends out to >100 kpc diameter Globular clusters, open clusters –clusters of stars distributed throughout halo

4 Our Galaxy – The Milky Way

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8 F 2MASS Galactic chart

9 Stellar populations: Baade’s observations Population I (‘pop one’) –Blue –Galactic disk, open clusters Population II (‘pop two’) –Red –Galactic globular clusters –Elliptical galaxies –Bulges of spiral galaxies

10 Stellar populations: Current thinking Pop I –Z= 0.01 – 0.04 –circular orbits within plane of Galaxy –young: Myr to 10 Gyr Pop II –stellar halo: Z<0.002 (minimum observed: Z=2x10 -6 ) –bulge Z ~< 0.02 –eccentric orbits (high-velocity stars) –old: 12 to 15 Gyr, so only low mass are visible Pop III –Theoretical idea = the first stars

11 The Galactic Disk List of topics: Different wavelengths probe different components Populations as a fn of position Gas in the disk The scale height Spiral arms

12 The Galactic Disk: Basics Majority of visible matter is in the disk –visible matter = stars, gas, dust Disk is dominated by visible matter – dark:visible matter is ~ 40:60 Observations: –Optical: stars; some obscuration by dust –near ir: stars; dust is now transparent –far ir: dust –radio: gas

13 Also find here an all sky interactive panorama in the optical

14 More nice pics at: Far-ir taken by IRAS: Dust Nearly the entire sky, as seen in infrared wavelengths and projected at one- half degree resolution, is shown in this image, assembled from six months of data from the Infrared Astronomical Satelite (IRAS). The bright horizontal band is the plane of the Milky Way, with the center of the Galaxy located at the center of the picture. (Because of its proximity, the Milky Way dominates our view of the entire sky, as seen in this image. IRAS data processed to show smaller regions of the sky, however, reveal thousands of sources beyond the Milky Way.) The colors represent infrared emission detected in three of the telescope's four wavelength bands (blue is 12 microns; green is 60 microns, and red is 100 microns). Hotter material appears blue or white while the cooler material appears red. The hazy, horizontal S-shaped feature that crosses the image is faint heat emitted by dust in the plane of the solar system. Celestial objects visible in the photo are regions of star formation in the constellation Ophiucus (directly above the galactic center) and Orion (the two brightest spots below the plane, far right). The Large Magellanic Cloud is the relatively isolated spot located below the plane, right of center. Black stripes are regions of the sky that were not scanned by the telescope in its first six months of operation.

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16 Milky Way in Different Wavebands Xray: –Observed by the ROSAT satellite. –X-ray emission is detected from hot, shocked gas +10° -10° 180° 0°0°90°270° The following 7 slides are based on Lisa Wright’s course at

17 Milky Way in Different Wavebands Optical: –Due to the obscuring effect of the interstellar dust, the light is primarily from stars within a ~1000 lt yrs of the Sun. –Dark patches are due to absorbing clouds of gas and dust Which can be seen in the molecular hydrogen and infrared maps as emitting regions. +10° -10° 180° 0°0°90°270°

18 Milky Way in Different Wavebands Near Infrared: –Most emission is from relatively cool giant K stars in the disk and the bulge of the Milky Way. –Interstellar dust does not strongly obscure emission at this wavelength. +10° -10° 180° 0°0°90°270°

19 Milky Way in Different Wavebands Infrared: –Most emission is thermal from interstellar dust warmed by starlight, including regions of star formation within the interstellar clouds. +10° -10° 180° 0°0°90°270°

20 Milky Way in Different Wavebands Molecular Hydrogen (H 2 ): -Mostly cold dense interstellar clouds -Star formation occurs in these clouds, producing infrared +10° -10° 180° 0°0°90°270°

21 Milky Way in Different Wavebands Atomic Hydrogen (HI) –Traced the gas of the interstellar medium. This gas is organised in to large diffuse clouds with sizes up to hundreds of light years across. +10° -10° 180° 0°0°90°270°

22 Milky Way in Different Wavebands Radio (407 MHz): –Emission from electrons moving through the magnetic fields, between stars, at nearly the speed of light. –Supernova shock waves accelerate the electrons to such high speed, producing lots of emission near these sources +10° -10° 180° 0°0° 90° 270°

23 The Sagittarius dwarf galaxy Stuff is still falling into our Galaxy

24 Simulation of a dwarf galaxy that fell in Like Canis Major dwarf, discovered in 2003

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26 Galactic Archaeology Observe positions and velocities of all stars in our galaxy Compare with simulations to constrain models Future experiments: –RAVE: just started, Australia –GAIA: satellite –WFMOS: proposed, UCL is involved

