Galaxies 626. Lecture 9 Metals (2) and the history of star formation from optical/uv observations

Transcription

1 Galaxies 626 Lecture 9 Metals (2) and the history of star formation from optical/uv observations

2 Measuring metals at high redshift

3 Metals at 6 How can we measure the ultra high z star formation? One robust way is to measure the accumulated metal content of the universe at high redshift. High z metals come from high z sources and provide an integrated measure of the preceding star formation. (Always going to be a lower limit since we will miss some of the metals, ionization corrections, etc.)

4 Lower ionization stages At z > 5 we lose the CIV and then the SiIV from the optical window. But we can still measure lower ionization lines (OI, CII and SiII) since these are at shorter wavelengths (around 1300A). These lines are primarily found in the high column density neutral H systems at low redshifts but they may also start to be found in the more diffuse gas at ultra high z if the ionization parameter starts to drop. SO: CAN WE FIND THESE LINES??

5 How do we measure metals at high z? Apart from the neutral hydrogen of the Lyman forest, we have only a very limited number of absorption lines that we can detect in the intergalactic gas : Most of the information on the lowest density component comes from CIV with limited ionization information from the SiIV and CII lines outside the forest and other lines that lie in the forest (e.g. SiIII, CIII) and some information on the hotter gas from OVI but OVI lies in the Lyman forest. For the higher density gas most of the low redshift information comes from DLA metallicity measurements. However, at high z the forest saturates and we can no longer find the DLAs. We CAN still measure OI, CII and SiII if we can identify the systems somehow.

6 The Sloan quasars provide bright z>5 targets

7 DLA type systems at high redshift Absorption systems exist at z ~ 6 but we cant measure the corresponding HI

8 Star formation history & metal enhancement z = OI lines at high resolution. HIRES observations R ~ 60,000 (G. Becker et al. 2006) z = z = z =

9 Star formation history & metal enhancement in DLA like systems Wolfe et al Rao et al Becker et al Songaila 2005 Songaila and Cowie 2006

10 Extension to z = 6 Near infrared spectroscopy of z > 6 SDSS quasars extends the measurement of the minimum Ω(CIV) out to z ~ 6. Ryan Weber et al astro ph/ (VLT ISAAC) Songaila Simcoe 2006, astro ph/ (Gemini GNIRS) Simcoe 2006

11 Metal Census DLAs have reasonable metallicity but low Ω (HI) Ly forest has low metallicity but dominates Ω (HI) Predicted metallicity from previous SF is ~ 1/30th So where are remaining metals?

12 Star formation history & metal enhancement Flat SFR normalized to z=2 Metal production at z >5 Metal production from SFR in LBGs at z = 2 Metal production at z > 10 Adelberger 2005 IGM metallicity at z = 2 Schaye et al IGM metallicity at z = 4 5

13 Already somewhat of a problem! How do we produce that many metals before z = 6? (Though this is a rather crude calculation.)

14 Star formation history & metal enhancement: star formation history from color breaks Integrated total star formation C

15 Star formation history & metal enhancement Wolfe et al Rao et al Songaila 2005 Songaila 2006 Becker et al. 2006

16 The actual drop is still uncertain but evidence suggests a decline in star formation density by a factor of between 4 and 6 between z = 3 and z = 6, assuming no large change in the luminosity function. BUT there are major caveats: Cosmic variance is expected to be 50% for a field the size of the UDF: could the UDF just be very underdense? Is there a lot of contamination in the low end LF? (This goes the other way ) There is some evidence for a change in the LF (more faint galaxies) i.e are we measuring most of the stars?

17 We can see the metal forest to high redshift (z of 6) The universal metal content in high and low ions at z near 6 is similar to that of the z=3 universe?. Implies we are seeing the relics of the highest redshift star formation? The overall star formation history of the history of IGM metal enhancement requires more star formation than we are seeing in the galaxy searches. probably missing the smaller galaxies that dominate the star formation at these times.

