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Current Topics
Lyman Break Galaxies
Dr Elizabeth Stanway
([email protected])
Current Topics: Lyman Break Galaxies - Lecture 4
Topic Summary
• Star Forming Galaxies and the Lyman Line
• Lyman Break Galaxies at z<4
• Lyman Break Galaxies at z>4
• Lyman Break Galaxies at z>7
• Reionisation, SFH and Luminosity
Functions
Current Topics: Lyman Break Galaxies - Lecture 4
Ground vs Space-Based Surveys
• HST can reach objects 0.7-1mag (2-3 times) fainter in the same length
of time
• Ground-based 8m telescopes have larger fields of view (by a factor of
about 4)
• So which is more efficient at finding high-z galaxies?
• The faint end of the Schecter Luminosity function (L<<L*) can be
approximated as power law (i.e. N(L)  LdA dz)
• So N8m/NHST=(L8m/LHST) (A8m/AHST)
 If  is steeper than about -1.2, then HST always wins (I.e depth is
more useful than area)
HST has higher resolution, but 8m telescopes are ‘cheaper’
Current Topics: Lyman Break Galaxies - Lecture 4
Surveys of z>4 LBGs
GOODS
(The Great Observatories
Origins Deep Survey)
SDF/SXDF
V-drops
Z-drops
I-drops
Subaru 8m telescope V-drops
R-drops
Hubble Space
Telescope
I-drops
BDF/ERGS
ESO Very Large
Telescopes (8m)
R-drops
Z-drops
I-drops
Cluster Lensing
Surveys
Keck / HST
I-drops
J-drops
Z-drops
UKIDSS
UK Infrared
Telescope (4m)
I-drops
Y-drops
Z-drops
J-drops
Current Topics: Lyman Break Galaxies - Lecture 4
Stellar populations
• As at z=3, most
information is
derived from SED
fitting.
• Unconfused
Spitzer data is
essential for this at
z>4
• Detailed results
are model
dependent
• General results
are model
independent
Verma et al, 2007
Current Topics: Lyman Break Galaxies - Lecture 4
SFRe-t/
• Sometimes both a new starburst and an old population are needed to fit
a galaxy
• As at z=3, some stars seem as old as the universe, but time scales are
shorter, so the constraints are tighter
Current Topics: Lyman Break Galaxies - Lecture 4
Eyles et al, 2005
Old Stars at z=6
Old Stars at z~6
z=5.83
Too
Young
for Ly
line
Older
than
universe
• Sometimes both a new starburst and an old population are needed to fit
a galaxy
• As at z=3, some stars seem as old as the universe, but time scales are
shorter, so the constraints are tighter
Current Topics: Lyman Break Galaxies - Lecture 4
Comparisons with z=3
• Using a z~5
HST v-drop
sample
• GOODS field
=> extremely
deep
• Using an SMC
(i.e. low
metallicity)
extinction law
• Using Spitzer
data
Current Topics: Lyman Break Galaxies - Lecture 4
Comparisons with z=3
fraction
Age:
At z=3,
age~300Myr
At z=5,
age~30Myr
Log (Age)
If Z=Z,
then age~3Myr
 Galaxies are
younger
(Verma et al 2007)
Current Topics: Lyman Break Galaxies - Lecture 4
Comparisons with z=3
fraction
Stellar Mass:
At z=3,
mass~1010M
At z=5,
Mass ~ 2x109M
Log (Mass)
Independent of
metallicity
 Galaxies are
smaller
(Verma et al 2007)
Current Topics: Lyman Break Galaxies - Lecture 4
Comparisons with z=3
Star Formation
Rate:
At z=3,
SFR~50M/yr
At z=5,
SFR ~ 50M/yr
fraction
If Z=Z,
SFR~600M/yr
=> Galaxies are
forming stars
at about the
same rate
Log (SFR)
Current Topics: Lyman Break Galaxies - Lecture 4
Comparisons with z=3
Dust:
At z=3,
Av~0.6 mags
At z=5,
Av ~ 0.3 mags
fraction
If Z=Z,
Av~0.6 mags
Current Topics: Lyman Break Galaxies - Lecture 4
=> High z
galaxies are
less dusty
Av
Ferguson et al 2004
Sizes and Morphologies
Current Topics: Lyman Break Galaxies - Lecture 4
• Galaxies at high-z have a smaller
projected size.
