Teplitz

Report
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?
◦ 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
the New Telescopes?
Reionization
z=1100
neutral Intergalactic
Medium (IGM)
Recombination
“Dark Ages”
z=?
Ionizing sources - What are they?
•
HI ionized by photons with energy
greather than 13.6 eV
•
•
 < 912 angstroms
“Lyman continuum”
Reionization
z=6
(LC or LyC)
Galaxies
QSOs
z=3
He II Reionization
z=0
Present Day
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
QSOs
Galaxies
Star formation probably caused
reionization!
QSO contribution from LF
Total ionizing bg from
Lya forest opacity
QSO proximity effect
Inferred stellar contribution
Inoue et al. (2006)
Galaxies contain lots of dust and HI:
how can LC escape?
Interactions
Feedback
LyC absorbed by Gas and Dust
Why UV?
•
•
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
Z=6
Z=3
Z=1.3
Z=0.7
The escape fraction: fesc
Δ(fν,1500/fν,750
)
Intrinsic
LyC
1500Å
Intrinsic
3-10
Dust
~2
IGM
2
Total
20-50
3-4 mags
Dust Reddened
E(B-V)=0.2
IGM Absorption
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.
Deharveng et al. (2001
Leitherer et al. (1995)
UV spectroscopy from space
Astro-2

Measure
close
to
the
Lyman
break
Potential
study
galaxy
or
IGM
properties
with the
same
Extremely
challenging
with
current
FUSE
results
for
local
galaxies
aretechnology
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
<observation
1%
(stack
limit;
Bridget
et
al.
2010)
Borthakur
etto
al.
Cy20:
local
LBG-As
with
COS
HST optical
Image
FUV (F150LP)
Slitless
Spectrum
λ
→
z~3 Lyman Break Galaxies from the Ground
Spectroscopy & Narrow-band imaging
Steidel et al. (2001)
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

NB imaging of SSA22 field, many NB detections


Possible spatial offset of LC from FUV


Iwata et al. (2008) and Shapley et al. (2009)
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)

UV Imaging with HST
GOODS-B
•SBC/FUV imaging of HDF, UDF
•Deep fields: Stack limit, fesc,rel <
8%
Far-UV
•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%
LyC?

•Siana
al. (2010)
Follow-up
NB et
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)
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?
The limits of what we can do with HST
97% of unobscured UV luminosity density
Reddy & Steidel 2009
Gravitational Lensing
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
Foreground Lyα?
CII 1334
Alavi, Siana, et al. (2012, in prep)
•
WFC3/UVIS F225W, F275W, F336W
•
•
Treasury science benefits
•
•
•
90 orbits in Cycle 19; covers NIR FOV; 3 separate ORIENTs
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
•Population of extremely strong
emission-line galaxies
oEW_rest > 200 Å and a surface
density of 1 arcmin-2 .
oThe emission-line selection
allows an efficient search for
extremely low metallicity galaxies
(XMPGs)
Atek et al.
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
WISP
Hα/[OIII] Flux Ratios
WFC3 Infrared Spectroscopic Parallels
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.
Lack of bright, low
Hα/[OIII] galaxies
Colbert et al. in prep
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.
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 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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