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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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slide1

Current Topics

Lyman Break Galaxies

Dr Elizabeth Stanway

([email protected])

Current Topics: Lyman Break Galaxies - Lecture 2

slide2

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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