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

2. Other Galaxies at z=3
Lyman Break Galaxies are selected to be UV-bright
Strongly star forming
Not too much dust extinction
They can’t account for all the material at z=3, so other techniques must fill in the gaps:
DLAs
Narrow Band Surveys
Sub-millimeter or Infrared selection
Current Topics: Lyman Break Galaxies - Lecture 3

3. UV-Dark Material: DLAs
The spectra of some very high redshift galaxies show dense, massive clouds of hydrogen along the line of sight
These ‘Damped Lyman- Absorbers’ must be UV-dark galaxies at intermediate redshifts
Prochaska et al (2001)
Current Topics: Lyman Break Galaxies - Lecture 3

4. Submillimeter Galaxies (SMGs)
The UV is heavily extincted
The light is absorbed by dust grains and re-emitted at far-IR and submillimetre wavelengths
Most of the galaxy’s light can be emitted at >100m
These frequencies are difficult to observe due to atmospheric effects
Current Topics: Lyman Break Galaxies - Lecture 3

5. Submillimeter Galaxies (SMGs)
At 1 mm, the distance is offset by the shape of the SED
This is known as a ‘negative K-correction’
In theory z=10 sources are as easily observed as z=1 in the 850m atmospheric window
z=1
z=10
Current Topics: Lyman Break Galaxies - Lecture 3

6. Submillimeter Galaxies (SMGs)
In practice, Submillimetre galaxies (SMGs) are hard to detect, and harder still to find redshifts for
But many probably lie at z=2-3 and each has a huge SFR (hundreds or thousands of solar masses /year)
Smail, Blain, Chapman et al, 2003
Current Topics: Lyman Break Galaxies - Lecture 3

7. Completing the z~3 Picture
Using molecular line emission at z=3, could probe cool gas
“low-excitation lines will map out a larger fraction of the ISM in these galaxies and…study in detail the spacially resolved kinematic structure of most of the gas…which resides in the cold phase” (Carilli & Blain 2002)
CO emitting galaxies may contribute significant mass and star formation
New telescopes such as ALMA, SKA and the EVLA will be crucial for completing the picture at z=3 and above.
Current Topics: Lyman Break Galaxies - Lecture 3

8. Topic Summary Star Forming Galaxies and the Lyman- Line
Lyman Break Galaxies at z<4
Lyman Break Galaxies at z>4
Extending the method to higher redshift
Properties of LBGs at high z
Shedding light on the high z universe
Lyman Breaks at z>7, SFH and Reionisation
Current Topics: Lyman Break Galaxies - Lecture 3

9. 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 - Lecture 3

10. Extending the LBG method to higher redshifts
At z=3-4, the Lyman break is bracketed by UGR filters
At z=5, the Lyman break falls just short of the I band
At z=6, it is about to enter the ZAB band R I
ZAB
Current Topics: Lyman Break Galaxies - Lecture 3

11. RIZ selection at z=5 and z=6
Current Topics: Lyman Break Galaxies - Lecture 3

12. RIZ selection at z=5 and z=6
BUT at these wavelengths, filters overlap and are far from standardised.
Current Topics: Lyman Break Galaxies - Lecture 3

13. Filters V-drop filters R-drop filters
Current Topics: Lyman Break Galaxies - Lecture 3

14. Redshift selection as a function of filter
Low z galaxy
High z galaxy
Current Topics: Lyman Break Galaxies - Lecture 3

15. Redshift selection as a function of filter
z~5 V- and R-drops
z~6 I-drops
Number density and redshift distribution depend on filters used
=> Results from surveys are not directly comparable
Current Topics: Lyman Break Galaxies - Lecture 3

16. Contamination
As well as problems from intermediate z galaxies, also have problems with cool stars
M, L and T-class stars are very red in the same bands as z=5 and z=6 LBGs
Can identify stars with HST data (morphology), or very deep infrared data (colour)
Problem if the survey is ground based or objects are faint.
Current Topics: Lyman Break Galaxies - Lecture 3

17. The effect of Ly line emission
The gradual change in colour with redshift is due to movement of the Lyman- break through the filter
Typical spectrum flat in f => f-2 (c=)
When the Lyman- break is halfway through the filter, the average flux in the filter is a factor of 2 lower than in a filter longwards of the break.
=> The object will appear 0.7 mags fainter in that filter
Spectrum flat in f
99% at z>5.5
Current Topics: Lyman Break Galaxies - Lecture 3

18. The effect of Ly line emission
The presence of a line affects the measured magnitude.
If W0=20Å, then Wobs=132Å at z=5.6
If the filter is 1000Å wide, then the line contributes ~10% of the flux
If half the filter is damped by Ly- forest, the line contributes ~20% of the flux
The exact contribution depends on the transmission of the Ly forest, width of filter and strength of line
Å * (1+z)
Current Topics: Lyman Break Galaxies - Lecture 3