18 Galaxies 626 The galaxy formation history: optical and UV observations

19 The cosmic star formation history We are now going to move on to mapping the history of the galaxy/star formation from the post reionization epoch to the present time We want to model star formation histories and make sure they are consistent with all aspects of the local distributions The goal is to understand how everything comes together

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21 Cosmic Star Formation History Various probes of the global SF rate: ρ (z) M yr 1 comoving Mpc 3 * UV continuum (U band surveys, GALEX, LBGs) Hα and [O II] emission in spectroscopic surveys mid IR dust emission 1.4 GHz radio emission No simple `best method : each has pros and cons (dust extinction, sample depth, z range, and physical calibration uncertainties) Each has a different time sensitivity to main sequence activity, so if SFR not uniform, we do not expect the same answers for the same sources For example, Hα is produced by the most luminous stars, since these produce the highest number of ionizing photons The UV continuum is dominated by A or B stars, which are longer lived So, at the beginning of a burst, there would be more Hα, which would then die away as time passes

22 Time Dependence of Various SF Diagnostics In a burst model, each SF diagnostic arises from a component of the stellar population whose lifetime is different, so there is no single best one Radio continuum is thought to arise from SN remnants and offers the potential of a dust free diagnostic Burst model

23 In calculating star/galaxy formation histories we must remember the uncertainty introduced by the IMF of the stars!

24 Importance of Initial Mass Functions w Salpeter > 1 M reproduces colors and Hα properties of spirals Mass fraction per log mass bin IMF < 1 M makes minor contribution to light but is very important for mass inventory (Salpeter diverges at low mass end) Mass (M ) A clipped IMF at <1 solar mass would not look any different in terms of the galaxy properties

25 If both the local value and the cumulative star formation are calibrated with the same IMF, then at least the description is consistent However, the IMF may not be invariant; one could be averaging over a lot of IMFs over time, so the IMF uncertainties are significant

26 Comparison of UV & Hα for Same Local Galaxies UV(2000Å) c.f. Hα (corrected for extinction via Balmer line ratio) Scatter cannot be explained with a dispersion in IMFs and metallicities Suggests evidence for non uniform SF histories &/or significant dust complications Myr; 10 30% mass

27 Cosmic SFH: Calibration Kennicutt 1998 Ann Rev A&A 36, 189 (Salpeter IMF) UV continuum ( Å) : Pro: Extensive datasets over 0<z<6; know stellar evolutionary tracks & w/imf, # of stars in a given mass interval, so know light produced each λ Con: dust! IMF dependence 2. Line emission (Hα, [O II]) : Pro: Very sensitive probe, available to z~2 (lose in thermal IR) Con: strong IMF dependence ( 3); dust (use Balmer decrement to correct); excitation uncertainties [OII] 3. Far IR emission ( µm) : Pro: Independent method, available for obscured sources to high z: Con: uncertain source of dust heating (AGN/SF?); primarily applicable in starbursts due to detection limits at high z; bolometric FIR flux required

28 Some Popular Dust Extinction Laws Losing light due to dust and have to know how to correct for it as a function of wavelength to estimate the SF Screen model has a shallower extinction curve because some parts are opaque, other parts aren t, and the opaque parts don t affect the color and so have no wavelength dependence

29 Finding Star Forming Galaxies at z=2.5 6 ionizing photons get absorbed The Lyman continuum discontinuity is particularly powerful for isolating star forming, high redshift galaxies From the ground, we have access to the redshift range z=2.5 6 in the micron range IG gas becomes denser and more neutral as go to higher z, so higher effect of scattering photons out of galaxy spectrum to 1216 A

30 Photometric selection: Expectations Real Data (10 field) Spectral energy distributions allow us to predict where distant SF galaxies lie in color color diagrams such as (U G vs G R)

31 Spectroscopic Confirmation at Keck See hot star lines Yellow: window function (what expect based on selection)

32 HST images of spectroscopically confirmed Lyman break galaxies with z>2 in Hubble Deep Field North reveal small physical scalelengths and irregular morphologies

33 Extending the Technique 1 < z < 4 Lyman break Balmer break (no real reason to do Balmer break because will select against v. blue objs)

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35 Optical Star Formation History (1+z)2 Salpeter IMF to 0.1 solar mass (1+z) 0.8 Wilson et al Steidel et al. 1999

36 End