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  1. The Search for Forming Galaxies Chris O’Dea Space Telescope Science Institute • Acknowledgements: • Mauro Giavalisco • Harry Ferguson

  2. Outline • Hierarchical Galaxy Formation • Star Formation & Stellar Evolution • Searches for Forming Galaxies • Narrow Band Optical Searches • GPS Quasars • High-Z Radio Galaxies • The Hubble Deep Fields • Lyman-Break Galaxies • Sub-mm/IR • Star Formation History of the Universe

  3. Hierarchical Galaxy Formation (Virgo consortium)

  4. Hierarchical Galaxy Formation: The Paradigm • At recombination (z~1160), the universe is very homogeneous & smooth • There is a spectrum of density perturbations – gravitational potential fluctuations are independent of length scale • Low mass clumps collapse first and merge to form galaxies • Larger scale structure builds slowly as galaxies form - groups, clusters, super clusters. e.g., Kauffmann etal. 1993, MNRAS, 264, 201

  5. Blow up of dark matter density in the region around a rich cluster in a simulation of a ΛCDM universe at z=0. Jenkins etal 1998, ApJ, 499, 20

  6. Numerical models of structure formation in 4 cosmologies. (dark matter density is plotted). • All simulations are normalized to reproduce the abundance of rich galaxy clusters today. • However, the power spectrum of the simulated dark matter distribution is not consistent with that of observed galaxies. Jenkins etal 1998, ApJ, 499, 20

  7. Star Formation & Stellar Evolution

  8. Star Formation • Evolution of the UV-Optical SED of a continuous star burst. • The SED brightens in the UV around 3 Myr and then reddens only slightly with time. 1 solar mass/yr with solar metals and Salpeter IMF 1-100 M⊙ (Starburst99 code).

  9. Star Formation • Evolution of the UV-Optical SED of an instantaneous star burst. • The SED brightens in the UV around 2 Myr and then reddens and fades as the stars evolve. 106 M⊙ burst with solar metals and Salpeter IMF 1-100 M⊙ (Starburst99 code).

  10. SED of Instantaneous Burst • Broadband spectrum of instantaneous burst reddens and dims are the population evolves (massive hot stars die first). Devriendt etal. 1999, A&A, 350, 381

  11. Star Formation in a Merger • N-Body simulation of evolution of galaxies with dusty starbursts showing old stellar population. Mass distribution of old stars projected onto (x,y) plane at each time T for the merger model. Each frame is 105 kpc. Merger is prograde-retrograde. (Bekki & Shioya 2001, ApJS, 134, 241).

  12. Star Formation in a Merger • N-Body simulation of evolution of galaxies with dusty starbursts showing gas and new stars. Mass distribution of gas and new stars projected onto (x,y) plane at each time T for the merger model. Each frame is 105 kpc. Merger is prograde-retrograde. (Bekki & Shioya 2001, ApJS, 134, 241).

  13. Star Formation in a Merger Star formation rate depends on the accumulation of dense gas in the central region. Time evolution of star formation rate in solar masses/yr in the merger. Time evolution of gas mass accumulated within the central regions. (Bekki & Shioya 2001, ApJS, 134, 241).

  14. Star Formation in a Merger • Time dependence of SED depends on time dependence of star formation rate. • IR and sub-mm luminosity increases during peak of star formation (when gas is efficiently transported to galaxy center). • In later stages, gas is rapidly consumed, and UV and IR luminosity declines. Spectral energy distribution of a merger as a function of time. Model includes gas and dust. Time given in Gyr. (Bekki & Shioya 2001, ApJS, 134, 241). 104 Å = 1μ.

  15. Star Formation in a Merger • Effect of dust is to remove UV light and re-radiate in the IR. Spectral energy distribution of a merger (top) with gas and dust, and (bottom) without. Corresponds to maximum SFR in the merger. Bekki & Shioya 2001, ApJS, 134, 241. 104 Å = 1μ.

  16. Integrated Spectra of Galaxies • Spectra reflect the large difference in SFR as a function of Hubble type. Fluxes Normalized at 5500 Å. (Kennicutt 1992, ApJS, 79, 255)

  17. SRF vs Hubble Type • Line EQW scales with stellar birthrate parameter (b) and Hubble type. From a large sample of nearby spiral galaxies (Kennicutt 1998, ARAA,36, 189).

  18. Narrow Band Searches • A proto galaxy forming stars at a rate of 100 M⊙/yr should produce a Lyα luminosity ~ 1043 ergs/s (e.g., Thompson etal, 1995, AJ, 110, 963). • Yet, with some exceptions (see next viewgraph) Lyα from possible proto galaxies is rarely detected in deep narrow band searches (Thompson etal 1995; Stern & Spinrad, 1999, PASP, 111, 1475) • This implies that the galaxies are obscured by dust.

