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

2. 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 Stanway

3. 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 Stanway

4. 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 Stanway

5. The History of High-z studies
Current Topics: Lyman Break Galaxies - Elizabeth Stanway

6. The History of High-z studies
Practical limits starting to be important
The highest redshift galaxy has been increasing steadily in distance for ~20 yrs
Current Topics: Lyman Break Galaxies - Elizabeth Stanway

7. The History of High-z studies
Universe 1Gyr old
Universe 1/8 current age
Universe 1/4 current age
Universe half current age
Now: Universe 13.7 Gyr
Current Topics: Lyman Break Galaxies - Elizabeth Stanway

8. The History of High-z studies
Z=3 => age about 2 Gyr
z=3 LBGs
Universe 1/8 current age
~ 2 Billion years after the Big Bang
Current Topics: Lyman Break Galaxies - Elizabeth Stanway

9. 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 Stanway

10. 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 Stanway

11. 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
Flux
Wavelength
Current 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
44 magnitudes of dimming at z=1
47 magnitudes of dimming at z=3
48 magnitudes of dimming at z=5
Current Topics: Lyman Break Galaxies - Elizabeth Stanway

13. Depth vs Area?
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
Current Topics: Lyman Break Galaxies - Elizabeth Stanway

14. 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 Stanway

15. Building a Galaxy TMS~10Gyr*(M/M) -2.5
30 Myr
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
300 Myr
10 Gyr
15 Gyr
Blue
Red
Current Topics: Lyman Break Galaxies - Elizabeth Stanway

16. Types of Galaxy SED
Old/Red
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.
Young/Blue
Rest-UV
Current Topics: Lyman Break Galaxies - Elizabeth Stanway

17. 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 Stanway

18. Hydrogen Emission Lines
OIII
The Balmer Series and Oxygen lines dominate the optical spectrum of a star forming galaxy
OII H H H H
The Lyman series emerges in the ultraviolet.
The Lyman- emission line can emit up to 1% of the galaxy’s bolometric flux, but ….
Current Topics: Lyman Break Galaxies - Elizabeth Stanway

19. Hydrogen Emission Lines
OIII
The Balmer Series and Oxygen lines dominate the optical spectrum of a star forming galaxy
OII H H H H
The Lyman series emerges in the ultraviolet.
The Lyman- emission line can emit up to 1% of the galaxy’s bolometric flux, but ….
Current Topics: Lyman Break Galaxies - Elizabeth Stanway

20. Hydrogen Emission Lines
Ly
Å
OIII
OII H H H
Ly H
The Lyman series emerges in the ultraviolet.
The Lyman- emission line can emit up to 1% of the galaxy’s bolometric flux, but ….
Current Topics: Lyman Break Galaxies - Elizabeth Stanway

21. The Asymmetric Lyman- Line
Low z
The Lyman- line is intrinsically symmetric
Higher z
At high-z the line always appears asymmetric and broadened
Current Topics: Lyman Break Galaxies - Elizabeth Stanway

22. The Asymmetric Lyman- Line
Star formation drives galaxy-scale winds (Adelberger et al 2003)
Lyman- is resonantly scattered by the winds
Blue Wing is scattered by outflowing galactic winds
Red wing is broadened by back-scattered light
Wind
v = 0
v =
+300 km/s
v =
-300 km/s
Current Topics: Lyman Break Galaxies - Elizabeth Stanway

23. The Asymmetric Lyman- Line
v/c = z/(1+z)
=> 300km/s wind broadens line by about 5Å FWHM at z=3
Blue Wing is scattered by outflowing galactic winds
Red wing is broadened by back-scattered light
Wind
v = 0
v =
+300 km/s
v =
-300 km/s
Current Topics: Lyman Break Galaxies - Elizabeth Stanway

24. The Lyman- Forest
… Lyman-a is also seen in absorption wherever there are clouds of hydrogen
Source
Observer
z=0 z*
Ly
Åz*)
Current Topics: Lyman Break Galaxies - Elizabeth Stanway

25. The Lyman- Forest
… Lyman-a is also seen in absorption wherever there are clouds of hydrogen
Source
Observer
z=0 z1 z*
Ly
Å
z1)
Åz*)
Current Topics: Lyman Break Galaxies - Elizabeth Stanway

26. The Lyman- Forest
… Lyman-a is also seen in absorption wherever there are clouds of hydrogen
Source
Observer
z=0 z4 z3 z2 z1 z*
Ly
Å
z3)
Å
z1)
Åz*)
Å
z4)
Å
z2)
Current Topics: Lyman Break Galaxies - Elizabeth Stanway

27. The Lyman- Forest
At low z almost all of a galaxy’s Lyman continuum flux reaches us
Current Topics: Lyman Break Galaxies - Elizabeth Stanway

28. The Lyman- Forest
Above z=3, the fraction of galaxy flux reaching us declines rapidly
Current Topics: Lyman Break Galaxies - Elizabeth Stanway

29. The Lyman- Forest
Beyond z=5.5, <1% of the galaxy’s flux gets through the IGM
Current Topics: Lyman Break Galaxies - Elizabeth Stanway

30. The Lyman- Forest Low z Higher z Lyman- Forest
Current Topics: Lyman Break Galaxies - Elizabeth Stanway

31. Properties of High-z Galaxies
Young galaxies at high-z are:
Current Topics: Lyman Break Galaxies - Elizabeth Stanway

32. Properties of High-z Galaxies
Young galaxies at high-z are:
Dominated by O and B stars
Current Topics: Lyman Break Galaxies - Elizabeth Stanway

33. 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 Stanway

34. 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 Stanway

35. 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 Stanway

36. 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 Stanway

37. 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 Stanway

38. Current Topics: Lyman Break Galaxies - Elizabeth Stanway

39. 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
Continuum
Ionising
Radiation
UV Continuum
Lyman-α
Break
912Å
Break
Current Topics: Lyman Break Galaxies - Elizabeth Stanway

40. The Lyman Break Technique
Red
If the filters bracket the breaks, then the galaxies show extreme colours
Blue
Red
Current Topics: Lyman Break Galaxies - Elizabeth Stanway

41. 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-2.0
For galaxies at 5.6<z<7.0, i'- z'>1.3
Current Topics: Lyman Break Galaxies - Elizabeth Stanway

42. Narrow Band Surveys A magnitude is the average flux in a filter
Sky Emission
A magnitude is the average flux in a filter
If half the filter is suppressed by Ly-a forest, the galaxy appears faint
Narrow Band
Broad Band
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 - Elizabeth Stanway

43. 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 Stanway

44. 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 Stanway

45. 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 Stanway

46. 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
Current Topics: Lyman Break Galaxies - Elizabeth Stanway