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Current Topics: Lyman Break Galaxies - Elizabeth Stanway1

Current Topics

Lyman Break Galaxies

Dr Elizabeth Stanway([email protected])


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.


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


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

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The History of High-z studies



The highest redshiftgalaxy has beenincreasing steadily indistance for ~20 yrs



Universe half current age

Universe 1/4 current age


Universe 1Gyr old

Now: Universe 13.7 Gyr




~ 2 Billion years after theBig Bang

z=3 LBGs

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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


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


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

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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 *

• At z=1, dL=6634 Mpc• At z=3, dL=25840 Mpc• At z=5, dL=47590 Mpc

=> Distant galaxies are very FAINT

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• 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?


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


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


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

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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


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



Hα

HβHγ

Hδ

OIII

OII



The Balmer Seriesand Oxygen linesdominate the opticalspectrum of a starforming galaxy



Hα

HβHγ

Hδ

OIII

OII

Lyα

1215.67 Å



Lyβ

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The Asymmetric Lyman-α LineLow z

Higher z

TheLyman-αline isintrinsicallysymmetric

At high-z theline alwaysappearsasymmetricandbroadened


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


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


The Lyman-α Forest

SourceObserver

1216Å∗(1+z*)

z*z=0

Lyα

… Lyman-a is also seen in absorption whereverthere are clouds of hydrogen

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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


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


The Lyman-α Forest

At low z almost all of agalaxy’s Lyman continuumflux reaches us


The Lyman-α Forest

Above z=3, the fraction ofgalaxy flux reaching usdeclines rapidly

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The Lyman-α Forest

Beyond z=5.5, <1% of thegalaxy’s flux gets throughthe IGM


The Lyman-α Forest

Low z

Higher zLyman-α Forest


Properties of High-z Galaxies

• Young galaxies at high-z are:



• Young galaxies at high-z are:– Dominated by O and B stars

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• Young galaxies at high-z are:– Dominated by O and B stars– Bright in the ultraviolet



• 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 key observable characteristics:– They have asymmetric Lyman-α emission

lines

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• They have key observable characteristics:– They have asymmetric Lyman-α emission

lines– Flux is suppressed shortward of Lyman-α


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.


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…


The Lyman Break TechniqueRed

RedBlue

If the filters bracket thebreaks, then the galaxiesshow extreme colours

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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


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


Narrow Band Surveys• But what line

have youdetected?

• Could be:– OIII at 5007A– OII at 3727A– Lyman-α at

1216A• Need

spectroscopicfollow-up


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

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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


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