1. z > 6 Surveys Represent the Current Frontier Motivation: - census of earliest galaxies (z=6,  =0.95 Gyr) - contribution of SF to cosmic reionization - constraints on early mass assembly - planning effective use of future facilities (ELTs, JWST) Developing complementary optical/IR techniques: - Lyman break dropouts - Ly  emitters - strong gravitational lensing by galaxy clusters

2. Some Key Issues How effective are the various high z selection methods? - L*(z=6)  i~26 where spectroscopy is hard - spectroscopic samples biased to include strong L  - great reliance on photometric redshifts Is there a decline in the UV luminosity density 3<z<6? - results are in some disagreement - differing trends in continuum drops & L  emitters Significant stellar masses for post-burst z~6 galaxies - how reliable are the stellar masses? - inconsistent with declining SF observed 6<z<10? - does this imply an early intense period of activity? - in conflict with hierarchical models?

3. Continuum sources probed via dropout technique z-dropout Traditional dropout technique poorly-suited for z>6 galaxies: - significant contamination (cool stars, z~2 passive galaxies) - spectroscopic verification impractical below ~few L* i-drop volumes : UDF (2.6 10 4 ), GOODS-N/S (5.10 5 ), Subaru (10 6 ) Mpc 3 flux limits: UDF z<28.5, GOODS z<25.6, Subaru z<25.4 Stanway et al (2003)

4. Reducing Contamination from z~2 Passive Galaxies z~2 passive galaxies Addition of a precise optical-infrared color (z - J) can, in addition to the (i - z) dropout cut, assist in rejecting z~2 passive galaxy contaminants. (Stanway et al 2004) (z – J) (i – z) 5.7 < z < 6.5 This contamination is ~10% at z~25.6 but is negligible at UDF limit (z~28.5)

5. Contamination by cool Galactic dwarfs - more worrisome HST half-light radius R h more effective than broad-band colors Contamination at bright end (z<25.6) is significant (30-40%) L dwarfs E/S0 UDF z<25.6 (Stanway et al 2004)

6. ACS dropouts: Luminosity Dependent Evolution? Bouwens et al (2006, ~500 sources at z=6!!!) propose L-dependent evolution - decline in abundance over 3<z<6 mostly for luminous sources – finally hierarchical growth?? If correct, this affects z-dependent integrated SF density measures corrected to some fiducial luminosity z=3

7. Decline in  UV over 3<z<6 has been controversial Bouwens et al 2005 Ap J 624, L5 Poisson errors fail to account for dispersion in claimed number of z~6 i-drops, because of varying ways of accounting for contamination plus cosmic variance (10% in GOODS; 40% in UDF) Bunker et al 2004 Giavalisco et al 2004

8. Results from Subaru HST offers superior photometry & resolution (important for stellar contamination) but SuPrimeCam has much bigger field (each pointing = 2  GOODS-N+S) Additional photometric bands developed to sort stellar contamination Shioya et al (2005): used intermediate band filters @ 709nm, 826nm to estimate stellar contamination in z~5 and z~6 samples respectively Shimasaku et al (2005) split z-band into two intermediate filters z B, z R - to measure UV continuum slope These studies confirm decline indicated via HST studies

9. z~6 dropouts from Subaru SDF dataset > 2  GOODS N+S; cosmic variance ~ 25% Confirm  5 abundance drop from z~3 to 6 (c.f. Bunker et al, HST) Luminosity dependent trends - more evolution in massive galaxies? Remember: this is observed number not dust-corrected SFR

10. The Spitzer Space Telescope Revolution A modest 60cm cooled telescope can see the most distant known objects and provide crucial data on their assembled stellar masses! IRAC camera has 4 channels at 3.6, 4.5, 5.8 and 8  m corresponding to 0.5-1  m at z~7! Egami et al (2005) - characterization of a lensed z~6.8 galaxy Eyles et al (2005) - old stars at z~6 Yan et al (2005) - masses at z~5 and z~6 Mobasher et al (2005) - a galaxy > 10 11 M  at z~6?

11. Spitzer detections of i-drops at z=6 #1 z=5.83 #3 z=5.78 4 i-drops in GOODS-S confirmed spectroscopically at Keck Ly  emission consistent with SFR > 6 M  yr -1 IRAC detections from GOODS Super-Deep Legacy Program Eyles et al (2005) MNRAS 364, 443

12. Spectral Energy Distributions of i-drops #1 z=5.83#3 z=5.78 Spitzer + Ly  emission constrains present & past star formation Ages > 100 Myr, probable 250-650 Myr (but Universe is only 1 Gyr old!!! (7.5<z F <13.5) Stellar masses: 2-4  10 10 M  (>20% L*) VLT K Look at lines!!!!!

