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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 Lyman Break Galaxies at z>7 Reionisation, SFH and Luminosity Functions.

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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 - Lecture 4

  2. Topic Summary • Star Forming Galaxies and the Lyman- Line • Lyman Break Galaxies at z<4 • Lyman Break Galaxies at z>4 • Lyman Break Galaxies at z>7 • Reionisation, SFH and Luminosity Functions Current Topics: Lyman Break Galaxies - Lecture 4

  3. Ground vs Space-Based Surveys • HST can reach objects 0.7-1mag (2-3 times) fainter in the same length of time • Ground-based 8m telescopes have larger fields of view (by a factor of about 4) • So which is more efficient at finding high-z galaxies? • The faint end of the Schecter Luminosity function (L<<L*) can be approximated as power law (i.e. N(L)  LdA dz) • So N8m/NHST=(L8m/LHST) (A8m/AHST) • If  is steeper than about -1.2, then HST always wins (I.e depth is more useful than area) HST has higher resolution, but 8m telescopes are ‘cheaper’ Current Topics: Lyman Break Galaxies - Lecture 4

  4. Surveys of z>4 LBGs Current Topics: Lyman Break Galaxies - Lecture 4

  5. Stellar populations • As at z=3, most information is derived from SED fitting. • Unconfused Spitzer data is essential for this at z>4 • Detailed results are model dependent • General results are model independent Verma et al, 2007 Current Topics: Lyman Break Galaxies - Lecture 4

  6. Old Stars at z=6 • Sometimes both a new starburst and an old population are needed to fit a galaxy • As at z=3, some stars seem as old as the universe, but time scales are shorter, so the constraints are tighter Eyles et al, 2005 SFRe-t/ Current Topics: Lyman Break Galaxies - Lecture 4

  7. Old Stars at z~6 Too Young for Ly line z=5.83 • Sometimes both a new starburst and an old population are needed to fit a galaxy • As at z=3, some stars seem as old as the universe, but time scales are shorter, so the constraints are tighter Older than universe Current Topics: Lyman Break Galaxies - Lecture 4

  8. Comparisons with z=3 • Using a z~5 HST v-drop sample • GOODS field => extremely deep • Using an SMC (i.e. low metallicity) extinction law • Using Spitzer data Current Topics: Lyman Break Galaxies - Lecture 4

  9. Comparisons with z=3 Age: At z=3, age~300Myr At z=5, age~30Myr If Z=Z, then age~3Myr • Galaxies are younger (Verma et al 2007) fraction Log (Age) Current Topics: Lyman Break Galaxies - Lecture 4

  10. Comparisons with z=3 Stellar Mass: At z=3, mass~1010M At z=5, Mass ~ 2x109M Independent of metallicity • Galaxies are smaller (Verma et al 2007) fraction Log (Mass) Current Topics: Lyman Break Galaxies - Lecture 4

  11. Comparisons with z=3 Star Formation Rate: At z=3, SFR~50M/yr At z=5, SFR ~ 50M/yr If Z=Z, SFR~600M/yr => Galaxies are forming stars at about the same rate fraction Log (SFR) Current Topics: Lyman Break Galaxies - Lecture 4

  12. Comparisons with z=3 Dust: At z=3, Av~0.6 mags At z=5, Av ~ 0.3 mags If Z=Z, Av~0.6 mags => High z galaxies are less dusty fraction Av Current Topics: Lyman Break Galaxies - Lecture 4

  13. Sizes and Morphologies • Galaxies at high-z have a smaller projected size. • Most of this is due to evolution in physical size rather than angular scale factor • Up to z~5, the size evolution is as expected for a fixed mass • Morphologies are often irregular and complex Ferguson et al 2004 Current Topics: Lyman Break Galaxies - Lecture 4

  14. Sizes and Morphologies • Galaxies at high-z have a smaller projected size. • Most of this is due to evolution in physical size rather than angular scale factor • Up to z~5, the size evolution is as expected for a fixed mass • Morphologies are often irregular and complex Current Topics: Lyman Break Galaxies - Lecture 4

  15. Spectroscopy at z~5 Spectroscopy at z~5 is challenging, but not impossible In 5 hours on an 8m telescope get good S/N on lines and reasonable detections of continuum flux The night sky is growing brighter but is still reasonable Current Topics: Lyman Break Galaxies - Lecture 4

  16. Spectroscopy at z~6 35 hours with Gemini 6 hours with Keck Spectroscopy at z~6 is extremely difficult Sources are typically 1 mag fainter at z=6 than at z=5 Continuum is only detected in exceptional or lensed galaxies Current Topics: Lyman Break Galaxies - Lecture 4