• Most of this is due to evolution in
physical size rather than angular
scale factor
• Up to z~5, the size evolution is as
expected for a fixed mass
• Morphologies are often irregular
and complex
Sizes and Morphologies
• Galaxies at high-z have a smaller
projected size.
• Most of this is due to evolution in
physical size rather than angular
scale factor
• Up to z~5, the size evolution is as
expected for a fixed mass
• Morphologies are often irregular
and complex
Current Topics: Lyman Break Galaxies - Lecture 4
Spectroscopy at z~5
Spectroscopy at z~5 is challenging, but not impossible
In 5 hours on an 8m telescope get good S/N on lines
and reasonable detections of continuum flux
The night sky is growing brighter but is still reasonable
Current Topics: Lyman Break Galaxies - Lecture 4
Spectroscopy at z~6
35 hours with Gemini
6 hours with Keck
Spectroscopy at z~6 is extremely difficult
Sources are typically 1 mag fainter at z=6 than at z=5
Continuum is only detected in exceptional or lensed galaxies
Current Topics: Lyman Break Galaxies - Lecture 4
The Rest-Ultraviolet
No Ly lines
Too
Blue
• Rest-UV slope is an age indicator:
– young=blue, old=red
• But many z~5 galaxies seem too blue
Current Topics: Lyman Break Galaxies - Lecture 4
Line emitters
The Rest-Ultraviolet
No Ly lines
Too
Blue
Line emitters
• Steep Rest-UV slope (blue of f-2) could indicate zero age, Pop
III, top-heavy initial mass function …
=> New physics! Interpretation still unclear
Current Topics: Lyman Break Galaxies - Lecture 4
Lyman- Equivalent Widths
z~6
i’-drops
(DEIMOS)
z~5
50% of z>5
sources have
EW>0Å
25% have
EW>30Å
• At z~5 the distribution of Lyman-a line strengths is similar to that at
z~3
• At z~6 see more high EW lines - selection function? More hot stars?
Dust effects? New physics?
Current Topics: Lyman Break Galaxies - Lecture 4
Other spectral lines and
outflows
• Stacking together ~50
z~5 galaxies, can start
to see other lines:
• CIV, SiIV and OI are
starting to be visible
• Velocity offsets =>
similar winds to z~3
• Work still in progress!
OI
Current Topics: Lyman Break Galaxies - Lecture 4
SIV
• In a few lensed cases, can
identify lines in individual
spectra
• This example is 6x the typical
z~5 LBG brightness
• It is also lensed!
• Strong interstellar lines
• No Ly => older than typical,
more dusty or more evolved
• Psychotic cases like this can’t
really describe the whole
population
Current Topics: Lyman Break Galaxies - Lecture 4
Dow-Hygelund et al, 2005
Other spectral lines and
outflows
Non-LBGs at z=5-6
• As at z=3, LBGs don’t show the whole picture
at z=5
• Some star forming sources are going to be
too faint to be detected as LBGs
– Narrowband detected galaxies (LAEs)
– Lensed galaxies
– GRB Host galaxies
• Some galaxies won’t be star forming
– Sub-mm galaxies
– DLAs
– Molecular Line Emitter galaxies
Current Topics: Lyman Break Galaxies - Lecture 4
The Whole Picture at z=5?
• How many galaxies at these
redshifts are UV-dark?
• Searching z=5 LBG clusters for
UV-dark material might be the
way forward
• Initial results are promising z=5 CO emission detected near
z=5 LBGs (Stanway et al, 2008)
• If typical, similar galaxies could
contribute a significant fraction
of the total galaxy mass in highz clusters and a large amount
of obscured star formation.
