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Current Topics

Current Topics. Lyman Break Galaxies Dr Elizabeth Stanway (E.R.Stanway@Bristol.ac.uk). Topic Summary. Star Forming Galaxies and the Lyman-  Line Lyman Break Galaxies at z <4 Lyman Break Galaxies at z >4 The Star Formation History of the Universe and Reionisation

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Current Topics

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  1. Current Topics Lyman Break Galaxies Dr Elizabeth Stanway (E.R.Stanway@Bristol.ac.uk) Current Topics: Lyman Break Galaxies - Elizabeth Stanway

  2. Topic Summary • Star Forming Galaxies and the Lyman- Line • Lyman Break Galaxies at z<4 • Lyman Break Galaxies at z>4 • The Star Formation History of the Universe and Reionisation • This course will be assessed through a 1 hour examination including mathematical and essay questions Current Topics: Lyman Break Galaxies - Elizabeth Stanway

  3. Recommended Reading • Steidel, Pettini & Hamilton, 1995, AJ, 110, 2519 • Carilli & Blain, 2002, ApJ, 569, 605 • Verma et al, 2007, MNRAS, 377, 1024 • Bouwens et al, 2007, ApJ, 670, 928 • Stanway et al, 2008, ApJ, 687, L1 Current Topics: Lyman Break Galaxies - Elizabeth Stanway

  4. A few definitions … • In these lectures • LBG = Lyman Break Galaxy • LAE = Lyman Alpha Emitter • HST = Hubble Space Telescope • Gyr = 1 Billion Years (Myr = 1 million yrs) • z = redshift • Z = metallicity • z’ or zAB are broadband filters Current Topics: Lyman Break Galaxies - Elizabeth Stanway

  5. The History of High-z studies Current Topics: Lyman Break Galaxies - Elizabeth Stanway

  6. The History of High-z studies The highest redshift galaxy has been increasing steadily in distance for ~20 yrs Current Topics: Lyman Break Galaxies - Elizabeth Stanway

  7. The History of High-z studies Universe 1Gyr old Universe 1/8 current age Universe 1/4 current age Universe half current age Now: Universe 13.7 Gyr Current Topics: Lyman Break Galaxies - Elizabeth Stanway

  8. The History of High-z studies z=3 LBGs Universe 1/8 current age ~ 2 Billion years after the Big Bang Current Topics: Lyman Break Galaxies - Elizabeth Stanway

  9. Why Push So Far Back? • We are now starting to probe the last major phase transition in the universe - reionisation • We’re within a few generations of the earliest galaxies forming • Unevolved galaxies are simpler - easier to understand - and so help shape theory Current Topics: Lyman Break Galaxies - Elizabeth Stanway

  10. Why Push So Far Back? • Lyman break galaxies are star-forming so directly measure how exciting a place the universe is • Lyman break galaxies are relatively bright and so easy to study • Lyman break galaxies are relatively easy to find Current Topics: Lyman Break Galaxies - Elizabeth Stanway

  11. But Why is it so difficult? • Redshift equation: (obs)=em) * (1+z) => Distant galaxies are very RED • The night sky is also very red => the sky background is much higher for high-z galaxies Flux Wavelength Current Topics: Lyman Break Galaxies - Elizabeth Stanway

  12. But Why is it so difficult? • Distance Modulus equation: m = M - 5 log (dL/10pc) • Luminosity Distance equation: dL = (1+z) * c/H0 * • At z=1, dL=6634 Mpc • At z=3, dL=25840 Mpc • At z=5, dL=47590 Mpc => Distant galaxies are very FAINT Current Topics: Lyman Break Galaxies - Elizabeth Stanway

  13. Depth vs Area? • The Luminosity Function (LF) of a galaxy population relates number of objects seen to volume/area observed • Most galaxies follow a Schecter (1973) function: N(L) dA  (L/L*)e-(L/L*) dA • When L<<L*, this approximates a power law: N(L) dA  LdA => Increasing area of observation leads to increase in galaxy sample BUT: since the power law is steep, increasing the depth usually increase sample size more quickly Current Topics: Lyman Break Galaxies - Elizabeth Stanway

  14. Building a Galaxy • Every galaxy is made of stars • Lower mass stars live longer • More massive stars are more luminous => burn more quickly TMS~10Gyr*(M/M)-2.5 M Blue Red Current Topics: Lyman Break Galaxies - Elizabeth Stanway

