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Current Topics: Lyman Break Galaxies - Elizabeth Stanway1
Current Topics
Lyman Break Galaxies
Dr Elizabeth Stanway([email protected])
Current Topics: Lyman Break Galaxies - Elizabeth Stanway2
Topic Summary
• Star Forming Galaxies and the Lyman- Line• Lyman Break Galaxies at z<4• Lyman Break Galaxies at z>4
• You are required to answer at least one shortanswer question on this topic in the exam
• Credit will be given in the essay question forcorrect citation of scientific literature.
Current Topics: Lyman Break Galaxies - Elizabeth Stanway3
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 Stanway4
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
2
Current Topics: Lyman Break Galaxies - Elizabeth Stanway5
The History of High-z studies
Current Topics: Lyman Break Galaxies - Elizabeth Stanway6
The History of High-z studies
The highest redshiftgalaxy has beenincreasing steadily indistance for ~20 yrs
Current Topics: Lyman Break Galaxies - Elizabeth Stanway7
The History of High-z studies
Universe half current age
Universe 1/4 current age
Universe 1/8 current age
Universe 1Gyr old
Now: Universe 13.7 Gyr
Current Topics: Lyman Break Galaxies - Elizabeth Stanway8
The History of High-z studies
Universe 1/8 current age
~ 2 Billion years after theBig Bang
z=3 LBGs
3
Current Topics: Lyman Break Galaxies - Elizabeth Stanway9
Why Push SoFar Back?
• We are now starting toprobe the last majorphase transition in theuniverse - reionisation
• We’re within a fewgenerations of theearliest galaxiesforming
• Unevolved galaxiesare simpler - easier tounderstand - and sohelp shape theory
Current Topics: Lyman Break Galaxies - Elizabeth Stanway10
Why Push SoFar Back?
• Lyman break galaxiesare star-forming sodirectly measure howexciting a place theuniverse is
• Lyman break galaxiesare relatively brightand so easy to study
• Lyman break galaxiesare relatively easy tofind
Current Topics: Lyman Break Galaxies - Elizabeth Stanway11
But Why is it so difficult?• Redshift equation:
λ(obs)=λ(em) * (1+z)=> Distant galaxies are very RED
• The night sky isalso very red=> the skybackground ismuch higher forhigh-z galaxies
Flux
WavelengthCurrent Topics: Lyman Break Galaxies - Elizabeth Stanway
12
But Why is it so difficult?• Distance Modulus equation:
m = M - 5 log (dL/10pc)
• Luminosity Distance equation:dL = (1+z) * c/H0 *
• At z=1, dL=6634 Mpc• At z=3, dL=25840 Mpc• At z=5, dL=47590 Mpc
=> Distant galaxies are very FAINT
4
Current Topics: Lyman Break Galaxies - Elizabeth Stanway13
• The Luminosity Function (LF) of a galaxypopulation relates number of objects seen tovolume/area observed
• Most galaxies follow a Schecter (1973) function:N(L) dA ∝ (L/L*)α e-(L/L*) dA
• When L<<L*, this approximates a power law:N(L) dA ∝ Lα dA
=> Increasing area of observation leads to increasein galaxy sample
BUT: since the power law is steep, increasing thedepth usually increase sample size more quickly
Depth vs Area?
Current Topics: Lyman Break Galaxies - Elizabeth Stanway14
Building aGalaxy
• Every galaxy ismade of stars
• Lower mass starslive longer
• More massive starsare more luminous=> burn more quickly
TMS~10Gyr*(M/M) -2.5
M
Blue Red
Current Topics: Lyman Break Galaxies - Elizabeth Stanway15
Building aGalaxy
• TMS~10Gyr*(M/M) -2.5
• Old galaxies aredominated by A-M starsand have 4000A breaks
• Young galaxies aredominated by short-livedO and B stars and areUV-bright
10 Gyr
Blue Red
300 Myr
30 Myr
15 Gyr
Current Topics: Lyman Break Galaxies - Elizabeth Stanway16
Types ofGalaxy SED
• Old galaxies aredominated by A-Mstars and have 4000Åbreaks
• Young galaxies aredominated by short-lived O and B stars andare UV-bright
• Younger galaxies alsoshow strong emissionlines, powered by starformation.
