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Lyman- α Galaxies at High Redshift. James E. Rhoads (Space Telescope Science Institute) with Sangeeta Malhotra, Steve Dawson, Arjun Dey, Buell Jannuzi, Emily Landes, Hy Spinrad, Dan Stern, JunXian Wang, and Chun Xu. Lyman α as a Signpost of Primordial Galaxies.

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Lyman galaxies at high redshift

Lyman-α Galaxies at High Redshift

James E. Rhoads

(Space Telescope Science Institute)


Sangeeta Malhotra,

Steve Dawson, Arjun Dey, Buell Jannuzi, Emily Landes, Hy Spinrad, Dan Stern, JunXian Wang, and Chun Xu

Lyman as a signpost of primordial galaxies
Lyman α as a Signpost of Primordial Galaxies

We are looking for the first galaxies to form from primordial gas. Such objects should have…

  • Young stars, with high ultraviolet luminosities.

  • Considerable amounts of gas.

  • Very low abundances of heavy elements, implying

    • Stars that are hotter than “normal” (for fixed mass);

    • Low dust abundances.

Lyman line emission
Lyman α Line Emission

  • Ionizing flux + gas → 2 Lyman α photons for every 3 ionizing photons absorbed by hydrogen.

  • In principle, up to 6-7% of a young galaxy’s luminosity may emerge in the Lyman α line (for a Salpeter IMF).

The narrowband search method
The Narrowband Search Method

  • We take pictures of a large area of sky in both broad and narrow filters.

  • Emission line sources appear bright in narrow filters (demand 5σ).

  • The blue “veto filter” eliminates foreground emission line objects (demand < 2σ).

Windows for narrowband surveys
Windows for Narrowband Surveys

Red lines mark our survey filter wavelengths.

Line searches are highly efficient in far red!

The large area lyman alpha survey lala an overview
The Large Area Lyman Alpha Survey (LALA): an Overview

z=4.5: Rhoads et al 2000, Malhotra & Rhoads 2002, Dawson et al 2004 in prep;

z=5.7: Rhoads & Malhotra 2001, Rhoads et al 2003, Wang et al 2004 in prep.

z=6.5: Rhoads et al 2004

Physical nature of ly galaxies
Physical Nature of Ly-α Galaxies

These are young star forming galaxies:

  • High equivalent widths require youth, plus either low metallicity or top-heavy IMF (Malhotra & Rhoads 2002)

  • AGN are ruled out by a combination of X-ray observations (see JunXian Wang’s talk [48.14] in this session) and optical spectroscopy (see Steve Dawson’s talk [48.04] in this session).

Candidate z 6 5 lala galaxies
Candidate z=6.5 LALA galaxies


All data from NOAO 4m telescopes; NB918 stack is 24 hours’ integration.

Lala j142442 24 353400 2 at z 6 535
LALA J142442.24+353400.2 at z=6.535

  • Gemini spectrum shows an Asymmetric line and no continuum.

  • Nod and shuffle helps eliminate the possibility of other lines if [OIII] (5007)

    (Rhoads et al. 2004; Gemini spectrum reduced by Chun Xu.)

Physical properties of this galaxy
Physical properties of this galaxy

  • Line luminosity 1043 erg s-1.

  • Star formation rate: 11 Msun / year

    • Based on Lyman-α line + Kennicutt IMF + Case B recombination… Some modelling uncertainty.

  • Equivalent width > 530Å observer frame (2σ lower bound from photometry)

    • Rest frame > 70 Å (naively)

    • Boosting z’ flux to account for IGM absorption under Madau’s (1995) prescription gives > 40Å

  • Spatially unresolved in 1” seeing  < 6 kpc.

May you live in interesting times
“May you live in interesting times…”

  • The z = 6.5 universe was just 6% of its present age.

  • Moreover, this precedes the redshift of the SDSS quasars with Gunn-Peterson troughs.

  • Thus the 9200Å window affords a view of late reionization.

The lyman reionizationtest
The Lyman-α ReionizationTest

Ionized IGM

Continuum Photons




Lyman α photons

The lyman reionization test
The Lyman-α Reionization Test

Neutral IGM

Continuum Photons




Lyman α photons

(Miralda-Escude 1998; Miralda-Escude & Rees 1998;

Haiman & Spaans 1999; Loeb & Rybicki 1999)

Constraining reionization with lala
Constraining Reionization with LALA

  • Lyman α emitters at z=5.7 and 6.5 suggest zre > 6.5.

    (Rhoads & Malhotra 2001; ApJ Lett 563, L5; Hu et al 2002; Rhoads et al 2003; Rhoads et al 2004)

  • Large equiv. width of the LALA object  line attenuation factor is a few at most.

Figure: z=5.7 Ly-α sources from Rhoads et al 2003

Charting reionization
Charting Reionization

Current evidence: Combine the Lyman α and Gunn-Peterson tests so far to study the evolution of the mass avgeraged neutral fraction, x:

There is no contradiction between the GP effect at z=6.2 and the Ly α at z=6.56.

(Figure from Rhoads et al 2003)

Lyman test details
Lyman α Test Details

  • HII regions: R>1.2 pMpc  line center τ0< 1.

    • Residual neutrals in HII region matter.

  • τ < τ0 in the red wing of lines (Haiman 2002)

  • Evolution affects galaxy counts… LBG comparison samples could resolve this.

  • Gas motions alter Ly-α radiative transfer (Santos 2003).

  • Bottom line: Factors of ~ 3 reduction in Ly-α luminosity are unavoidable in neutral universe. Reduction larger in many models but not all.

Star formation and metal production
Star Formation and Metal Production

  • Using LF constraint from HCM 6a  ~3e-2 Msun/yr/Mpc3 star formation, and ~ 200 Msun/Myr/Mpc3 of metal production… comparable to z ~ 4 numbers from UV continuum!

  • Using instead lens+blank slit data of Santos et al could reduce both numbers by a factor ~ 30.

  • Compare SFRD estimates of 5.2e-4 (Kodaira et al, z=6.5 Ly-α sample); 4.6e-4 (Stanway et al, z ~ 6 LBG sample); neither integrates a LF much below the observed limit.

  • The usual conclusion: More good data wanted!


  • Lyman-α provides an efficient way of finding young galaxies at high redshift.

  • These galaxies afford a new probe of reionization, complementing the Gunn-Peterson test.

  • Ly-α from LALA z=6.535 galaxy requires either a largely ionized IGM, or an intrinsic redshift of the line relative to systemic velocity.

  • Remaining uncertainties in Lyman-α reionization test can be overcome using statistical samples.