19. Ly emission: Worked Example
Say emission line has
flux=2x10-17 ergs/s/cm2
Line has W0=20Å
Line is at z=5.6
Filter is 2000Å wide, centred on line emission
What is the line contribution and apparent broadband magnitude?
Å * (1+z)
W0=Wobs/(1+z) => Wobs = 20*6.6 = 132Å
Filter is 2000Å wide, but at z>5, the effective Lyman break is 100%, I.e. only 1000Å is measuring flux.
Have 1000Å of continuum flux and line flux equivalent to 132Å. Line contibution is 132/( ) = 12%
The galaxy will appear 12% brighter and is more likely to be detected
Current Topics: Lyman Break Galaxies - Lecture 3

20. Ly emission: Worked Example
Say emission line has
flux=2x10-17 ergs/s/cm2
Line has W0=20Å
Line is at z=5.6
Filter is 2000Å wide , centred on line emission
What is the line contribution and apparent broadband magnitude?
Å * (1+z)
Continuum flux density = line flux / Wobs= 1.5x10-19 ergs/s/cm2/Å
This is per unit wavelength (i.e. f). AB magnitudes are defined in f.
f = f d/d, c=, d=1./2 d => f= 2/c f
f = ((8000x8000) / 3x1018) * f = 3.2x10-30 ergs/s/cm2/Hz
AB mag = -2.5 log(f) = 25.1
But galaxy will appear -2.5 log (2) = 0.7 mag fainter in this filter
Current Topics: Lyman Break Galaxies - Lecture 3

21. The effect of Ly line emission
If line emission is in the R band (4<z<5.1), R-I is decreased.
If it is in I (5.1<z<6.1), both R-I and I-Z are affected.
But if colour selection criteria are relaxed, get more contamination
=> Difficult to be both complete and uncontaminated
Current Topics: Lyman Break Galaxies - Lecture 3

22. 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 - Lecture 3

23. 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 - Lecture 3

24. Ground vs Space-Based Surveys
HST can reach objects 0.7-1mag (2-3 times) fainter in the same length of time
Ground-based 8m telescopes have larger fields of view (by a factor of about 4)
So which is more efficient at finding high-z galaxies?
The faint end of the Schecter Luminosity function (L<<L*) can be approximated as power law (i.e. N(L)  LdA dz)
So N8m/NHST=(L8m/LHST) (A8m/AHST)
If  is steeper than about -1.2, then HST always wins (I.e depth is more useful than area)
HST has higher resolution, but 8m telescopes are ‘cheaper’
Current Topics: Lyman Break Galaxies - Lecture 3

25. Surveys of z>4 LBGs GOODS Hubble Space Telescope SDF/SXDF
(The Great Observatories Origins Deep Survey)
Hubble Space Telescope
V-drops I-drops
Z-drops
SDF/SXDF
Subaru 8m telescope
V-drops R-drops
I-drops
BDF/ERGS
ESO Very Large Telescopes (8m)
R-drops I-drops
Cluster Lensing Surveys
Keck / HST
I-drops Z-drops
J-drops
UKIDSS
UK Infrared Telescope (4m)
I-drops Z-drops
Y-drops J-drops
Current Topics: Lyman Break Galaxies - Lecture 3

26. Stellar populations
As at z=3, most information is derived from SED fitting.
Unconfused Spitzer data is essential for this at z>4
Detailed results are model dependent
General results are model independent
Verma et al, 2007
Current Topics: Lyman Break Galaxies - Lecture 3

27. Old Stars at z=6 SFRe-t/
Eyles et al, 2005
SFRe-t/
Sometimes both a new starburst and an old population are needed to fit a galaxy
As at z=3, some stars seem as old as the universe, but time scales are shorter, so the constraints are tighter
Current Topics: Lyman Break Galaxies - Lecture 3

28. Old Stars at z~6
Too Young for Ly line
z=5.83
Older than universe
Sometimes both a new starburst and an old population are needed to fit a galaxy
As at z=3, some stars seem as old as the universe, but time scales are shorter, so the constraints are tighter
Current Topics: Lyman Break Galaxies - Lecture 3

29. Comparisons with z=3 Using a z~5 HST v-drop sample
GOODS field => extremely deep
Using an SMC (i.e. low metallicity) extinction law
Using Spitzer data
Current Topics: Lyman Break Galaxies - Lecture 3

30. Comparisons with z=3 Age: At z=3, age~300Myr fraction At z=5,
If Z=Z,
then age~3Myr
Galaxies are younger
(Verma et al 2007)
fraction
Log (Age)
Current Topics: Lyman Break Galaxies - Lecture 3

