Searching for the first galaxies

Junxian WangUniversity of Science and Technology of

China

Beijing, June. 2008

Warm greetings to KIAA-PKUfrom CFA@USTC

Z=0.158

How to find high redshift galaxies?

Look very hard Get lucky Look next to something else Watch the fireworks Look smart (LBG, Lyman-α galaxies,

submm) get some help etc

Credit: Mark Dickinson

Galaxy Clusters as a“Cosmic Telescopes”

Lyman α from Young 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 laterThere are now over a dozen research groups,Over thousands candidate Lyman- galaxies,Over hundreds spectroscopically confirmedUp to a redshift of 6.96

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

Iye et al. 2006

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

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

Large line to continuum ratios are common. (Malhotra & Rhoads 2002, ApJ Lett 565, L71):

Very hot stars? Accretion power (i.e, Active Galactic

Nuclei)? Continuum preferentially suppressed

by dust? (Neufeld 1991; Hansen & Oh 2005)

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

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

Reionization: a phase transition.

The detection of Gunn-Peterson trough(s) in z > 6 quasars show neutral IGM at z~6. (Becker et al. 2001, Fan et al. 2002.)

This implies a qualitative change: enough photons existed after z=6 to ionize the IGM, but not before.

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.

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.

Madau Plot

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

Medium 80 8.56

High 4 1.08

How does this compare? Other galaxies at similar redshift have

masses ~ 109-10 solar masses. These are consistent with our lowest line strength

objects, which are also the brightest, and thus easier to detect in a normal survey.

The higher line strength objects are much fainter, which is why we only found them when we looked for the emission line.

Fainter usually means smaller, and we see this in their lower mass.

Milky Way ~ 1011 solar masses; ~ 10 billion years old.

Extension to redshifts z > 7

Z-Band Dropout behind cluster

H

JZ

NB 1.06

Credit: Wei Zheng

Blank sky search for

Lyman alpha lines

Wait for JWST?

Thank you!