current topics lecture2 - university of bristol

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Current Topics: Lyman Break Galaxies - Elizabeth Stanway

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

• You are required to answer at least onequestion on this topic in the exam


Lecture 1 Summary• Starburst galaxies are UV-bright, dominated by hot,

young massive stars• They have a spectrum dominated by Lyman-α in the

ultraviolet• Lyman-α is characteristically asymmetric due to galaxy-

scale outflows• Absorption by the intervening IGM suppresses flux

shortwards of Lyman-α• The degree of suppression increases with redshift• This leads to a characteristic spectral break


The Lyman Break TechniqueRed

RedBlue

If the filters bracket thebreaks, then the galaxiesshow extreme colours(Steidel, Pettini & Hamilton 1995)

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The Steidel et al LBG Sample

• “Searches for galaxies atz>3 have beenspectacularly unsuccessfulup to now”

• “The combined statisticaleffects of…intervening gasare guaranteed to producean effective Lymancontinuum decrement”

• The red U-G and blue G-Rcolours of a galaxy at z=3should readily differentiate itfrom other objects in thefield.”

(Steidel, Pettini & Hamilton 1995)



• Method confirmed spectroscopically using theHale 5m telescope

• They targeted QSO fields in order to studyknown peaks in the matter distribution at highredshift

Lyα

CIVLyβ

z=3.2



• By 2001, over 1000 LBGs at z=3-4 had beenspectroscopically confirmed by the CalTechgroup

• Access to the Keck telescopes was crucial tothis survey (sensitivity, resolution)

• This sample still forms the most completeanalysis of star forming galaxies at thisredshift

• In recent years, the same group has beenextending their survey to z=1-3


LBGs at z<3• By selecting galaxies with less extreme

colours, you can select lower redshift galaxiesat the cost of higher contamination

• Expect higher metallicities/older stellar pops.

LBGs

BX

BM

BM

LBGs

BX

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The Stellar Populations of LBGs• We select for

rest-UV =>age<500Myr

• But is there anolder stellar popin the samegalaxy?

• Needmeasurements atλ>4000Å todetermine.

• At z=3, this is K-band

10 Myr

100 Myr

1 Gyr


The Stellar Populations of LBGs

Age

Dust

Most LBGS at z=3 are afew x 100Myr old


The Stellar Populations of LBGs

Age

Dust

A minority are veryyoung indeed

A few (~12%) arevery old (>1 Gyr)


Stellar Popsat z~2

• At z=2, the 4000Åbreak lies in theJ-band

• It’s easier tomeasure the SEDin the rest optical

• At this redshift theuniverse is mucholder => olderstellar pops?

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Stellar Popsat z~2

• ~25% of galaxiesare older than 1Gyr

• BUT, most are stilla few x 100Myr old

• LBG selection isidentifying thesame, star-formingpopulation at z=2 &z=3

• Some must havebeen forming starsat z>5

404 1278 15 128

321 1015 1015 286

255 255 227 1015

10 719 8 905

10 15 509 2750


Morphology and Size

• Almost all LBGsare unresolvedfrom the ground

• Typical size:~0.3 arcsec~2.5 kpc (comoving)

• LBGs show avariety ofmorphologies inHST data


Morphology and Size

• Some are:– Disk Galaxies


Morphology and Size

• Some are:– Disk Galaxies– Interacting systems

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Morphology and Size

• Some are:– Disk Galaxies– Interacting systems– Compact galaxies


Morphology and Size

• Some are:– Disk Galaxies– Interacting systems– Compact galaxies– Star forming knots

in a larger system


Morphology and Size

• Some are:– Disk Galaxies– Interacting systems– Compact galaxies– Star forming knots

in a larger system• Most Have:

– Irregular ordisruptedmorphologies

=> Triggered StarFormation


Dust in LBGs• UV light is

scattered moreefficiently bydust than opticallight

• The scatteredradiation is re-emitted in the IR

• The exactextinction curveis metallicity andlocal physicsdependent

Lyα

E(B-V)=A(B)-A(V)

=A(4000)-A(4500)

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Dust in LBGs

A typical LBG at z=1-3 has 0.15 magnitudes of dust inE(B-V) => a factor of 5 extinction at 1500Å.

This is determined by a combination of SED fitting andline ratios (e.g. Hα to Lyα, or OII to OIII)


Dust v Age

In general olderLBGs appear tobe less dusty

i.e. they havelower E(B-V)values.

Is this intrinsic ora selectioneffect?


Dust v Age

EXPECTEDPHYSICS:

Older galaxies willhave processedmore gas into stars

More supernovae

More stellar winds

=> More dust!


