Lyman  Emitters in Historical and Cosmic Perspective

1. Lyman  Emitters in Historical and Cosmic Perspective Esther Hu University of Hawaii, Institute for Astronomy Understanding Lyman  Emitters, MPIA 7 October 2008

2. Overview • Historical Context of Search for early, forming galaxies, and explorations of the distant universe • Development of large-scale Lyman alpha surveys • The current status of investigations at high redshift • Revisiting the topic of low-z emission-line galaxies • Have we found analogues to the high-z Lyman alpha galaxies that we can study at lower redshifts?

3. Detecting Distant Galaxies I • Status 1980s: typical redshift limit z~0.2 for normal, star-forming galaxies • Discovery of quasars in the late 1950s, and realization that radio galaxies were often distant objects these unusual objects held the redshift record until 1997 • In 1985, this was about z~3.78

4. Theory Predictions • These galaxies should be faint, because they are distant (uncertainties because of cosmic geometry – Ho, qo and galaxy properties) • Early star formation should produce strong emission from Hydrogen – easiest way to find these galaxies is to look for Lyman a (1216 Å) • Lyα emission was proposed as a signpost of primitive galaxies in formation (Partridge & Peebles 1967)

5. Strategy • Galaxies aren’t randomly distributed; the best place to look for a galaxy is next to another galaxy. • We can’t identify high-z galaxies, but we do know about high-z quasars, which are believed to have host galaxies and which have strong hydrogen Lyman a emission. • Use known high-z quasars as markers in redshift space, and look around them in a narrowband filter centered on the quasar’s Lyman a emission. • Check whether emission-line objects are high-z galaxies at the same redshift as the quasar by taking spectra

6. First Try • 1985 Djorgovksi, Spinrad, McCarthy and Strauss observe the z=3.215 quasar PKS1614+051 – and find a Lyman a companion galaxy! • But, none of their other tries are successful

7. The Observational Horizon In recent years the limits of our observational horizon have been rapidly pushed back – but the most distant detected sources are often atypical. • The highest redshift galaxies have been found by: • Radio Surveys • Surveys of QSO fields. • Lensing Surveys • Narrow-band Selection • Or some combination of these methods. Stanway 2005

8. What was Wrong? • This was a radio quasar (about 10% of quasars); the close-in companion has something to do with radio sources – not star-forming galaxies • Observations assumed that galaxies at z~3 would be like galaxies today – just as massive; predicted amount of Lyman a scales with galaxy size • Consequently, nobody looked deeply enough

9. How was this interpreted? • The failure to detect Lyman a emission from early star formation led many scientists to conclude that dust in star-forming regions would always successfully block ultraviolet light. • They concluded that Lyman a emission could never be used to find early, star-forming galaxies and this method fell out of favor.

10. Interim Status • A number of searches on the 2-4-m class telescopes were unsuccessful in finding Lyman alpha emitters (Pritchet 1994) • However, there were suggestions (Cowie 1988) that deeper Lyman a searches might turn up candidates.

11. Detecting Distant Galaxies II • 1991 Workshop on Distant Quasars hosted by the Space Telescope Science Institute • By then, more than a dozen quasars at z~4.5 had been found (redshift record: z=4.89) • This was approaching the redshift range of forming galaxies – the first generation of star formation dust might be dust-free • Already some evidence from infrared observations on the ground, and Hubble Space Telescope images that galaxy properties might have changed substantially in the past

12. Discovery of z=4.55 Galaxies Deeper targeted search succeeds (left panel 20hr narrowband on 2.2-m telescpe)

13. Detecting Distant Galaxies III • In the early 1990s, only a few groups persisted in the searches for the field Lyman  emission-line galaxies (e.g. Hawaii and CADIS surveys) • First successful field galaxy detections of Lyman  galaxies at z~ 3.4 Cowie & Hu 1997 (LRIS imager on Keck I 10=m telescope with a narrow-band filter). • Roughly at the same time, color-based samples at z~ 3 begin to be available (Steidel et al.) The two methods are complementary. • This led to an enormous flourishing of Lyman  searches over the subsequent decade, aided by the larger format mosaic CCD cameras and the large telescopes

14. The Observational Horizon In recent years the limits of our observational horizon have been rapidly pushed back – but the most distant detected sources are often atypical. • The highest redshift galaxies have been found by: • Radio Surveys • Surveys of QSO fields. • Lensing Surveys • Narrow-band Selection • Or some combination of these methods. Stanway 2005

15. Reaching Yet More Distant Galaxies • More distant galaxies are harder, because the Lyman  line is fainter — but it’s also harder to be sure that it is the Lyman  line that you are looking at and not some foreground emission line. • Additional criteria to make the high-z searches easier: • Color Selection • Emission-line profile • The problems of foreground contamination mean that you do need a confirming spectrum to check the emission-line profile. • Will come back to the continuing issue of possible foreground contaminants if there is time at the end.

