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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 Reionisation and the Star Formation History of the Universe

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Current Topics

Lyman Break Galaxies

Dr Elizabeth Stanway

([email protected])

Current Topics: Lyman Break Galaxies - Lecture 2


Topic Summary

  • Star Forming Galaxies and the Lyman- Line

  • Lyman Break Galaxies at z<4

  • Lyman Break Galaxies at z>4

  • Reionisation and the Star Formation History of the Universe

  • There will be a 1 hr examination on this topic

Current Topics: Lyman Break Galaxies - Lecture 2


Lecture 1 summary
Lecture 1 Summary

  • Starburst galaxies are UV-bright, dominated by hot, young massive stars

  • They have a spectrum dominated by Lyman- in the ultraviolet

  • 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

  • This leads to a characteristic spectral break

Current Topics: Lyman Break Galaxies - Lecture 2


The lyman break technique
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 2


The lyman break technique1
The Lyman Break Technique

Red

If the filters bracket the breaks, then the galaxies show extreme colours

(Steidel, Pettini & Hamilton 1995)

Blue

Red

Current Topics: Lyman Break Galaxies - Lecture 2


The steidel et al lbg sample
The Steidel et al LBG Sample

  • “Searches for galaxies at z>3 have been spectacularly unsuccessful up to now”

  • “The combined statistical effects of…intervening gas are guaranteed to produce an effective Lyman continuum decrement”

  • “The red U-G and blue G-R colours of a galaxy at z=3 should readily differentiate it from other objects in the field.”

    (Steidel, Pettini & Hamilton 1995)

Current Topics: Lyman Break Galaxies - Lecture 2


The steidel et al lbg sample1
The Steidel et al LBG Sample

Ly

z=3.2

  • Method confirmed spectroscopically using the Hale 5m telescope

  • They targeted QSO fields in order to study known peaks in the matter distribution at high redshift

Ly

CIV

Current Topics: Lyman Break Galaxies - Lecture 2


The steidel et al lbg sample2
The Steidel et al LBG Sample

  • By 2001, over 1000 LBGs at z=3-4 had been spectroscopically confirmed by the CalTech group

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

  • This sample still forms the most complete analysis of star forming galaxies at this redshift

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

Current Topics: Lyman Break Galaxies - Lecture 2


Lbgs at z 3
LBGs at z<3

  • By selecting galaxies with less extreme colours, you can select lower redshift galaxies at the cost of higher contamination

  • Expect higher metallicities/older stellar pops.

BX

LBGs

LBGs

BM

BX

BM

Current Topics: Lyman Break Galaxies - Lecture 2


The stellar populations of lbgs
The Stellar Populations of LBGs

  • We select for rest-UV => age<500Myr

  • But is there an older stellar pop in the same galaxy?

  • Need measurements at >4000Å to determine.

  • At z=3, this is K-band

1 Gyr

10 Myr

100 Myr

Current Topics: Lyman Break Galaxies - Lecture 2


The stellar populations of lbgs1
The Stellar Populations of LBGs

Most LBGS at z=3 are a few x 100Myr old

Age

Dust

Current Topics: Lyman Break Galaxies - Lecture 2


The stellar populations of lbgs2
The Stellar Populations of LBGs

A few (~12%) are very old (>1 Gyr)

Age

A minority are very young indeed

Dust

Current Topics: Lyman Break Galaxies - Lecture 2


Stellar pops at z 2
Stellar Pops at z~2

  • At z=2, the 4000Å break lies in the J-band

  • It’s easier to measure the SED in the rest optical

  • At this redshift the universe is much older => older stellar pops?

