The Near Infrared Background Excess and Star Formation in the HUDF

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The Near Infrared Background Excess and Star
Formation in the HUDF

• Rodger Thompson
• Steward Observatory
• University of Arizona

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Blameless Collaborators

• Mark Dickinson
• Daniel Eisenstein
• Xiaohui Fan
• Garth Illingworth
• Rob Kennicutt
• Marcia Rieke

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Topics

• The near infrared background excess
• The lack thereof
• Star formation history of the HUDF

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Near Infrared Background Excess

• Claims of a Near InfraRed Background (NIRBE) of
  70 nW m-2 sr-1, not due to known galaxies, stars
  or zodiacal light, that peaks at 1.4-1.6 mm.
• Resolved objects in the NUDF and NHDF contribute
  6-7 nW m-2 sr-1, a factor of 10 below the claimed
  background.
• Fluctuations in deep 2MASS images claimed as
  evidence for a population of very high redshift
  (10-15) Pop. III stars. (Kashlinsky et al. 2006)

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Implications of the NIRB

• Most popular model for the NIRB is the light from
  the high redshift Pop. III stars that reionized
  the universe.
• Requires that the total number of baryons turned
  into stars in the first 3 of the age of the
  universe be greater than or equal to the total
  number of baryons converted to stars in the
  remaining 97.
• The metals produced by this conversion must be
  hidden in black holes.
• There must be no x-ray producing accretion onto
  the black holes.
• The NIRB must not interact with TeV emission from
  distant blazars.

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Fluctuation Analysis of the NICMOS UDF F160W Image
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Results of the Fluctuation Analysis

• The fluctuations observed in the 2MASS field can
  be completely accounted for by the redshift 0-7
  galaxies such as those observed in the NUDF
• There is no need for an excess population of high
  redshift Pop.III stars to account for the
  fluctuations
• Fluctuations have been removed as evidence for a
  NIRBE at 1.6 mm

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The IRTS NIRBE

• Wide field of view spectrometer
• Aperture almost 17 times the size of the NUDF
• Zodiacal light and contributions from sources
  determined from models
• After subtraction of modeled components, 70 out
  of 330 nW m-2 sr-1 remain and is attributed to a
  NIRBE

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The NIRBE According to IRTS
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IRTS vs NICMOS FLUX ALLOCATIONS All
fluxes in nW m-2 Sr-1
Observed
Modeled
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Differences

• The zodiacal component determined by medianed
  images in the NUDF exceeds the IRTS modeled
  component by 100 nW m-2 sr-1.
• Dwek et al. 2006 point out that the IRTS spectrum
  is better fit by a zodiacal spectrum than a high
  z Pop.III spectrum.
• The IRTS NIRBE is most likely due to an under
  estimate of the zodiacal light component by the
  model.

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Caveats

• A NIRBE component that is flat on scales of
  greater than 100 would be mistaken for zodiacal
  light in our reduction.
• At odds with CMB predictions
• A NIRBE component that is clumped on the order of
  several arc minutes could be missed by our two
  small fields.
• Archival proposal to check other fields
• However the light in a NIRB can not be
  distributed in the same manner as the light from
  baryonic matter at redshifts of 6 and less.

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Scattering of UV Light at High Z

• Emission from massive Pop. III stars will be
  primarily shortward of 912 Å and will be degraded
  into Ly a photons.
• In a metal and dust free gas they can scatter to
  large distances and become smooth on scales of
  10-100 arc seconds.

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Smoothing on 10 arc second Scales
Portion of the NUDF at 1.6 mm
Same portion with background in 10 gaussians
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Star Formation History in the NICMOS UDF
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The F775W Mag. vs Redshift
AGN
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Star Formation Rates

• Star formation rate determined from the rest
  frame 1500 Å flux via the Madau relation.
• The flux is measured from the selected SED
  without extinction to produce an extinction
  corrected SFR.

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Star Formation Intensity Distribution

• The star formation intensity x is the SFR in M?
  per year per proper square kpc.
• The distribution function h(x) is the sum of all
  proper areas in an x interval, divided by that
  interval and divided by the comoving volume
  defined by the field and redshift interval.
• Under this definition SFR is the first moment of
  h(x) SFR ?x h(x) dx

Lanzetta et al. 1999, ASP Conf. Ser. 191, 223
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Star Formation Density
Redshift 1
95 complete
60 complete
Log(h(x))
Starburst
Log Star Formation Intensity x in M? per year per
kpc2
About 80 of the stars are formed in a starburst
region
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Application of the Distribution

• The SFR is calculated for every pixel that is
  part of a galaxy.
• Assumes a uniform SED and extinction within a
  galaxy
• Assumes that the rest frame 1500 Å light is
  distributed in the same way as the observed flux
  in the ACS F775W band.

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The Observed h(x)
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Star Formation History of the NUDF
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Comparison with the NHDF
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Conclusions

• Fluctuations have been removed as evidence for a
  NIRBE at 1.6 mm.
• The IRTS NIRBE is probably zodiacal flux.
• Any NIRBE must be either maximally flat or
  maximally clumped.
• The star formation history of the universe is
  roughly constant from z1-6.
• The vast majority of star formation occurs in a
  minority of galaxies at any one time.