The Large Area Lyman- Survey (LALA)
Junxian WangUniversity of Science and Technology of
China
Beijing, July. 2008
LALA Collaborators
ASU: Sangeeta Malhotra, James Rhoads, Steven Finkelstein, Norman Grogin
China: JunXian Wang, Chun Xu, Baltimore: Norbert Pirzkal, Katarina
Kovac Tucson: Buell Jannuzi, Arjun Dey,
Michael Brown Berkeley Diaspora: Hy Spinrad, Dan
Stern, Steve Dawson
outline
introduction to LAEs (Lyman- emitting galaxies) and the Large Area Lyman-Alpha survey (LALA)
Physical properties of LAEs
Constraints on cosmic re-ionization
What are LAEs?
Lyman-Alpha Emitting galaxies
Why study LAEs? Lyα gives an easy way to spot high-z 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
The Gunn-Peterson Test and LAEs
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 properties Very rare, bright.
Common, faint.
Redshift coverage
Continuous. Discrete from ground; continuous above atmosphere.
How to detect LAEs?
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).
The Narrowband Search Method
take images in both broad and narrow filters.
Emission line sources appear faint or absent in broad filter
Selection criteria 5 detection in
narrow band 0.75mag color
excess 4 color
excess <2 detection
in veto band
Success rate up to >70%
Contaminants include variable sources, asteroids, satellite trails, noise spikes in NB, foreground emission line galaxies ([OIII], [OII], etc).
LALA z=6.5 Source
Gemini GMOS 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, ApJ; Gemini spectrum reduced by Chun Xu.)
Blank sky spectral search for LAEs
Integrated field unit
Multi-slit masks + narrow band filter
Long slits (behind strong lensing)
Blank sky search for
Lyman alpha lines
LBG vs LAE ?
Origin of the Lyman break
Steidel & Hamilton 1992
LBG in E-CDFS, R=22.8, z=3.38 strong Ly emission (EW=60Å, SFRUV ≥350
M/yr) numerous chemical absorption features (6 hr
IMACS exposure)
Ly
SiII
OI/SiII
CIIFeII
SiIV
SiII
CIV
MUSYCGawiser et al 2005
Windows for Narrowband Surveys
Z=6.9
LALA filters FWHM ~ 80Å (trade-off between
sensitivity and volume)
Z ~ 4.5, 6559Å, 6611Å, 6650Å, 6692Å, 6730Å
Z ~ 5.7, 8150Å, 8230Å
Z ~ 6.5, 9180Å
LALA Survey Overview
z Volume (Field) Sensitivity Candidates,
Spectroscopic Success rate
4.5 (5 filters)
1.4x106 Mpc
(Bootes, Cetus,CDF-S)
1.7x10-17 ergs/s/cm2 400; > 70%
5.7 (2 filters)
4 x105 Mpc
(Bootes, CDF-S)
1x10-17 ergs/s/cm2 ~50; ~70%
6.5 (1 filter)
1.5x105 Mpc (Bootes, CDF-S)
2x10-17 ergs/s/cm2 3; 1 of 3 confirmed.
LBG (broad band dropout)
LAE (narrow band excess)
Large volume Small volume
continuous redshift certain redshifts, but deeper
Hard to identify Easy to identify
sensitive to UV continuum
sensitive to Ly line
Luminous galaxies Fainter galaxies
trace the large scale structure
A Large Scale Structure at z~6
Spatial distribution of z=5.75 galaxies in the CDF-S region. (Wang et al. 2005, ApJL)
Lyman- SurveysA 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.
Physical Properties of Ly-α Galaxies
numerous LAEs with EWs > 200 Å stellar populations are expected to
produce peak EWs 100Å~200Å (Charlot & Fall 1993), EW ~ 80 Å for a normal stellar population.
Very hot stars? Accretion power (i.e, Active Galactic
Nuclei)? Continuum preferentially suppressed by
dust? (Neufeld 1991; Hansen & Oh 2005)
A Bright High Equivalent Width Galaxy
None of 101 imaged Ly emitters were detected in X-ray individually
neither in stacked images Left: all Ly emitters (effective
exposure time 11.2 Ms) Right: Ly emitter with Ly EW >
240Å
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
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)
The role of dust: reduce the line EW
Ly photons
Continuum photonsLy photons take longer path to escape, thus are more likely to be absorbed by smoothly distributed dust.
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
Two populations of LAEs?
Finkelstein et al. 2008
Ages and Masses We found the best-fit ages and masses for different
categories of Lyman alpha galaxies:
Ly line strength Age (Myr)Stellar Mass (108 solar
masses; 100,000,000*mass of Sun)
Low 200 23.75
High 4 1.08
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 ScatteringDark Ages
Galaxies, Clusters, etc.
Reionization
G. Djorgovski
First Galaxies
Dawson et al. 2007
Charting ReionizationCurrent 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.
Thank you!