Current Topics
Lyman Break Galaxies
Dr Elizabeth Stanway
(E.R.Stanway@Bristol.ac.uk)
Current Topics: Lyman Break Galaxies - Elizabeth Stanway
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Topic Summary
• Star Forming Galaxies and the Lyman-
Line
• Lyman Break Galaxies at z<4
• Lyman Break Galaxies at z>4
• The Star Formation History of the Universe
and Reionisation
• This course will be assessed through a 1
hour examination including mathematical and
essay questions
Current Topics: Lyman Break Galaxies - Elizabeth Stanway
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Recommended Reading
• Steidel, Pettini & Hamilton, 1995, AJ, 110, 2519
• Carilli & Blain, 2002, ApJ, 569, 605
• Verma et al, 2007, MNRAS, 377, 1024
• Bouwens et al, 2007, ApJ, 670, 928
• Stanway et al, 2008, ApJ, 687, L1
Current Topics: Lyman Break Galaxies - Elizabeth Stanway
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A few definitions …
• In these lectures
– LBG = Lyman Break Galaxy
– LAE = Lyman Alpha Emitter
– HST = Hubble Space Telescope
– Gyr = 1 Billion Years (Myr = 1 million yrs)
– z = redshift
– Z = metallicity
– z’ or zAB are broadband filters
Current Topics: Lyman Break Galaxies - Elizabeth Stanway
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The History of High-z studies
Current Topics: Lyman Break Galaxies - Elizabeth Stanway
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The History of High-z studies
The highest redshift
galaxy has been
increasing steadily in
distance for ~20 yrs
Current Topics: Lyman Break Galaxies - Elizabeth Stanway
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The History of High-z studies
Universe 1Gyr old
Universe 1/8 current age
Universe 1/4 current age
Universe half current age
Now: Universe 13.7 Gyr
Current Topics: Lyman Break Galaxies - Elizabeth Stanway
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The History of High-z studies
z=3 LBGs
Universe 1/8 current age
~ 2 Billion years after the
Big Bang
Current Topics: Lyman Break Galaxies - Elizabeth Stanway
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Why Push So
Far Back?
• We are now starting to
probe the last major
phase transition in the
universe - reionisation
• We’re within a few
generations of the
earliest galaxies
forming
• Unevolved galaxies
are simpler - easier to
understand - and so
help shape theory
Current Topics: Lyman Break Galaxies - Elizabeth Stanway
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Why Push So
Far Back?
• Lyman break galaxies
are star-forming so
directly measure how
exciting a place the
universe is
• Lyman break galaxies
are relatively bright
and so easy to study
• Lyman break galaxies
are relatively easy to
find
Current Topics: Lyman Break Galaxies - Elizabeth Stanway
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But Why is it so difficult?
• Redshift equation:
• The night sky is
also very red
=> the sky
background is
much higher for
high-z galaxies
Flux
(obs)=em) * (1+z)
=> Distant galaxies are very RED
Current Topics: Lyman Break Galaxies - Elizabeth Stanway
Wavelength
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But Why is it so difficult?
• Distance Modulus equation:
m = M - 5 log (dL/10pc)
• Luminosity Distance equation:
dL = (1+z) * c/H0 *

z
0
dz '
 m (1  z' )   k (1  z ' )   
3
2
• At z=1, dL=6634 Mpc

• At z=3, dL=25840 Mpc
• At z=5, dL=47590 Mpc
=> Distant galaxies are very FAINT
Current Topics: Lyman Break Galaxies - Elizabeth Stanway
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Depth vs Area?
• The Luminosity Function (LF) of a galaxy
population relates number of objects seen to
volume/area observed
• Most galaxies follow a Schecter (1973) function:
N(L) dA  (L/L*)e-(L/L*) dA
• When L<<L*, this approximates a power law:
N(L) dA  LdA
=> Increasing area of observation leads to increase
in galaxy sample
BUT: since the power law is steep, increasing the
depth usually increase sample size more quickly
Current Topics: Lyman Break Galaxies - Elizabeth Stanway
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Building a
Galaxy
• Every galaxy is
made of stars
• Lower mass stars
live longer
• More massive stars
are more luminous
=> burn more quickly
TMS~10Gyr*(M/M)
M
-2.5
Blue
Current Topics: Lyman Break Galaxies - Elizabeth Stanway
Red
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Building a
Galaxy
• TMS~10Gyr*(M/M)
30 Myr
-2.5
300 Myr
• Old galaxies are
dominated by A-M stars
and have 4000A breaks
• Young galaxies are
dominated by short-lived
O and B stars and are
UV-bright
10 Gyr
15 Gyr
Blue
Current Topics: Lyman Break Galaxies - Elizabeth Stanway
Red
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Types of
Galaxy SED
Old/Red
• Old galaxies are
dominated by A-M stars
and have 4000Å
breaks
• Young galaxies are
dominated by shortlived O and B stars and
are UV-bright
• Younger galaxies also
show strong emission
lines, powered by star
formation.