Old/Red
Young/Blue Rest-UV
5
Current Topics: Lyman Break Galaxies - Elizabeth Stanway17
Hydrogen Emission Lines• Flux from star
formation exciteselectrons in atoms
• The most abundantatom in the universeis Hydrogen
• As an electronrelaxes from anexcited state, it emitsa photon
• Each transition emitsat a particularwavelength
• The easiest transitionto excite is Lyman-α
The Balmer seriesemerges in the opticaland so is known as‘Hydrogen-α’ etc forhistorical reasons
Current Topics: Lyman Break Galaxies - Elizabeth Stanway18
Hydrogen Emission Lines
Hα
HβHγ
Hδ
OIII
OII
•The Lyman series emerges in the ultraviolet.
•The Lyman-α emission line can emit up to 1% of the galaxy’sbolometric flux, but ….
The Balmer Seriesand Oxygen linesdominate the opticalspectrum of a starforming galaxy
Current Topics: Lyman Break Galaxies - Elizabeth Stanway19
Hydrogen Emission Lines
Hα
HβHγ
Hδ
OIII
OII
•The Lyman series emerges in the ultraviolet.
•The Lyman-α emission line can emit up to 1% of the galaxy’sbolometric flux, but ….
The Balmer Seriesand Oxygen linesdominate the opticalspectrum of a starforming galaxy
Current Topics: Lyman Break Galaxies - Elizabeth Stanway20
Hydrogen Emission Lines
Hα
HβHγ
Hδ
OIII
OII
Lyα
1215.67 Å
•The Lyman series emerges in the ultraviolet.
•The Lyman-α emission line can emit up to 1% of the galaxy’sbolometric flux, but ….
Lyβ
6
Current Topics: Lyman Break Galaxies - Elizabeth Stanway21
The Asymmetric Lyman-α LineLow z
Higher z
TheLyman-αline isintrinsicallysymmetric
At high-z theline alwaysappearsasymmetricandbroadened
Current Topics: Lyman Break Galaxies - Elizabeth Stanway22
The Asymmetric Lyman-α LineBlue Wing isscattered byoutflowinggalactic winds
Red wing isbroadened byback-scatteredlight
Star formation drivesgalaxy-scale winds(Adelberger et al 2003)
Lyman-α is resonantlyscattered by the winds
Wind
v = 0 v =+300 km/s
v =-300 km/s
Current Topics: Lyman Break Galaxies - Elizabeth Stanway23
The Asymmetric Lyman-α LineBlue Wing isscattered byoutflowinggalactic winds
Red wing isbroadened byback-scatteredlight
Wind
v = 0 v =+300 km/s
v =-300 km/s
Δv/c = Δz/(1+z)
=> 300km/s windbroadens line by about5Å FWHM at z=3
Current Topics: Lyman Break Galaxies - Elizabeth Stanway24
The Lyman-α Forest
SourceObserver
1216Å∗(1+z*)
z*z=0
Lyα
… Lyman-a is also seen in absorption whereverthere are clouds of hydrogen
7
Current Topics: Lyman Break Galaxies - Elizabeth Stanway25
The Lyman-α Forest
SourceObserver
1216Å∗(1+z*)
z*z=0 z1
1216Å∗(1+z1)
Lyα
… Lyman-a is also seen in absorption whereverthere are clouds of hydrogen
Current Topics: Lyman Break Galaxies - Elizabeth Stanway26
The Lyman-α Forest… Lyman-a is also seen in absorption wherever
there are clouds of hydrogen
SourceObserver
1216Å∗(1+z*)
z*z=0 z1z2z3z4
1216Å∗(1+z1)
1216Å∗(1+z2)
1216Å∗(1+z3)
1216Å∗(1+z4)
Lyα
Current Topics: Lyman Break Galaxies - Elizabeth Stanway27
The Lyman-α Forest
At low z almost all of agalaxy’s Lyman continuumflux reaches us
Current Topics: Lyman Break Galaxies - Elizabeth Stanway28
The Lyman-α Forest
Above z=3, the fraction ofgalaxy flux reaching usdeclines rapidly
8
Current Topics: Lyman Break Galaxies - Elizabeth Stanway29
The Lyman-α Forest
Beyond z=5.5, <1% of thegalaxy’s flux gets throughthe IGM
Current Topics: Lyman Break Galaxies - Elizabeth Stanway30
