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Characterizing Star Formation at z>7: the Current Frontier

Constraining Star Formation at z>7 Dan Stark (Caltech) Collaborators: Richard Ellis, Johan Richard, Avi Loeb, Eiichi Egami, Graham Smith, Andy Bunker, Jean-Paul Kneib, Mike Santos A Century of Cosmology, San Servolo, Italy, 27 August 2007.

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Characterizing Star Formation at z>7: the Current Frontier

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  1. Constraining Star Formation at z>7 Dan Stark (Caltech)Collaborators: Richard Ellis, Johan Richard, Avi Loeb, Eiichi Egami, Graham Smith, Andy Bunker, Jean-Paul Kneib, Mike SantosA Century of Cosmology, San Servolo, Italy, 27 August 2007

  2. Characterizing Star Formation at z>7: the Current Frontier • Motivation: • Probing some of the earliest galaxies (t<750 Myr) • how does the global star formation rate density evolve? • what is the density and luminosity distribution of earliest star-forming systems? • what are physical properties (e.g. metallicity, stellar mass) of galaxies at the highest redshifts? • contribution of early star formation to reionization • what were the objects that reionized the IGM? • what is the redshift distribution of the reionizing systems? • if star-forming galaxies, were the galaxies primarily low luminosity? • note: current mode of observational study of z>7 universe is exploratory in nature

  3. Decline in Star Formation Rate Density? Bouwens et al., 2007 arXiv:0707:2080 Does decline in SFRD extend to higher redshifts?

  4. Old stars at z~5-6 Implies Star Formation at z>6 Measurement of stellar mass density allows constraints to be placed on required SF at z>5-6 to assemble galaxies (Stark & Ellis 2005, Stark et al. 2006, Yan et al. 2006, Eyles et al. 2006) z = 5.554 1.1x1011 M Stark et al. (2007a) ApJ, 659, 84 Assembled stellar mass density at z~5-6 requires greater star formation rate density at z>6 than has been observed thus far. How do we directly observe missing sources at z>6?

  5. Evidence for Luminosity-Dependent Evolution Bouwens et al. 2006, ApJ, 653, 53 Bouwens & Illingworth, 2006, Nature Bouwens et al (2006, 2007) and Yoshida et al. (2006) propose L-dependent evolution - decline in abundance over 3<z<6 mostly for luminous sources Observations suggest star formation increasingly dominated by low luminosity sources for z>6?

  6. Beyond z~7 with Strong Gravitational Lensing z=5.6 z = 6.8 Utilizing strong magnification (10-30) of clusters, probe much fainter than other methods in small areas (<0.1 arcmin2 /cluster)

  7. Spectroscopic Surveys for Lensed Lyman-alpha Emitters From arclet spectroscopy the location of the “critical lines” is known precisely for z=1 and for z=5 A Keck survey in the optical discovered 11 candidate LAEs between 2.2 < z < 5.6 -- Santos et al. ApJ 606, 683 (2004)

  8. Low Luminosity z~10 Ly Emitters: Critical Line Mapping With Keck NIRSPEC Wavelength sensitivity (1.5hr) Cluster critical line for zS > 7 J-band 10-17 cgs NIRSPEC slit positions • 9 clusters completed between 04-05 (Stark et al. 2007b, ApJ, 663,10) • Clusters have well-defined mass models & deep ACS imaging • Obs. sensitivity ~ 0.6-2 x10-17 cgs; magn. > 10-30 throughout • Sky area observed: 0.3 arcmin2; V(comoving)~30 Mpc3 • 6 promising lensed emitter candidates (>5) • 8.6 < z < 10.2; L ~ 2 - 50. 1041 cgs; SFR ~ 0.2 -5 M yr-1

  9. Candidate Ly Emitters 8.6 < z < 10.2; L ~ 2 - 50. 1041 cgs; SFR ~ 0.2 - 5 M yr-1 Recognize burden of proof that these are z~10 emitters is high! (see discussion in Stern et al. 2000ab, Bremer et al. 2004, G. Smith et al. 2006) Each detection is > 5, seen in independent exposures/visits

  10. Spectroscopic Elimination of Interlopers If J-band emission is H… z~0.9 Ly, [OII], and [OIII] observable with optical spectroscopy

  11. Spectroscopic Elimination of Interlopers • Archival LRIS spectroscopy (Santos et al 2004) from 4000-9400Å available for all candidates • No emission lines detected: candidates are probably not H or [OII] • H-band spectroscopy obtained for 3 candidates with NIRSPEC • No emission lines detected: 2/6 candidates are probably not H, [O III] • J-band spectroscopy available for all candidates with NIRSPEC • No additional emission lines detected: candidates are most likely not[OIII] 4959 • optical broadband? one candidate has marginal z-band detection • remove from high-z sample At least 2 candidates are most easily explained as z~10 sources, and an additional 3 candidates have strong case to be at z~10.