Current Topics: Lyman Break Galaxies - Elizabeth Stanway
Young/
Blue
Rest-UV
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Hydrogen Emission Lines
• Flux from star
formation excites
electrons in atoms
• The most abundant
atom in the universe
is Hydrogen
• As an electron
relaxes from an
excited state, it emits
a photon
• Each transition emits
at a particular
wavelength
• The easiest transition
to excite is Lyman-
Current Topics: Lyman Break Galaxies - Elizabeth Stanway
The Balmer series
emerges in the optical
and so is known as
‘Hydrogen-’ etc for
historical reasons
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Hydrogen Emission Lines
OIII
The Balmer Series
and Oxygen lines
dominate the optical
spectrum of a star
forming galaxy
OII
H
H
H
H
•The Lyman series emerges in the ultraviolet.
•The Lyman- emission line can emit up to 1% of the galaxy’s
bolometric flux, but ….
Current Topics: Lyman Break Galaxies - Elizabeth Stanway
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Hydrogen Emission Lines
OIII
The Balmer Series
and Oxygen lines
dominate the optical
spectrum of a star
forming galaxy
OII
H
H
H
H
•The Lyman series emerges in the ultraviolet.
•The Lyman- emission line can emit up to 1% of the galaxy’s
bolometric flux, but ….
Current Topics: Lyman Break Galaxies - Elizabeth Stanway
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Hydrogen Emission Lines
OIII
Ly
Å
OII
H
Ly
H
H
H
•The Lyman series emerges in the ultraviolet.
•The Lyman- emission line can emit up to 1% of the galaxy’s
bolometric flux, but ….
Current Topics: Lyman Break Galaxies - Elizabeth Stanway
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The Asymmetric Lyman- Line
Low z
Higher z
The
Lyman-
line is
intrinsicall
y
symmetric
At high-z the
line always
appears
asymmetric
and
broadened
Current Topics: Lyman Break Galaxies - Elizabeth Stanway
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The Asymmetric Lyman- Line
Blue Wing is
scattered by
outflowing
galactic winds
Red wing is
broadened by
back-scattered
light
Star formation drives
galaxy-scale winds
(Adelberger et al 2003)
Lyman- is resonantly
scattered by the winds
Wind
v=0
v=
+300 km/s
v=
-300 km/s
Current Topics: Lyman Break Galaxies - Elizabeth Stanway
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The Asymmetric Lyman- Line
Blue Wing is
scattered by
outflowing
galactic winds
Red wing is
broadened by
back-scattered
light
v/c = z/(1+z)
=> 300km/s wind
broadens line by about
5Å FWHM at z=3
Wind
v=0
v=
+300 km/s
v=
-300 km/s
Current Topics: Lyman Break Galaxies - Elizabeth Stanway
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The Lyman- Forest
… Lyman-a is also seen in absorption wherever
there are clouds of hydrogen
Observer
Source
z*
z=0
Ly
Å
z*)
Current Topics: Lyman Break Galaxies - Elizabeth Stanway
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The Lyman- Forest
… Lyman-a is also seen in absorption wherever
there are clouds of hydrogen
Observer
z=0
Source
z1
z*
Ly
Å
z1)
Å
z*)
Current Topics: Lyman Break Galaxies - Elizabeth Stanway
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The Lyman- Forest
… Lyman-a is also seen in absorption wherever
there are clouds of hydrogen
Observer
z=0
Source
z4
z3
z2
z1
z*
Ly
Å
z3)
Å
z4)
Current Topics: Lyman Break Galaxies - Elizabeth Stanway
Å
z1)
Å
z2)
Å
z*)
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The Lyman- Forest
At low z almost all of a
galaxy’s Lyman continuum
flux reaches us
Current Topics: Lyman Break Galaxies - Elizabeth Stanway
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The Lyman- Forest
Above z=3, the fraction of
galaxy flux reaching us
declines rapidly
Current Topics: Lyman Break Galaxies - Elizabeth Stanway
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The Lyman- Forest
Beyond z=5.5, <1% of the
galaxy’s flux gets through
the IGM
Current Topics: Lyman Break Galaxies - Elizabeth Stanway
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The Lyman- Forest
Low z
Lyman- Forest
Current Topics: Lyman Break Galaxies - Elizabeth Stanway
Higher z
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Properties of High-z Galaxies
• Young galaxies at high-z are:
Current Topics: Lyman Break Galaxies - Elizabeth Stanway
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Properties of High-z Galaxies
• Young galaxies at high-z are:
– Dominated by O and B stars
Current Topics: Lyman Break Galaxies - Elizabeth Stanway
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Properties of High-z Galaxies
• Young galaxies at high-z are:
– Dominated by O and B stars
– Bright in the ultraviolet
Current Topics: Lyman Break Galaxies - Elizabeth Stanway
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Properties of High-z Galaxies
• Young galaxies at high-z are:
– Dominated by O and B stars
– Bright in the ultraviolet
– Drive strong galactic winds
Current Topics: Lyman Break Galaxies - Elizabeth Stanway
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Properties of High-z Galaxies
• Young galaxies at high-z are:
– Dominated by O and B stars
– Bright in the ultraviolet
– Drive strong galactic winds
• They have key observable characteristics:
Current Topics: Lyman Break Galaxies - Elizabeth Stanway
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Properties of High-z Galaxies
• Young galaxies at high-z are:
– Dominated by O and B stars
– Bright in the ultraviolet
– Drive strong galactic winds
• They have key observable characteristics:
– They have asymmetric Lyman- emission
lines
Current Topics: Lyman Break Galaxies - Elizabeth Stanway
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Properties of High-z Galaxies
• Young galaxies at high-z are:
– Dominated by O and B stars
– Bright in the ultraviolet
– Drive strong galactic winds
• They have key observable characteristics:
– They have asymmetric Lyman- emission
lines
– Flux is suppressed shortward of Lyman-
Current Topics: Lyman Break Galaxies - Elizabeth Stanway
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Methods of Identifying
High z Galaxies
Lyman Break
Surveys
Narrow Band
Surveys
Gravitational Lensing
Surveys
• Identifies sources
with bright UV
continuum emission.
