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Current Topics

Current Topics. Lyman Break Galaxies Dr Elizabeth Stanway (E.R.Stanway@Bristol.ac.uk). 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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Current Topics

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  1. Current Topics Lyman Break Galaxies Dr Elizabeth Stanway (E.R.Stanway@Bristol.ac.uk) Current Topics: Lyman Break Galaxies - Lecture 2

  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 Current Topics: Lyman Break Galaxies - Lecture 2

  3. 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

  4. 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

  5. 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

  6. 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

  7. 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

  8. 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

  9. 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

  10. 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

  11. 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

  12. 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

  13. 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

  14. 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

  15. 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

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

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

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

  19. 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

  20. 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

  21. 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

  22. 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

  23. 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

  24. 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

  25. 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

  26. 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

  27. 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

  28. 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

  29. 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

  30. 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

  31. 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

  32. 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

  33. 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

  34. 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

  35. 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

  36. 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

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

  38. 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

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

  40. 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

  41. 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

  42. 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

  43. 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

  44. 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

  45. 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

  46. 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

  47. 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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