1. Searching for the first galaxies Junxian Wang University of Science and Technology of China Beijing, June. 2008 Warm greetings to KIAA-PKU from CFA@USTC

2. Z=0.158

3. Why study high redshift galaxies  We Can! (definition of high-z)  It’s Fun!  Watch cosmic history! Credit: Mark Dickinson

4. How to find high redshift galaxies?  Look very hard  Get lucky  Look next to something else  Watch the fireworks  Look smart (LBG, Lyman-α galaxies, submm)  get some help  etc Credit: Mark Dickinson

5. Galaxy Clusters as a “Cosmic Telescopes”

7. Lyman α from Young Galaxies Young galaxies forming their first stars produce copious ionizing radiation, hence strong Lyman-  emission. (Partridge and Peebles 1967) In principle, up to 6-7% of a young galaxy’s luminosity may emerge in the Lyman α line (for a Salpeter IMF). High z LAEs not detected until 30 years later There are now over a dozen research groups, Over thousands candidate Lyman-  galaxies, Over hundreds spectroscopically confirmed Up to a redshift of 6.96

8. The Narrowband Search Method  take images in both broad and narrow filters.  Emission line sources appear faint or absent in broad filter  The blue “ veto filter ” eliminates foreground emission line objects (demand < 2σ).

9. The Narrowband Search Method  take images in both broad and narrow filters.  Emission line sources appear faint or absent in broad filter

10. Iye et al. 2006

11. LBG vs LAE ?

12. Origin of the Lyman break Steidel & Hamilton 1992

14. LBG in E-CDFS, R=22.8, z=3.38 strong Ly  emission (EW=60Å, SFR UV ≥350 M  /yr) numerous chemical absorption features (6 hr IMACS exposure) Ly  SiII OI/SiII CII FeII SiIV SiII CIV MUSYC Gawiser et al 2005

15. Windows for Narrowband Surveys Z=6.9

16. LBG (broad band dropout)LAE (narrow band excess) Large volumeSmall volume continuous redshiftcertain redshifts, but deeper Hard to identifyEasy to identify sensitive to UV continuum sensitive to Ly  line Luminous galaxiesFainter galaxies trace the large scale structure

17. A Large Scale Structure at z~6  Spatial distribution of z=5.75 galaxies in the CDF-S region. (Wang et al. 2005, ApJL)

18. Lyman-  Surveys A partial listing of Lyman-  surveys since the first discovered field Ly-  galaxies: z < 4: Hu et al 1998, Kudritzki et al 2000, Stiavelli & Scarlatta 2003, Fynbo et al, Palunas et al, 4 < z < 5: LALA; Venemans et al 2002; Ouchi et al 2002; 5 < z < 6: LALA, Hu et al 2003; Ajiki et al 2003, 2003; Wang et al 2005; Ouchi et al 2005; Santos et al 2004; Martin & Sawicki 2004; 6 < z < 7: Hu et al 2002, Kodaira et al 2003, Taniguchi et al 2004, LALA (Rhoads et al 2004), Cuby et al 2003, Tran et al 2004, Santos et al 2004, Stern et al 2005. 7 < z < 9: Several surveys in progress, no confirmed detections yet.

19. Physical Properties of Ly-α Galaxies Large line to continuum ratios are common. (Malhotra & Rhoads 2002, ApJ Lett 565, L71):  Very hot stars?  Accretion power (i.e, Active Galactic Nuclei)?  Continuum preferentially suppressed by dust? (Neufeld 1991; Hansen & Oh 2005)

20. Lyman-α to X-ray ratios  Individual Lyman-α emitters are consistent with some but not all Type-II QSOs, and most are consistent with Seyfert IIs.  The composite Ly-α to X-ray ratio strongly rules out a large fraction of AGN in the Ly-α sample. Wang et al 2004, ApJ Letters 608, L21

21. Composite Ly-α Galaxy Spectrum Optical spectra show no sign of C IV or HeII lines. These would be expected for AGN. (Dawson et al 2004, ApJ 617, 707)

22. The role of dust: reduce the line EW Ly  photons Continuum photons Ly  photons take longer path to escape, thus are more likely to be absorbed by smoothly distributed dust.

