Finding z 6.5 galaxies with HST’s WFC3 and their implication on reionization
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Transcript of Finding z 6.5 galaxies with HST’s WFC3 and their implication on reionization
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 IRLBGsDataResultsSFRShechter (Luminosity) FunctionReionization
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
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
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
http://www.stsci.edu/hst/proposing/documents/primer/Ch_49.html#1924814
Transmission efficiency for relevant filters
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
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.
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)
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.
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???
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
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.
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)
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
Extra results Z~8 results (y-drops)
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