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Current Topics: Lyman Break Galaxies - Lecture 3 Current Topics Lyman Break Galaxies Dr Elizabeth Stanway

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Presentation on theme: "Current Topics: Lyman Break Galaxies - Lecture 3 Current Topics Lyman Break Galaxies Dr Elizabeth Stanway"— Presentation transcript:

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

2 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 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

3 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)

4 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 galaxys light can be emitted at >100 m These frequencies are difficult to observe due to atmospheric effects

5 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

6 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

7 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.

8 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

9 Current Topics: Lyman Break Galaxies - Lecture 3 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…

10 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 Z AB band RI Z AB

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

12 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.

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

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

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

16 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.

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

18 Current Topics: Lyman Break Galaxies - Lecture 3 The presence of a line affects the measured magnitude. If W 0 =20Å, then W obs =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 The effect of Ly line emission 1215.67Å * (1+z)

19 Current Topics: Lyman Break Galaxies - Lecture 3 Say emission line has flux=2x10 -17 ergs/s/cm 2 Line has W 0 =20Å Line is at z=5.6 Filter is 2000Å wide, centred on line emission What is the line contribution and apparent broadband magnitude? Ly emission: Worked Example 1215.67Å * (1+z) W 0 =W obs /(1+z) => W obs = 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

20 Current Topics: Lyman Break Galaxies - Lecture 3 Say emission line has flux=2x10 -17 ergs/s/cm 2 Line has W 0 =20Å Line is at z=5.6 Filter is 2000Å wide, centred on line emission What is the line contribution and apparent broadband magnitude? Ly emission: Worked Example 1215.67Å * (1+z) Continuum flux density = line flux / W obs = 1.5x10 -19 ergs/s/cm 2 /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) / 3x10 18 ) * f = 3.2x10 -30 ergs/s/cm 2 /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

21 Current Topics: Lyman Break Galaxies - Lecture 3 The effect of Ly line emission If line emission is in the R band (4 Difficult to be both complete and uncontaminated

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

23 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

24 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< { "@context": "http://schema.org", "@type": "ImageObject", "contentUrl": "http://images.slideplayer.com/3/782327/slides/slide_24.jpg", "name": "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.", "description": "The faint end of the Schecter Luminosity function (L<

25 Current Topics: Lyman Break Galaxies - Lecture 3 Surveys of z>4 LBGs GOODS (The Great Observatories Origins Deep Survey) Hubble Space Telescope V-drops I-drops Z-drops SDF/SXDFSubaru 8m telescope V-drops R-drops I-drops BDF/ERGSESO Very Large Telescopes (8m) R-drops I-drops Z-drops Cluster Lensing Surveys Keck / HST I-drops Z-drops J-drops UKIDSSUK Infrared Telescope (4m) I-drops Z-drops Y-drops J-drops

26 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

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

28 Current Topics: Lyman Break Galaxies - Lecture 3 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=5.83 Too Young for Ly line Older than universe

29 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

30 Current Topics: Lyman Break Galaxies - Lecture 3 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) Log (Age) fraction

31 Current Topics: Lyman Break Galaxies - Lecture 3 Comparisons with z=3 Stellar Mass: At z=3, mass~10 10 M At z=5, Mass ~ 2x10 9 M Independent of metallicity Galaxies are smaller (Verma et al 2007) Log (Mass) fraction

32 Current Topics: Lyman Break Galaxies - Lecture 3 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 Log (SFR) fraction

33 Current Topics: Lyman Break Galaxies - Lecture 3 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 fraction

34 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

35 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

36 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

37 Current Topics: Lyman Break Galaxies - Lecture 3 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 Gemini 6 hours with Keck

38 Current Topics: Lyman Break Galaxies - Lecture 3 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

39 Current Topics: Lyman Break Galaxies - Lecture 3 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

40 Current Topics: Lyman Break Galaxies - Lecture 3 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

41 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! SIVOI

42 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 cant really describe the whole population Dow-Hygelund et al, 2005

43 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

44 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


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