Why study high redshift galaxies We Can! (definition of high-z) It’s Fun! Watch cosmic history! Credit: Mark Dickinson
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 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 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 (broad band dropout)LAE (narrow band excess) Large volumeSmall volume continuous redshiftcertain redshifts, but deeper Hard to identifyEasy to identify sensitive to UV continuum sensitive to Ly line Luminous galaxiesFainter 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- 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-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 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 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 Scattering Dark 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 propertiesVery rare, bright.Common, faint. Redshift coverage Continuous.Discrete from ground; continuous above atmosphere.
Charting Reionization Current 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 strengthAge (Myr) Stellar Mass (10 8 solar masses; 100,000,000*mass of Sun) Low20023.75 Medium808.56 High41.08
How does this compare? Other galaxies at similar redshift have masses ~ 10 9-10 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 ~ 10 11 solar masses; ~ 10 billion years old.
Why is this interesting? Compared to the Milky Way, the LAE’s are much smaller. Consistent with hierarchical clustering theory of galaxy growth. Compared with other high-redshift galaxies: Our ages and masses are consistent with other studies of similar objects One study derived smaller masses than ours, but their galaxies were fainter, so our results are consistent.
Windows in the atmospheric OH spectrum continue into the J and H bands, though narrower. Newest NIR cameras have A sufficient for plausible LBG and Ly- searches. Especially with the help of strong lensing Several efforts under way … Horton et al 2004 (DAzLE project): VLT + DAzLE) z ~ 7.7 Smith et al (see Barton et al 2004): Gemini + NIRI, z ~ 8.2 Stark et al. 2007: Keck +NIRSPEC 6 candidates between z=8.7 and z=10.2 Willis et al ( “ ZEN ” project): VLT +ISAAC, z ~ 8.8 Cuby et al: VLT +ISAAC, z ~ 8.8 Nilsson et al: ELVIS @ VISTA
Z-Band Dropout behind cluster H JZ NB 1.06 Credit: Wei Zheng
Spectroscopic Followup Approximately 12 bright z-band dropout candidates at AB < 25 VLT/ISAAC, low-resolution (6-8 objects), short exposure (1-2 hr) Gemini-S/GNIRS medium resolution (4 objects) None was confirmed yet