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Current Topics
Lyman Break Galaxies
Dr Elizabeth Stanway
([email protected])
Current Topics: Lyman Break Galaxies - Lecture 2
Topic Summary
• Star Forming Galaxies and the Lyman-
Line
• Lyman Break Galaxies at z<4
• Lyman Break Galaxies at z>4
• Reionisation and the Star Formation History
of the Universe
• There will be a 1 hr examination on this topic
Current Topics: Lyman Break Galaxies - Lecture 2
Lecture 1 Summary
• Starburst galaxies are UV-bright, dominated by hot,
young massive stars
• They have a spectrum dominated by Lyman- in the
ultraviolet
• Lyman- is characteristically asymmetric due to galaxyscale outflows
• Absorption by the intervening IGM suppresses flux
shortwards of Lyman-
• The degree of suppression increases with redshift
• This leads to a characteristic spectral break
Current Topics: Lyman Break Galaxies - Lecture 2
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 - Lecture 2
Lyman-α
Break
UV Continuum
The Lyman Break Technique
Red
If the filters bracket the
breaks, then the galaxies
show extreme colours
(Steidel, Pettini & Hamilton 1995)
Current Topics: Lyman Break Galaxies - Lecture 2
Blue
Red
The Steidel et al LBG Sample
• “Searches for galaxies at
z>3 have been
spectacularly unsuccessful
up to now”
• “The combined statistical
effects of…intervening gas
are guaranteed to produce
an effective Lyman
continuum decrement”
• “The red U-G and blue G-R
colours of a galaxy at z=3
should readily differentiate it
from other objects in the
field.”
(Steidel, Pettini & Hamilton 1995)
Current Topics: Lyman Break Galaxies - Lecture 2
The Steidel et al LBG Sample
Ly
z=3.2
Ly
CIV
• Method confirmed spectroscopically using the
Hale 5m telescope
• They targeted QSO fields in order to study
known peaks in the matter distribution at high
redshift
Current Topics: Lyman Break Galaxies - Lecture 2
The Steidel et al LBG Sample
• By 2001, over 1000 LBGs at z=3-4 had been
spectroscopically confirmed by the CalTech
group
• Access to the Keck telescopes was crucial to
this survey (sensitivity, resolution)
• This sample still forms the most complete
analysis of star forming galaxies at this
redshift
• In recent years, the same group has been
extending their survey to z=1-3
Current Topics: Lyman Break Galaxies - Lecture 2
LBGs at z<3
• By selecting galaxies with less extreme
colours, you can select lower redshift
galaxies at the cost of higher contamination
• Expect higher metallicities/older stellar pops.
BX
LBGs
BX
BM
Current Topics: Lyman Break Galaxies - Lecture 2
LBGs
BM
The Stellar Populations of LBGs
• We select for
rest-UV =>
age<500Myr
• But is there an
older stellar pop
in the same
galaxy?
• Need
measurements at
>4000Å to
determine.
• At z=3, this is Kband
Current Topics: Lyman Break Galaxies - Lecture 2
1 Gyr
10 Myr
100 Myr
The Stellar Populations of LBGs
Age
Most LBGS at z=3 are a
few x 100Myr old
Dust
Current Topics: Lyman Break Galaxies - Lecture 2
The Stellar Populations of LBGs
Age
A few (~12%) are
very old (>1 Gyr)
A minority are very
young indeed
Dust
Current Topics: Lyman Break Galaxies - Lecture 2
Stellar Pops
at z~2
• At z=2, the 4000Å
break lies in the
J-band
• It’s easier to
measure the SED
in the rest optical
• At this redshift the
universe is much
older => older
stellar pops?
Current Topics: Lyman Break Galaxies - Lecture 2
Stellar Pops
at z~2
• ~25% of galaxies
are older than 1Gyr
• BUT, most are still
a few x 100Myr old
• LBG selection is
identifying the
same, star-forming
population at z=2 &
z=3
• Some must have
been forming stars
at z>5
404
1278
15
128
321
1015
1015
286
255
255
227
1015
10
719
8
905
10
15
509
2750
Current Topics: Lyman Break Galaxies - Lecture 2
Morphology and Size
• Almost all LBGs
are unresolved
from the ground
• Typical size:
~0.3 arcsec
~2.5 kpc (comoving)
• LBGs show a
variety of
morphologies in
HST data
Current Topics: Lyman Break Galaxies - Lecture 2
Morphology and Size
• Some are:
– Disk Galaxies
Current Topics: Lyman Break Galaxies - Lecture 2
Morphology and Size
• Some are:
– Disk Galaxies
– Interacting systems
Current Topics: Lyman Break Galaxies - Lecture 2
Morphology and Size
• Some are:
– Disk Galaxies
– Interacting systems
– Compact galaxies
Current Topics: Lyman Break Galaxies - Lecture 2
Morphology and Size
• Some are:
–
–
–
–
Disk Galaxies
Interacting systems
Compact galaxies
Star forming knots
in a larger system
Current Topics: Lyman Break Galaxies - Lecture 2
Morphology and Size
• Some are:
–
–
–
–
Disk Galaxies
Interacting systems
Compact galaxies
Star forming knots
in a larger system
• Most Have:
– Irregular or
disrupted
morphologies
=> Triggered Star
Formation
Current Topics: Lyman Break Galaxies - Lecture 2
Velocity Maps and Morphology
• Emission lines occur at
known wavelengths
• Offsets from those
wavelengths indicate
movement in the emitting
source
• At z=3, sources are
spatially resolved - can
measure velocity profiles
across source
• Done with ‘Integral Field
Spectroscopy’ looking at
Hemission
e.g. Using OSIRIS on Keck
(Law et al 2007)
Current Topics: Lyman Break Galaxies - Lecture 2
QuickTime™ and a
decompressor
are needed to see this picture.
