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NICMOS Measurements of the Near Infrared Background

This paper addresses the nature of the near infrared background. We investigate whether there is an excess background at 1.4 microns, what is the source of the near infrared background and whether that background after the subtraction of all known sources contains the signature of high redshift objects (Z>10). Based on NICMOS observations in the Hubble Ultra Deep Field and the Northern Hubble Deep Field we find that there is no excess in the background at 1.4 microns and that the claimed excess is due to inaccurate models of the zodiacal background. We find that the near infrared background is now spatially resolved and is dominated by galaxies in the redshift range between 0.5 and 1.5. We find no signature than can be attributed to high redshift sources after subtraction of all known sources either in the residual background or in the fluctuations of the residual background. We show that the color of the fluctuations from both NICMOS and Spitzer observations are consistent with low redshift objects and inconsistent with objects at redshifts greater than 10. It is most likely that the residual fluctuation power after source subtraction is due to the outer regions of low redshift galaxies that are below the source detection limit and therefore not removed during the source subtraction.


-Introduction
The nature of the near infrared background is the subject of intense current investigation. Much of this interest centers on whether the near infrared background contains the signature of very high redshift (Z > 10) sources. Claims for such a signature have been spurred by the claim of an excess in the background with a sharp cutoff to the blue of 1.4µ [1] and from fluctuation analyses of deep Spitzer data [2,4]. These findings are in contrast to earlier analyses of NICMOS images from the Northern Hubble Deep Field (NHDF) [5] and the Hubble Ultra Deep Field (HUDF) [6] where no excess was found.

-The Near Infrared Background Excess
Observations with Infrared Telescope in Space [1] found a near infrared background at 1.4µm of 70 nw m −2 sr −1 after subtraction of modeled zodiacal flux and modeled contributions from stars and galaxies. Observations in the NHDF and hUDF [7,5,6] measured a total contribution from stars and galaxies of 7-12 nw m −2 sr −1 after subtraction of a zodiacal background measured from a median of all of the images taken in the field. A later analysis [8] showed that the total powers measured in both investigations were essentially identical, therefore the discrepancy was not due to instrumental effects. The difference is in the subtracted zodiacal light. The measured zodiacal light in the NICMOS images is greater than the modeled zodiacal light in [1] by almost exactly the claimed excess. This led to the conclusion that the claimed excess did not exist and is the result of the inadequacy of the zodiacal models to accurately predict the zodiacal flux. The error in the model was relatively modest (28%), however, since the source flux is so small, 2% of the zodiacal flux, the error led to a significant excess of flux not due to zodiacal or known sources. The analysis in [8] removes the false 1.4µ excess as evidence for a high redshift component to the observed near infrared background.

-Nature of the Near Infrared Background
The sources in the zodiacal subtracted NICMOS images in the NHDF and NUDF provide all of the measured power. Photometric redshifts [5,6] show that the majority of power is provided by galaxies in the redshift range between 0.5 and 2.0. From these measurements we conclude that the observed near infrared background is now resolved into galaxies and is primarily due to galaxies at relatively low redshifts. This extragalactic background is a small percentage of the overall background which is due to zodiacal reflected emission from nearby dust.

-The Source Subtracted Background
Having determined that the near infrared background is due to resolved galaxies in the redshift range between 0.5 and 2.0 we can then ask about the nature of the background after all known resolved sources have been subtracted. The source positions and extents were determined by an optimal extraction technique [9] that utilizes both the ACS and NICMOS images in all of the six bands. The source subtracted image was then produced by setting all of the pixels identified as being a source to zero. This procedure only removed 7% of the pixels from the image. Note that the method in [9] is a single pixel criterion. In the extraction we utilized SExtractor [10] to impose the additional criterion that a source must contain at least 3 contiguous pixels. Neither of these techniques extends the source beyond the region where a single pixel meets the source detection criterion. This is important in the analysis that follows.
1. Fluctuation Analysis. -We use a fluctuation analysis to investigate whether the residual background after source subtraction contains a signal from a distribution of sources that are fainter than the single source detection limit. The fluctuation analysis is based on a 2 dimensional Fourier transform of the background image and is described in detail in Apendix A of [8]. The results of the analysis on the F160W HUDF image are shown in Figure 1. The figure for the F110W images is essentially identical. It is clear from the difference at longer wavelengths between the all sources subtracted power  [11] in 7 different 2MASS calibration fields. The photon Poisson noise for the all sources included and brighter than 20. mag deleted curves is smaller than the symbol sizes. The noise in the all sources deleted curve can be estimated from the deviations from a smooth curve. and the gaussian noise power that there is signal in the residual background after source subtraction.