  19. Extended Lyα Emission • Two large, bright, diffuse Lyα blobs in a protocluster region at z~3.09 • The blobs are similar to those seen around powerful radio galaxies, but these are radio-weak. • They could be excited by obscured AGN or they could be large cooling-flows. (Steidel etal, 2000, ApJ, 532, 170)

  20. High z GPS Quasars • A significant fraction of radio-loud quasars at high z (>2) tend to be GPS. • GPS quasars tend to be at high z (>2) • Possibly, the high z quasars are GPS because the radio sources are confined to small scales (<100 pc) due to dense gas in the host circumnuclear region. • The presence of the dense gas necessary to confine a powerful quasar (> 1010 M⊙), suggests that the host is a proto galaxy. (O’Dea 1998, PASP,110, 493)

  21. Radio Galaxies (Carilli 2000)

  22. Radio Galaxies at High z • Powerful radio galaxies are detectable out to high z. • They are generally bright L* Ellipticals with old stellar populations rather than proto galaxies. Van Breugel etal. 1999, ApJ, 518, L61

  23. The Hubble Deep Fields

  24. HDF Census ~3000 Galaxies at U,B,V,I ~1700 Galaxies at J, H ~300 Galaxies at K ~9 Galaxies at 3.2mm ~50 Galaxies at 6.7 or 15mm ~5 Sources at 850mm 0 Sources at 450mm or 2800mm ~16 Sources at 8.5 GHz ~150 Measured redshifts ~30 Galaxies with spectroscopic z > 2 <20 Main-sequence stars to I = 26.3 ~2 Supernovae 0-2 Strong gravitational lenses 6 X-ray sources Ferguson, Dickinson & Williams 2000, ARAA, 38, 667

  25. Advantages and disadvantages of a pencil-beam survey • Normalized by galaxy luminosity function. Shows the number of L* volumes. • Volume is smallest at low z where most of cosmic time passes. (Ferguson etal. 2000, ARAA, 38, 667)

  26. Galaxy Counts • Galaxy number counts favor ΛCDM cosmologies. • Galaxies are more numerous than simple no-evolution models (esp at U) Ferguson etal 2000, ARAA, 38,667

  27. WFPC2 & NICMOS Imaging • Selected galaxies from the HDF-N at a range of z. Left – B, V, I; Right – I, J, H. • Morphologies are similar in both optical and near-IR. Ferguson etal. 2000, ARAA, 38, 667

  28. Galaxy Morphologies • Higher fraction of irregular & peculiar galaxies than seen locally. • Qualitatively supports hierarchical galaxy formation. • LSB galaxies and bursting dwarf galaxies don’t dominate the counts. Abraham et al. 1996, Baugh et al. 1996, Ferguson & Babul 1998…

  29. Galaxy Sizes at z~3 • The galaxies at z~3 are small but luminous, with half-light radii 1.8 <r1/2< 6.5 h kpc and absolute magnitudes -21.5 > M(B) > -23. Blue magnitude vs half-light radius for High-Z HDF galaxies and a representative sample of local galaxies.(Lowenthal etal 1997, ApJ, 481, 673)

  30. F814W

  31. F606W

  32. F450W

  33. F300W

  34. STIS 2300Ǻ

  35. STIS 1600Å

  36. Lyman Break Galaxies

  37. Lyman-Break Galaxies Color selection of star-forming galaxies from the 912 Å continuum discontinuity • Effects of cosmic opacity… • Photoelectric absorption • Line blanketing • … and moderate dust obscuration • Makes identification of distant galaxies “easy” with optical/near-IR multi-band imaging • Very efficient: ~90% at z~3, 50% at z~4 • Current best way to test ideas on galaxy formation

  38. Spectral Features due to Hydrogen (Valenti 2001)

  39. Lyman-Break selection (Giavalisco 2001)

  40. Lyman-Break selection (Giavalisco 2001)

  41. Expected colors of high z Lyman break galaxies are well defined, and not sensitive to reddening. Steidel etal 1999, ApJ, 519, 1

  42. Steidel etal 1999, ApJ, 519, 1

  43. Color color plot of real data. • 207/29,000 satisfy the color selection criteria. • Blue circles are objects with spectroscopic 3.7<z<4.8. And yellow objects are interlopers. Steidel etal 1999, ApJ, 519, 1

  44. Lyman-Break Technique • NOT photometric redshift • Just effective set of selection criteria • Requires follow-up spectroscopic identification to be useful

  45. Keck-LRIS spectra • Rs<25.5 • Texp~2-4 hr • Δλ~12 Å • Similar to local SF galaxies • Richness of features from: • Interstellar gas • Nebular gas • Stars • Presence of OB stars • Varying Lyα Giavalisco 2001

  46. Keck-LRIS spectra • Rs<25.5 • Texp~2-4 hr • Δλ~12 Å • Similar to local SF galaxies • Richness of features from: • Interstellar gas • Nebular gas • Stars • Presence of OB stars • Varying Lyα Giavalisco 2001

  47. Large survey • Results of spectroscopic follow up of color selected LBGs. • The two samples are consistent with having similar colors. Steidel etal 1999, ApJ, 519, 1

  48. The Nature of LBGs • What is the link between LBGs and the local populations? • Are LBGs small sub-galactic systems that will merge to form more massive galaxies, as predicted by hierarchical cosmologies (CDM)? • What is their mass distribution? • Regardless, their stars must be old • Can they be the progenitors of the spheroids? • What is their metallicity? • What are their stellar mass and age?

  49. HST morphology • Observed mostly only faint LBGs (m>m*) • Small size: r1/2~1-3 kpc • Dispersion of properties: both disk-like and spheroid-like observed • Rest-UV and rest-optical morphologies similar