Current Topics: Lyman Break Galaxies - Lecture 3 Current Topics Lyman Break Galaxies Dr Elizabeth Stanway ([email protected])

Current Topics: Lyman Break Galaxies - Lecture 3

Current Topics

Lyman Break Galaxies

Dr Elizabeth Stanway([email protected])


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

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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

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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




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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.


Topic Summary

• Star Forming Galaxies and the Lyman- Line• Lyman Break Galaxies at z<4• Lyman Break Galaxies at z>4

– Extending the method to higher redshift– Properties of LBGs at high z– Shedding light on the high z universe

• Lyman Breaks at z>7, SFH and Reionisation


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…


Extending the LBG method to higher redshifts

• At z=3-4, the Lyman break is bracketed by UGR filters

• At z=5, the Lyman break falls just short of the I band

• At z=6, it is about to enter the ZAB band

R I ZAB


RIZ selection at z=5 and z=6


RIZ selection at z=5 and z=6

BUT at these wavelengths, filters overlap and are far from standardised.


Filters

V-drop filters

R-drop filters


Redshift selection as a function of filter

Low z galaxyHigh z

galaxy


Redshift selection as a function of filter

• Number density and redshift distribution depend on filters used=> Results from surveys are not directly comparable

z~5 V- and R-drops

z~6 I-drops


Contamination• As well as problems from

intermediate z galaxies, also have problems with cool stars

• M, L and T-class stars are very red in the same bands as z=5 and z=6 LBGs

• Can identify stars with HST data (morphology), or very deep infrared data (colour)

• Problem if the survey is ground based or objects are faint.


• The gradual change in colour with redshift is due to movement of the Lyman- break through the filter

• Typical spectrum flat in f => f-2 (c=)

• When the Lyman- break is halfway through the filter, the average flux in the filter is a factor of 2 lower than in a filter longwards of the break.

=> The object will appear 0.7 mags fainter in that filter

The effect of Ly line emission

Spectrum flat in f

99% at z>5.5


• The presence of a line affects the measured magnitude.

• If W0=20Å, then Wobs=132Å at z=5.6

• If the filter is 1000Å wide, then the line contributes ~10% of the flux

• If half the filter is damped by Ly- forest, the line contributes ~20% of the flux

• The exact contribution depends on the transmission of the Ly forest, width of filter and strength of line


1215.67Å * (1+z)


• Say emission line has flux=2x10-17 ergs/s/cm2

• Line has W0=20Å• Line is at z=5.6• Filter is 2000Å wide, centred on

line emission• What is the line contribution and

apparent broadband magnitude?

Ly emission: Worked Example1215.67Å * (1+z)

• W0=Wobs/(1+z) => Wobs = 20*6.6 = 132Å

• Filter is 2000Å wide, but at z>5, the effective Lyman break is 100%, I.e. only 1000Å is measuring flux.

Have 1000Å of continuum flux and line flux equivalent to 132Å. Line contibution is 132/(1000+132) = 12%

The galaxy will appear 12% brighter and is more likely to be detected


• Say emission line has flux=2x10-17 ergs/s/cm2

• Line has W0=20Å• Line is at z=5.6• Filter is 2000Å wide , centred on line

emission• What is the line contribution and

apparent broadband magnitude?

Ly emission: Worked Example1215.67Å * (1+z)

• Continuum flux density = line flux / Wobs= 1.5x10-19 ergs/s/cm2/Å

• This is per unit wavelength (i.e. f). AB magnitudes are defined in f.

f = f d/d, c=, d=1./2 d => f= 2/c f

f = ((8000x8000) / 3x1018) * f = 3.2x10-30 ergs/s/cm2/Hz

AB mag = -2.5 log(f) - 48.6 = 25.1

But galaxy will appear -2.5 log (2) = 0.7 mag fainter in this filter



• If line emission is in the R band (4<z<5.1), R-I is decreased.

• If it is in I (5.1<z<6.1), both R-I and I-Z are affected.

• But if colour selection criteria are relaxed, get more contamination

=> Difficult to be both complete and uncontaminated


Narrow Band Surveys

• A magnitude is the average flux in a filter

• If half the filter is suppressed by Ly-a forest, the galaxy appears faint

• If an emission line fills the filter, the galaxy will seem bright

• By comparing flux in a narrow band with flux in a broadband, you can detect objects with strong line emission

Broad Band

Narrow Band

Sky Emission


Narrow Band Surveys

• But what line have you detected?

• Could be:– OIII at 5007A– OII at 3727A– Lyman- at 1216A

• Need spectroscopic follow-up


Ground vs Space-Based Surveys

• HST can reach objects 0.7-1mag (2-3 times) fainter in the same

length of time

• Ground-based 8m telescopes have larger fields of view (by a

factor of about 4)

• So which is more efficient at finding high-z galaxies?