16. SED Color and Line Properties of High-Redshift Objects

17. Comparison of stacked colors ofz=5.7 emitters with z=5.7 quasar SED of Galaxies consistent with Ly Forest Absorption in Quasar Spectrum

18. Z=5.18 CHANDRA source comp’d with SDSS z=5.01 Quasar COMPARISON OF SLOAN SELECTED QUASAR WITH Z = 5.18 CHANDRA SOURCE IN HDF-N

19. Continuum break Red stars O II O III HDF SSA22 Equivalent width N(8150) < 24 samples -- Z=5.7 selection

20. Spectroscopic z = 5.7 (solid boxes) SSA22 field to N(AB)=25.1 19 spectroscopic Ly a emitters

21. Composite of Deimos Spectra Hu et al. (2004) R=2700 spectra allow us to easily distinguish OII and OIII emitters instrument profile Important to spectroscopically confirm candidates as Ly (profile)

22. 1% Night Sky Filter profile Keck LRIS spectrum

23. z=5.74 Galaxies with Keck Narrowband Imaging Searches on a 10-m Telescope

24. Optical (B thr z’ & narrowband) Images of z=5.74 Gal. 8185/105 I z’ R V B z=5.74 galaxy (SSA22-HCM1)

25. Spectral Energy Distribution

26. The Observational Horizon In recent years the limits of our observational horizon have been rapidly pushed back – but the most distant detected sources are often atypical. • The highest redshift galaxies have been found by: • Radio Surveys • Surveys of QSO fields. • Lensing Surveys • Narrow-band Selection • Or some combination of these methods. Stanway 2005

27. z=6.56 Galaxy behind A370 Cluster

28. Z=6.56 Galaxy comp’d in narrowband vs. R z=6.56 Galaxy Behind A370 NARROW BAND (strong Ly aemission) R BAND (no galaxy detected)

29. Redshift distribution of spectroscopically identified objects in Hawaii fields.

30. Redshift distribution of spectroscopically identified objects by field.

31. Distribution of z~5.7 and z~6.5 Ly Galaxies in A370 field.

32. Distribution of z~5.7 and z~6.5 Ly Galaxies in SSA22 field.

33. Cosmic Variance • Is a problem for analysis (both spatial & redshift distribution varies from field to field) - an issue for surveys with single field coverage, and limited areas • Will be a difficulty for spectroscopic surveys and lensing surveys

34. Summary--- The large samples of z=5.7 and 6.5 galaxies that have been obtained over multiple wide fields demonstrate large field-tp-field variations (factors of 4). Problems in interpretation of results based single fields – luminosity function, luminosity function evolution Problems for surveys with limited sample coverage – spectroscopic surveys, lensing surveys, new IR surveys For robust results, use a number of fields; make sure all objects are spectroscopically confirmed.

35. Ultra-Strong Emission-Line Galaxies • The OTHER emitters we find in narrow band searches • Low redshift (z<1.5) galaxies found with [OIII], [OII] or Ha • Extremely low metallicity galaxies • Possible low-z analogues of the high-z Lyman a emitters.

36. GOODS-NB816 selected [OIII] emitter at z = 0.64 One of the lowest metallicity galaxies known ( Kakazu, Cowie, & Hu 2007) 12 + log(O/H) = 7.27 = 1/40Z [7.17 – 739} →Comparable to the most minimum metallicity found locally • [e.g. I Zw 18; 12 + log(O/H) = 7.1 – 7.2 Compact object NB816 HST/ACS(B, V, I) 12.5” x 12.5” 6.5 hours Keck/DEIMOS 2D spectrum

37. Composite of Deimos Spectra Hu et al. (2004) R=2700 spectra allow us to easily distinguish OII and OIII emitters instrument profile

38. Lya [OII] [OIII] Color-color plot: Lya Galaxies can be distinguished from [OIII] and [OII] emitters and red objects

39. Ly  [OII] [OIII] H Low Redshift Systems with High EW(a complete sample in a 0.5 square degree field)

40. Ultra-Strong Emission Line Galaxy(Low-z, High-Equivalent Width) Strong [OIII] Emitter selected in the Deep Narrow-Band Filter Images

41. Ultra-Strong Emission Line Galaxy(Low-z, High-Equivalent Width) Strong [OIII] Emitter selected in the Deep Narrow-Band Filter Images

42. Ultra-Strong Emission Line Galaxy(Low-z, High-Equivalent Width) Weak [OII], high ionization ([NeIII], [OIII] 4363)

45. Intriguing similarities to the LAEsLine dominated, little extinctionSimilar luminosities (case B)Similar fraction of star formationQuestion is whether these objects haveL alpha emission?

46. Summary • The low metallicity extreme emission line galaxies at z<1 may be the low redshift analogs • Needs confirmed with UV spectroscopy! • Finally... On to z=10 with the new IR arrays!!