Current Topics Lyman Break Galaxies Dr Elizabeth Stanway
Current Topics Lyman Break Galaxies Dr Elizabeth Stanway (E. R. Stanway@Bristol. ac. uk) Current Topics: Lyman Break Galaxies - Lecture 4
Topic Summary • Star Forming Galaxies and the Lyman Line • Lyman Break Galaxies at z<4 • Lyman Break Galaxies at z>7 • Reionisation, SFH and Luminosity Functions Current Topics: Lyman Break Galaxies - Lecture 4
Ground vs Space-Based Surveys • HST can reach objects 0. 7 -1 mag (2 -3 times) fainter in the same length of time • Ground-based 8 m 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 d. A dz) • So N 8 m/NHST=(L 8 m/LHST) (A 8 m/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 8 m telescopes are ‘cheaper’ Current Topics: Lyman Break Galaxies - Lecture 4
Surveys of z>4 LBGs GOODS (The Great Observatories Origins Deep Survey) SDF/SXDF V-drops Z-drops I-drops Subaru 8 m telescope V-drops R-drops Hubble Space Telescope I-drops BDF/ERGS ESO Very Large Telescopes (8 m) R-drops Z-drops I-drops Cluster Lensing Surveys Keck / HST I-drops J-drops Z-drops UKIDSS UK Infrared Telescope (4 m) I-drops Y-drops Z-drops J-drops Current Topics: Lyman Break Galaxies - Lecture 4
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
SFR e-t/ Eyles et al, 2005 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 Current Topics: Lyman Break Galaxies - Lecture 4
Old Stars at z~6 z=5. 83 Too Young for Ly line Older than universe • 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 Current Topics: Lyman Break Galaxies - Lecture 4
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
Comparisons with z=3 fraction Age: At z=3, age~300 Myr At z=5, age~30 Myr Log (Age) If Z=Z , then age~3 Myr Galaxies are younger (Verma et al 2007) Current Topics: Lyman Break Galaxies - Lecture 4
Comparisons with z=3 fraction Stellar Mass: At z=3, mass~1010 M At z=5, Mass ~ 2 x 109 M Log (Mass) Independent of metallicity Galaxies are smaller (Verma et al 2007) Current Topics: Lyman Break Galaxies - Lecture 4
Comparisons with z=3 Star Formation Rate: At z=3, SFR~50 M /yr At z=5, SFR ~ 50 M /yr fraction If Z=Z , SFR~600 M /yr => Galaxies are forming stars at about the same rate Log (SFR) Current Topics: Lyman Break Galaxies - Lecture 4
Comparisons with z=3 Dust: At z=3, Av~0. 6 mags At z=5, Av ~ 0. 3 mags fraction If Z=Z , Av~0. 6 mags Current Topics: Lyman Break Galaxies - Lecture 4 => High z galaxies are less dusty Av
Ferguson et al 2004 Sizes and Morphologies Current Topics: Lyman Break Galaxies - Lecture 4 • 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
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
Spectroscopy at z~5 is challenging, but not impossible In 5 hours on an 8 m 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
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
The Rest-Ultraviolet No Ly lines Too Blue • Rest-UV slope is an age indicator: – young=blue, old=red • But many z~5 galaxies seem too blue Current Topics: Lyman Break Galaxies - Lecture 4 Line emitters
The Rest-Ultraviolet No Ly lines Too Blue Line emitters • Steep Rest-UV slope (blue of f -2) could indicate zero age, Pop III, top-heavy initial mass function … => New physics! Interpretation still unclear Current Topics: Lyman Break Galaxies - Lecture 4
Lyman- Equivalent Widths z~6 i’-drops (DEIMOS) z~5 50% of z>5 sources have EW>0Å 25% have EW>30Å • 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
Other spectral lines and outflows • Stacking together ~50 z~5 galaxies, can start to see other lines: • CIV, Si. IV and OI are starting to be visible • Velocity offsets => similar winds to z~3 • Work still in progress! OI Current Topics: Lyman Break Galaxies - Lecture 4 SIV
• In a few lensed cases, can identify lines in individual spectra • This example is 6 x 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 Current Topics: Lyman Break Galaxies - Lecture 4 Dow-Hygelund et al, 2005 Other spectral lines and outflows
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
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 highz clusters and a large amount of obscured star formation. Current Topics: Lyman Break Galaxies - Lecture 4
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 submm wavelengths • 80 telescopes at 5000 m • 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
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
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
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
Bradley et al 2008 Lensed LBGs at z>7 Current Topics: Lyman Break Galaxies - Lecture 4 • z=7. 6 candidate galaxy • z-drop • J-drop • 100 Myr old • No dust • Lensed
HST and WFC 3 • In 2009 HST was serviced and a new camera was installed: WFC 3 • This gave HST much better resolution, field of view and sensitivity in the nearinfrared • Can now effectively extend the LBG technique to higher redshifts • Spectroscopic follow-up remains a problem Current Topics: Lyman Break Galaxies - Lecture 4
LBGs at Higher Redshifts WFC 3 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
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
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
The Rest UV spectral Slope • AGN have spectra described by a power law, z’ Y J H L i. e L • In the rest-frame ultraviolet, star forming galaxies also show powerlaw 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 Current Topics: Lyman Break Galaxies - Lecture 4 z=7 galaxy Magnitude gives the flux in J and H => f. J and f. H Know the central wavelengths of J and H => J and H LJ/LH = f. J/f. H ( J
The Rest UV spectral Slope LJ/LH = f. J/f. H ( 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 AB mag = -2. 5 log(f )-48. 6 z’ Y J z=7 galaxy - App. mag, defined in f J-H = -2. 5 log(f. J)-48. 6 - (-2. 5 log(f. H)-48. 6) - Colour is (mag) J-H = -2. 5 log (f. J/f. H) = -2. 5 log (( J - Using spectral index J-H = -2. 5 (- -2) log ( J H Simplifying J-H = 0. 16 magnitudes Current Topics: Lyman Break Galaxies - Lecture 4
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
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
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 Current Topics: Lyman Break Galaxies - Lecture 4 Bouwens et al (2010) < -3 slopes may indicate that z=7 galaxies have very low metallicity
Cosmic Evolution of Star Formation Property z=1 -3 z=5 -6 z>7 Age ~200 Myr ~50 Myr May be younger Mass few x 1010 M ~109 M No data Metallicity 0. 3 -0. 5 Z ~0. 2 Z May be very low - Pop III Size (half light radius) 1. 5 -2 kpc ~1 kpc ~0. 5 kpc scales as comoving M* -21. 1 z=5 : -20. 7 z=5 : -20. 2 -19. 9? Faint end Slope -1. 6 may be steeper No data Dust E(B-V)~0. 2 Probably less dusty No data Star Formation Rate ~30 M /yr Current Topics: Lyman Break Galaxies - Lecture 4 ~30 M /yr
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
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
- Slides: 40