Current Topics: Lyman Break Galaxies - Lecture 4
Future Millimeter Observations
• The Atacama Large
Millimeter Array (ALMA)
begins commissioning this
year
• It will be fully online by
about 2013
• It observes at mm and submm wavelengths
• 80 telescopes at 5000m
• Will be sensitive to dust
emission, CO and other
strong emission lines (e.g.
[CII]) to very high z
Current Topics: Lyman Break Galaxies - Lecture 4
Quick Time™ and a
decompressor
are needed to s ee this pic ture.
QuickTime™ and a
decompressor
are needed to see this picture.
Gamma-Ray Bursts
• Some star formation will be going on in
galaxies too faint to detect as LBGs
• Where massive stars are forming, some
small number can go supernova
• In certain circumstances, supernovae
are associated with extraordinarily
luminous, highly beamed flashes of
gamma rays
• These are known as Gamma Ray
Bursts (GRBs) and can be used as
tracers of low mass star formation
• At high redshifts, a GRB will show up as
a dropout (i.e. selected like an LBG), but
will fade rapidly with time
• The most distant objects known in the
Universe are GRBs (z=8.3)
Current Topics: Lyman Break Galaxies - Lecture 4
QuickTime™ an
decompresso
are needed to see this p
Lensing as a tool at high redshift
• In rare cases, can use
intervening galaxy
clusters as gravitational
lenses - gives spatial
information, boosted
signal-to-noise, near-IR
spectroscopy
• 2 known strongly lensed
LBGs at z~5
• Only provides
information on rare
sources - not average
sources
• Requires lens
reconstruction
z=4.9
QuickTime™ and a
decompressor
are needed to see this picture.
Swinbank et al (2009)
Current Topics: Lyman Break Galaxies - Lecture 4
z=6.5
candidate
LBGs at z>6
• Beyond z=6, the
Lyman break moves
into the infrared
• Resolution and
sensitivity are poor
• Need lensing to
stand realistic
chance of detecting
objects from ground
• NO
spectroscopically
confirmed galaxies
beyond z=6.96
Current Topics: Lyman Break Galaxies - Lecture 4
Bradley et al 2008
Lensed LBGs at z>7
Current Topics: Lyman Break Galaxies - Lecture 4
• z=7.6
candidate
galaxy
• z-drop
• J-drop
• 100 Myr
old
• No dust
• Lensed
HST and WFC3
• In 2009 HST was serviced
and a new camera was
installed: WFC3
• This gave HST much better
resolution, field of view and
sensitivity in the nearinfrared
• Can now effectively extend
the LBG technique to
higher redshifts
• Spectroscopic follow-up
remains a problem
Current Topics: Lyman Break Galaxies - Lecture 4
QuickTime™ and a
decompressor
are needed to see thi s picture.
QuickTime™ and a
decompressor
are needed to see this picture.
LBGs at Higher Redshifts
WFC3 on HST can find z-drops (z~7), Y-drops (z~8) and
maybe J-drops (z~10) but can’t confirm them
Current Topics: Lyman Break Galaxies - Lecture 4
LBGs at Higher Redshifts
z’-drop
candidates
at z~7
Bunker et al (2009), see also Bouwens+ Oesch+ Castellano+ Wilkins+ etc, etc
(About 20 papers in Sep-Dec 2009)
Current Topics: Lyman Break Galaxies - Lecture 4
Size Evolution to z>7
• Galaxies at z=7
continue to get
smaller
• This scales as
size  (1+z)-1.12 ±
0.17 , consistent with
constant comoving
sizes
• Most z=7 candidates
very compact
(Oesch et al 2010)
Current Topics: Lyman Break Galaxies - Lecture 4
The Rest UV spectral Slope
• AGN have spectra
described by a power law,
z’
Y
J
H
L   i.e L  
• In the rest-frame
ultraviolet, star forming
galaxies also show powerlaw spectra
• The slope of the power law
depends on the
temperature of the emitting
source
• This power law slope can
be measured using
broadband photometry
Current Topics: Lyman Break Galaxies - Lecture 4
z=7 galaxy
Magnitude gives the flux in J
and H => fJ and fH
Know the central wavelengths
of J and H => J and H
LJ/LH = fJ/fH  (J
The Rest UV spectral Slope
LJ/LH = fJ/fH  (J
Example: A source has a spectral
slope =-2.5 - calculate the J-H
colour in AB mags, given central
wavelengths of 1.2m and 1.6m
for J and H respectively
z’
Y
J
H
z=7 galaxy
AB mag = -2.5 log(f)-48.6
- App. mag, defined in f
J-H = -2.5 log(fJ)-48.6 - (-2.5 log(fH)-48.6)
- Colour is (mag)
J-H = -2.5 log (fJ/fH) = -2.5 log ((J 
- Using spectral index
J-H = -2.5 (--2) log (J
Simplifying
J-H = 0.16 magnitudes
Current Topics: Lyman Break Galaxies - Lecture 4
Rest-UV Spectral Slope
• AGN have ≈-1 at all
redshifts
• Zero-age, star forming
galaxies with normal
stellar populations have
≈-2
• Dust or age will make this
slope redder (i.e.