-Nature of the Fluctuation Sources
Several studies have attributed the residual fluctuations after source subtraction to very high redshift (z > 10) sources. Fluctuations in deep 2MASS calibration images have been attributed to high redshift sources [11] as well as fluctuations in deep Spitzer images [2,4]. We will address the 2MASS and Spitzer images separately.
1. 2MASS Fluctuations. -In [2] all detectable sources in 7 deep H band 2MASS calibration field images were subtracted out down to an equivalent AB magnitude of 20 and a fluctuation analysis performed on each of the images. The average of the fluctuations in the images is shown as the dashed line in Figure 1. To test whether the remaining fluctuations were due to high redshift objects we subtracted sources in the NICMOS F160W HUDF images down to the same limiting magnitude. Only 10 sources out of the 5000 sources in the NICMOS image were at or brighter than the subtraction limit in the 2MASS images. The fluctuations from the 20th magnitude or brighter subtracted image are shown by the diamonds in Figure 1. They are consistent with the 2MASS fluctuations over the common region of spatial wavelengths. Next all of the sources were subtracted in the NICMOS image and the analysis performed again. The result is shown by the asterisks. The all source subtracted fluctuations are significantly below the 20th magnitude or brighter subtracted fluctuations indicating that the observed sources in the much deeper NICMOS image can easily account for the fluctuations. All of these sources have redshifts less than 7 and the predominant power comes from sources in the redshift range between 0.5 and 2.0 as would be expected from the analysis of the sources that provide the majority of the near infrared background power. The conclusion is that the observed fluctuations in the 2MASS source subtracted image are due to low redshift objects below the 2MASS detection limit and do not indicate the presence of very high redshift objects. Details of this analysis are given in [8].

-HUDF and Spitzer Fluctuations
We next turn to the fluctuations in the HUDF [8,12] and Spitzer [2,3,4] images to see if they require the presence of high redshift sources.
1. HUDF Fluctuations. -In addition to the fluctuations in the F160W NICMOS HUDF image shown in Figure 1 we also analyzed the F110W image for fluctuations. The spatial spectrum of the F110W fluctuations are almost identical to the F160W fluctuations. Using the predominant SEDs in the HUDF we next calculated the expected ratio of fluctuations in the F110W and F160W NICMOS bands versus redshift as well as the Spitzer 3.6 and 4.5 micron bands. The results of the calculation are shown in Figure 2. The observed ratio of the NICMOS band fluctuations given by the horizontal dashed line is inconsistent with sources with redshifts greater than eight so we conclude that the fluctuations in the source subtracted NICMOS images are not due to high redshift sources. We consider the most likely source of the residual fluctuations to be the extended regions of the observed sources that were too faint to be detected by our source subtraction technique.
2. Spitzer Fluctuations. -We show in [12] that the degree of source subtraction in the NICMOS HUDF and the Spitzer images used in [3,4] are essentially equal and it is therefore legitimate to make a comparison. Due to the long wavelength of the Spitzer bands the Lyman break does not enter them even for redshifts as high as 15 so their ratio is not a sensitive indicator of the redshift of the residual fluctuations. The ratio of the NICMOS F160W to Spitzer 3.6 micron fluctuations, however, indicates that the ratio is incompatible with redshifts above 10. We therefore conclude that the residual fluctuations in the Spitzer images are not evidence for the presence of very high redshift objects.
3. Comparison of Spatial Wavelength Spectra. -The point was made in [4] that the spatial spectrum of the residual fluctuations in the Spitzer images at spatial scales larger than 5 arc seconds is evidence for a high redshift population. In Figure 3 we compare the spatial spectrum of the NICMOS residual fluctuations which we have shown to be due to low redshift objects to the observed Spitzer fluctuations. The fluctuations were normalized to be equal at 10 arc seconds. Within the noise the spatial spectra are identical, removing the argument that the observed spatial spectrum of the residual Spitzer fluctuations require a high redshift origin.

-Conclusions
We conclude that the Near Infrared Extragalactic Background has been resolved and is due primarily to normal galaxies at redshifts near 1. The claimed excess at 1.4 microns is false and arose from the inadequacy of zodiacal models to predict the background level to the accuracy need to determine the true source flux. We further conclude that none of the properties of the fluctuation spectrum after source subtraction in any of the 2MASS, NICMOS or Spitzer images require very high redshift (z > 10) objects to account for them. * * * This article is based on data from observations with the NASA/ESA Hubble Space Telescope obtained at the Space Telescope Science Institute, which is operated by the