  15. Building a Galaxy • TMS~10Gyr*(M/M)-2.5 • Old galaxies are dominated by A-M stars and have 4000A breaks • Young galaxies are dominated by short-lived O and B stars and are UV-bright 30 Myr 300 Myr 10 Gyr 15 Gyr Blue Red Current Topics: Lyman Break Galaxies - Elizabeth Stanway

  16. Types of Galaxy SED Old/Red • Old galaxies are dominated by A-M stars and have 4000Å breaks • Young galaxies are dominated by short-lived O and B stars and are UV-bright • Younger galaxies also show strong emission lines, powered by star formation. Young/Blue Rest-UV Current Topics: Lyman Break Galaxies - Elizabeth Stanway

  17. Hydrogen Emission Lines • Flux from star formation excites electrons in atoms • The most abundant atom in the universe is Hydrogen • As an electron relaxes from an excited state, it emits a photon • Each transition emits at a particular wavelength • The easiest transition to excite is Lyman- The Balmer series emerges in the optical and so is known as ‘Hydrogen-’ etc for historical reasons Current Topics: Lyman Break Galaxies - Elizabeth Stanway

  18. Hydrogen Emission Lines OIII OII The Balmer Series and Oxygen lines dominate the optical spectrum of a star forming galaxy H H H H • The Lyman series emerges in the ultraviolet. • The Lyman- emission line can emit up to 1% of the galaxy’s bolometric flux, but …. Current Topics: Lyman Break Galaxies - Elizabeth Stanway

  19. Hydrogen Emission Lines OIII OII The Balmer Series and Oxygen lines dominate the optical spectrum of a star forming galaxy H H H H • The Lyman series emerges in the ultraviolet. • The Lyman- emission line can emit up to 1% of the galaxy’s bolometric flux, but …. Current Topics: Lyman Break Galaxies - Elizabeth Stanway

  20. Hydrogen Emission Lines Ly Å OIII OII H H H Ly H • The Lyman series emerges in the ultraviolet. • The Lyman- emission line can emit up to 1% of the galaxy’s bolometric flux, but …. Current Topics: Lyman Break Galaxies - Elizabeth Stanway

  21. The Asymmetric Lyman- Line Low z The Lyman- line is intrinsically symmetric At high-z the line always appears asymmetric and broadened Higher z Current Topics: Lyman Break Galaxies - Elizabeth Stanway

  22. The Asymmetric Lyman- Line Star formation drives galaxy-scale winds (Adelberger et al 2003) Lyman- is resonantly scattered by the winds Red wing is broadened by back-scattered light Blue Wing is scattered by outflowing galactic winds Wind v = +300 km/s v = 0 v = -300 km/s Current Topics: Lyman Break Galaxies - Elizabeth Stanway

  23. The Asymmetric Lyman- Line v/c = z/(1+z) => 300km/s wind broadens line by about 5Å FWHM at z=3 Red wing is broadened by back-scattered light Blue Wing is scattered by outflowing galactic winds Wind v = +300 km/s v = 0 v = -300 km/s Current Topics: Lyman Break Galaxies - Elizabeth Stanway

  24. The Lyman- Forest … Lyman-a is also seen in absorption wherever there are clouds of hydrogen Source Observer z* z=0 Ly Åz*) Current Topics: Lyman Break Galaxies - Elizabeth Stanway

  25. The Lyman- Forest … Lyman-a is also seen in absorption wherever there are clouds of hydrogen Source Observer z1 z* z=0 Ly Å z1) Åz*) Current Topics: Lyman Break Galaxies - Elizabeth Stanway

  26. The Lyman- Forest … Lyman-a is also seen in absorption wherever there are clouds of hydrogen Source Observer z4 z3 z2 z1 z* z=0 Ly Å z3) Å z1) Åz*) Å z2) Å z4) Current Topics: Lyman Break Galaxies - Elizabeth Stanway

  27. The Lyman- Forest At low z almost all of a galaxy’s Lyman continuum flux reaches us Current Topics: Lyman Break Galaxies - Elizabeth Stanway

  28. The Lyman- Forest Above z=3, the fraction of galaxy flux reaching us declines rapidly Current Topics: Lyman Break Galaxies - Elizabeth Stanway

  29. The Lyman- Forest Beyond z=5.5, <1% of the galaxy’s flux gets through the IGM Current Topics: Lyman Break Galaxies - Elizabeth Stanway

  30. The Lyman- Forest Low z Lyman- Forest Higher z Current Topics: Lyman Break Galaxies - Elizabeth Stanway

  31. Properties of High-z Galaxies • Young galaxies at high-z are: Current Topics: Lyman Break Galaxies - Elizabeth Stanway

