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Current Topics
Lyman Break Galaxies
Dr Elizabeth Stanway
([email protected])
Current Topics: Lyman Break Galaxies - Lecture 3
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 3
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 3
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 3
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 3
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 3
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 3
Topic Summary
• Star Forming Galaxies and the Lyman-
Line
• Lyman Break Galaxies at z<4
• Lyman Break Galaxies at z>4
– Extending the method to higher redshift
– Properties of LBGs at high z
– Shedding light on the high z universe
• Lyman Breaks at z>7, SFH and
Reionisation
Current Topics: Lyman Break Galaxies - Lecture 3
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 3
Lyman-α
Break
UV Continuum
Extending the LBG method to
higher redshifts
R
I
ZAB
Current Topics: Lyman Break Galaxies - Lecture 3
• At z=3-4, the
Lyman break is
bracketed by UGR
filters
• At z=5, the Lyman
break falls just
short of the I band
• At z=6, it is about
to enter the ZAB
band
RIZ selection at z=5 and z=6
Current Topics: Lyman Break Galaxies - Lecture 3
RIZ selection at z=5 and z=6
BUT at these wavelengths, filters overlap and are far from
standardised.
Current Topics: Lyman Break Galaxies - Lecture 3
Filters
V-drop
filters
R-drop
filters
Current Topics: Lyman Break Galaxies - Lecture 3
Redshift selection as a
function of filter
High z
galaxy
Current Topics: Lyman Break Galaxies - Lecture 3
Low z
galaxy
Redshift selection as a
function of filter
z~5 V- and R-drops
z~6 I-drops
• Number density and redshift distribution depend on filters
used
=> Results from surveys are not directly comparable
Current Topics: Lyman Break Galaxies - Lecture 3
Contamination
• As well as problems from
intermediate z galaxies,
also have problems with
cool stars
• M, L and T-class stars are
very red in the same
bands as z=5 and z=6
LBGs
• Can identify stars with
HST data (morphology),
or very deep infrared data
(colour)
• Problem if the survey is
ground based or objects
are faint.
Current Topics: Lyman Break Galaxies - Lecture 3
The effect of Ly line emission
Spectrum
flat in f
99% at
z>5.5
Current Topics: Lyman Break Galaxies - Lecture 3
• The gradual change in
colour with redshift is due
to movement of the
Lyman- break through
the filter
• Typical spectrum flat in f
=> f-2 (c=)
• When the Lyman- break
is halfway through the
filter, the average flux in
the filter is a factor of 2
lower than in a filter
longwards of the break.
=> The object will appear
0.7 mags fainter in that
filter
The effect of Ly line emission
1215.67Å
* (1+z)
Current Topics: Lyman Break Galaxies - Lecture 3
• The presence of a line
affects the measured
magnitude.
• If W0=20Å, then Wobs=132Å
at z=5.6
• If the filter is 1000Å wide,
then the line contributes
~10% of the flux
• If half the filter is damped by
Ly- forest, the line
contributes ~20% of the flux
• The exact contribution
depends on the
transmission of the Ly
forest, width of filter and
strength of line
Ly emission: Worked Example
1215.67Å
* (1+z)
• Say emission line has
flux=2x10-17 ergs/s/cm2
• Line has W0=20Å
• Line is at z=5.6
• Filter is 2000Å wide, centred on
line emission
• What is the line contribution and
apparent broadband magnitude?
• W0=Wobs/(1+z) => Wobs = 20*6.6 = 132Å
• Filter is 2000Å wide, but at z>5, the effective Lyman break is 100%, I.e.
only 1000Å is measuring flux.
 Have 1000Å of continuum flux and line flux equivalent to 132Å. Line
contibution is 132/(1000+132) = 12%
 The galaxy will appear 12% brighter and is more likely to be detected
Current Topics: Lyman Break Galaxies - Lecture 3
Ly emission: Worked Example
1215.67Å
* (1+z)
•
Say emission line has
flux=2x10-17 ergs/s/cm2
•
•
•
Line has W0=20Å
Line is at z=5.6
Filter is 2000Å wide , centred on line
emission
•
What is the line contribution and
apparent broadband magnitude?
• Continuum flux density = line flux / Wobs= 1.5x10-19 ergs/s/cm2/Å
• This is per unit wavelength (i.e. f). AB magnitudes are defined in f.
f = f d/d,
c=,
d=1./2 d => f= 2/c f
f = ((8000x8000) / 3x1018) * f = 3.2x10-30 ergs/s/cm2/Hz
AB mag = -2.5 log(f) - 48.6 = 25.1
But galaxy will appear -2.5 log (2) = 0.7 mag fainter in this filter
Current Topics: Lyman Break Galaxies - Lecture 3
The effect of Ly line emission
• If line emission is in the
R band (4<z<5.1), R-I is
decreased.
• If it is in I (5.1<z<6.1),
both R-I and I-Z are
affected.
• But if colour selection
criteria are relaxed, get
more contamination
=> Difficult to be both
complete and
uncontaminated
Current Topics: Lyman Break Galaxies - Lecture 3
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 - Lecture 3
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 - Lecture 3
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 3
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 3
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 3
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 3
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 3
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 3
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
Current Topics: Lyman Break Galaxies - Lecture 3
(Verma et al 2007)
Comparisons with z=3
fraction
Stellar Mass:
At z=3,
mass~1010M
Log (Mass)
At z=5,
Mass ~
2x109M
Independent
of metallicity
 Galaxies
are smaller
(Verma et al 2007)
Current Topics: Lyman Break Galaxies - Lecture 3
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
Log (SFR)
Current Topics: Lyman Break Galaxies - Lecture 3
=> Galaxies are
forming stars
at about the
same rate
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 3
=> High z
galaxies are
less dusty
Av
Ferguson et al 2004
Sizes and Morphologies
Current Topics: Lyman Break Galaxies - Lecture 3
• 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 3
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 3
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 3
The Rest-Ultraviolet
No Ly lines
Too
Blue
Line emitters
• 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 3
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 3
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 3
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!
Current Topics: Lyman Break Galaxies - Lecture 3
OI
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 3
Dow-Hygelund et al, 2005
Other spectral lines and
outflows
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
• The sample looked at varies with survey filters
and characteristics
• Lyman- emission can affect measured
magnitudes and galaxy selection
• With increasing redshift see:
–
–
–
–
Decreasing metallicity
Decreasing dust extinction
Decreasing age
Decreasing mass
Current Topics: Lyman Break Galaxies - Lecture 3
Lecture Summary
• Spectroscopy is beginning to probe absorption lines,
finding:
– similar velocity outflows to z~3
– similar Lyline distribution at z~5
– stronger Lya lines at z~6
• Very blue rest-UV spectra are hinting at changes in the
nature of star formation
• LBGs at every redshift are used to characterise evolution
in star formation density and the mechanisms and
environment for star formation
• But, as at z=3, LBGs are not the whole story
• Knowledge of star formation properties is essential for
understanding galaxy evolution
Current Topics: Lyman Break Galaxies - Lecture 3