• The faint end of the Schecter Luminosity function (L<<L*) can be

approximated as power law (i.e. N(L) LdA dz)

• So N8m/NHST=(L8m/LHST) (A8m/AHST)

If is steeper than about -1.2, then HST always wins (I.e depth is

more useful than area)

HST has higher resolution, but 8m telescopes are ‘cheaper’


Surveys of z>4 LBGsGOODS(The Great Observatories Origins Deep Survey)

Hubble Space Telescope

V-drops I-drops

Z-drops

SDF/SXDF Subaru 8m telescope V-drops R-drops

I-drops

BDF/ERGS ESO Very Large Telescopes (8m)

R-drops I-drops

Z-drops

Cluster Lensing Surveys

Keck / HST I-drops Z-drops

J-drops

UKIDSS UK Infrared Telescope (4m)

I-drops Z-drops

Y-drops J-drops


Stellar populations

• As at z=3, most information is derived from SED fitting.

• Unconfused Spitzer data is essential for this at z>4

• Detailed results are model dependent

• General results are model independent

Verma et al, 2007


Old Stars at z=6

• Sometimes both a new starburst and an old population are needed to fit a galaxy• As at z=3, some stars seem as old as the universe, but time scales are shorter,

so the constraints are tighter

SFRe-t/

Eyl

es

et a

l, 2

005


Old Stars at z~6

• Sometimes both a new starburst and an old population are needed to fit a galaxy• As at z=3, some stars seem as old as the universe, but time scales are shorter,

so the constraints are tighter

z=5.83Too Young for Ly line

Older than universe


Comparisons with z=3

• Using a z~5 HST v-drop sample

• GOODS field => extremely deep

• Using an SMC (i.e. low metallicity) extinction law

• Using Spitzer data



Age:At z=3,age~300Myr

At z=5,age~30Myr

If Z=Z,then age~3Myr

Galaxies are younger

(Verma et al 2007)

Log (Age)

frac

tion



Stellar Mass:At z=3,

mass~1010M

At z=5,Mass ~

2x109M

Independent of metallicity

Galaxies are smaller

(Verma et al 2007)

Log (Mass)

frac

tion



Star Formation Rate:

At z=3,

SFR~50M/yr

At z=5,

SFR ~ 50M/yr

If Z=Z,

SFR~600M/yr

=> Galaxies are forming stars at about the same rate

Log (SFR)

frac

tion



Dust:

At z=3,Av~0.6 mags

At z=5,Av ~ 0.3 mags

If Z=Z,Av~0.6 mags

=> High z galaxies are less dusty

Av

frac

tion


Sizes and Morphologies

• Galaxies at high-z have a smaller projected size.

• Most of this is due to evolution in physical size rather than angular scale factor

• Up to z~5, the size evolution is as expected for a fixed mass

• Morphologies are often irregular and complex

Fer

guso

n et

al 2

004


Sizes and Morphologies

• Galaxies at high-z have a smaller projected size.

• Most of this is due to evolution in physical size rather than angular scale factor

• Up to z~5, the size evolution is as expected for a fixed mass

• Morphologies are often irregular and complex


Spectroscopy at z~5

Spectroscopy at z~5 is challenging, but not impossible

In 5 hours on an 8m telescope get good S/N on lines and reasonable detections of continuum flux

The night sky is growing brighter but is still reasonable


Spectroscopy at z~6

Spectroscopy at z~6 is extremely difficult

Sources are typically 1 mag fainter at z=6 than at z=5

Continuum is only detected in exceptional or lensed galaxies

35 hours with Gemini6 hours with Keck


The Rest-Ultraviolet

• Rest-UV slope is an age indicator: – young=blue, old=red

• But many z~5 galaxies seem too blue

Line emitters

No Ly lines

Too Blue


The Rest-Ultraviolet

• Steep Rest-UV slope (blue of f-2) could indicate zero age, Pop III, top-heavy initial mass function …

=> New physics! Interpretation still unclear

Line emitters

No Ly lines

Too Blue


Lyman- Equivalent Widths

i’-drops (DEIMOS)

• At z~5 the distribution of Lyman-a line strengths is similar to that at z~3

• At z~6 see more high EW lines - selection function? More hot stars? Dust effects? New physics?

50% of z>5 sources have EW>0Å

25% have EW>30Å

z~6 z~5


Other spectral lines and outflows

• Stacking together ~50 z~5 galaxies, can start to see other lines:

• CIV, SiIV and OI are starting to be visible

• Velocity offsets => similar winds to z~3

• Work still in progress! SIVOI


Other spectral lines and outflows

• In a few lensed cases, can identify lines in individual spectra

• This example is 6x the typical z~5 LBG brightness

• It is also lensed!• Strong interstellar lines• No Ly => older than

typical, more dusty or more evolved

• Psychotic cases like this can’t really describe the whole population

Dow

-Hyg

elun

d et

al,

2005


Lecture Summary• LBGs at z>4 are significantly harder to find than

those at z<4• LBGs at z~6 are a lot harder than z~5• The sample looked at varies with survey filters

and characteristics• Lyman- emission can affect measured

magnitudes and galaxy selection• With increasing redshift see:

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


Lecture Summary• Spectroscopy is beginning to probe absorption lines,

finding: – similar velocity outflows to z~3– similar Lyline distribution at z~5– stronger Lya lines at z~6

• 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

• But, as at z=3, LBGs are not the whole story• Knowledge of star formation properties is essential for

understanding galaxy evolution

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Current Topics: Lyman Break Galaxies - Lecture 3 Current Topics Lyman Break Galaxies Dr Elizabeth Stanway ([email protected])