Current Topics

Current Topics: Lyman Break Galaxies - Lecture 4

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

• Lyman Break Galaxies at z>7

• Reionisation, SFH and Luminosity Functions


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


Comparisons with z=3Age: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


Comparisons with z=3Stellar 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


Comparisons with z=3Star 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


Comparisons with z=3

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


Non-LBGs at z=5-6

• As at z=3, LBGs don’t show the whole picture at z=5

• Some star forming sources are going to be too faint to be detected as LBGs– Narrowband detected galaxies (LAEs)– Lensed galaxies– GRB Host galaxies

• Some galaxies won’t be star forming– Sub-mm galaxies– DLAs– Molecular Line Emitter galaxies


The Whole Picture at z=5?• How many galaxies at these

redshifts are UV-dark?

• Searching z=5 LBG clusters for UV-dark material might be the way forward

• Initial results are promising - z=5 CO emission detected near z=5 LBGs (Stanway et al, 2008)

• If typical, similar galaxies could contribute a significant fraction of the total galaxy mass in high-z clusters and a large amount of obscured star formation.


Future Millimeter Observations

• The Atacama Large Millimeter Array (ALMA) begins commissioning this year

• It will be fully online by about 2013

• It observes at mm and sub-mm wavelengths

• 80 telescopes at 5000m• Will be sensitive to dust

emission, CO and other strong emission lines (e.g. [CII]) to very high z

QuickTime™ and a decompressor

are needed to see this picture.




Gamma-Ray Bursts• Some star formation will be going on in

galaxies too faint to detect as LBGs• Where massive stars are forming, some

small number can go supernova• In certain circumstances, supernovae

are associated with extraordinarily luminous, highly beamed flashes of gamma rays

• These are known as Gamma Ray Bursts (GRBs) and can be used as tracers of low mass star formation

• At high redshifts, a GRB will show up as a dropout (i.e. selected like an LBG), but will fade rapidly with time

• The most distant objects known in the Universe are GRBs (z=8.3)




Lensing as a tool at high redshift

• In rare cases, can use intervening galaxy clusters as gravitational lenses - gives spatial information, boosted signal-to-noise, near-IR spectroscopy

• 2 known strongly lensed LBGs at z~5

• Only provides information on rare sources - not average sources

• Requires lens reconstruction



Swinbank et al (2009)

z=4.9


LBGs at z>6• Beyond z=6, the

Lyman break moves into the infrared

• Resolution and sensitivity are poor

• Need lensing to stand realistic chance of detecting objects from ground

• NO spectroscopically confirmed galaxies beyond z=6.96

z=6.5 candidate


Lensed LBGs at z>7

• z=7.6 candidate galaxy

• z-drop• J-drop• 100 Myr

old• No dust• Lensed

Bra

dle

y e

t al 2

008


HST and WFC3

• In 2009 HST was serviced and a new camera was installed: WFC3

• This gave HST much better resolution, field of view and sensitivity in the near-infrared

• Can now effectively extend the LBG technique to higher redshifts

• Spectroscopic follow-up remains a problem






LBGs at Higher Redshifts

WFC3 on HST can find z-drops (z~7), Y-drops (z~8) and maybe J-drops (z~10) but can’t confirm them


LBGs at Higher Redshifts

Bunker et al (2009), see also Bouwens+ Oesch+ Castellano+ Wilkins+ etc, etc

(About 20 papers in Sep-Dec 2009)

z’-drop candidates at z~7


Size Evolution to z>7

• Galaxies at z=7 continue to get smaller

• This scales as size (1+z)-1.12 ± 0.17, consistent with constant comoving sizes

• Most z=7 candidates very compact

(Oesch et al 2010)


The Rest UV spectral Slope

• AGN have spectra described by a power law,

L i.e L

• In the rest-frame ultraviolet, star forming galaxies also show power-law spectra

• The slope of the power law depends on the temperature of the emitting source

• This power law slope can be measured using broadband photometry

z’Y J H

Magnitude gives the flux in J and H => fJ and fH

Know the central wavelengths of J and H => J and H

LJ/LH = fJ/fH (J

z=7 galaxy


The Rest UV spectral Slope

z’Y J H

z=7 galaxy

AB mag = -2.5 log(f)-48.6 - App. mag, defined in f

J-H = -2.5 log(fJ)-48.6 - (-2.5 log(fH)-48.6) - Colour is (mag)

J-H = -2.5 log (fJ/fH) = -2.5 log ((J - Using spectral index

J-H = -2.5 (--2) log (JSimplifying

J-H = 0.16 magnitudes

LJ/LH = fJ/fH (J

Example: A source has a spectral slope =-2.5 - calculate the J-H colour in AB mags, given central wavelengths of 1.2m and 1.6m for J and H respectively


Rest-UV Spectral Slope

• AGN have ≈-1 at all redshifts

• Zero-age, star forming galaxies with normal stellar populations have ≈-2

• Dust or age will make this slope redder (i.e. shallower)

• Within the LBG population the spectral slope is seen to evolve with z => age evolution? Dust evolution?



Bouwens et al (2010)


Rest-UV slope at z = 7 - 8

• At z~7, candidate galaxies are very blue, particularly faint galaxies

< -3 is very hard to explain with any ‘normal’ (Population II) stellar population

Bouw

ens et al (2010)




Rest-UV slope at z = 7 - 8• Pop III stars are defined as having very low or zero metallicity

• With no metals, they have fewer ways to emit radiation (i.e. cool down)

• They can become hotter, and more massive (supported by radiation pressure)

• Hotter galaxies have bluer spectral slopes

Bouwens et al (2010)

< -3 slopes may indicate that z=7 galaxies have very low metallicity


Cosmic Evolution of Star FormationProperty z=1-3 z=5-6 z>7

Age ~200 Myr ~50 Myr May be younger

Mass few x 1010 M ~109 M No data

Metallicity 0.3-0.5 Z ~0.2 Z May be very low - Pop III

Size (half light radius)

1.5-2 kpc ~1kpc

scales as comoving

~0.5 kpc

M* -21.1 z=5 : -20.7

z=5 : -20.2

-19.9?

Faint end Slope -1.6 may be steeper No data

Dust E(B-V)~0.2 Probably less dusty No data

Star Formation Rate

~30 M/yr ~30 M/yr ~30 M/yr


Ensemble Properties of LBGs• At z=2-4, you can study individual galaxies in detail• At z=5-6, and more so at z>7, this becomes much

harder• Studying an individual galaxy only tells you about its

immediate environment• By looking about the ensemble properties of galaxies

you can study the universe as a whole => observational cosmology

• By using a common selection method (LBGs), you are comparing like-for-like across cosmic time

=> Insights into galaxy formation, the star formation histoy of the Universe and Reionisation


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• With increasing redshift see:

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

• These traits extend to z~7-8• Very blue rest-UV spectra are hinting at changes in the

nature of star formation• But, as at z=3, LBGs are not the whole story

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