searching for the first galaxies junxian wang university of science and technology of china beijing,...
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Searching for the first galaxies
Junxian WangUniversity of Science and Technology of
China
Beijing, June. 2008
Warm greetings to KIAA-PKUfrom CFA@USTC
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 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
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
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.
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.