1. Observing Complexities of Reionization James Rhoads STScI Mini-Workshop on The End of the Dark Ages: From First Light to Reionization In collaboration with Sangeeta Malhotra, Nor Pirzkal, Chun Xu, Junxian Wang, Steve Dawson, Steve Finkelstein, Nimish Hathi, Dan Stern, Arjun Dey, Buell Jannuzi, Hy Spinrad, Katarina Kovac, Emily Landes, and other members of the GRAPES and PEARS project teams

2. Complexities of reionization Complex reionization history: nonmonotonic or multiple reionization. Spatially inhomogeneous reionization: How do we find empirical evidence for spatial variations in –Ionizing source density, –Redshift of reionization

3. History of Reionization: Observational tools CMBR: Integral of n e along line of sight. QSO spectra: GP trough gives constraint near z QSO ; saturates at 1%. Lyman-  galaxies: Neutral fraction constraints near 50% at z Lya. Scale > ~ 1pMpc. 21 cm: Mapping of neutral regions provided foregrounds can be subtracted. GRBs: Damping wing gives neutral fraction at z GRB. Issue: Degeneracy with host ISM.

4. Observational tools: Prospects at Higher Redshift CMBR: All redshifts, all the time. QSO spectra: Bright QSOs get rarer rapidly at high redshift…. Big NIR surveys may help? Lyman-  galaxies: Intrinsic LF evolution seems weak; good prospects for high z. 21 cm: Promising so long as T S <> T CMB GRBs: Observable at high z if present. Theory suggests, should be present. Observationally, z max increasing fast (Totani’s talk) though not as fast as some theory papers predicted!

5. Lyman  at higher redshift? From ground, Ly  is observed through select “windows”. These continue into the near-IR, but are ever- narrower as redshift rises. Technical challenges for narrowbands: f/ratio.

6. Mapping Spatially Inhomogeneous Reionization Bright z > 6 quasars are much too rare to probe transverse structure of ionized bubbles with Gunn-Peterson measurements. Lyman α galaxies are much more numerous and are sensitive to neutral gas in the IGM. Mapping line emission from neutral and ionized regions would probe topology directly. Possible issues here: Signal strength; foregrounds; velocity effects.

7. The Lyman-  ReionizationTest To Observer Ionized IGM Young starburst Lyman-  photons Continuum Photons

8. The Lyman-  Test To Observer Neutral IGM Young starburst Lyman-  photons Continuum Photons (Miralda-Escude 1998; Miralda-Escude & Rees 1998; Haiman & Spaans 1999; Loeb & Rybicki 1999)

9. Lyman-  Luminosity Functions Luminosity function fits on all available data at z=5.7 and 6.5 –Santos et al. 2004, Taniguchi et al. 2004, Rhoads et al. 2004, Kurk et al. 2004, Tran et al. 2004, Hu et al. 2002, Hu et al. 2004, Ajiki et al. 2004, Rhoads et al. 2003, Rhoads & Malhotra 2001 (few tens of nights on large telescopes) z = 6.5 plot shows two hypotheses: –z = 5.7 LF, or –z = 5.7 LF reduced by a factor of 3 in luminosity to approximate IGM absorption. No evidence for neutral IGM! Malhotra & Rhoads 2004.

10. Lyman  Volume Test Reminder: (Number density) x (Volume per Bubble) = ionized volume fraction. Malhotra & Rhoads 2006 (includes appropriate modifications for overlap and spatial correlation.)

11. Mapping Inhomogeneities with Lyman  Galaxies

13. Topology of Reionization from Lyman Alpha Galaxies The overlap phase is a topological change in the ionized gas distribution. Use topological statistics-- the Genus number Figure after Gott, Weinberg, & Melott 1987

14. Topology of Reionization from Lyman Alpha Galaxies The overlap phase is a topological change in the ionized gas distribution. Use topological statistics-- the Genus number The 3D Genus number quantifies whether a two phase medium is dominated by islands of one phase embedded in a sea of the other, or whether both phases percolate. 2D version exists too. Useful for HI as well as Lyman- . –(Rhoads+, 2006)

15. Requirements for a Lyman-α Topological Test Ionized bubbles large enough to yield  1.2 pMpc Multiple sources per bubble (to allow genus measurements unbiased by discreteness). Control sample (Lyman break galaxies) to determine intrinsic topology of galaxy distribution. (Rhoads 2006)

16. Expectations for Genus Statistics Cartoon version: Start with negative genus from early bubbles. Central overlap phase: Percolation, multiply connected… Late phase: Islands of dense, still-neutral gas. (Rhoads 2006)

17. Observational Methods Narrowband imaging –z(Ly α) = 5.7, 6.5, etc. –Δz~0.1 & area ~ 1 sq. deg -> 10 6 cMpc 3 –Sensitivity ~ 10 -17 erg/cm 2 /sec (5σ) -> luminosity ~5 10 42 erg/s –Good for 2D tests at fixed redshift windows. –With massive spectroscopy, good for 3D test also. Fabry-Perot imaging; integral field spectroscopy; space-based grisms… Skip the followup step?