  17. The Rest-Ultraviolet • Rest-UV slope is an age indicator: • young=blue, old=red • But many z~5 galaxies seem too blue No Ly lines Too Blue Line emitters Current Topics: Lyman Break Galaxies - Lecture 4

  18. The Rest-Ultraviolet • Steep Rest-UV slope (blue of f-2) could indicate zero age, Pop III, top-heavy initial mass function … => New physics! Interpretation still unclear No Ly lines Too Blue Line emitters Current Topics: Lyman Break Galaxies - Lecture 4

  19. Lyman- Equivalent Widths z~6 50% of z>5 sources have EW>0Å 25% have EW>30Å z~5 i’-drops (DEIMOS) • At z~5 the distribution of Lyman-a line strengths is similar to that at z~3 • At z~6 see more high EW lines - selection function? More hot stars? Dust effects? New physics? Current Topics: Lyman Break Galaxies - Lecture 4

  20. Other spectral lines and outflows • Stacking together ~50 z~5 galaxies, can start to see other lines: • CIV, SiIV and OI are starting to be visible • Velocity offsets => similar winds to z~3 • Work still in progress! OI SIV Current Topics: Lyman Break Galaxies - Lecture 4

  21. Other spectral lines and outflows • In a few lensed cases, can identify lines in individual spectra • This example is 6x the typical z~5 LBG brightness • It is also lensed! • Strong interstellar lines • No Ly => older than typical, more dusty or more evolved • Psychotic cases like this can’t really describe the whole population Dow-Hygelund et al, 2005 Current Topics: Lyman Break Galaxies - Lecture 4

  22. Non-LBGs at z=5-6 • As at z=3, LBGs don’t show the whole picture at z=5 • Some star forming sources are going to be too faint to be detected as LBGs • Narrowband detected galaxies (LAEs) • Lensed galaxies • GRB Host galaxies • Some galaxies won’t be star forming • Sub-mm galaxies • DLAs • Molecular Line Emitter galaxies Current Topics: Lyman Break Galaxies - Lecture 4

  23. The Whole Picture at z=5? • How many galaxies at these redshifts are UV-dark? • Searching z=5 LBG clusters for UV-dark material might be the way forward • Initial results are promising - z=5 CO emission detected near z=5 LBGs (Stanway et al, 2008) • If typical, similar galaxies could contribute a significant fraction of the total galaxy mass in high-z clusters and a large amount of obscured star formation. Current Topics: Lyman Break Galaxies - Lecture 4

  24. Future Millimeter Observations • The Atacama Large Millimeter Array (ALMA) begins commissioning this year • It will be fully online by about 2013 • It observes at mm and sub-mm wavelengths • 80 telescopes at 5000m • Will be sensitive to dust emission, CO and other strong emission lines (e.g. [CII]) to very high z Current Topics: Lyman Break Galaxies - Lecture 4

  25. Gamma-Ray Bursts • Some star formation will be going on in galaxies too faint to detect as LBGs • Where massive stars are forming, some small number can go supernova • In certain circumstances, supernovae are associated with extraordinarily luminous, highly beamed flashes of gamma rays • These are known as Gamma Ray Bursts (GRBs) and can be used as tracers of low mass star formation • At high redshifts, a GRB will show up as a dropout (i.e. selected like an LBG), but will fade rapidly with time • The most distant objects known in the Universe are GRBs (z=8.3) Current Topics: Lyman Break Galaxies - Lecture 4

  26. Lensing as a tool at high redshift • In rare cases, can use intervening galaxy clusters as gravitational lenses - gives spatial information, boosted signal-to-noise, near-IR spectroscopy • 2 known strongly lensed LBGs at z~5 • Only provides information on rare sources - not average sources • Requires lens reconstruction z=4.9 Swinbank et al (2009) Current Topics: Lyman Break Galaxies - Lecture 4

  27. z=6.5 candidate LBGs at z>6 • Beyond z=6, the Lyman break moves into the infrared • Resolution and sensitivity are poor • Need lensing to stand realistic chance of detecting objects from ground • NO spectroscopically confirmed galaxies beyond z=6.96 Current Topics: Lyman Break Galaxies - Lecture 4

  28. Lensed LBGs at z>7 • z=7.6 candidate galaxy • z-drop • J-drop • 100 Myr old • No dust • Lensed Bradley et al 2008 Current Topics: Lyman Break Galaxies - Lecture 4

