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Warm greetings to KIAA-PKU from CFA@USTC. Searching for the first galaxies. Junxian Wang University of Science and Technology of China Beijing, June. 2008. Z=0.158. How to find high redshift galaxies?. Look very hard Get lucky

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searching for the first galaxies

Warm greetings to KIAA-PKU


Searching for the first galaxies

Junxian Wang

University of Science and Technology of China

Beijing, June. 2008

how to find high redshift galaxies
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

lyman from young galaxies
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 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 narrowband search method
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 method1
The Narrowband Search Method
  • take images in both broad and narrow filters.
  • Emission line sources appear faint or absent in broad filter
origin of the lyman break
Origin of the Lyman break

Steidel & Hamilton 1992


LBG in E-CDFS, R=22.8, z=3.38strong Ly emission (EW=60Å, SFRUV ≥350 M/yr) numerous chemical absorption features (6 hr IMACS exposure)









Gawiser et al 2005

Ly 

a large scale structure at z 6
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 surveys
Lyman- Surveys

A 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 galaxies
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 to x ray ratios
Lyman-α to X-ray ratios

Wang et al 2004, ApJ Letters 608, L21

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.

composite ly galaxy spectrum
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)

AGN fraction < 10%

the role of dust reduce the line ew
The role of dust: reduce the line EW

Ly photons

Continuum photons

Ly 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
The role of dust: enhance the line EW

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.

Ly photons

UV photons

Hansen & Oh 2006

a brief history of the universe
A Brief History of the Universe

Big Bang

  • 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: ?

Last Scattering

Dark Ages

First Galaxies


Galaxies, Clusters, etc.

G. Djorgovski

reionization a phase transition
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.
charting reionization
Charting Reionization

There is no contradiction between the GP effect at z=6.2 and the Ly α at z=6.5.

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

ages and masses
Ages and Masses
  • We found the best-fit ages and masses for different categories of Lyman alpha galaxies:
how does this compare
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.
z band dropout behind cluster
Z-Band Dropout behind cluster


NB 1.06



Credit: Wei Zheng