Dust in LBGs
• UV light is
scattered more
efficiently by
dust than optical
light
• The scattered
radiation is reemitted in the IR
• The exact
extinction curve
is metallicity and
local physics
dependent
Current Topics: Lyman Break Galaxies - Lecture 2
Ly
E(B-V)=A(B)-A(V)
=A(4000)-A(4500)
Dust in LBGs
A typical LBG at z=1-3 has 0.15 magnitudes of dust in
E(B-V) => a factor of 5 extinction at 1500Å.
This is determined by a combination of SED fitting and
line ratios (e.g. H to Ly, or OII to OIII)
Current Topics: Lyman Break Galaxies - Lecture 2
Dust v Age
In general older
LBGs appear to
be less dusty
i.e. they have
lower E(B-V)
values.
Is this intrinsic or
a selection
effect?
Current Topics: Lyman Break Galaxies - Lecture 2
Dust v Age
EXPECTED
PHYSICS:
Older galaxies will
have processed
more gas into stars
More supernovae
More stellar winds
=> More dust!
Current Topics: Lyman Break Galaxies - Lecture 2
Dust v Age
SELECTION
EFFECT:
A younger object
will be more UV
luminous => can
be suppressed
more by dust
before dropping
out of selection
Current Topics: Lyman Break Galaxies - Lecture 2
Interstellar and Stellar Lines
• Typical luminosity of
LBGs at z=3 is R=25.5
(AB)
• An 8m telescope takes
1hr to get to S/N=5 at
R=24 in good
conditions
• To get a factor of 5
fainter => 25hrs!
=> Look at average
properties of stacks of
galaxies
Current Topics: Lyman Break Galaxies - Lecture 2
Interstellar and Stellar Lines
• Stacking ~1000
galaxies, can see
absorption and emission
lines from:
– Hot stars
– Interstellar medium
– Outflowing winds
• Can measure the
velocity offsets between
components
• Can measure metallicity
• Can measure wind
properties
Current Topics: Lyman Break Galaxies - Lecture 2
Winds and Outflows
• Lyman- is
redshifted with
respect to nebular
emission lines
• The interstellar
medium is blueshifted with
respect to nebular
emission lines
Lyman- is
heavily absorbed
The galaxy is
driving outflows
Current Topics: Lyman Break Galaxies - Lecture 2
Equivalent Widths
=
Wobs
• Wobs = Integrated line flux / Continuum flux density
• The width of continuum in Angstroms that must be
integrated to equal flux in line
Current Topics: Lyman Break Galaxies - Lecture 2
Equivalent Widths
=
Wobs
• Wobs = Integrated line flux / Continuum flux density
• Consider the rest frame
– Integrated flux in line increases by 1/4r2
– Continuum flux density increases by 1/4r2 * (1+z)
– Rest frame EW: W0 = Wobs / (1+z)
Current Topics: Lyman Break Galaxies - Lecture 2
Winds and Outflows
Ly
escapes
galaxy
Ly
absorbed
by ISM
Current Topics: Lyman Break Galaxies - Lecture 2
• Lyman- is
redshifted with
respect to nebular
emission lines
• The interstellar
medium is blueshifted with respect
to nebular emission
lines
 Lyman- is heavily
absorbed
 The galaxy is driving
outflows
Winds and
Outflows
• The sources with
strongest Lyman-
emission have the
weakest ISM absorption
• By contrast, the stellar
SIV feature is insensitive
to Lyman- strength
=> Decrease in covering
fraction of neutral
material with increasing
Ly- strength
Current Topics: Lyman Break Galaxies - Lecture 2
LBGs and
AGN
• LBGs are
massive galaxies
for their redshift
• Massive galaxies
at low z host
AGN
• Only 4% of LBGs
show evidence
for AGN
Current Topics: Lyman Break Galaxies - Lecture 2
LBGs and AGN
AGN
HeII
CIV
NV
LBG
Current Topics: Lyman Break Galaxies - Lecture 2
• AGN are quite
easy to identify
in the rest-UV,
even at lowish
S/N
• NV at 1240Å
• CIV at 1550Å
• HeII at 1640Å
Metallicity
Indicators
• Metallicity is
measured from
the ratio of
emission and
absorption lines
in spectra
• Unfortunately,
most of the wellcalibrated
indicators are in
the rest-frame
optical
Current Topics: Lyman Break Galaxies - Lecture 2
Metallicity indicators with redshift
Current Topics: Lyman Break Galaxies - Lecture 2
Rest-Frame Optical Spectra
• At z=3, the rest-frame
optical falls in the
observed near-infrared
(>1m)
• Spectroscopy is harder
and only a few sources
can be observed
• The H[OII] and [OIII]
emission lines can give
Star Formation Rate
indicators independent of
dust
• Their ratio can also
indicate AGN/QSO activity
Current Topics: Lyman Break Galaxies - Lecture 2
QuickTime™ and a
decompressor
are needed to see this picture.