27 E or S or Irr? E0 E1 E2 E3 E4 E6 E7

28 NGC E or S or Irr? S0 SBO S SB Sa Sb Sc

29 E or S or Irr? Irr I Irr II

30 \ E or S or Irr? S0 SBO S SB S0 1 S0 2 S0 3

31 M81 E or S or Irr? S0 SBO S SB Sa Sb Sc

32 NGC E or S or Irr? S0 SBO S SB SBa SBb SBc

33 NGC Copyright David Hogg E or S or Irr? E0 E1 E2 E3 E4 E5 E6 E7

34 E or S or Irr? Irr I Irr II

35 M51 gallery/html/im0063.htmlwww.noao.edu/image_ gallery/html/im0063.html E or S or Irr? S0 SBO S SB Sa Sb Sc

36 Copyright David Hogg cosmo.nyu.edu/hogg/rc3/cosmo.nyu.edu/hogg/rc3/ E or S or Irr? S0 SBO S SB SBa SBb SBc

37 E or S or Irr? S0 SBO S SB Sa Sb Sc

38 E or S or Irr? E0 E1 E2 E3 E4 E5 E6 E7 Or dE3

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41 Surface Brightness Profiles What is the total luminosity of a galaxy? –At what radius to stop adding up? –Atmosphere and instrument limit radius Surface brightness profile = apparent surface brightness as fn of radius –Apparent surface brightness = energy from source, per unit area of telescope, per arcsec 2 –Usually for a given waveband, e.g. R-band Usually assume –ellipticals: de Vaucouleurs profile: I(R) = exp(-(r/r 0 ) 1/4 ) –spirals: de Vaucouleurs bulge + exponential disk exponential disk: I(R) = exp(-r/r 0 )

42 Boroson 1981ApJS B

43 Local luminosity function The 2dF galaxy survey (astro-ph/ )

44 The Galaxy Luminosity Function Number of galaxies of given L per unit V Schechter function fits well –  (L) = (n * / L * ) (L/L * )  exp(-L/L * ) –Where  ~ -1.1, L * ~10 10 L solar, n * ~ 0.01 Mpc -3 (depending on selection criteria of galaxies) L * is a typical galaxy luminosity n * is a typical galaxy density –what is a typical intergalactic distance? n -1/3 = 5 Mpc N= Integral of  (L) dL diverges as L(min) -> 0 –not physical but emphasises large number of faint galaxies

45 Dependence of  (L) on galaxy type From the 2dF survey astro-ph/

46 The Tully-Fisher relation Relates luminosity to line-width for spirals v(r) for our galaxy is typical of spirals –Observe other galaxies using HI or HII

47 Rotation curves of spirals are flat- Dark matter halos (or MOND?)

48 M(r) = v 2 r / G

49 The Tully-Fisher relation Relates luminosity to line-width for spirals –Tully & Fisher 1977 v(r) shape for our galaxy is typical of spirals –Observe other galaxies using HI or HII –call v rot the flattened velocity We observe L = v rot  –  ~2.5 in b-band –  ~ 4 in the infrared What would you expect for  –Assume M / L is constant –Assume L = L o  r  (with L  same for all gals)

50 local calibrators Virgo Ursa major

51 The Faber Jackson relation Relates luminosity and central velocity dispersion of ellipticals Stars in ellipticals seem to be virialised –all move in random directions, velocities v –velocity dispersion =  v = 1/2 –virial theorem: 2 K.E.=-P.E. so  v 2 / M/R Faber-Jackson relation: L =  v 4 The Fundamental Plane: –plot galaxies in 3d: L,  v, R –find R =  v -2 L 1.25

52 The D n -  relation Another observed relation for ellipticals –has 2 params and better correlation than FJ D n = diameter within which mean surface brightness is greater than some number Tight correlation due to fundamental plane Galaxies A and B are at the same distance with the same size r 0, but A is brighter than B. –Which has the larger D n ? – Recall I(R) = exp(-(r/r 0 ) 1/4 )

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54 Measurements of D n at UCL 2MASS galaxy, work by Alex Abate (UCL) and Lachlan Campbell (Sydney) D n is 2.23" and the definition of D n here is diameter enclosing 17.7 mag/sq arcsec. mean surface brightness in radius r radius 1/4 de Vaucouleurs fit

55 Mass-to-light in ellipticals Virial theorem: M =  v 2 R The fundamental plane: R =  v -2 L 1.25 Suppose (M/L) = L  –Find  The bigger the luminosity, the bigger the mass, the more dark matter

56 Spiral galaxies Blue spiral arms, young, high Z Older disk population, medium Z Old low mass Pop II stars in bulge and spheroid Decrease in Z with increasing r –e.g. bulge metallicity is higher than solar

57 Elliptical galaxies Little sign of gas or recent star formation Stellar populations are old and red (Pop II) –(B-V ~ 1) –most light from red giants Small E/S0 gals have low metal content –Large E/S0 galaxies are relatively metal rich


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