The Lyman Continuum Escape Fraction

Harry Teplitz,

Brian Siana, &

Claudia Scarlata

OutlineMotivation

◦ 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 requiredWhat 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 the New Telescopes?

z=0

z=6

z=1100

z = ?

Reionization Recombination

Reionization

Present Day

“Dark Ages”

He II Reionizationz=3

Ionizing sources - What are they?

neutral Intergalactic Medium (IGM)

• HI ionized by photons with energy greather than 13.6 eV

• < 912 angstroms

• “Lyman continuum”

(LC or LyC)

QSOsGalaxies

Inoue et al. (2006)

QSOs

Galaxies

Data points are measurements from Lyman-α forest.

QSO Contribution to Ionizing Background

QSO contribution from LFTotal ionizing bg from

Lya forest opacityQSO proximity effect

Inferred stellar contribution

• 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!

Galaxies contain lots of dust and HI:how can LC escape?

Interactions Feedback

LyC absorbed by Gas and Dust

Why UV?

Z=6 Z=3 Z=1.3 Z=0.7

• Required to measure LyC at z<3

• LyC is absorbed by intervening HI; 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

The escape fraction: fesc

Intrinsic

Dust ReddenedE(B-V)=0.2

IGM Absorption

1500ÅLyCΔ(fν,1500/fν,750)

Intrinsic 3-10

Dust ~2

IGM 2

Total 20-503-4 mags

1. fesc = fraction of lyman continuum photons which escape galaxy.

2. fesc,rel = fraction of lyman continuum photons which escape galaxy divided by fraction of 1500Å photons escaping galaxy.

UV spectroscopy from space

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

Leitherer et al. (1995)Deharveng et al. (2001)

Astro-2

Slitless Spectrum

λ→

HST optical Image

FUV (F150LP)

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 ◦ Steidel et al. (2001), Shapley et al.

(2006)◦ Bogosavljevic et 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

R-band Ly-a NB LC NB

The deepest UV observations with HST

Understanding the escape of Lyman continuum photons from galaxies

350 orbits in 6 programs (Teplitz & Siana)

fesc(z~1)=0

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?

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?

97% of unobscured UV luminosity density

Reddy & Steidel 2009

The limits of what we can do with HST

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

F625W F275W(LyC)

Lyα CII 1334Foreground Lyα?

Alavi, Siana, et al. (2012, in prep)

Gravitational Lensing

• 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)

High EW sources

Atek et al. (2011)

• Population of extremely strong emission-line galaxieso EW_rest > 200 Å and a surface

density of 1 arcmin-2 .o 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 LyC study of low-z high-ew Ha emitter

WISPWFC3 Infrared Spectroscopic Parallels Hα/[OIII] Flux Ratios

Lack of bright, low Hα/[OIII]

galaxies

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.Colbert et al. in prep

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

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

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 remainIf one of the New Telescopes includes UV

capability, it will provide the opportunity for needed progress◦Will require better sensitivity and detector

performance