current topics

Current Topics: Lyman Break Galaxies - Lecture 5

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

• Lyman Break Galaxies at z>4

• Lyman Break Galaxies at z>7

• Reionisation, SFH and Luminosity Functions


LBGs at z>7

Bunker et al (2009), see also Bouwens+ Oesch+ Castellano+ Wilkins+ etc, etc

(About 20 papers in Sep-Dec 2009)

z’-drop candidates at z~7


Size Evolution to z>7

• Galaxies at z=7 continue to get smaller

• This scales as size (1+z)-1.12 ± 0.17, consistent with constant comoving sizes

• Most z=7 candidates very compact

(Oesch et al 2010)


The Rest UV spectral Slope

• AGN have spectra described by a power law,

L i.e L

• In the rest-frame ultraviolet, star forming galaxies also show power-law spectra

• The slope of the power law depends on the temperature of the emitting source

• This power law slope can be measured using broadband photometry

z’Y J H

Magnitude gives the flux in J and H => fJ and fH

Know the central wavelengths of J and H => J and H

LJ/LH = fJ/fH (J

z=7 galaxy


Rest-UV Spectral Slope

• AGN have ≈-1 at all redshifts

• Zero-age, star forming galaxies with normal stellar populations have ≈-2

• Dust or age will make this slope redder (i.e. shallower)

• Within the LBG population the spectral slope is seen to evolve with z => age evolution? Dust evolution?

QuickTime™ and a decompressor

are needed to see this picture.

Bouwens et al (2010)


Rest-UV slope at z = 7 - 8

• At z~7, candidate galaxies are very blue, particularly faint galaxies

< -3 is very hard to explain with any ‘normal’ (Population II) stellar population

Bouw

ens et al (2010)




Rest-UV slope at z = 7 - 8• Pop III stars are defined as having very low or zero metallicity

• With no metals, they have fewer ways to emit radiation (i.e. cool down)

• They can become hotter, and more massive (supported by radiation pressure)

• Hotter galaxies have bluer spectral slopes

Bouwens et al (2010)

< -3 slopes may indicate that z=7 galaxies have very low metallicity


Ensemble Properties of LBGs• At z=2-4, you can study individual galaxies in detail• At z=5-6, and more so at z>7, this becomes much

harder• Studying an individual galaxy only tells you about its

immediate environment• By looking about the ensemble properties of galaxies

you can study the universe as a whole => observational cosmology

• By using a common selection method (LBGs), you are comparing like-for-like across cosmic time

=> Insights into galaxy formation, the star formation histoy of the Universe and Reionisation


Luminosity Functions

z~4z~5

z~6

• The number counts of galaxies changes as a function of luminosity

• This is described by a Schecter function

N(L) dA (L/L*)e-(L/L*) dA

• At low-z this parameterises the galaxy mass distribution

• The function has three important parameters:

– Characteristic luminosity, L* or M* (~26.5 at z=6)

– Faint end slope, – Normalisation, *

(Bouwens et al, 2007)




• Number counts are affected by incompleteness and contamination• There is degeneracy in the parameter fitting=> The exact values at high z are still uncertain, but…

– The typical magnitude of the population is decreasing at high z => younger, smaller galaxies

– The faint end slope appears steeper at high z => more faint galaxies compared to bright galaxies

– At any given luminosity there are fewer z~6 galaxies than z~3 galaxies(Bouwens et al, 2007)

Luminosity Functions

z~4

z~5z~6

z~3


LF Results at z >7

•At z>7 there are fewer galaxies and we don’t probe the faint end slope

•The Luminosity function is continuing to evolve - there are fewer Lyman Break galaxies as you move to higher redshifts, but the fraction that are faint increases Bouwens et al (2010)


Luminosity Function Implications

• At earlier times, star formation in the Universe is increasingly dominated by small, hard to detect galaxies

• The fraction missed by a magnitude limited survey is increasing

• The more massive galaxies we see at z=3 are increasingly rare at higher z - star formation is occuring in less massive, less mature regions (i.e. lower metallicity? less dusty?)

• A Schecter function still describes the distribution reasonably well out to z=6 - star formation may still be tracing the mass distribution despite the short-lived starbursts

• Models for hierarchical merging suggest that the typical luminosity is evolving to follow the typical galaxy mass at a given redshift


Cosmic Evolution of Star FormationProperty z=1-3 z=5-6 z>7

Age ~200 Myr ~50 Myr May be younger

Mass few x 1010 M ~109 M No data

Metallicity 0.3-0.5 Z ~0.2 Z May be very low - Pop III

Size (half light radius)

1.5-2 kpc ~1kpc

scales as comoving

~0.5 kpc

M* -21.1 z=5 : -20.7

z=5 : -20.2

-19.9?

Faint end Slope -1.6 may be steeper No data

Dust E(B-V)~0.2 Probably less dusty No data

Star Formation Rate

~30 M/yr ~30 M/yr ~30 M/yr


The Star Formation History of the Universe

• LBGs are star forming galaxies

• If there was other star formation at the same z it would be detected UNLESS it is extincted

• So LBGs can be used to measure the star formation history of the universe modulo dust extinction



• This was first done using LBGs in the mid-1990s using Lyman Break Galaxies at z=3-4 by Piero Madau

• As a result, the Star Formation History of the Universe is usually shown on a diagram known as the ‘Madau Plot’

• Early work showed that star formation peaked around z=1, but it was unclear what happened at higher redshifts

(Steidel et al 1999)


The SFH out to z=6

GOODS extended this work to z=6

(for bright galaxies)

The Star Formation Rate Density out to z=6 shows steep evolution, particularly when only bright galaxies are considered


Uncertainties in the Madau PlotTo get to star formation rate density you need:

• Number of objects per unit volume

• Star formation rate per object

You have:

• Number of galaxies (Complete sample? Contamination?)

