The Near Infrared Background Excess and Star Formation in the HUDF

Rodger ThompsonSteward ObservatoryUniversity of Arizona

Blameless Collaborators

Mark DickinsonDaniel EisensteinXiaohui FanGarth IllingworthRob KennicuttMarcia Rieke

Topics

The near infrared background excess The lack thereof

Star formation history of the HUDF

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 m.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)

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.

Fluctuation Analysis of the NICMOS UDF F160W Image

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 NUDFThere is no need for an excess population of high redshift Pop.III stars to account for the fluctuationsFluctuations have been removed as evidence for a NIRBE at 1.6 m

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 modelsAfter subtraction of modeled components, 70 out of 330 nW m-2 sr-

1 remain and is attributed to a NIRBE

The NIRBE According to IRTS

0

50

100

150

200

250

300

350

Total Zodiacal Sources Background

IRTSNHDF

IRTS vs NICMOS FLUX ALLOCATIONS All fluxes in nW m-2 Sr-1

Observed

Modeled

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.

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.

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 photons.In a metal and dust free gas they can scatter to large distances and become smooth on scales of 10-100 arc seconds.

Smoothing on 10 arc second Scales

Portion of the NUDF at 1.6 m Same portion with backgroundin 10” gaussians

Star Formation History in the NICMOS UDF

The F775W Mag. vs Redshift

AGN

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.

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

Star Formation Density

Log Star Formation Intensity x in M per year per kpc2

Log(h(x))

60% complete

95%complete

Redshift = 1

Starburst

About 80% of the stars are formed in a “starburst region”

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 galaxyAssumes that the rest frame 1500 Å light is distributed in the same way as the observed flux in the ACS F775W band.

The Observed h(x)

Star Formation History of the NUDF

Comparison with the NHDF

Conclusions

Fluctuations have been removed as evidence for a NIRBE at 1.6 m.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 z=1-6.The vast majority of star formation occurs in a minority of galaxies at any one time.