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