Current Topics: Lyman Break Galaxies - Lecture 2 Current Topics Lyman Break Galaxies Dr Elizabeth Stanway ([email protected])

Current Topics: Lyman Break Galaxies - Lecture 2

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

• There will be a 1 hr examination on this topic


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


The Lyman Break TechniqueThe Steidel, Pettini & Hamilton (1995) Lyman Break Method

Ionising

RadiationUV Continuum

Lyman

Continuum

912ÅBreak

Lyman-αBreak

• 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…


The Lyman Break TechniqueRed

RedBlue

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

(Steidel, Pettini & Hamilton 1995)


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)



• 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

CIVLy

z=3.2



• 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


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.

LBGs

BX

BM

BM

LBGs

BX


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

10 Myr

100 Myr

1 Gyr


The Stellar Populations of LBGs

Age

Dust

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


The Stellar Populations of LBGs

Age

Dust

A minority are very young indeed

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


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?


Stellar Pops at z~2

• ~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

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 LBGs are unresolved from the ground

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

• LBGs show a variety of morphologies in HST data


Morphology and Size

• Some are:– Disk Galaxies


Morphology and Size

• Some are:– Disk Galaxies– Interacting systems


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 or disrupted

morphologies

=> Triggered Star Formation


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 Hemission

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

QuickTime™ and a decompressor

are needed to see this picture.


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)


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)


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?


Dust v Age

EXPECTED PHYSICS:

Older galaxies will have processed more gas into stars

More supernovae

More stellar winds

=> More dust!


Dust v Age

SELECTION EFFECT:

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


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


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


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


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


Equivalent Widths

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

– Integrated flux in line increases by 1/4r2

– Continuum flux density increases by 1/4r2 * (1+z)– Rest frame EW: W0 = Wobs / (1+z)

=

Wobs


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


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


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


LBGs and 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

NV

CIV

HeII

LBG

AGN


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


Metallicity indicators with redshift


Rest-Frame Optical Spectra

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

• 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)


Rest-Frame Optical Spectra





Rest-optical spectra (Law et al 2007)Rest-UV spectra


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


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


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

QuickTime™ and aTIFF (Uncompressed) decompressor


Prochaska et al (2001)


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 >100m

• These frequencies are difficult to observe due to atmospheric effects




Submillimeter Galaxies (SMGs)

• 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 850m atmospheric window

z=1

z=10


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




are needed to see this picture.QuickTime™ and a

TIFF (Uncompressed) decompressorare needed to see this 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.


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.

Documents

Current Topics: Lyman Break Galaxies - Lecture 2 Current Topics Lyman Break Galaxies Dr Elizabeth Stanway ([email protected])