31. Comparisons with z=3 Stellar Mass: At z=3, mass~1010M fraction
Mass ~ 2x109M
Independent of metallicity
Galaxies are smaller
(Verma et al 2007)
fraction
Log (Mass)
Current Topics: Lyman Break Galaxies - Lecture 3

32. Comparisons with z=3 fraction Log (SFR) Star Formation Rate: At z=3,
SFR~50M/yr
At z=5,
SFR ~ 50M/yr
If Z=Z,
SFR~600M/yr
=> Galaxies are forming stars at about the same rate
fraction
Log (SFR)
Current Topics: Lyman Break Galaxies - Lecture 3

33. Comparisons with z=3 fraction Av Dust: At z=3, Av~0.6 mags At z=5,
If Z=Z,
=> High z galaxies are less dusty
fraction Av
Current Topics: Lyman Break Galaxies - Lecture 3

34. Sizes and Morphologies
Galaxies at high-z have a smaller projected size.
Most of this is due to evolution in physical size rather than angular scale factor
Up to z~5, the size evolution is as expected for a fixed mass
Morphologies are often irregular and complex
Ferguson et al 2004
Current Topics: Lyman Break Galaxies - Lecture 3

35. Sizes and Morphologies
Galaxies at high-z have a smaller projected size.
Most of this is due to evolution in physical size rather than angular scale factor
Up to z~5, the size evolution is as expected for a fixed mass
Morphologies are often irregular and complex
Current Topics: Lyman Break Galaxies - Lecture 3

36. Spectroscopy at z~5
Spectroscopy at z~5 is challenging, but not impossible
In 5 hours on an 8m telescope get good S/N on lines and reasonable detections of continuum flux
The night sky is growing brighter but is still reasonable
Current Topics: Lyman Break Galaxies - Lecture 3

37. Spectroscopy at z~6 Spectroscopy at z~6 is extremely difficult
35 hours with Gemini
6 hours with Keck
Spectroscopy at z~6 is extremely difficult
Sources are typically 1 mag fainter at z=6 than at z=5
Continuum is only detected in exceptional or lensed galaxies
Current Topics: Lyman Break Galaxies - Lecture 3

38. The Rest-Ultraviolet Rest-UV slope is an age indicator:
No Ly lines
Too Blue
Line emitters
Rest-UV slope is an age indicator:
young=blue, old=red
But many z~5 galaxies seem too blue
Current Topics: Lyman Break Galaxies - Lecture 3

39. The Rest-Ultraviolet No Ly lines Line emitters
Too Blue
Line emitters
Steep Rest-UV slope (blue of f-2) could indicate zero age, Pop III, top-heavy initial mass function …
=> New physics! Interpretation still unclear
Current Topics: Lyman Break Galaxies - Lecture 3

40. Lyman- Equivalent Widths
z~6
50% of z>5 sources have EW>0Å
25% have EW>30Å
z~5
i’-drops (DEIMOS)
At z~5 the distribution of Lyman-a line strengths is similar to that at z~3
At z~6 see more high EW lines - selection function? More hot stars? Dust effects? New physics?
Current Topics: Lyman Break Galaxies - Lecture 3

41. Other spectral lines and outflows
Stacking together ~50 z~5 galaxies, can start to see other lines:
CIV, SiIV and OI are starting to be visible
Velocity offsets => similar winds to z~3
Work still in progress! OI
SIV
Current Topics: Lyman Break Galaxies - Lecture 3

42. Other spectral lines and outflows
In a few lensed cases, can identify lines in individual spectra
This example is 6x the typical z~5 LBG brightness
It is also lensed!
Strong interstellar lines
No Ly => older than typical, more dusty or more evolved
Psychotic cases like this can’t really describe the whole population
Dow-Hygelund et al, 2005
Current Topics: Lyman Break Galaxies - Lecture 3

43. Lecture Summary
LBGs at z>4 are significantly harder to find than those at z<4
LBGs at z~6 are a lot harder than z~5
The sample looked at varies with survey filters and characteristics
Lyman- emission can affect measured magnitudes and galaxy selection
With increasing redshift see:
Decreasing metallicity
Decreasing dust extinction
Decreasing age
Decreasing mass
Current Topics: Lyman Break Galaxies - Lecture 3

44. Lecture Summary
Spectroscopy is beginning to probe absorption lines, finding:
similar velocity outflows to z~3
similar Lyline distribution at z~5
stronger Lya lines at z~6
Very blue rest-UV spectra are hinting at changes in the nature of star formation
LBGs at every redshift are used to characterise evolution in star formation density and the mechanisms and environment for star formation
But, as at z=3, LBGs are not the whole story
Knowledge of star formation properties is essential for understanding galaxy evolution
Current Topics: Lyman Break Galaxies - Lecture 3