  32. Properties of High-z Galaxies • Young galaxies at high-z are: • Dominated by O and B stars Current Topics: Lyman Break Galaxies - Elizabeth Stanway

  33. Properties of High-z Galaxies • Young galaxies at high-z are: • Dominated by O and B stars • Bright in the ultraviolet Current Topics: Lyman Break Galaxies - Elizabeth Stanway

  34. Properties of High-z Galaxies • Young galaxies at high-z are: • Dominated by O and B stars • Bright in the ultraviolet • Drive strong galactic winds Current Topics: Lyman Break Galaxies - Elizabeth Stanway

  35. Properties of High-z Galaxies • Young galaxies at high-z are: • Dominated by O and B stars • Bright in the ultraviolet • Drive strong galactic winds • They have key observable characteristics: Current Topics: Lyman Break Galaxies - Elizabeth Stanway

  36. Properties of High-z Galaxies • Young galaxies at high-z are: • Dominated by O and B stars • Bright in the ultraviolet • Drive strong galactic winds • They have key observable characteristics: • They have asymmetric Lyman- emission lines Current Topics: Lyman Break Galaxies - Elizabeth Stanway

  37. Properties of High-z Galaxies • Young galaxies at high-z are: • Dominated by O and B stars • Bright in the ultraviolet • Drive strong galactic winds • They have key observable characteristics: • They have asymmetric Lyman- emission lines • Flux is suppressed shortward of Lyman- Current Topics: Lyman Break Galaxies - Elizabeth Stanway

  38. Current Topics: Lyman Break Galaxies - Elizabeth Stanway

  39. The Lyman Break Technique The Steidel, Pettini & Hamilton (1995) Lyman Break Method • At z=3, about 50% of the Lyman continuum is transmitted • This leads to a ‘break’ in the spectrum • So consider what would happen if you place filters either side of the Lyman- and Lyman limit breaks… Lyman Continuum Ionising Radiation UV Continuum Lyman-α Break 912Å Break Current Topics: Lyman Break Galaxies - Elizabeth Stanway

  40. The Lyman Break Technique Red If the filters bracket the breaks, then the galaxies show extreme colours Blue Red Current Topics: Lyman Break Galaxies - Elizabeth Stanway

  41. The Dropout Technique • At z>4, the Lyman forest absorption reaches near 100%  only one break is detected • A source will be detected in filters above the break but ‘drop-out’ of filters below it • V-drops  z > 4.5 • R-drops  z > 5. • I-drops  z > 5.8 Starburst at z=6 f-2.0 For galaxies at 5.6<z<7.0, i'- z'>1.3 Current Topics: Lyman Break Galaxies - Elizabeth Stanway

  42. Narrow Band Surveys Sky Emission Narrow Band • A magnitude is the average flux in a filter • If half the filter is suppressed by Ly-a forest, the galaxy appears faint Broad Band • If an emission line fills the filter, the galaxy will seem bright • By comparing flux in a narrow band with flux in a broadband, you can detect objects with strong line emission Current Topics: Lyman Break Galaxies - Elizabeth Stanway

  43. Narrow Band Surveys • But what line have you detected? • Could be: • OIII at 5007A • OII at 3727A • Lyman- at 1216A • Need spectroscopic follow-up Current Topics: Lyman Break Galaxies - Elizabeth Stanway

  44. Lecture Summary (I) • Building a sample of high z galaxies gives vital information on the state of the early universe • It requires the right balance between depth and area - because the LF is steep, depth is usually preferred • Starburst galaxies are UV-bright, dominated by hot, young massive stars • They have a rich spectrum of emission lines, dominated by: • oxygen and Balmer series lines in the optical • Lyman series lines in the ultraviolet Current Topics: Lyman Break Galaxies - Elizabeth Stanway

  45. Lecture Summary (II) • Lyman- is characteristically asymmetric due to galaxy-scale outflows • Absorption by the intervening IGM suppresses flux shortwards of Lyman- • The degree of suppression increases with redshift • A few percent at z=1 • 50% at z=3 • More than 99% by z=5.5 • This leads to a characteristic spectral break Current Topics: Lyman Break Galaxies - Elizabeth Stanway

  46. Lecture Summary (III) • Galaxies at high-z are selected by: • Narrow band surveys • Selecting for presence of strong emission lines • Uses improved background between skylines • Prone to contamination • Lyman break galaxy surveys • Selecting on the presence of a 912A or 1216A break • Based on broad-band photometry Current Topics: Lyman Break Galaxies - Elizabeth Stanway

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