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The Lyman Continuum Escape Fraction

The Lyman Continuum Escape Fraction. Harry Teplitz , Brian Siana , & Claudia Scarlata. Outline. Motivation the Lyman continuum ( LyC ) escape fraction is a key parameter in the study of reionization Why UV?

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The Lyman Continuum Escape Fraction

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  1. The Lyman Continuum Escape Fraction Harry Teplitz, Brian Siana, & Claudia Scarlata

  2. Outline • Motivation • the Lyman continuum (LyC) escape fraction is a key parameter in the study of reionization • Why UV? • LyC is best measured at z<3, where IGM absorption is lower, and thus UV is required • What has been done with HST, and what are the limits of what we can still do? • Lensing clusters, rare objects, and stacking • What do we need for progress with one of theNewTelescopes?

  3. Reionization z=1100 Recombination neutral Intergalactic Medium (IGM) “Dark Ages” z = ? Ionizing sources - What are they? • HI ionized by photons with energy greather than 13.6 eV •  < 912 angstroms • “Lyman continuum” (LC or LyC) Reionization z=6 z=3 He II Reionization Galaxies QSOs z=0 Present Day

  4. QSO Contribution to Ionizing Background Data points are measurements from Lyman-α forest. • QSOs are prodigious soures of ionizing radiation • Lyman Continuum (LC) <912 AA • Dominate ionizing flux at z<2 • Steep decline in number of QSOs at z>3 • Star formation probably caused reionization! QSOs Galaxies QSO contribution from LF Total ionizing bg from Lya forest opacity QSO proximity effect Inferred stellar contribution Inoue et al. (2006)

  5. Galaxies contain lots of dust and HI:how can LC escape? Interactions Feedback LyC absorbed by Gas and Dust

  6. Why UV? • Required to measure LyC at z<3 • LyC is absorbed by interveningHI; Can’t measure fesc at z~6 because of intervening IGM • avg IGM transmission ~ 50% at z=2.7, but 90% at z=1.5 • Reduced scatter in IGM transparency and foreground contamination • Halpha accessible from the ground Z=6 Z=1.3 Z=0.7 Z=3

  7. The escape fraction: fesc LyC 1500Å Intrinsic Dust Reddened E(B-V)=0.2 IGM Absorption • fesc = fraction of lyman continuum photons which escape galaxy. • fesc,rel = fraction of lyman continuum photons which escape galaxy divided by fraction of 1500Å photons escaping galaxy.

  8. UV spectroscopy from space Deharveng et al. (2001) Leitherer et al. (1995) Astro-2 • Measure close to the Lyman break • Potential to study galaxy or IGM properties with the same observation • Extremely challenging with current technology • FUSE results for local galaxies are controversial • HST/COS limited by high resolution • HST/SBC limited by slitless operation • Our HST/SBC study of LBG analogs at z~0.7 showed fesc,rel < 1% (stack limit; Bridget et al. 2010) • Borthakur et al. Cy20: local LBG-As with COS Slitless Spectrum HST optical Image FUV (F150LP) λ→

  9. R-band Ly-a NB LC NB z~3 Lyman Break Galaxies from the GroundSpectroscopy & Narrow-band imaging Lyman Break Galaxies (LBGs): UV-selected, star forming galaxies at z>3 • Spectra of LBGs show shockingly high fesc,rel ~ 1 • Steidelet al. (2001), Shapley et al. (2006) • Bogosavljevicet al. (2009) have many more spectra (100+), with ~10% fesc detected Steidel et al. (2001) • NB imaging of SSA22 field, many NB detections • Iwata et al. (2008) and Shapley et al. (2009) • Possible spatial offset of LC from FUV • Some resulting from foreground contamination • Very high fesc in Ly-a emitters

  10. fesc(z~1)=0 The deepest UV observations with HST • Understanding the escape of Lyman continuum photons from galaxies • 350 orbits in 6 programs (Teplitz & Siana)