• Rest-UV flux (after dust extinction)

• Redshift selection function (Survey and model dependent)

Uncertainties:

• How much UV flux has been absorbed by dust?

• How much is emitted by galaxies below your selection limit?

• How do star formation rate and rest-UV flux relate?



(Bouwens et al, 2007)

The LF has a steep faint end slope at high-z

The fainter you integrate down the Luminosity Function, the more flux you’ll see

Even to faint magnitudes the SFRD is still dropping at high redshift



Significant uncertainty in star formation density

General consensus:

The SFRD is either steady beyond z=2 or declining slowly

It declines rapidly beyond z=6

Metallicity, age and duty cycle are all important parameters Verma et al, 2007


But is this a complete picture?

• Until recently most models predicted SF peaking much earlier

• LBGs are selected to be rest-UV bright

• How complete is the picture of the z=3 universe they paint?

• Do they even map out all the star formation?

• What about UV-dark or dusty material?

(Springel & Hernquist, 2003)

Many Models predict SFR should peak at z>6


Sub-mm and Dust-obscured Galaxies• Rest-UV flux is reprocessed and reemitted in the far-infrared by

dust• At z~2, 25% of the far-infrared luminosity in the universe is seen in

IR-detected DOGs• Luminous sub-mm galaxies are rare but can have SFRs of 100s of

M/yr



• Numbers of SMGs are known to peak at z=2-3 (the epoch of galaxy mergers)

• At these redshifts, dust obscured galaxies might contribute 50% or more of the cosmic SFRD


Including Obscured Galaxies in the Madau Plot

• Newer SFH models include feedback from QSOs and gas

• Models tend not to consider dust obscuration

• Predict SFH peaking at z=3-4

• Possible that SMGs could contribute to this picture, particularly at z=1-3

Nagamine et al (2009)


Reionisation

• Lyman break galaxies are star-forming so directly measure the star formation properties of the universe

• At z=7 they are starting to probe a transition known as ‘reionisation’ when the galaxy went from largely neutral to largely ionised


Reionisation and the End of the Dark Ages

• After the Big Bang, the universe cooled and recombined – leaving a neutral universe. At this time, all rest-frame UV light is absorbed by neutral hydrogen the Cosmic Dark Ages

• The IGM in the local universe is highly ionised

• So when and how did the universe reionise? What ended the Dark Ages?

• The WMAP satellite studied the optical depth to the CMB data which suggests that zreion~9-12

•The failure of the LAE Luminosity Function to evolve implies zreion>6

• However, the observation of a Gunn-Peterson trough in the spectra of SDSS z=6 QSOS implies zreion~6.2 (or at least a large neutral fraction)

• Could z~6 starbursts contribute to reionisation?


Reionisation - Evidence from WMAP



• The CMB has been streaming through the Universe since Recombination

• The mean free path of CMB photons will depend on the distance the radiation travels through a neutral vs ionised medium

• WMAP has measured the CMB power spectrum, constraining cosmological properties

• One of these, , is the optical depth of CMB photons to reionisation

• After five years of data, the best fitting value suggests zreion=10.8 ± 1.4


Reionisation - Evidence from SDSS

• Damping of the spectrum due to Lyman-alpha forest lines rapidly increases with increasing redshift

• This can be seen in the spectra of distant QSOs seen in the SDSS

• Beyond z=6.4 large regions of the spectrum are seen with zero flux

• These are known as ‘Gunn-Peterson troughs’ and indicate that the universe is at least partly neutral beyond z=6




Reionisation - Evidence from LAEs

• ‘Gunn-Peterson’ absorption (i.e. due to neutral gas) has broad damping wings

• Therefore if there’s neutral gas surrounding a Lyman-alpha emitter, the line can be suppressed, even though it’s longwards of 1216*(1+z) A

• In a neutral universe you expect to see a smaller number of Lyman-alpha emitters detected


Reionisation - Evidence from LAEs

• ‘Gunn-Peterson’ absorption (i.e. due to neutral gas) has broad damping wings

• Therefore if there’s neutral gas surrounding a Lyman-alpha emitter, the line can be suppressed, even though it’s longwards of 1216*(1+z) A

• In a neutral universe you expect to see a smaller number of Lyman-alpha emitters detected

• There is some evidence for this at z=6.6 Lya Luminosity Function

(Lya LF)

Ouchi+ in prep

~30%

Pure lum. evolution

Pure num. evolution


Reionisation - Evidence from LBGs

• The neutral IGM is ionised by UV-flux

• The dominant source of UV-flux in the universe is star formation

• The effect of UV-flux on the universe depends on – Clumpy IGM– Escape of UV-photons– Temperature of IGM– Cosmology

• Can determine a critical Star Formation Density that will ionise the Universe given some values for these parameters

• By integrating the LBG LF we measure the total UV flux and can compare it with this critical values


Reionisation - Evidence from LBGs

• Can calculate the ionised fraction of the universe due to contribution of LBG galaxies given certain assumptions

• For reasonable assumptions about a warm IGM, it is possible to fit the data (within errors) and ionise the universe with z=7 LBGs

• BUT LBGs are scarcer at higher redshifts - is this a problem at z~10?



Oesch et al 2010

Current best fit

=0.087+-0.017


Lecture Summary• With increasing redshift see:

– Decreasing metallicity– Decreasing dust extinction– Decreasing age– Decreasing mass

• 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

• This could be critical for understanding the star formation history of the Universe and Reionisation

current topics

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high z

fewer z

detailat z

daat lowz

faint galaxies

fewer galaxies

individual galaxies

galaxies changes