Observing Complexities of ReionizationJames Rhoads

STScI Mini-Workshop on The End of the Dark Ages: From First Light to

Reionization

In collaboration withSangeeta 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

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

History of Reionization: Observational tools

• CMBR: Integral of ne along line of sight.• QSO spectra: GP trough gives constraint near

zQSO; saturates at 1%.• Lyman- galaxies: Neutral fraction constraints

near 50% at zLya. Scale > ~ 1pMpc .• 21 cm: Mapping of neutral regions provided

foregrounds can be subtracted.• GRBs: Damping wing gives neutral fraction at

zGRB. Issue: Degeneracy with host ISM.

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 TS <> TCMB

• GRBs: Observable at high z if present. Theory suggests, should be present. Observationally, zmax increasing fast (Totani’s talk) though not as fast as some theory papers predicted!

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.

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.

The Lyman- ReionizationTest

To

Observer

Ionized IGM

Young starburst

Lyman- photons

Continuum Photons

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)

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.

Lyman Volume Test• Reminder:

(Number density) x (Volume per Bubble) = ionized volume fraction.

• Malhotra & Rhoads 2006 (includes appropriate modifications for overlap and spatial correlation.)

Mapping Inhomogeneities with Lyman Galaxies

Mapping Inhomogeneities with Lyman Galaxies

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

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)

Requirements for a Lyman-α Topological Test

• Ionized bubbles large enough to yield < 1 R > 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)

Expectations for Genus StatisticsCartoon version:• Start with negative

genus from early bubbles.

• Central overlap phase: Percolation, multiply connected…

• Late phase: Islands of dense, still-neutral gas.

(Rhoads 2006)

Observational Methods• Narrowband imaging

– z(Ly α) = 5.7, 6.5, etc.– Δz~0.1 & area ~ 1 sq. deg -> 106 cMpc3

– Sensitivity ~ 10-17 erg/cm2/sec (5σ) -> luminosity ~5 1042 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?

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

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.

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 !

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.

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

A Spiral galaxy at z=0.3

Direct image | Dispersed image

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.

High redshift galaxiesz=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.

Reliability of (i-z) selection • 80% for (i-z) > 0.9

• 96% for (i-z) > 1.3

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%

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.

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

Luminosity function at the overdensity

• Star-formation rate density for this over-dense region is 2-4x10-2 MO/Mpc3/year

• Volume well known.• This is enough to drive

re-ionization in this “local” over-density.

Solid: GRAPES spectroscopic sample. Open: Corrected for incompleteness.

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.

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.

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!)

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.