  12. Confirming Galaxies at z>8 1. Asymmetric Line Profile? Long integrations needed.Only look for in best candidates. Should be potentially feasible with MOSFIRE. 2. Lensed Counterimage? Could confirm redshift, but difficult with longslit spectrograph. Stern et al. 2005, ApJ

  13. Contribution to Reionization? # density required for reionizaton Consider range: fesc ~ 0.02-0.5 t ~250-575 Myr None are real All are real If even 1-3 of the 6 candidates is at z~10, low luminosity galaxies may play a dominant role in cosmic reionization Key uncertainty: if candidates are at z~10, are observed densities characteristic given large cosmic variance?

  14. A surprisingly large abundance of z~10 Lya Emitters? Stark, Loeb, & Ellis, 2007, ApJ, in press Observed densities of z~10 LAEs model for z~10 LF assuming increase in star formation efficiency model for z~10 LF assuming LLya-M mapping unchanged from z~5.7/6.6 • abundances of z~10 Lya emitters not consistent with extrapolation from z~6 • require more Lya photons per unit halo mass, achievable through 1) increase in star formation efficiency or 2) change in IMF • possibilities tantalizing but more work needed to confirm candidates and additional survey volume needed to confirm densities.

  15. Predictions for future LBG surveys at z>7 Stark, Loeb, & Ellis 2007, ApJ, in press Ground-based telescope regime • Upcoming surveys (e.g. VISTA / WFC3) should be able to detect large number of LBGs at z~7-8. • If increase in density suggested by z~9-10 lensed LAE candidates proves false, probing to z~10 may be much more difficult, especially from the ground

  16. Summary • Strong lensing surveys are finding an abundant population of candidate faint Ly emitters at z~8-10 with SFR <1 M yr-1 - a population which may contribute significantly to reionization. • Exhaustive spectroscopic and imaging follow-up supports hypothesis that many of lensed Lya emitters are at z~10 but additional follow-up still required. Final confirmation is especially difficult at z>7! • Implied densities of z~9 LAEs still very uncertain, but if correct, would require increase in Lya photon output per unit halo mass from z~6. • Even with conservative assumptions, new instruments should result in reasonably large samples of galaxies at z~7-8 in the next few years. If increase in density implied in lensed LAE survey proves false, z~10 objects may be difficult to find before JWST, especially for (non-lensing) ground-based imaging surveys

  17. Lensed z>7 Lyman-Break Galaxies UDF flux limit Bouwens & Illingworth, 2006 Only most luminous systems probed at z~7.5!

  18. Predictions for Lya Emitters at z>7 z~10 z~7-8 • Upcoming/ongoing surveys should be able to detect LAEs at z~7-8 • Expected Poisson and cosmic variance will complicate conclusions about reionization. • Probing to z~10 may be much more difficult, unless there is upward evolution in LAE LF (as implied in lensed LAE searches).

  19. HST+Spitzer Survey for Lensed z>7 LBGs MS1358 - H Critical line Magnification > 2 mags Richard, Stark, Ellis et al, in prep. • 11 NICMOS pointings in 6 lensing clusters (4 orbits J/F110W, 5 orbits H/F160W) • ACS/F850LP imaging of all clusters • K-band ground based imaging with Keck/NIRC + Subaru/MOIRCS • Deep IRAC imaging for all clusters

  20. Candidate z~7.5 LBGs Bouwens et al Lensed candidates z-J J “effective” magnitudes • 10 candidate z-drops in the 6 clusters surveyed with H ~ 26 - 26.8 • Implied SFR ~ 0.1 - 2 M yr-1 (unlensed) • Time allocated to follow up these spectroscopically with NIRSPEC

  21. Source Plane and Image Plane Position of source relative to lens is crucial! No rings: giant arcs with counter images The Real Sky What Observer Sees In the general case (here elliptical lens) as background source moves closer in alignment, multiple images, some highly magnified appear. Lines where magnification is maximized are known as `caustics’ in the source plane, `critical lines’ in the image plane.

  22. Bulk of candidates unlikely to be z~2 interlopers z ~ 2 quiescent Current limit Possible limit in warm mission z ~ 8 star-forming Stacked IRAC limit for 8 unconfused candidates gives upper limit at 3.6 microns rejecting passive z~2 population as primary population

  23. Implied Luminosity Function z~7.5 All 10 z-drops are real None is real Bouwens & Illingworth z~7.4 UDF Lensed LBG survey probes faintward of UDF

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