  29. HST and WFC3 • In 2009 HST was serviced and a new camera was installed: WFC3 • This gave HST much better resolution, field of view and sensitivity in the near-infrared • Can now effectively extend the LBG technique to higher redshifts • Spectroscopic follow-up remains a problem Current Topics: Lyman Break Galaxies - Lecture 4

  30. LBGs at Higher Redshifts WFC3 on HST can find z-drops (z~7), Y-drops (z~8) and maybe J-drops (z~10) but can’t confirm them Current Topics: Lyman Break Galaxies - Lecture 4

  31. LBGs at Higher Redshifts z’-drop candidates at z~7 Bunker et al (2009), see also Bouwens+ Oesch+ Castellano+ Wilkins+ etc, etc (About 20 papers in Sep-Dec 2009) Current Topics: Lyman Break Galaxies - Lecture 4

  32. Size Evolution to z>7 • Galaxies at z=7 continue to get smaller • This scales as size  (1+z)-1.12 ± 0.17, consistent with constant comoving sizes • Most z=7 candidates very compact (Oesch et al 2010) Current Topics: Lyman Break Galaxies - Lecture 4

  33. The Rest UV spectral Slope Y J H z’ • AGN have spectra described by a power law, Li.e L • In the rest-frame ultraviolet, star forming galaxies also show power-law spectra • The slope of the power law depends on the temperature of the emitting source • This power law slope can be measured using broadband photometry z=7 galaxy Magnitude gives the flux in J and H => fJ and fH Know the central wavelengths of J and H => J and H LJ/LH = fJ/fH (J Current Topics: Lyman Break Galaxies - Lecture 4

  34. The Rest UV spectral Slope LJ/LH = fJ/fH (J Example: A source has a spectral slope =-2.5 - calculate the J-H colour in AB mags, given central wavelengths of 1.2m and 1.6m for J and H respectively Y J H z’ AB mag = -2.5 log(f)-48.6 - App. mag, defined in f J-H = -2.5 log(fJ)-48.6 - (-2.5 log(fH)-48.6) - Colour is (mag) J-H = -2.5 log (fJ/fH) = -2.5 log ((J - Using spectral index J-H = -2.5 (--2) log (J Simplifying J-H = 0.16 magnitudes z=7 galaxy Current Topics: Lyman Break Galaxies - Lecture 4

  35. Rest-UV Spectral Slope • AGN have ≈-1 at all redshifts • Zero-age, star forming galaxies with normal stellar populations have ≈-2 • Dust or age will make this slope redder (i.e. shallower) • Within the LBG population the spectral slope is seen to evolve with z => age evolution? Dust evolution? Bouwens et al (2010) Current Topics: Lyman Break Galaxies - Lecture 4

  36. Rest-UV slope at z = 7 - 8 Bouwens et al (2010) • At z~7, candidate galaxies are very blue, particularly faint galaxies •  < -3 is very hard to explain with any ‘normal’ (Population II) stellar population Current Topics: Lyman Break Galaxies - Lecture 4

  37. Rest-UV slope at z = 7 - 8 • Pop III stars are defined as having very low or zero metallicity • With no metals, they have fewer ways to emit radiation (i.e. cool down) • They can become hotter, and more massive (supported by radiation pressure) • Hotter galaxies have bluer spectral slopes Bouwens et al (2010)  < -3 slopes may indicate that z=7 galaxies have very low metallicity Current Topics: Lyman Break Galaxies - Lecture 4

  38. Cosmic Evolution of Star Formation Current Topics: Lyman Break Galaxies - Lecture 4

  39. Ensemble Properties of LBGs • At z=2-4, you can study individual galaxies in detail • At z=5-6, and more so at z>7, this becomes much harder • Studying an individual galaxy only tells you about its immediate environment • By looking about the ensemble properties of galaxies you can study the universe as a whole => observational cosmology • By using a common selection method (LBGs), you are comparing like-for-like across cosmic time => Insights into galaxy formation, the star formation histoy of the Universe and Reionisation Current Topics: Lyman Break Galaxies - Lecture 4

  40. Lecture Summary • LBGs at z>4 are significantly harder to find than those at z<4 • LBGs at z~6 are a lot harder than z~5 • With increasing redshift see: • Decreasing metallicity • Decreasing dust extinction • Decreasing age • Decreasing mass • These traits extend to z~7-8 • Very blue rest-UV spectra are hinting at changes in the nature of star formation • But, as at z=3, LBGs are not the whole story Current Topics: Lyman Break Galaxies - Lecture 4

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