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Page 1: Current Topics: Lyman Break Galaxies - Elizabeth Stanway 1 Current Topics Lyman Break Galaxies Dr Elizabeth Stanway (E.R.Stanway@Bristol.ac.uk)

Current Topics: Lyman Break Galaxies - Elizabeth Stanway1

Current Topics

Lyman Break Galaxies

Dr Elizabeth Stanway([email protected])

Page 2: Current Topics: Lyman Break Galaxies - Elizabeth Stanway 1 Current Topics Lyman Break Galaxies Dr Elizabeth Stanway (E.R.Stanway@Bristol.ac.uk)

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• The Star Formation History of the Universe

and Reionisation

• This course will be assessed through a 1 hour examination including mathematical and essay questions

Page 3: Current Topics: Lyman Break Galaxies - Elizabeth Stanway 1 Current Topics Lyman Break Galaxies Dr Elizabeth Stanway (E.R.Stanway@Bristol.ac.uk)

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

Page 4: Current Topics: Lyman Break Galaxies - Elizabeth Stanway 1 Current Topics Lyman Break Galaxies Dr Elizabeth Stanway (E.R.Stanway@Bristol.ac.uk)

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

Page 5: Current Topics: Lyman Break Galaxies - Elizabeth Stanway 1 Current Topics Lyman Break Galaxies Dr Elizabeth Stanway (E.R.Stanway@Bristol.ac.uk)

Current Topics: Lyman Break Galaxies - Elizabeth Stanway5

The History of High-z studies

Page 6: Current Topics: Lyman Break Galaxies - Elizabeth Stanway 1 Current Topics Lyman Break Galaxies Dr Elizabeth Stanway (E.R.Stanway@Bristol.ac.uk)

Current Topics: Lyman Break Galaxies - Elizabeth Stanway6

The History of High-z studies

The highest redshift galaxy has been increasing steadily in distance for ~20 yrs

Page 7: Current Topics: Lyman Break Galaxies - Elizabeth Stanway 1 Current Topics Lyman Break Galaxies Dr Elizabeth Stanway (E.R.Stanway@Bristol.ac.uk)

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

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Current Topics: Lyman Break Galaxies - Elizabeth Stanway8

The History of High-z studies

Universe 1/8 current age

~ 2 Billion years after the Big Bang

z=3 LBGs

Page 9: Current Topics: Lyman Break Galaxies - Elizabeth Stanway 1 Current Topics Lyman Break Galaxies Dr Elizabeth Stanway (E.R.Stanway@Bristol.ac.uk)

Current Topics: Lyman Break Galaxies - Elizabeth Stanway9

Why Push So Far Back?

• We are now starting to probe the last major phase transition in the universe - reionisation

• We’re within a few generations of the earliest galaxies forming

• Unevolved galaxies are simpler - easier to understand - and so help shape theory

Page 10: Current Topics: Lyman Break Galaxies - Elizabeth Stanway 1 Current Topics Lyman Break Galaxies Dr Elizabeth Stanway (E.R.Stanway@Bristol.ac.uk)

Current Topics: Lyman Break Galaxies - Elizabeth Stanway10

Why Push So Far Back?

• Lyman break galaxies are star-forming so directly measure how exciting a place the universe is

• Lyman break galaxies are relatively bright and so easy to study

• Lyman break galaxies are relatively easy to find

Page 11: Current Topics: Lyman Break Galaxies - Elizabeth Stanway 1 Current Topics Lyman Break Galaxies Dr Elizabeth Stanway (E.R.Stanway@Bristol.ac.uk)

Current Topics: Lyman Break Galaxies - Elizabeth Stanway11

But Why is it so difficult?

• Redshift equation:(obs)=em) * (1+z)

=> Distant galaxies are very RED

• The night sky is also very red => the sky background is much higher for high-z galaxies

Flu

x

Wavelength

Page 12: Current Topics: Lyman Break Galaxies - Elizabeth Stanway 1 Current Topics Lyman Break Galaxies Dr Elizabeth Stanway (E.R.Stanway@Bristol.ac.uk)

Current Topics: Lyman Break Galaxies - Elizabeth Stanway12

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

dz'Ωm (1+ z')3 + Ωk (1+ z')2 + ΩΛ

0

z

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Current Topics: Lyman Break Galaxies - Elizabeth Stanway13

• The Luminosity Function (LF) of a galaxy population relates number of objects seen to volume/area observed

• Most galaxies follow a Schecter (1973) function: N(L) dA (L/L*) e-(L/L*) dA

• When L<<L*, this approximates a power law: N(L) dA L dA

=> Increasing area of observation leads to increase in galaxy sample

BUT: since the power law is steep, increasing the depth usually increase sample size more quickly

Depth vs Area?