Rest-optical spectra (Law et al 2007)
Rest-Frame Optical Spectra
QuickTime™ and a
decompressor
are needed to see this picture.
Rest-UV spectra
Current Topics: Lyman Break Galaxies - Lecture 2
QuickTime™ and a
decompressor
are needed to see this picture.
Rest-optical spectra (Law et al 2007)
Metallicity
• But rest-optical
emission lines can
be used to
determine
metallicities
• R23=[OII+OIII]/H
• [O/H]=8.8
• LBGs at z=3
have
Z~0.2-0.8Z
Current Topics: Lyman Break Galaxies - Lecture 2
Other Galaxies at z=3
• Lyman Break Galaxies are selected to be UVbright
 Strongly star forming
 Not too much dust extinction
• They can’t account for all the material at z=3,
so other techniques must fill in the gaps:
– DLAs
– Narrow Band Surveys
– Sub-millimeter or Infrared selection
Current Topics: Lyman Break Galaxies - Lecture 2
UV-Dark Material: DLAs
• The spectra of some
very high redshift
galaxies show dense,
massive clouds of
hydrogen along the
line of sight
• These ‘Damped
Lyman- Absorbers’
must be UV-dark
galaxies at
intermediate redshifts
QuickTime™ and a
TIFF (Uncompressed) decompressor
are needed to see this picture.
Prochaska et al (2001)
Current Topics: Lyman Break Galaxies - Lecture 2
Submillimeter Galaxies (SMGs)
• The UV is heavily
extincted
• The light is absorbed
by dust grains and reemitted at far-IR and
submillimetre
wavelengths
• Most of the galaxy’s
light can be emitted at
>100m
• These frequencies are
difficult to observe due
to atmospheric effects
Current Topics: Lyman Break Galaxies - Lecture 2
QuickTime™ and a
decompressor
are needed to see this picture.
Submillimeter Galaxies (SMGs)
• At 1 mm, the
distance is offset
by the shape of
the SED
• This is known as
a ‘negative Kcorrection’
• In theory z=10
sources are as
easily observed
as z=1 in the
850m
atmospheric
window
Current Topics: Lyman Break Galaxies - Lecture 2
z=1
z=10
Submillimeter Galaxies (SMGs)
• In practice,
Submillimetre galaxies
(SMGs) are hard to
detect, and harder still
to find redshifts for
• But many probably lie
at z=2-3 and each has
a huge SFR
(hundreds or
thousands of solar
masses /year)
QuickTime™ and a
TIFF (Uncompressed) decompressor
are needed to see this picture.
QuickTime™ and a
TIFF (Uncompressed) decompressor
are needed to see this picture.
QuickTime™ and a
TIFF (Uncompressed) decompressor
are needed to see this picture.
Smail, Blain, Chapman et al, 2003
Current Topics: Lyman Break Galaxies - Lecture 2
Completing the z~3 Picture
• Using molecular line emission at z=3, could probe
cool gas
• “low-excitation lines will map out a larger fraction of
the ISM in these galaxies and…study in detail the
spacially resolved kinematic structure of most of the
gas…which resides in the cold phase” (Carilli & Blain
2002)
• CO emitting galaxies may contribute significant mass
and star formation
• New telescopes such as ALMA, SKA and the EVLA
will be crucial for completing the picture at z=3 and
above.
Current Topics: Lyman Break Galaxies - Lecture 2
Lecture Summary
• LBGs at z=3 and below are selected in the UGR
colour-colour plane
• They are very faint compared to local galaxies =>
difficult to observe
• These galaxies have been followed up in great
detail and their properties are now well
understood
• These properties include stellar ages,
metallicities, outflows, morphology, AGN fraction,
star formation history and dust extinction.
• But z=3 LBGs do not present a complete picture
of the universe at this redshift.
Current Topics: Lyman Break Galaxies - Lecture 2