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Current Topics

Current Topics. Lyman Break Galaxies Dr Elizabeth Stanway (E.R.Stanway@Bristol.ac.uk). Other Galaxies at z=3. Lyman Break Galaxies are selected to be UV-bright Strongly star forming Not too much dust extinction

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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 3

  2. 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 3

  3. 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 3

  4. 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 3

  5. 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 3

  6. 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 3

  7. 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 3

  8. 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 Current Topics: Lyman Break Galaxies - Lecture 3

  9. 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 3

  10. 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 Current Topics: Lyman Break Galaxies - Lecture 3

  11. RIZ selection at z=5 and z=6 Current Topics: Lyman Break Galaxies - Lecture 3

  12. RIZ selection at z=5 and z=6 BUT at these wavelengths, filters overlap and are far from standardised. Current Topics: Lyman Break Galaxies - Lecture 3

  13. Filters V-drop filters R-drop filters Current Topics: Lyman Break Galaxies - Lecture 3

  14. Redshift selection as a function of filter Low z galaxy High z galaxy Current Topics: Lyman Break Galaxies - Lecture 3

  15. Redshift selection as a function of filter z~5 V- and R-drops • Number density and redshift distribution depend on filters used => Results from surveys are not directly comparable z~6 I-drops Current Topics: Lyman Break Galaxies - Lecture 3

  16. 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. Current Topics: Lyman Break Galaxies - Lecture 3

  17. The effect of Ly line emission Spectrum flat in f • 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 99% at z>5.5 Current Topics: Lyman Break Galaxies - Lecture 3

  18. The effect of Ly line emission • 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) Current Topics: Lyman Break Galaxies - Lecture 3

  19. Ly emission: Worked Example 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? • 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 Current Topics: Lyman Break Galaxies - Lecture 3

  20. Ly emission: Worked Example 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? • 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 Current Topics: Lyman Break Galaxies - Lecture 3

  21. The effect of Ly line emission • 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 Current Topics: Lyman Break Galaxies - Lecture 3

  22. Narrow Band Surveys Sky Emission Narrow Band • A magnitude is the average flux in a filter • If half the filter is suppressed by Ly-a forest, the galaxy appears faint Broad Band • 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 Current Topics: Lyman Break Galaxies - Lecture 3

  23. Narrow Band Surveys • But what line have you detected? • Could be: • OIII at 5007A • OII at 3727A • Lyman- at 1216A • Need spectroscopic follow-up Current Topics: Lyman Break Galaxies - Lecture 3

  24. 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)  LdA 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’ Current Topics: Lyman Break Galaxies - Lecture 3

  25. Surveys of z>4 LBGs Current Topics: Lyman Break Galaxies - Lecture 3

  26. 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 Current Topics: Lyman Break Galaxies - Lecture 3

  27. 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 Eyles et al, 2005 SFRe-t/ Current Topics: Lyman Break Galaxies - Lecture 3

  28. Old Stars at z~6 Too Young for Ly line z=5.83 • 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 Older than universe Current Topics: Lyman Break Galaxies - Lecture 3

  29. 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 Current Topics: Lyman Break Galaxies - Lecture 3

  30. Comparisons with z=3 Age: At z=3, age~300Myr At z=5, age~30Myr If Z=Z, then age~3Myr • Galaxies are younger (Verma et al 2007) fraction Log (Age) Current Topics: Lyman Break Galaxies - Lecture 3

  31. Comparisons with z=3 Stellar Mass: At z=3, mass~1010M At z=5, Mass ~ 2x109M Independent of metallicity • Galaxies are smaller (Verma et al 2007) fraction Log (Mass) Current Topics: Lyman Break Galaxies - Lecture 3

  32. Comparisons with z=3 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 fraction Log (SFR) Current Topics: Lyman Break Galaxies - Lecture 3

  33. 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 fraction Av Current Topics: Lyman Break Galaxies - Lecture 3

  34. 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 Ferguson et al 2004 Current Topics: Lyman Break Galaxies - Lecture 3

  35. 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 Current Topics: Lyman Break Galaxies - Lecture 3

  36. 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 Current Topics: Lyman Break Galaxies - Lecture 3

  37. Spectroscopy at z~6 35 hours with Gemini 6 hours with Keck 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 Current Topics: Lyman Break Galaxies - Lecture 3

  38. The Rest-Ultraviolet • Rest-UV slope is an age indicator: • young=blue, old=red • But many z~5 galaxies seem too blue No Ly lines Too Blue Line emitters Current Topics: Lyman Break Galaxies - Lecture 3

  39. 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 No Ly lines Too Blue Line emitters Current Topics: Lyman Break Galaxies - Lecture 3

  40. Lyman- Equivalent Widths z~6 50% of z>5 sources have EW>0Å 25% have EW>30Å z~5 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? Current Topics: Lyman Break Galaxies - Lecture 3

  41. 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! OI SIV Current Topics: Lyman Break Galaxies - Lecture 3

  42. 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-Hygelund et al, 2005 Current Topics: Lyman Break Galaxies - Lecture 3

  43. 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 Current Topics: Lyman Break Galaxies - Lecture 3

  44. Lecture Summary • Spectroscopy is beginning to probe absorption lines, finding: • similar velocity outflows to z~3 • similar Lyline 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 Current Topics: Lyman Break Galaxies - Lecture 3

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