Page 14: Current Topics: Lyman Break Galaxies - Elizabeth Stanway 1 Current Topics Lyman Break Galaxies Dr Elizabeth Stanway (E.R.Stanway@Bristol.ac.uk)

Current Topics: Lyman Break Galaxies - Elizabeth Stanway14

Building a Galaxy

• Every galaxy is made of stars

• Lower mass stars live longer

• More massive stars are more luminous => burn more quickly

TMS~10Gyr*(M/M) -2.5

M

Blue Red

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Building a Galaxy

• TMS~10Gyr*(M/M) -2.5

• Old galaxies are dominated by A-M stars and have 4000A breaks

• Young galaxies are dominated by short-lived O and B stars and are UV-bright

10 Gyr

Blue Red

300 Myr

30 Myr

15 Gyr

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Types of Galaxy SED

• Old galaxies are dominated by A-M stars and have 4000Å breaks

• Young galaxies are dominated by short-lived O and B stars and are UV-bright

• Younger galaxies also show strong emission lines, powered by star formation.

Old/Red

Young/Blue Rest-UV

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Hydrogen Emission Lines• Flux from star

formation excites electrons in atoms

• The most abundant atom in the universe is Hydrogen

• As an electron relaxes from an excited state, it emits a photon

• Each transition emits at a particular wavelength

• The easiest transition to excite is Lyman-

The Balmer series emerges in the optical and so is known as ‘Hydrogen-’ etc for historical reasons

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Hydrogen Emission Lines

H

HH

H

OIII

OII

•The Lyman series emerges in the ultraviolet.

•The Lyman- emission line can emit up to 1% of the galaxy’s bolometric flux, but ….

The Balmer Series and Oxygen lines dominate the optical spectrum of a star forming galaxy

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Hydrogen Emission Lines

H

HH

H

OIII

OII

•The Lyman series emerges in the ultraviolet.

•The Lyman- emission line can emit up to 1% of the galaxy’s bolometric flux, but ….

The Balmer Series and Oxygen lines dominate the optical spectrum of a star forming galaxy

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Hydrogen Emission Lines

H

HH

H

OIII

OII

Ly

Å

•The Lyman series emerges in the ultraviolet.

•The Lyman- emission line can emit up to 1% of the galaxy’s bolometric flux, but ….

Ly

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The Asymmetric Lyman- LineLow z

Higher z

The Lyman- line is intrinsically symmetric

At high-z the line always appears asymmetric and broadened

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The Asymmetric Lyman- LineBlue Wing is scattered by outflowing galactic winds

Red wing is broadened by back-scattered light

Star formation drives galaxy-scale winds (Adelberger et al 2003)

Lyman- is resonantly scattered by the winds

Wind

v = 0v = +300 km/s

v = -300 km/s

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The Asymmetric Lyman- LineBlue Wing is scattered by outflowing galactic winds

Red wing is broadened by back-scattered light

Wind

v = 0v = +300 km/s

v = -300 km/s

v/c = z/(1+z)

=> 300km/s wind broadens line by about 5Å FWHM at z=3

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The Lyman- Forest

SourceObserver

Åz*)

z*z=0

Ly

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

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The Lyman- Forest

SourceObserver

Åz*)

z*z=0 z1

Åz1)

Ly

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

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The Lyman- Forest… Lyman-a is also seen in absorption wherever there are clouds

of hydrogen

SourceObserver

Åz*)

z*z=0 z1z2z3z4

Åz1)

Åz2)

Åz3)

Åz4)

Ly

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The Lyman- Forest

At low z almost all of a galaxy’s Lyman continuum flux reaches us

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The Lyman- Forest

Above z=3, the fraction of galaxy flux reaching us declines rapidly

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The Lyman- Forest

Beyond z=5.5, <1% of the galaxy’s flux gets through the IGM

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The Lyman- Forest

Low z

Higher zLyman- Forest

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Properties of High-z Galaxies

• Young galaxies at high-z are:

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Properties of High-z Galaxies

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

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Properties of High-z Galaxies

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

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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

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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:

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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- emission

lines

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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- emission

lines– Flux is suppressed shortward of Lyman-

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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.

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The Lyman Break TechniqueThe Steidel, Pettini & Hamilton (1995) Lyman Break Method

Ionising

RadiationUV Continuum

Lyman

Continuum

912ÅBreak

Lyman-αBreak

• 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…

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The Lyman Break TechniqueRed

RedBlue

If the filters bracket the breaks, then the galaxies show extreme colours

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The Dropout Technique

● At z>4, the Lyman forest absorption reaches near 100% only one break is detected

● A source will be detected in filters 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

f

For galaxies at 5.6<z<7.0, i'- z'>1.3

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Narrow Band Surveys

• A magnitude is the average flux in a filter

• If half the filter is suppressed by Ly-a 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

Broad Band

Narrow Band

Sky Emission

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Current Topics: Lyman Break Galaxies - Elizabeth Stanway43

Narrow Band Surveys

• But what line have you detected?

• Could be:– OIII at 5007A– OII at 3727A– Lyman- at 1216A

• Need spectroscopic follow-up

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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, dominated by:– oxygen and Balmer series lines in the optical– Lyman series lines in the ultraviolet

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Lecture Summary (II)• Lyman- is characteristically asymmetric due to galaxy-

scale outflows

• Absorption by the intervening IGM suppresses flux shortwards of Lyman-

• 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

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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