shallower)
• Within the LBG population
the spectral slope is seen
to evolve with z => age
evolution? Dust evolution?
QuickTime™ and a
decompressor
are needed to see this picture.
Bouwens et al (2010)
Current Topics: Lyman Break Galaxies - Lecture 4
Rest-UV slope at z = 7 - 8
• At z~7, candidate galaxies are very blue, particularly faint
galaxies
 < -3 is very hard to explain with any ‘normal’ (Population
II) stellar population
Current Topics: Lyman Break Galaxies - Lecture 4
Bouwens et al (2010)
QuickTime™ and a
decompressor
are needed to see this picture.
Rest-UV slope at z = 7 - 8
• Pop III stars are defined as
having very low or zero
metallicity
• With no metals, they have
fewer ways to emit radiation
(i.e. cool down)
• They can become hotter,
and more massive
(supported by radiation
pressure)
• Hotter galaxies have bluer
spectral slopes
Current Topics: Lyman Break Galaxies - Lecture 4
Bouwens et al (2010)
 < -3 slopes may indicate
that z=7 galaxies have very
low metallicity
Cosmic Evolution of Star Formation
Property
z=1-3
z=5-6
z>7
Age
~200 Myr
~50 Myr
May be
younger
Mass
few x 1010 M
~109 M
No data
Metallicity
0.3-0.5 Z
~0.2 Z
May be very
low - Pop III
Size (half light
radius)
1.5-2 kpc
~1kpc
~0.5 kpc
scales as comoving
M*
-21.1
z=5 : -20.7
z=5 : -20.2
-19.9?
Faint end Slope
-1.6
may be steeper
No data
Dust
E(B-V)~0.2
Probably less dusty No data
Star Formation
Rate
~30 M/yr
~30 M/yr
Current Topics: Lyman Break Galaxies - Lecture 4
~30 M/yr
Ensemble Properties of LBGs
• At z=2-4, you can study individual galaxies in detail
• At z=5-6, and more so at z>7, this becomes much
harder
• Studying an individual galaxy only tells you about its
immediate environment
• By looking about the ensemble properties of galaxies
you can study the universe as a whole =>
observational cosmology
• By using a common selection method (LBGs), you
are comparing like-for-like across cosmic time
=> Insights into galaxy formation, the star formation
histoy of the Universe and Reionisation
Current Topics: Lyman Break Galaxies - Lecture 4
Lecture Summary
• LBGs at z>4 are significantly harder to find than
those at z<4
• LBGs at z~6 are a lot harder than z~5
• With increasing redshift see:
– Decreasing metallicity
– Decreasing dust extinction
– Decreasing age
– Decreasing mass
• These traits extend to z~7-8
• Very blue rest-UV spectra are hinting at changes in the
nature of star formation
• But, as at z=3, LBGs are not the whole story
Current Topics: Lyman Break Galaxies - Lecture 4