Dust v Age

SELECTIONEFFECT:

A younger objectwill be more UVluminous => canbe suppressedmore by dustbefore droppingout of selection

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Interstellar and Stellar Lines

• Typical luminosity ofLBGs at z=3 is R=25.5(AB)

• An 8m telescope takes1hr to get to S/N=5 atR=24 in goodconditions

• To get a factor of 5fainter => 25hrs!

=> Look at averageproperties of stacks ofgalaxies


Interstellar and Stellar Lines• Stacking ~1000

galaxies, can seeabsorption and emissionlines from:– Hot stars– Interstellar medium– Outflowing winds

• Can measure thevelocity offsets betweencomponents

• Can measure metallicity• Can measure wind

properties


Winds and Outflows• Lyman-α is

redshifted withrespect to nebularemission lines

• The interstellarmedium is blue-shifted withrespect to nebularemission lines

⇒Lyman-α isheavily absorbed

⇒The galaxy isdriving outflows


Equivalent Widths

• Wobs = Integrated line flux / Continuum flux density• The width of continuum in Angstroms that must be

integrated to equal flux in line

=

Wobs

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Equivalent Widths

• Wobs = Integrated line flux / Continuum flux density• Consider the rest frame

– Integrated flux in line increases by 1/4πr2

– Continuum flux density increases by 1/4πr2 * (1+z)– Rest frame EW: W0 = Wobs / (1+z)

=

Wobs


Winds and Outflows• Lyman-α is

redshifted withrespect to nebularemission lines

• The interstellarmedium is blue-shifted with respectto nebular emissionlines

⇒Lyman-α is heavilyabsorbed

⇒The galaxy is drivingoutflows

Lyαescapesgalaxy

Lyαabsorbedby ISM


Winds andOutflows

• The sources withstrongest Lyman-αemission have theweakest ISM absorption

• By contrast, the stellarSIV feature is insensitiveto Lyman-α strength

=> Decrease in coveringfraction of neutralmaterial with increasingLy-α strength


LBGs andAGN

• LBGs aremassive galaxiesfor their redshift

• Massive galaxiesat low z hostAGN

• Only 4% of LBGsshow evidencefor AGN

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LBGs and AGN

• AGN are quiteeasy to identifyin the rest-UV,even at lowishS/N

• NV at 1240Å• CIV at 1550Å• HeII at 1640Å

NV

CIVHeII

LBG

AGN


MetallicityIndicators

• Metallicity ismeasured fromthe ratio ofemission andabsorption linesin spectra

• Unfortunately,most of the well-calibratedindicators are inthe rest-frameoptical


Metallicity indicators with redshift


Rest-optical spectra

• Rest-optical is observed in the near-IR• This is MUCH harder than optical observing• Only a few bright objects and emission lines

can be observed

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Metallicity

• But rest-opticalemission lines canbe used todeterminemetallicities

• R23=[OII+OIII]/Hβ• [O/H]=8.8• LBGs at z=3

haveZ~0.2-0.8Z


The Star Formation History ofthe Universe

• LBGs are star forminggalaxies

• If there was other starformation at the samez it would be detectedUNLESS it is extincted

• So LBGs can be usedto measure the starformation history of theuniverse modulo dustextinction


The Star Formation History ofthe Universe


But is this a complete picture?

• LBGs are selectedto be rest-UV bright

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

• Do they even mapout all the starformation?

• What about UV-darkor dusty material? (Springel & Hernquist, 2003)

Modelspredict SFRshould peakat z>6

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UV-Dark Material: DLAs

• The spectra of somevery high redshiftgalaxies show dense,massive clouds ofhydrogen along theline of sight

• These ‘DampedLyman-α Absorbers’must be UV-darkgalaxies atintermediate redshifts

Prochaska et al (2001)


UV-Dark Material: SMGs• The UV is heavily

extincted by dust• The light is absorbed

and re-emitted at far-IR and submillimetrewavelengths

• Submillimetre galaxies(SMGs) are hard todetect, and harder stillto find redshifts for

• But many probably lieat z=2-3 and each hasa huge SFR

Smail, Blain, Chapman et al, 2003


Completing the z~3 Picture

• Using molecular line emission at z=3, could probecool gas

• “low-excitation lines will map out a larger fraction ofthe ISM in these galaxies and…study in detail thespacially resolved kinematic structure of most of thegas…which resides in the cold phase” (Carilli & Blain2002)

• CO emitting galaxies may contribute significant massand star formation

• New telescopes such as ALMA, SKA and the EVLAwill be crucial for completing the picture at z=3 andabove.


Lecture Summary• LBGs at z=3 and below are selected in the UGR

colour-colour plane• They are very faint compared to local galaxies =>

difficult to observe• These galaxies have been followed up in great

detail and their properties are now wellunderstood

• These properties include stellar ages,metallicities, outflows, morphology, AGN fraction,star formation history and dust extinction.

• But z=3 LBGs do not present a complete pictureof the universe at this redshift.

current topics lecture2 - university of bristol

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