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Current Topics: Lyman Break Galaxies - Lecture 2 Current Topics Lyman Break Galaxies Dr Elizabeth Stanway

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Presentation on theme: "Current Topics: Lyman Break Galaxies - Lecture 2 Current Topics Lyman Break Galaxies Dr Elizabeth Stanway"— Presentation transcript:

1 Current Topics: Lyman Break Galaxies - Lecture 2 Current Topics Lyman Break Galaxies Dr Elizabeth Stanway

2 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

3 Current Topics: Lyman Break Galaxies - Lecture 2 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

4 Current Topics: Lyman Break Galaxies - Lecture 2 The Lyman Break Technique The Steidel, Pettini & Hamilton (1995) Lyman Break Method Ionising Radiation UV Continuum Lyman Continuum 912Å Break Lyman-α Break 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…

5 Current Topics: Lyman Break Galaxies - Lecture 2 The Lyman Break Technique Red Blue If the filters bracket the breaks, then the galaxies show extreme colours (Steidel, Pettini & Hamilton 1995)

6 Current Topics: Lyman Break Galaxies - Lecture 2 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)

7 Current Topics: Lyman Break Galaxies - Lecture 2 The Steidel et al LBG Sample 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 Ly z=3.2

8 Current Topics: Lyman Break Galaxies - Lecture 2 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

9 Current Topics: Lyman Break Galaxies - Lecture 2 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 LBGs BX

10 Current Topics: Lyman Break Galaxies - Lecture 2 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

11 Current Topics: Lyman Break Galaxies - Lecture 2 The Stellar Populations of LBGs Age Dust Most LBGS at z=3 are a few x 100Myr old

12 Current Topics: Lyman Break Galaxies - Lecture 2 The Stellar Populations of LBGs Age Dust A minority are very young indeed A few (~12%) are very old (>1 Gyr)

13 Current Topics: Lyman Break Galaxies - Lecture 2 Stellar Pops at z~2 At z=2, the 4000Å break lies in the J-band Its easier to measure the SED in the rest optical At this redshift the universe is much older => older stellar pops?

14 Current Topics: Lyman Break Galaxies - Lecture 2 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>

15 Current Topics: Lyman Break Galaxies - Lecture 2 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

16 Current Topics: Lyman Break Galaxies - Lecture 2 Morphology and Size Some are: –Disk Galaxies

17 Current Topics: Lyman Break Galaxies - Lecture 2 Morphology and Size Some are: –Disk Galaxies –Interacting systems

18 Current Topics: Lyman Break Galaxies - Lecture 2 Morphology and Size Some are: –Disk Galaxies –Interacting systems –Compact galaxies

19 Current Topics: Lyman Break Galaxies - Lecture 2 Morphology and Size Some are: –Disk Galaxies –Interacting systems –Compact galaxies –Star forming knots in a larger system

20 Current Topics: Lyman Break Galaxies - Lecture 2 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

21 Current Topics: Lyman Break Galaxies - Lecture 2 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)

22 Current Topics: Lyman Break Galaxies - Lecture 2 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)

23 Current Topics: Lyman Break Galaxies - Lecture 2 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)

24 Current Topics: Lyman Break Galaxies - Lecture 2 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?

25 Current Topics: Lyman Break Galaxies - Lecture 2 Dust v Age EXPECTED PHYSICS: Older galaxies will have processed more gas into stars More supernovae More stellar winds => More dust!

26 Current Topics: Lyman Break Galaxies - Lecture 2 Dust v Age SELECTION EFFECT: A younger object will be more UV luminous => can be suppressed more by dust before dropping out of selection

27 Current Topics: Lyman Break Galaxies - Lecture 2 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

28 Current Topics: Lyman Break Galaxies - Lecture 2 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

29 Current Topics: Lyman Break Galaxies - Lecture 2 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

30 Current Topics: Lyman Break Galaxies - Lecture 2 Equivalent Widths W obs = Integrated line flux / Continuum flux density The width of continuum in Angstroms that must be integrated to equal flux in line = W obs

31 Current Topics: Lyman Break Galaxies - Lecture 2 Equivalent Widths W obs = Integrated line flux / Continuum flux density Consider the rest frame –Integrated flux in line increases by 1/4 r 2 –Continuum flux density increases by 1/4 r 2 * (1+z) –Rest frame EW: W 0 = W obs / (1+z) = W obs

32 Current Topics: Lyman Break Galaxies - Lecture 2 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

33 Current Topics: Lyman Break Galaxies - Lecture 2 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

34 Current Topics: Lyman Break Galaxies - Lecture 2 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

35 Current Topics: Lyman Break Galaxies - Lecture 2 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

36 Current Topics: Lyman Break Galaxies - Lecture 2 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

37 Current Topics: Lyman Break Galaxies - Lecture 2 Metallicity indicators with redshift

38 Current Topics: Lyman Break Galaxies - Lecture 2 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)

39 Current Topics: Lyman Break Galaxies - Lecture 2 Rest-Frame Optical Spectra Rest-optical spectra (Law et al 2007)Rest-UV spectra

40 Current Topics: Lyman Break Galaxies - Lecture 2 Metallicity But rest-optical emission lines can be used to determine metallicities R 23 =[OII+OIII]/H [O/H] =8.8 LBGs at z=3 have Z~ Z

41 Current Topics: Lyman Break Galaxies - Lecture 2 Other Galaxies at z=3 Lyman Break Galaxies are selected to be UV- bright Strongly star forming Not too much dust extinction They cant 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

42 Current Topics: Lyman Break Galaxies - Lecture 2 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)

43 Current Topics: Lyman Break Galaxies - Lecture 2 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 galaxys light can be emitted at >100 m These frequencies are difficult to observe due to atmospheric effects

44 Current Topics: Lyman Break Galaxies - Lecture 2 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 850 m atmospheric window z=1 z=10

45 Current Topics: Lyman Break Galaxies - Lecture 2 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

46 Current Topics: Lyman Break Galaxies - Lecture 2 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.

47 Current Topics: Lyman Break Galaxies - Lecture 2 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.


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