23. The role of dust: enhance the line EW Ly  photons UV photons Ly  photons can be scattered off at the surface of cold dust clumps, thus could avoid being absorbed by dust grains, while the continuum could be severely attenuated. Hansen & Oh 2006

24. A Brief History of the Universe  Last scattering: z=1089, t=379,000 yr  Today: z=0, t=13.7 Gyr  Reionization: z=6-20, t=0.2-1 Gyr  First galaxies: ? Big Bang Last Scattering Dark Ages Galaxies, Clusters, etc. Reionization G. Djorgovski First Galaxies

25. Reionization: a phase transition.  The detection of Gunn- Peterson trough(s) in z > 6 quasars show neutral IGM at z~6. (Becker et al. 2001, Fan et al. 2002.)  This implies a qualitative change: enough photons existed after z=6 to ionize the IGM, but not before.

26. Comparing the Ly-  and Gunn- Peterson Tests Gunn- Peterson Lyman α Threshold neutral fraction in uniform IGM 10 -4 0.1 In nonuniform IGM 10 -2 > 0.1 Source propertiesVery rare, bright.Common, faint. Redshift coverage Continuous.Discrete from ground; continuous above atmosphere.

27. Charting Reionization Current evidence: Combine the Lyman α and Gunn- Peterson tests so far to study the evolution of the mass averaged neutral fraction, x: There is no contradiction between the GP effect at z=6.2 and the Ly α at z=6.5.

28. Madau Plot

29. Ages and Masses  We found the best-fit ages and masses for different categories of Lyman alpha galaxies: Ly  line strengthAge (Myr) Stellar Mass (10 8 solar masses; 100,000,000*mass of Sun) Low20023.75 Medium808.56 High41.08

30. How does this compare?  Other galaxies at similar redshift have masses ~ 10 9-10 solar masses.  These are consistent with our lowest line strength objects, which are also the brightest, and thus easier to detect in a normal survey.  The higher line strength objects are much fainter, which is why we only found them when we looked for the emission line.  Fainter usually means smaller, and we see this in their lower mass.  Milky Way ~ 10 11 solar masses; ~ 10 billion years old.

31. Why is this interesting?  Compared to the Milky Way, the LAE’s are much smaller.  Consistent with hierarchical clustering theory of galaxy growth.  Compared with other high-redshift galaxies:  Our ages and masses are consistent with other studies of similar objects  One study derived smaller masses than ours, but their galaxies were fainter, so our results are consistent.

32. Extension to redshifts z > 7

33.  Windows in the atmospheric OH spectrum continue into the J and H bands, though narrower.  Newest NIR cameras have A  sufficient for plausible LBG and Ly-  searches.  Especially with the help of strong lensing  Several efforts under way …  Horton et al 2004 (DAzLE project): VLT + DAzLE) z ~ 7.7  Smith et al (see Barton et al 2004): Gemini + NIRI, z ~ 8.2  Stark et al. 2007: Keck +NIRSPEC 6 candidates between z=8.7 and z=10.2  Willis et al ( “ ZEN ” project): VLT +ISAAC, z ~ 8.8  Cuby et al: VLT +ISAAC, z ~ 8.8  Nilsson et al: ELVIS @ VISTA

34. Z-Band Dropout behind cluster H JZ NB 1.06 Credit: Wei Zheng

35. Spectroscopic Followup  Approximately 12 bright z-band dropout candidates at AB < 25  VLT/ISAAC, low-resolution (6-8 objects), short exposure (1-2 hr)  Gemini-S/GNIRS medium resolution (4 objects)  None was confirmed yet

36. Blank sky search for Lyman alpha lines

40. Wait for JWST?

41. Thank you!