(E.R.Stanway@Bristol.ac.uk) Current Topics Lyman Break Galaxies Dr Elizabeth Stanway (E.R.Stanway@Bristol.ac.uk) Current Topics: Lyman Break Galaxies - Lecture 3

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

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

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

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 Current Topics: Lyman Break Galaxies - Lecture 3

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

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

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

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

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

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

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

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

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

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

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

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

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

Ly emission: Worked Example 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? 1215.67Å * (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 Current Topics: Lyman Break Galaxies - Lecture 3

Ly emission: Worked Example 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? 1215.67Å * (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 Current Topics: Lyman Break Galaxies - Lecture 3

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

Narrow Band Surveys A magnitude is the average flux in a filter Sky Emission A magnitude is the average flux in a filter If half the filter is suppressed by Ly-a forest, the galaxy appears faint Narrow Band 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

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

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

Surveys of z>4 LBGs GOODS Hubble Space Telescope SDF/SXDF (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 Cluster Lensing Surveys Keck / HST I-drops Z-drops J-drops UKIDSS UK Infrared Telescope (4m) I-drops Z-drops Y-drops J-drops Current Topics: Lyman Break Galaxies - Lecture 3

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

Old Stars at z=6 SFRe-t/ Eyles et al, 2005 SFRe-t/ 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 Current Topics: Lyman Break Galaxies - Lecture 3

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

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

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

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

Comparisons with z=3 fraction Log (SFR) 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

Comparisons with z=3 fraction Av Dust: At z=3, Av~0.6 mags At z=5, If Z=Z, => High z galaxies are less dusty fraction Av Current Topics: Lyman Break Galaxies - Lecture 3

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

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

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

Spectroscopy at z~6 Spectroscopy at z~6 is extremely difficult 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

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

The Rest-Ultraviolet No Ly lines Line emitters Too Blue Line emitters Steep Rest-UV slope (blue of f-2) could indicate zero age, Pop III, top-heavy initial mass function … => New physics! Interpretation still unclear Current Topics: Lyman Break Galaxies - Lecture 3

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

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

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

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

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