The Lyman-α Forest
Low z
Higher zLyman-α Forest
Current Topics: Lyman Break Galaxies - Elizabeth Stanway31
Properties of High-z Galaxies
• Young galaxies at high-z are:
Current Topics: Lyman Break Galaxies - Elizabeth Stanway32
Properties of High-z Galaxies
• Young galaxies at high-z are:– Dominated by O and B stars
9
Current Topics: Lyman Break Galaxies - Elizabeth Stanway33
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 Stanway34
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 Stanway35
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 Stanway36
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-α emission
lines
10
Current Topics: Lyman Break Galaxies - Elizabeth Stanway37
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-α emission
lines– Flux is suppressed shortward of Lyman-α
Current Topics: Lyman Break Galaxies - Elizabeth Stanway38
Methods of Identifying High z Galaxies
Narrow Band Surveys
Lyman Break Surveys
Gravitational Lensing Surveys
• Identifies sources with high equivalent widths in certain emission lines.• Narrow redshift range (typically Δz~0.1).
• Identifies sources with bright UV continuum emission. • Broad redshift range (typically Δz~0.3-0.5).
• Identifies strongly lensed sources • Often combined with other two methods.• Redshift range variable.
Current Topics: Lyman Break Galaxies - Elizabeth Stanway39
The Lyman Break TechniqueThe Steidel, Pettini & Hamilton (1995) Lyman Break Method
Ionising
Radiation UV Continuum
Lyman
Continuum
912ÅBreak
Lyman-αBreak
• At z=3, about 50% ofthe Lyman continuumis transmitted
• This leads to a ‘break’in the spectrum
• So consider whatwould happen if youplace filters either sideof the Lyman-α andLyman limit breaks…
Current Topics: Lyman Break Galaxies - Elizabeth Stanway40
The Lyman Break TechniqueRed
RedBlue
If the filters bracket thebreaks, then the galaxiesshow extreme colours
11
Current Topics: Lyman Break Galaxies - Elizabeth Stanway41
The Dropout Technique
● At z>4, the Lyman forestabsorption reaches near100% ⇒ only one break isdetected
● A source will be detected infilters above the break but‘drop-out’ of filters below it
● V-drops ⇒ z > 4.5
● R-drops ⇒ z > 5.
● I-drops ⇒ z > 5.8
Starburst at z=6
fλ∝λ−2.0
For galaxies at 5.6<z<7.0, i'- z'>1.3
Current Topics: Lyman Break Galaxies - Elizabeth Stanway42
Narrow Band Surveys
• A magnitude isthe average flux ina filter
• If half the filter issuppressed by Ly-a forest, thegalaxy appearsfaint
• 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
BroadBand
NarrowBand
SkyEmission
Current Topics: Lyman Break Galaxies - Elizabeth Stanway43
Narrow Band Surveys• But what line
have youdetected?
• Could be:– OIII at 5007A– OII at 3727A– Lyman-α at
1216A• Need
spectroscopicfollow-up
Current Topics: Lyman Break Galaxies - Elizabeth Stanway44
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, dominatedby:– oxygen and Balmer series lines in the optical– Lyman series lines in the ultraviolet
12
Current Topics: Lyman Break Galaxies - Elizabeth Stanway45
Lecture Summary (II)• Lyman-α is characteristically asymmetric due to
galaxy-scale outflows
• Absorption by the intervening IGM suppresses fluxshortwards of Lyman-α
• 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 Stanway46
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