  11. UV Imaging with HST • SBC/FUV imaging of HDF, UDF • Deep fields: Stack limit, fesc,rel < 8% • Teplitz et al. (2006); Siana et al. (2007) • FUV imaging of LBG-like galaxies z~1.3 • 5 orbits per target; AB>29, 3s • new stack limit fesc,rel < 1.8% • Siana et al. (2010) GOODS-B Far-UV • Follow-up NB detections (Shapley et al.) • 32 Orbits - WFC3/UVIS F336W; 30.0 mag/arcsec2 (1s, AB); • Deepest U-band image ever! • Keck spectroscopy rules out 5/6 detections! • Conclusion:LyC not from bright LBGs • Stay Tuned! (Siana et al. 2012, in prep) LyC?

  12. fesc evolves with redshift • High-z galaxy density suggests f_esc>20% to reionize the Universe • Multiple detections of high f_esc at z~3 • How does LyC escape in these galaxies?

  13. The limits of what we can do with HST 97% of unobscured UV luminosity density Reddy & Steidel 2009

  14. Gravitational Lensing

  15. Gravitational Lensing • Lensing magnification is the best (only?) way to study the faint galaxies that are likely to be the strongest LyC emitters • Limited by small volumes and uncertain lensing model • Siana et al. Cycle 18,20 • 30 orbits UVIS on Abell 1689 reaching 0.03 L* • Detection of LyC: NUV~27 AB; mag=82x F275W (LyC) F625W Lyα CII 1334 Foreground Lyα? Alavi, Siana, et al. (2012, in prep)

  16. WFC3/UVIS F225W, F275W, F336W • 90 orbits in Cycle 19;covers NIR FOV; 3 separate ORIENTs • Treasury science benefits • f(esc) at z~2 • Sub-galactic clumps at z~1;Star formation efficiency in LBGs • Teplitz et al (2012, in prep)

  17. High EW sources • Population of extremely strong emission-line galaxies • EW_rest > 200 Å and a surface density of 1 arcmin-2 . • The emission-line selection allows an efficient search for extremely low metallicity galaxies (XMPGs) Many are too faint for individual LyC detections even with HST: we will have to rely on stacking in CANDELS or future deep-wide surveys Cycle 20 program for LyCstudyof low-z high-ew Ha emitter Atek et al. (2011)

  18. WISP WFC3 Infrared Spectroscopic Parallels Hα/[OIII] Flux Ratios Orange region: Predicted single emission line sources, assuming: Hα > 3x10-16 ergs s-1 cm-2 & [OIII] > 1x10-16. Roughly a third of emitters will be single line. There are NO [OIII]-emitters where the reverse would be true (over 60 arcmin2). At >3x10-16 ergs s-1 cm-2 contamination from [OIII] for single line emitters will be low (0/37 sources), but more area needed. Lack of bright, low Hα/[OIII] galaxies Colbert et al. in prep

  19. The search for LyC in low-z galaxies • We would like to study LyC escape in local galaxies • Best resolution • Most ancillary information • Difficult with current technology • Where to place the COS aperture? • Cycle 20 program on FUSE LyC candidate will use UV imaging for positioning • Lower z limit imposed by blue cut off • Need far-UV (1000 AA) sensitivity for large area imaging detectors. Hayes et al. (2007) “production map” model of LyC in Haro 11

  20. Requirements for progress after HST • Increased UV sensitivity • Detect <0.1 L* without lensing • About 10x HST sensitivity at <3000 AA • Lower read noise • Imaging local galaxies at ~1000 AA • Substantially improved CTE • This is a major limitation of HST deep UV surveys • Slower rate of degradation? • Larger UV field of view • 3 to 10 times WFC3/UVIS • Capability for wide field UV survey • More UV filters • Probe more redshifts with imaging • Possibly narrow- or medium-bands, depending on redshift • Red cutoff is most important

  21. Conclusions/Summary • Understanding ionizing emissivity (LyC escape fraction) is a vital part of studying reionization • Best measured in the UV • We are obtaining significant results with HST, but many questions remain • If one of the New Telescopes includes UV capability, it will provide the opportunity for needed progress • Will require better sensitivity and detector performance

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