• Broad redshift
range (typically
Δz~0.3-0.5).
• Identifies sources
with high equivalent
widths in certain
emission lines.
• Narrow redshift
range (typically
Δz~0.1).
• Identifies strongly
lensed sources
• Often combined
with other two
methods.
• Redshift range
variable.
Current Topics: Lyman Break Galaxies - Elizabeth Stanway
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The Lyman Break Technique
The Steidel, Pettini & Hamilton (1995) Lyman Break Method
• At z=3, about 50% of
the Lyman continuum is
transmitted
• This leads to a ‘break’
in the spectrum
• So consider what
would happen if you
place filters either side
of the Lyman- and
Lyman limit breaks…
Lyman
Ionising
Continuum
Radiation
912Å
Break
Current Topics: Lyman Break Galaxies - Elizabeth Stanway
UV Continuum
Lyman-α
Break
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The Lyman Break Technique
Red
If the filters bracket the
breaks, then the galaxies
show extreme colours
Current Topics: Lyman Break Galaxies - Elizabeth Stanway
Blue
Red
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The Dropout Technique
●
●
At z>4, the Lyman forest
absorption reaches near
100%  only one break is
detected
Starburst at z=6
f-0
A source will be detected in
filters above the break but
‘drop-out’ of filters below it
●
V-drops  z > 4.5
●
R-drops  z > 5.
●
I-drops  z > 5.8
For galaxies at 5.6<z<7.0, i'- z'>1.3
Current Topics: Lyman Break Galaxies - Elizabeth Stanway
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Narrow Band Surveys
Sky
Emission • A magnitude is the
Narrow
Band
Broad
Band
average flux in a
filter
• If half the filter is
suppressed by Lya forest, the
galaxy appears
faint
• If an emission line fills the filter, the galaxy will seem bright
• By comparing flux in a narrow band with flux in a
broadband, you can detect objects with strong line emission
Current Topics: Lyman Break Galaxies - Elizabeth Stanway
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Narrow Band Surveys
• But what line
have you
detected?
• Could be:
– OIII at 5007A
– OII at 3727A
– Lyman- at
1216A
• Need
spectroscopic
follow-up
Current Topics: Lyman Break Galaxies - Elizabeth Stanway
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Lecture Summary (I)
• Building a sample of high z galaxies gives vital
information on the state of the early universe
• It requires the right balance between depth and area because the LF is steep, depth is usually preferred
• Starburst galaxies are UV-bright, dominated by hot,
young massive stars
• They have a rich spectrum of emission lines, dominated
by:
– oxygen and Balmer series lines in the optical
– Lyman series lines in the ultraviolet
Current Topics: Lyman Break Galaxies - Elizabeth Stanway
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Lecture Summary (II)
• Lyman- is characteristically asymmetric due to
galaxy-scale outflows
• Absorption by the intervening IGM suppresses flux
shortwards of Lyman-
• The degree of suppression increases with redshift
– A few percent at z=1
– 50% at z=3
– More than 99% by z=5.5
• This leads to a characteristic spectral break
Current Topics: Lyman Break Galaxies - Elizabeth Stanway
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Lecture Summary (III)
• Galaxies at high-z are selected by:
– Narrow band surveys
• Selecting for presence of strong emission lines
• Uses improved background between skylines
• Prone to contamination
– Lyman break galaxy surveys
• Selecting on the presence of a 912A or 1216A
break
• Based on broad-band photometry
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