Current Topics Topic Summary
• Star Forming Galaxies and the Lyman- Line • Lyman Break Galaxies at z<4 • Lyman Break Galaxies at z>4
• You are required to answer at least one short answer question on this topic in the exam Lyman Break Galaxies • Credit will be given in the essay question for correct citation of scientific literature. Dr Elizabeth Stanway ([email protected]) 1 2 Current Topics: Lyman Break Galaxies - Elizabeth Stanway Current Topics: Lyman Break Galaxies - Elizabeth Stanway
Recommended Reading A few definitions … • In these lectures – LBG = Lyman Break Galaxy • Steidel, Pettini & Hamilton, 1995, AJ, 110, 2519 – LAE = Lyman Alpha Emitter • Carilli & Blain, 2002, ApJ, 569, 605 – HST = Hubble Space Telescope • Verma et al, 2007, MNRAS, 377, 1024 – Gyr = 1 Billion Years (Myr = 1 million yrs) • Bouwens et al, 2007, ApJ, 670, 928 – z = redshift • Stanway et al, 2008, ApJ, 687, L1 – Z = metallicity
– z’ or zAB are broadband filters
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1 The History of High-z studies The History of High-z studies
The highest redshift galaxy has been increasing steadily in distance for ~20 yrs
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The History of High-z studies The History of High-z studies
Universe 1Gyr old
z=3 LBGs Universe 1/8 current age Universe 1/8 current age ~ 2 Billion years after the Universe 1/4 current age Big Bang
Universe half current age Now: Universe 13.7 Gyr
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2 Why Push So Why Push So Far Back? Far Back?
• We are now starting to • Lyman break galaxies probe the last major are star-forming so phase transition in the directly measure how universe - reionisation exciting a place the • We’re within a few universe is generations of the • Lyman break galaxies earliest galaxies are relatively bright forming and so easy to study • Unevolved galaxies • Lyman break galaxies are simpler - easier to are relatively easy to understand - and so find help shape theory
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But Why is it so difficult? But Why is it so difficult?
• Redshift equation: • Distance Modulus equation:
λ(obs)=λ(em) * (1+z) m = M - 5 log (dL/10pc) => Distant galaxies are very RED • Luminosity Distance equation:
• The night sky is dL = (1+z) * c/H0 * also very red
x • At z=1, d =6634 Mpc
u L
=> the sky l background is F • At z=3, dL=25840 Mpc much higher for • At z=5, dL=47590 Mpc high-z galaxies => Distant galaxies are very FAINT Wavelength 11 12 Current Topics: Lyman Break Galaxies - Elizabeth Stanway Current Topics: Lyman Break Galaxies - Elizabeth Stanway
3 Depth vs Area? Building a
• The Luminosity Function (LF) of a galaxy Galaxy population relates number of objects seen to volume/area observed • Every galaxy is • Most galaxies follow a Schecter (1973) function: made of stars N(L) dA (L/L*)α e-(L/L*) dA • Lower mass stars ∝ M • When L<
Building a Types of Old/Red Galaxy Galaxy SED
30 Myr • Old galaxies are -2.5 • TMS~10Gyr*(M/M) dominated by A-M 300 Myr stars and have 4000Å • Old galaxies are breaks • Young galaxies are dominated by A-M stars 10 Gyr and have 4000A breaks dominated by short- lived O and B stars and • Young galaxies are 15 Gyr are UV-bright Young/ dominated by short-lived • Younger galaxies also Blue Rest-UV O and B stars and are show strong emission UV-bright lines, powered by star formation. Blue Red
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4 Hydrogen Emission Lines Hydrogen Emission Lines • Flux from star formation excites OIII electrons in atoms • The most abundant The Balmer Series OII atom in the universe and Oxygen lines Hα is Hydrogen dominate the optical • As an electron spectrum of a star Hβ relaxes from an Hγ forming galaxy Hδ excited state, it emits a photon • Each transition emits The Balmer series at a particular emerges in the optical wavelength and so is known as •The Lyman series emerges in the ultraviolet. ‘Hydrogen-α’ etc for • The easiest transition historical reasons •The Lyman-α emission line can emit up to 1% of the galaxy’s to excite is Lyman-α bolometric flux, but …. 17 18 Current Topics: Lyman Break Galaxies - Elizabeth Stanway Current Topics: Lyman Break Galaxies - Elizabeth Stanway
Hydrogen Emission Lines Hydrogen Emission Lines
OIII Lyα OIII
The Balmer Series OII 1215.67 Å OII and Oxygen lines Hα Hα dominate the optical spectrum of a star Hβ Hβ Hγ Hγ forming galaxy Hδ Lyβ Hδ
•The Lyman series emerges in the ultraviolet. •The Lyman series emerges in the ultraviolet. •The Lyman-α emission line can emit up to 1% of the galaxy’s •The Lyman-α emission line can emit up to 1% of the galaxy’s bolometric flux, but …. bolometric flux, but …. 19 20 Current Topics: Lyman Break Galaxies - Elizabeth Stanway Current Topics: Lyman Break Galaxies - Elizabeth Stanway
5 The Asymmetric Lyman-α Line The Asymmetric Lyman-α Line
Red wing is Star formation drives Low z The Blue Wing is scattered by broadened by galaxy-scale winds Lyman- α outflowing back-scattered (Adelberger et al 2003) line is galactic winds light intrinsically Lyman-α is resonantly symmetric scattered by the winds
Wind Higher z At high-z the v = v = 0 line always +300 km/s appears v = asymmetric -300 km/s and broadened 21 22 Current Topics: Lyman Break Galaxies - Elizabeth Stanway Current Topics: Lyman Break Galaxies - Elizabeth Stanway
The Asymmetric Lyman-α Line The Lyman-α Forest