18. The 21 cm Line Test of Topology The 21cm line of neutral hydrogen is a promising alternative to Ly-α emission. Emission comes directly from the IGM  “sampling” and minimum bubble size are lesser concerns than with Ly-α. Thin slices in velocity space  2D topology; data cubes  3D topology. Concerns: Peculiar velocities? Foregrounds? …A clean test in principle, but may not be practical so soon. See also Gleser et al 2006, on astro/ph

19. Inhomogeneous reionization: The source term The census of ionizing sources in the z~6 universe is close to the requirements for reionization. Whether it is adequate, or not quite, depends on the faint end of the LF and on metallicity distribution of early stellar populations. See Fall’s talk; papers by Bunker et al, Bouwens et al, Stiavelli et al, …. The global census need not apply everywhere locally.

20. GRAPES (GRism ACS Program for Extragalactic Science) and PEARS (Probing Evolution And Reionization Spectroscopically) GRAPES team: S. Malhotra, J. Rhoads, N. Pirzkal, C. Xu, A. Cimatti, E. Daddi, H. Ferguson, J. Gardner, C. Gronwall, Z. Haiman, A. Koekemoer, A. Pasquali, N. Panagia, L. Petro, M. Stiavelli, Z. Tsvetanov, J. Walsh, R. Windhorst, H.J. Yan PEARS: most of the GRAPES team, plus N. Hathi, I. Ferreras, M. Kuemmel The deepest unbiased spectroscopy yet !

21. GRAPES & PEARS overview Advantages of the ACS Grism: Dark sky, great PSF, high efficiency. GRAPES: 40 orbits of HST ACS G800L grism spectroscopy in the Hubble Ultra Deep Field. Cycle 12. Fully reduced data is public, with a fun web interface! PEARS: 200 orbit HST Treasury program, cycle 14. 9 fields total: 40 more orbits on the UDF, plus 4 more fields in GOODS-N and 4 in GOODS-S, each to 20 orbit depth. Observations completed 2/2006; v. 1 data reduction finished. Further work in progress.

22. 40 orbits of UDF observations with the ACS grism Spectra for every source in the field. Good S/N continuum detections to I(AB) ~ 27.5. Ten times deeper than ground-based : Keck, Gemini, VLT About 15% of UDF sources ~ 1500 spectra with good s/n Spectral identification of every z=4-7 object to I(AB)=27.5 Moderate redshift ellipticals z~1-2 Emission line galaxies Reduced spectra available from HST archives: http://archive.stsci.edu/prepds/udf/udf_hlsp.html

23. A Spiral galaxy at z=0.3 Direct image | Dispersed image

24. GRAPES Experimental design (Pirzkal et al. 2004) Four orients: 0, 8, 90, 98 degrees orient to disentangle overlapping spectra. Good agreement demonstrates good wavelength and flux calibration PEARS design similar.

25. High redshift galaxies z=5.5, z=26.9 z=6.4, z=27.8 z=5.8, z=25.1 With GRAPES we can spectroscopically confirm LBGs to z’(AB)=27-28 depending on the redshift.

26. Reliability of (i-z) selection 80% for (i-z) > 0.9 96% for (i-z) > 1.3

27. Completeness: color-redshift plot The (i-z’) generally follows the expected color but there are some blue galaxies: all can be explained by a moderately strong Lyman-alpha emission. Incompleteness implied is about 4/23~20%

28. A spike in the Redshift distribution (Malhotra et al. 2005) Comparison of observed redshift distribution (histogram) vs. expected numbers The spike at z~6 is at least a factor of two over-dense.

29. Deep probe vs. Flat-wide probe Ly-alpha emitters at z=5.7-5.77 observed with mosaic at CTIO –(36’ = 13 pMpc = 80 cMpc) (Wang, Malhotra & Rhoads 2004) Inhomogeneous distribution –UDF is at the edge of it

30. Luminosity function at the overdensity Star-formation rate density for this over- dense region is 2- 4x10 -2 M O /Mpc 3 /year Volume well known. This is enough to drive re-ionization in this “local” over-density. Solid: GRAPES spectroscopic sample. Open: Corrected for incompleteness.

31. The Overdensity in PEARS? The PEARS survey gives us a factor of 5 in solid angle over GRAPES. Linear layout: Covers a gradient of density. Constrained by GOODS layout.

32. Future Prospects for Mapping Reionization Sources JWST: dropout search with NIRCAM + NIRSPEC followup DESTINY: a Dark Energy mission concept. Grism observations 0.85 to 1.7 microns, monitoring two regions for SNe Ia for 2 years. –Byproduct: 3 square degree survey, deeper than PEARS, and at higher redshift. Massive ground based surveys? Difficult in the NIR due to bright atmosphere.

33. First and Last word about Pop-III: Spectral slopes of UDF faint galaxies The composite spectrum of z=4-5 objects in the UDF is shown by the white line. The Lyman break sample (Shapley et al.) at z=3 is shown in yellow for comparison and one of the bluest nearby galaxies NGC 1705 is shown in blue. (Blue… but not Pop-III!)

34. Summary Reionization history: –A range of techniques will help determine reionization history. Want sensitivity to a range of neutral fractions at a range of redshifts. –Ly , GRB, 21cm extend well to high z. Inhomogeneity in HI fraction: –Bubble mapping with Ly  (current technology!), 21 cm (near future). –Use appropriate statistics… Topology. Source inhomogeneity: –Break surveys plus massive followup… or, –Grism surveys roll both steps into one! –Reduced GRAPES data are public. –Watch for PEARS results, from reionization to Galactic stars.