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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 The Star Formation History of the Universe and Reionisation - PowerPoint PPT Presentation
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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• The Star Formation History of the Universe
and Reionisation
• This course will be assessed through a 1 hour examination including mathematical and essay questions
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
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 redshift galaxy has been increasing steadily in distance 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 the Big Bang
z=3 LBGs
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
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
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
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
∫
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?
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
Current Topics: Lyman Break Galaxies - Elizabeth Stanway15
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
Current Topics: Lyman Break Galaxies - Elizabeth Stanway16
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
Current Topics: Lyman Break Galaxies - Elizabeth Stanway17
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
Current Topics: Lyman Break Galaxies - Elizabeth Stanway18
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
Current Topics: Lyman Break Galaxies - Elizabeth Stanway19
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
Current Topics: Lyman Break Galaxies - Elizabeth Stanway20
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
Current Topics: Lyman Break Galaxies - Elizabeth Stanway21
The Asymmetric Lyman- LineLow z
Higher z
The Lyman- line is intrinsically symmetric
At high-z the line always appears asymmetric and broadened
Current Topics: Lyman Break Galaxies - Elizabeth Stanway22
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
Current Topics: Lyman Break Galaxies - Elizabeth Stanway23
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
Current Topics: Lyman Break Galaxies - Elizabeth Stanway24
The Lyman- Forest
SourceObserver
Åz*)
z*z=0
Ly
… Lyman-a is also seen in absorption wherever there are clouds of hydrogen
Current Topics: Lyman Break Galaxies - Elizabeth Stanway25
The Lyman- Forest
SourceObserver
Åz*)
z*z=0 z1
Åz1)
Ly
… Lyman-a is also seen in absorption wherever there are clouds of hydrogen
Current Topics: Lyman Break Galaxies - Elizabeth Stanway26
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
Current Topics: Lyman Break Galaxies - Elizabeth Stanway27
The Lyman- Forest
At low z almost all of a galaxy’s Lyman continuum flux reaches us
Current Topics: Lyman Break Galaxies - Elizabeth Stanway28
The Lyman- Forest
Above z=3, the fraction of galaxy flux reaching us declines rapidly
Current Topics: Lyman Break Galaxies - Elizabeth Stanway29
The Lyman- Forest
Beyond z=5.5, <1% of the galaxy’s flux gets through the IGM
Current Topics: Lyman Break Galaxies - Elizabeth Stanway30
The Lyman- Forest
Low z
Higher zLyman- 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
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- emission
lines
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-
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
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…
Current Topics: Lyman Break Galaxies - Elizabeth Stanway40
The Lyman Break TechniqueRed
RedBlue
If the filters bracket the breaks, then the galaxies show extreme colours
Current Topics: Lyman Break Galaxies - Elizabeth Stanway41
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
Current Topics: Lyman Break Galaxies - Elizabeth Stanway42
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
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
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, dominated by:– oxygen and Balmer series lines in the optical– Lyman series lines in the ultraviolet
Current Topics: Lyman Break Galaxies - Elizabeth Stanway45
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
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