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Current Topics: Lyman Break Galaxies - Elizabeth Stanway 1 Current Topics Lyman Break Galaxies Dr Elizabeth Stanway

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Presentation on theme: "Current Topics: Lyman Break Galaxies - Elizabeth Stanway 1 Current Topics Lyman Break Galaxies Dr Elizabeth Stanway"— Presentation transcript:

1 Current Topics: Lyman Break Galaxies - Elizabeth Stanway 1 Current Topics Lyman Break Galaxies Dr Elizabeth Stanway

2 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

3 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

4 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 z AB are broadband filters

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

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

7 Current Topics: Lyman Break Galaxies - Elizabeth Stanway 7 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

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

9 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 Were within a few generations of the earliest galaxies forming Unevolved galaxies are simpler - easier to understand - and so help shape theory

10 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

11 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

12 Current Topics: Lyman Break Galaxies - Elizabeth Stanway 12 But Why is it so difficult? Distance Modulus equation: m = M - 5 log (d L /10pc) Luminosity Distance equation: d L = (1+z) * c/H 0 * At z=1, d L =6634 Mpc At z=3, d L =25840 Mpc At z=5, d L =47590 Mpc => Distant galaxies are very FAINT

13 Current Topics: Lyman Break Galaxies - Elizabeth Stanway 13 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< 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?

14 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 T MS ~10Gyr*(M/M ) -2.5 M BlueRed

15 Current Topics: Lyman Break Galaxies - Elizabeth Stanway 15 Building a Galaxy T MS ~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 BlueRed 300 Myr 30 Myr 15 Gyr

16 Current Topics: Lyman Break Galaxies - Elizabeth Stanway 16 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

17 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

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

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

20 Current Topics: Lyman Break Galaxies - Elizabeth Stanway 20 Hydrogen Emission Lines H H H H OIII OII Ly Å The Lyman series emerges in the ultraviolet. The Lyman- emission line can emit up to 1% of the galaxys bolometric flux, but …. Ly

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

22 Current Topics: Lyman Break Galaxies - Elizabeth Stanway 22 The Asymmetric Lyman- Line Blue 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 = 0 v = +300 km/s v = -300 km/s

23 Current Topics: Lyman Break Galaxies - Elizabeth Stanway 23 The Asymmetric Lyman- Line 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 v/c = z/(1+z) => 300km/s wind broadens line by about 5Å FWHM at z=3

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

25 Current Topics: Lyman Break Galaxies - Elizabeth Stanway 25 The Lyman- Forest Source Observer Å z * ) z*z* z=0 z1z1 Å z 1 ) Ly … Lyman-a is also seen in absorption wherever there are clouds of hydrogen

26 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 Å z * ) z*z* z=0 z1z1 z2z2 z3z3 z4z4 Å z 1 ) Å z 2 ) Å z 3 ) Å z 4 ) Ly

27 Current Topics: Lyman Break Galaxies - Elizabeth Stanway 27 The Lyman- Forest At low z almost all of a galaxys Lyman continuum flux reaches us

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

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

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

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

32 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

33 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

34 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

35 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:

36 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

37 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-

38 Current Topics: Lyman Break Galaxies - Elizabeth Stanway 38 Methods of Identifying High z Galaxies Narrow Band Surveys Identifies sources with high equivalent widths in certain emission lines. Narrow redshift range (typically Δz~0.1). Lyman Break Surveys Identifies sources with bright UV continuum emission. Broad redshift range (typically Δz~ ). Gravitational Lensing Surveys Identifies strongly lensed sources Often combined with other two methods. Redshift range variable.

39 Current Topics: Lyman Break Galaxies - Elizabeth Stanway 39 The Lyman Break Technique The Steidel, Pettini & Hamilton (1995) Lyman Break Method Ionising Radiation UV Continuum Lyman Continuum 912Å Break Lyman-α Break 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…

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

41 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 For galaxies at

42 Current Topics: Lyman Break Galaxies - Elizabeth Stanway 42 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

43 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

44 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

45 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

46 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


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