finding z 6.5 galaxies with hst’s wfc3 and their implication on reionization
DESCRIPTION
Finding z 6.5 galaxies with HST’s WFC3 and their implication on reionization. Mark Richardson. “Possible Low-Z starz in High-z z’-drop galaziez”. Outline. WFC3 in the IR LBGs Data Results SFR Shechter (Luminosity) Function Reionization. Note. - PowerPoint PPT PresentationTRANSCRIPT
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Finding z 6.5 galaxies with HST’s
WFC3 and their implication on reionization
Mark Richardson
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“Possible Low-Z starz in High-z z’-drop galaziez”
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Outline
WFC3 in the IRLBGsDataResultsSFRShechter (Luminosity) FunctionReionization
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Note
Paper 1 Probing ~ L* Lyman-Break Galaxies at z~7 in GOODS-South with WFC3 on HST
Paper 2 The Contribution of High Redshift Galaxies to Cosmic Reionization: New Results from Deep WFC3 Imaging of the Hubble Ultra Deep Field
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Hubble
Two previous detectors on Hubble used in these texts: ACS & NICMOS
ACS: Large detecting area, UV to NIR (~0.85μ), efficient
NICMOS: Small FOV, NIR (up to ~1.6μ)
http://www.edcheung.com/job/sm4/wfpc/wfpc.htm
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Hubble WFC3: Installed May 2009, Larger FOV than NICMOS, smaller FOV
than ACS; same spectral range as NICMOS Used Y,J,H bands with WFC3, although Paper 1 used Y(0.98μm)
whereas Paper 2 used Y(1.05μm). Note (in μm):
ACS B = 0.435 V = 0.606 i ~ 0.740 Z = 0.850
NICMOS Y ~ 1. J ~ 1.25 H ~ 1.6
WFC3 Y = 0.98 or 1.05 J = 1.25 H = 1.60
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http://www.stsci.edu/hst/proposing/documents/primer/Ch_49.html#1924814
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Transmission efficiency for relevant filters
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High Redshift Observations
How do we find high-z objects? Lyman-alpha emission (narrow band) Lyman-break (broad band) Gamma-Ray bursts (GR observatories)
Lyman-Break Galaxies: Cue: Mark meet Board … 3 Filters at low-z vs. 2 Filters at high-z
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Lyman break Galaxies
So for high-z (z > 5) galaxies: No detection below filter with 1216A(1+z):
Let’s call this a ‘UV detection’ Detections in and above filter with
1216A(1+z) Expect bluer colours in filters above
1216A(1+z) than most other sources.
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Data
Selection Criteria: Paper I: z-Y>0.8 Paper II: z-Y>1.3
T-L dwarf & Low-z rejection Criteria Paper I: Y-J~< 1.0 Paper II: z-Y ~>3.6(Y-J)-0.8 OR >2
Low-z rejection Criteria No UV detections
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Results
Paper I: Considered FOV of GOODS-South: 20
arcmin2
148 objects: 55 spurious, 79 have detections in B and V, 8 in i, 6 in z
8 in i are likely z~6 galaxies (some previously confirmed)
6 in z are likely z~7 galaxies. Further supported since objects not in MIPS 24μm (corresponds to ~3μm if z~7 correct)
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Results
Paper II: Considered FOV of HUDF: 4.18 arcmin2
110 objects: 35 spurious, 55 have detections in B and V, 8 in i, 12 in z
8 in i are likely z~6 galaxies (some previously confirmed)
10 in z are likely z~7 galaxies. One in z is likely a transient object (compare with NICMOS), another is likely a T or L dwarf.
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Determining UV flux & SFR
For z=7, LUV can be determined from Y Madau et al. 98 show that after enough time the equilibrium: LUV
= const*SFR is reached Paper I: SFRs in the range of 5-10 Mo/yr Paper II: SFRs in the range of 1- 4 Mo/yr with one object having
a SFR of 8Mo/yr --> Total SFR in field = 29.6 Mo/yr Assumptions???
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Schechter Function
We wish to describe the number density of galaxies with luminosity between L and L+dL:
Parameters: z~7 Φ* = 0.0011 Mpc-3 α = -1.73 MUV
* = -19.8
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Reionization
Cosmological history: recombination, reionization, today
Possible sources of reionization: AGN -- likely not: densities too low Star formation early in the Universe
But evidence of Luminosity function evolution seems to contradict this.
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Reionization
Madau et al 98 give the necessary SFR density to provide reionization:
ρSFR = (0.005Mo yr-1 Mpc-3/fesc)([1+z]/8)3(Ωbh702/0.0457)(C/5)
Considering Paper II, with a FOV of 4.2 arcmin2 and a z-range of 6.7-8.8, the 29.6 Mo/yr observed are taking place in a volume of ~18000Mpc giving a SFR density of ~ 0.0017Mo yr-1 Mpc -3
Considering the assumptions that go into this a value of 0.0035-0.004 Mo yr-1 Mpc-3 is more likely (if not higher)
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Results
Thus the observed UV flux is too low to account for reionization by factors of a few.
Possible resolution: fesc is very high, or faint end slope of Luminosity function is much steeper than given before.
Two last possible solutions: low metallicity or top-heavy IMF
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Extra results Z~8 results (y-drops)
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Sources Wilkens, Stephen M. et al, Probing ~L* Lyman-break Galaxies at z ~ 7 in GOODS-South
with WFC3 on HST, arXiv: 0910.1098v3, Dec 2009 Bunker, Andrew J. et al, The Contribution of high Redshift Galaxies to Cosmic Reionization:
New Results from Deep WFC3 Imaging of the Hubble Ultra Deep Field, arXiv:0909.2255v3, Dec 2009
Luminosity Function lecture from Phil Armitage, university of Colorado in Boulder, http://jila.colorado.edu/~pja/astr3830/index.html
http://www.astro.ku.dk/~jfynbo/LBG.html WFC3 info: http://www.edcheung.com/job/sm4/wfpc/wfpc.htm WFC3 vs ACS: http://www.stsci.edu/hst/proposing/documents/primer/Ch_49.html#1924814 ACS filters: http://adcam.pha.jhu.edu/instrument/filters/ GRB: Wikipedia