Blue Wing is Red wing is Δv/c = Δz/(1+z) … Lyman-a is also seen in absorption wherever there are clouds of hydrogen scattered by broadened by => 300km/s wind outflowing back-scattered broadens line by about galactic winds light 5Å FWHM at z=3 Observer Source z=0 z*
Wind Lyα
v = v = 0 +300 km/s
v = -300 km/s 1216Å∗
(1+z*)
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6 The Lyman-α Forest The Lyman-α Forest … Lyman-a is also seen in absorption wherever … Lyman-a is also seen in absorption wherever there are clouds of hydrogen there are clouds of hydrogen
Observer Source Observer Source z=0 z1 z* z=0 z4 z3 z2 z1 z*
Lyα Lyα
1216Å∗ 1216Å∗ 1216Å∗ (1+z1) (1+z3) (1+z1)
1216Å∗ 1216Å∗
(1+z*) 1216Å∗ 1216Å∗ (1+z*) (1+z4) (1+z2)
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The Lyman-α Forest The Lyman-α Forest
At low z almost all of a Above z=3, the fraction of galaxy’s Lyman continuum galaxy flux reaching us flux reaches us declines rapidly
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7 The Lyman-α Forest The Lyman-α Forest
Low z
Lyman-α Forest Beyond z=5.5, <1% of the Higher z galaxy’s flux gets through the IGM
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Properties of High-z Galaxies Properties of High-z Galaxies
• Young galaxies at high-z are: • Young galaxies at high-z are: – Dominated by O and B stars
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8 Properties of High-z Galaxies Properties of High-z Galaxies
• Young galaxies at high-z are: • Young galaxies at high-z are: – Dominated by O and B stars – Dominated by O and B stars – Bright in the ultraviolet – Bright in the ultraviolet – Drive strong galactic winds
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Properties of High-z Galaxies Properties of High-z Galaxies
• Young galaxies at high-z are: • Young galaxies at high-z are: – Dominated by O and B stars – Dominated by O and B stars – Bright in the ultraviolet – Bright in the ultraviolet – Drive strong galactic winds – Drive strong galactic winds • They have key observable characteristics: • They have key observable characteristics: – They have asymmetric Lyman-α emission lines
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9 Properties of High-z Galaxies Methods of Identifying High z Galaxies
• Young galaxies at high-z are: Lyman Break Narrow Band Gravitational Lensing – Dominated by O and B stars Surveys Surveys Surveys – Bright in the ultraviolet – Drive strong galactic winds • Identifies sources • Identifies strongly • Identifies sources • They have key observable characteristics: with high equivalent lensed sources with bright UV widths in certain • Often combined – They have asymmetric Lyman- emission continuum emission. α emission lines. with other two • Broad redshift lines • Narrow redshift methods. range (typically range (typically • Redshift range Δz~0.3-0.5). – Flux is suppressed shortward of Lyman-α Δz~0.1). variable.
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The Lyman Break Technique The Lyman Break Technique
The Steidel, Pettini & Hamilton (1995) Lyman Break Method Red
• At z=3, about 50% of the Lyman continuum is transmitted • This leads to a ‘break’ in the spectrum
• So consider what Lyman Ionising would happen if you Continuum UV Continuum place filters either side Radiation If the filters bracket the of the Lyman- and α breaks, then the galaxies Lyman limit breaks… Blue Red show extreme colours 912Å Lyman-α Break Break 39 40 Current Topics: Lyman Break Galaxies - Elizabeth Stanway Current Topics: Lyman Break Galaxies - Elizabeth Stanway
10 The Dropout Technique Narrow Band Surveys
Sky ● At z>4, the Lyman forest Emission • A magnitude is absorption reaches near Narrow the average flux in Band 100% ⇒ only one break is Starburst at z=6 a filter f ∝λ−2.0 Broad detected λ • If half the filter is Band suppressed by Ly- a forest, the ● A source will be detected in galaxy appears filters above the break but faint ‘drop-out’ of filters below it
● V-drops ⇒ z > 4.5
● R-drops ⇒ z > 5. • If an emission line fills the filter, the galaxy will seem bright For galaxies at 5.6
Narrow Band Surveys Lecture Summary (I)
• But what line • Building a sample of high z galaxies gives vital information on the state of the early universe have you detected? • It requires the right balance between depth and area - • Could be: because the LF is steep, depth is usually preferred – OIII at 5007A – OII at 3727A • Starburst galaxies are UV-bright, dominated by hot, young massive stars – Lyman-α at 1216A • They have a rich spectrum of emission lines, dominated • Need by: spectroscopic – oxygen and Balmer series lines in the optical follow-up – Lyman series lines in the ultraviolet
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11 Lecture Summary (II) Lecture Summary (III)
• Lyman-α is characteristically asymmetric due to galaxy-scale outflows • Galaxies at high-z are selected by: • Absorption by the intervening IGM suppresses flux – Narrow band surveys shortwards of Lyman-α • Selecting for presence of strong emission lines • Uses improved background between skylines • The degree of suppression increases with redshift • Prone to contamination – A few percent at z=1 – 50% at z=3 – Lyman break galaxy surveys – More than 99% by z=5.5 • Selecting on the presence of a 912A or 1216A break • This leads to a characteristic spectral break • Based on broad-band photometry
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