Current Topics: Lyman Break Galaxies - Lecture 2


Stellar pops at z 21
Stellar Pops at z~2

404

1278

15

128

  • ~25% of galaxies are older than 1Gyr

  • BUT, most are still a few x 100Myr old

  • LBG selection is identifying the same, star-forming population at z=2 & z=3

  • Some must have been forming stars at z>5

321

1015

1015

286

255

255

227

1015

10

719

905

8

10

15

509

2750

Current Topics: Lyman Break Galaxies - Lecture 2


Morphology and size
Morphology and Size

  • Almost all LBGs are unresolved from the ground

  • Typical size:

    ~0.3 arcsec

    ~2.5 kpc (comoving)

  • LBGs show a variety of morphologies in HST data

Current Topics: Lyman Break Galaxies - Lecture 2


Morphology and size1
Morphology and Size

  • Some are:

    • Disk Galaxies

Current Topics: Lyman Break Galaxies - Lecture 2


Morphology and size2
Morphology and Size

  • Some are:

    • Disk Galaxies

    • Interacting systems

Current Topics: Lyman Break Galaxies - Lecture 2


Morphology and size3
Morphology and Size

  • Some are:

    • Disk Galaxies

    • Interacting systems

    • Compact galaxies

Current Topics: Lyman Break Galaxies - Lecture 2


Morphology and size4
Morphology and Size

  • Some are:

    • Disk Galaxies

    • Interacting systems

    • Compact galaxies

    • Star forming knots in a larger system

Current Topics: Lyman Break Galaxies - Lecture 2


Morphology and size5
Morphology and Size

  • Some are:

    • Disk Galaxies

    • Interacting systems

    • Compact galaxies

    • Star forming knots in a larger system

  • Most Have:

    • Irregular or disrupted morphologies

      => Triggered Star Formation

Current Topics: Lyman Break Galaxies - Lecture 2


Velocity maps and morphology
Velocity Maps and Morphology

  • Emission lines occur at known wavelengths

  • Offsets from those wavelengths indicate movement in the emitting source

  • At z=3, sources are spatially resolved - can measure velocity profiles across source

  • Done with ‘Integral Field Spectroscopy’ looking at Hemission

    e.g. Using OSIRIS on Keck (Law et al 2007)

Current Topics: Lyman Break Galaxies - Lecture 2


Dust in lbgs
Dust in LBGs

  • UV light is scattered more efficiently by dust than optical light

  • The scattered radiation is re-emitted in the IR

  • The exact extinction curve is metallicity and local physics dependent

Ly

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

=A(4000)-A(4500)

Current Topics: Lyman Break Galaxies - Lecture 2


Dust in lbgs1
Dust in LBGs

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

This is determined by a combination of SED fitting and line ratios (e.g. H to Ly, or OII to OIII)

Current Topics: Lyman Break Galaxies - Lecture 2


Dust v age
Dust v Age

In general older LBGs appear to be less dusty

i.e. they have lower E(B-V) values.

Is this intrinsic or a selection effect?

Current Topics: Lyman Break Galaxies - Lecture 2


Dust v age1
Dust v Age

EXPECTED PHYSICS:

Older galaxies will have processed more gas into stars

More supernovae

More stellar winds

=> More dust!

Current Topics: Lyman Break Galaxies - Lecture 2


Dust v age2
Dust v Age

SELECTION EFFECT:

A younger object will be more UV luminous => can be suppressed more by dust before dropping out of selection

Current Topics: Lyman Break Galaxies - Lecture 2


Interstellar and stellar lines
Interstellar and Stellar Lines

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

  • An 8m telescope takes 1hr to get to S/N=5 at R=24 in good conditions

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

    => Look at average properties of stacks of galaxies

Current Topics: Lyman Break Galaxies - Lecture 2


Interstellar and stellar lines1
Interstellar and Stellar Lines

  • Stacking ~1000 galaxies, can see absorption and emission lines from:

    • Hot stars

    • Interstellar medium

    • Outflowing winds

  • Can measure the velocity offsets between components

  • Can measure metallicity

  • Can measure wind properties

Current Topics: Lyman Break Galaxies - Lecture 2


Winds and outflows
Winds and Outflows

  • Lyman- is redshifted with respect to nebular emission lines

  • The interstellar medium is blue-shifted with respect to nebular emission lines

  • Lyman- is heavily absorbed

  • The galaxy is driving outflows

Current Topics: Lyman Break Galaxies - Lecture 2


Equivalent widths
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

Current Topics: Lyman Break Galaxies - Lecture 2


Equivalent widths1
Equivalent Widths

  • Wobs = Integrated line flux / Continuum flux density

  • Consider the rest frame

    • Integrated flux in line increases by 1/4r2

    • Continuum flux density increases by 1/4r2 * (1+z)

    • Rest frame EW: W0 = Wobs / (1+z)

=

Wobs

Current Topics: Lyman Break Galaxies - Lecture 2


Winds and outflows1
Winds and Outflows

  • Lyman- is redshifted with respect to nebular emission lines

  • The interstellar medium is blue-shifted with respect to nebular emission lines

  • Lyman- is heavily absorbed

  • The galaxy is driving outflows

Ly escapes galaxy

Ly absorbed by ISM

Current Topics: Lyman Break Galaxies - Lecture 2


Winds and outflows2
Winds and Outflows

  • The sources with strongest Lyman- emission have the weakest ISM absorption

  • By contrast, the stellar SIV feature is insensitive to Lyman- strength

    => Decrease in covering fraction of neutral material with increasing Ly- strength

Current Topics: Lyman Break Galaxies - Lecture 2


Lbgs and agn
LBGs and AGN

  • LBGs are massive galaxies for their redshift

  • Massive galaxies at low z host AGN

  • Only 4% of LBGs show evidence for AGN

Current Topics: Lyman Break Galaxies - Lecture 2


Lbgs and agn1
LBGs and AGN

AGN

  • AGN are quite easy to identify in the rest-UV, even at lowish S/N

  • NV at 1240Å

  • CIV at 1550Å

  • HeII at 1640Å

HeII

CIV

NV

LBG

Current Topics: Lyman Break Galaxies - Lecture 2


Metallicity indicators
Metallicity Indicators

  • Metallicity is measured from the ratio of emission and absorption lines in spectra

  • Unfortunately, most of the well-calibrated indicators are in the rest-frame optical

Current Topics: Lyman Break Galaxies - Lecture 2


Metallicity indicators with redshift
Metallicity indicators with redshift

Current Topics: Lyman Break Galaxies - Lecture 2


Rest frame optical spectra
Rest-Frame Optical Spectra

  • At z=3, the rest-frame optical falls in the observed near-infrared (>1m)

  • Spectroscopy is harder and only a few sources can be observed

  • The H[OII] and [OIII] emission lines can give Star Formation Rate indicators independent of dust

  • Their ratio can also indicate AGN/QSO activity

Rest-optical spectra (Law et al 2007)

Current Topics: Lyman Break Galaxies - Lecture 2


Rest frame optical spectra1
Rest-Frame Optical Spectra

Rest-UV spectra

Rest-optical spectra (Law et al 2007)

Current Topics: Lyman Break Galaxies - Lecture 2


Metallicity
Metallicity

  • But rest-optical emission lines can be used to determine metallicities

  • R23=[OII+OIII]/H

  • [O/H]=8.8

  • LBGs at z=3 have

    Z~0.2-0.8Z

Current Topics: Lyman Break Galaxies - Lecture 2


Other galaxies at z 3
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 2


Uv dark material dlas
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 2


Submillimeter galaxies smgs
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 2


Submillimeter galaxies smgs1
Submillimeter Galaxies (SMGs)

z=1

  • 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=10

Current Topics: Lyman Break Galaxies - Lecture 2


Submillimeter galaxies smgs2
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 2


Completing the z 3 picture
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 2


Lecture summary
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 well understood

  • 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 picture of the universe at this redshift.

Current Topics: Lyman Break Galaxies - Lecture 2


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