13. Independent z~6 UDF Spitzer analysis 3 sources at z=5.9, Yan et al Ap J 634, 109 (2005) Confirms high stellar masses and prominent Balmer breaks

14. Spitzer detection of a resolved J-drop in UDF Criterion: (J – H) AB > 1.3 plus no detection in combined ACS While prominent detection in all 4 IRAC bands JD2: strong K/3.6  m break  potential high mass z~7 source Mobasher et al (2005) Ap J 635, 832

15. STARBURST99: z=6.6; E B-V =0.0; Z=0.02, z F >9 BC03: z=6.5; E B-V =0.0; Z=0.004, z F >9 Stellar Mass: 2-7  10 11 M  dependent on AGN contamination High mass, two breaks, but not confirmed spectroscopically – risk of foreground Mobasher et al (2005)

16. Uncertainty in Redshift and Stellar Mass ~ 25% chance of being z~2.5

17. Abundance of Massive Galaxies at z~6: A Crisis? Abundance of massive galaxies at z~6 with  CDM in terms of their implied halo masses, assuming Scalo IMF SF efficiency 20% Find a 10 13 M  halo in the tiny UDF is a problem! Yan et al Eyles et al Barkana & Loeb (2005) z = 5.8 z = 15 Mobasher et al

18. Summary Great progress using v,i,z,J-band drop outs to probe abundance of SF galaxies from 3<z<10: Bouwens et al discuss the properties of 506 I- band dropouts to z~29.5! In practice, these samples are contaminated by foreground stars, z~2 galaxies etc to an extent which remains controversial. We are unlikely to resolve this definitively with spectroscopy until era of ELTs. Comoving SF rate declines from z~3 to z~6 (and probably beyond) Contribution of lower luminosity systems less clear Spitzer’s IRAC can detect large numbers of z~5-6 galaxies and it seems many have high masses (one spectacularly so!) and signatures of mature stellar populations - implies earlier activity Reconciling mature galaxies at z~6 with little evidence for SF systems with 7<z<10 may turn out to be a very interesting result

19. Strong lensing & the hi-z Universe Zwicky (1937) predicted its utility From curiosity associated with verification of General Relativity to practical tool for cosmologists

20. Lensed pair dropout behind Abell2218: SED Implies Established Stellar Population @ z~7 Key parameters: SFR = 2.6 M  yr -1 M star ~ 5-10 10 8 M  z ~ 6.8  0.1 age 40 – 450 Myr (7 < z F < 12) Age > e-folding SF time  more luminous during active phase? (Egami et al 2005) Several groups are now surveying more lensing clusters - Given small search area, such sources may be very common

21. z > 6 Lyman  Surveys Complementary techniques: - narrow band imaging techniques (f  < 10 -17 cgs, L  < 5. 10 42 cgs, SFR~3 M  yr -1, V~2. 10 5 Mpc 3 ) at z=6 - lensed spectra (f  < 3.10 -19, L  < 10 41, SFR~0.1 M  yr -1, V <50 Mpc 3 ) Origin: ionizing flux absorbed by H gas  Ly  photons Lyman alpha emission: n=2  1, E=10.2eV, 1216Å Efficient: as much as 6-7% of young galaxy light may emerge in L  depending on IMF, metallicity etc. 1 M  yr -1 = 1.5 10 42 ergs sec -1 (Kennicutt 1998)(no dust, normal IMF)

22. Panoramic Imaging Camera on Subaru Megacam Suprime-Cam Can survey distant Universe for Lyman alpha emitters by constructing narrow-band filters and comparing with signal in suitably-chosen broad-band filters

23. Large Scale Structure @ z=5.7 via 515 Ly  emitters Ouchi et al 2005 Ap J 620 L1

24. Narrow bands in `quiet’ windows in night sky spectrum z(L  ) = 4.7 5.7 6.6 6.9 Requires panoramic imaging as  z range is small Airglow spectrum

25. Selection & spectroscopic verification of interlopers Hu et al (2003) z=5.7 survey Compare signal in nb filter with broad- band signal using Subaru Spectroscopic follow- up of candidates with Keck 5007Å 3727Å 1216Å

26. Example: Ly  Emitters at z=6.5 Very distant Subaru Ly  emitters: = (a)z=6.541, W = 130, SFR=9 (b)z=6.578, W = 330, SFR=5 Kodaira et al (2003) PASJ 55, 17 spectra

27. z=5.7 Ly  Luminosity Function Shimasaku et al astro-ph/0602614 Comprehensive Subaru nb survey of 725 arcmin 2 89 candidates 28/39 spec. confirmed ~230 Å - normal stellar popn. Malhotra & Rhoads 2004

28. Ly  Emitters at z~6.6 (Taniguchi et al 2005) Two color criteria: (z - NB921) > 1.0 and(i - z) > 1.3 Yields 58 candidates Spectra confirm 9-14 out of 20 (45-70%) Two key results: -L  emitters less significant than dropouts as contributors to SFR at z~6.6 -Yet an increasing fraction with increasing redshift (less evolution from z~3-6 than dropouts)