Extragalactic Radio Sources and the WMAP Cold spot
Lawrence Rudnick 1, Shea Brown2, Liliya R. Williams3
Department of Astronomy, University of Minnesota
116 Church St. SE, Minneapolis, MN 55455
ABSTRACT
We detect a dip of 20-45% in the surface brightness and number counts of
NVSS sources smoothed to a few degrees at the location of the WMAP cold spot.
The dip has structure on scales of ∼ 1◦ to 10◦. Together with independent all-sky
wavelet analyses, our results suggest that the dip in extragalactic brightness and
number counts and the WMAP cold spot are physically related, i.e., that the
coincidence is neither a statistical anomaly nor a WMAP foreground correction
problem. If the cold spot does originate from structures at modest redshifts, as
we suggest, then there is no remaining need for non-Gaussian processes at the
last scattering surface of the CMB to explain the cold spot. The late integrated
Sachs-Wolfe effect, already seen statistically for NVSS source counts, can now
be seen to operate on a single region. To create the magnitude and angular size
of the WMAP cold spot requires a ∼ 140 Mpc radius completely empty void at
z≤1 along this line of sight. This is far outside the current expectations of the
concordance cosmology, and adds to the anomalies seen in the CMB.
Subject headings: large-scale structure of the universe – cosmic microwave back-
ground – radio continuum: galaxies
1. Introduction
The detection of an extreme “cold spot” (Vielva et al. 2004) in the foreground-corrected
WMAP images was an exciting but unexpected finding. At 4◦ resolution, Cruz et al. (2005)
determine an amplitude of -73 µK, which reduces to -20 µK at ∼10◦ scales (Cruz et al.
1larry@astro.umn.edu
2brown@astro.umn.edu
3llrw@astro.umn.edu
http://arxiv.org/abs/0704.0908v2
– 2 –
2007). The non-gaussianity of this extreme region has been scrutinized, (Cayon, Jun &
Treaster 2005; Cruz et al. 2005, 2006, 2007) concluding that it cannot be explained by either
foreground correction problems or the normal Gaussian fluctuations of the CMB. Thus, the
cold spot seems to require a distinct origin – either primordial or local. Across the whole sky,
local mass tracers such as the optical Sloan Digital Sky Survey (SDSS, York et al. 2000) and
the radio NRAO VLA Sky Survey (NVSS, Condon et al. 1998) are seen to correlate with
the WMAP images of the CMB (Pietrobon, Balbi & Marinucci 2006; Cabre et al. 2006),
probably through the late integrated Sachs-Wolfe effect (ISW, Crittenden & Turok 1996).
McEwen et al. (2007) extended the study of radio source/CMB correlations by per-
forming a steerable wavelet analysis of NVSS source counts and WMAP images. They
isolated 18 regions that, as a group, contributed a significant fraction of the total NVSS-
ISW signal. Three of those 18 regions were additionally robust to the choice of wavelet form.
The centroid of one of those three robust correlated regions, (#16), is inside the 10◦ cold
spot derived from WMAP data alone (Cruz et al. 2007), although McEwen et al. (2007)
did not point out this association.
The investigations reported here were conducted independently and originally without
knowledge of the McEwen et al. (2007) analysis. However, our work is a posteriori in
nature, because we were specifically looking for the properties in the direction of the cold
spot. These results thus support and quantify the NVSS properties in the specific direction
of the cold spot, but should be considered along with the McEwen et al. (2007) analysis for
the purposes of an unbiased proof of a WMAP association.
2. Analysis and Characterization of the NVSS “dip”
We examined both the number counts of NVSS sources in the direction of the WMAP
cold spot and their smoothed brightness distribution. The NVSS 21 cm survey covers the
sky above a declination of -40◦ at a resolution of 45”. It has an rms noise of 0.45 mJy/beam
and is accompanied by a catalog of sources stronger than 2.5 mJy/beam. Because of the
short interferometric observations that went into its construction, the survey is insensitive to
diffuse sources greater than ≈ 15’ in extent. Convolution of the NVSS images to larger beam
sizes, as done here, shows the integrated surface brightness of small extragalactic sources;
this is very different than what would be observed by a single dish of equivalent resolution. In
the latter case, the diffuse structure of the Milky Way Galaxy dominates (e.g, Haslam et al.
(1981)), although it is largely invisible to the interferometer.
To explore the extragalactic radio source population in the direction of the WMAP
– 3 –
cold spot, we first show in Figure 1 the 50◦×50◦ region around the cold spot convolved to a
resolution of 3.4◦. Here, the region of the cold spot is seen to be the faintest region on the
image (minimum at lII ,bII = 207.8
◦, -56.3◦). At minimum, its brightness is 14 mK below
the mean, with an extent of ≈5◦. The WMAP cold spot thus picks out a special region in
the NVSS – at least within this 2500� region.
We examined the smoothed brightness distribution across the whole NVSS survey using
another averaging technique that reduces the confusion from the brightest sources. We first
pre-convolved the images to 800”, which fills all the gaps between neighboring sources, and
then calculated the median brightness in sliding boxes 3.4◦ on a side. The resulting image
is shown in Figure 2 which is in galactic coordinates centered at lII=180
◦. The dark regions
near the galactic plane are regions of the NVSS survey that were perturbed by the presence
of very strong sources. Note that the galactic plane itself, which dominates single dish maps,
is only detectable here between -20◦ < lII < 90
◦, where there is a local increase in the number
of small sources detectable by the interferometer.
To evaluate the NVSS brightness properties of the cold spot, we compared it with the
distribution of median brightnesses in two strips from this all sky map. The first strip was
in the north, taking everything above a nominal galactic latitude of 30◦ (More precisely,
we used the horizontal line in the Aitoff projection tangent to the 30◦ line at lII=180
The second strip was in the south, taking everything below a nominal galactic latitude of
-30◦, but only from 10◦ < lII <180
◦, to avoid regions near the survey limit of δ=-40◦. The
minimum brightness in the cold spot region (≈ 20 mK) is equal to the lowest values seen in
the 16,800 square degree area of the two strips, and is ≈ 30% below the mean (Figure 3).
Formally, the probability of finding this weakest NVSS spot within the ≈10◦ (diameter)
region of the WMAP cold spot is 0.6%. This a posteriori analysis thus is in agreement with
the statistical conclusions of McEwen et al. (2007) that the NVSS properties in this region
are linked to those of WMAP. We also note that the magnitude of the NVSS dip is at the
extreme, but not an outlier of the overall brightness distribution. We thus expect that less
extreme NVSS dips would also individually correlate with WMAP cold regions, although it
may be more difficult to separate those from the primordial fluctuations.
The NVSS brightness dip can be seen at a number of resolutions, and there is probably
more than one scale size present. At resolutions of 1◦, 3.4◦, and 10◦, we find that the dip is
≈ 60%, 30% and 10% of the respective mean brightness. At 10◦ resolution, the NVSS deficit
overlaps with another faint region about 10◦ to the west, while the average dip in brightness
then decreases from -14 mK (at 3.4◦) to -4 mK.
The dip in NVSS brightness in the WMAP cold spot region is not due to some peculiarity
– 4 –
of the NVSS survey itself. In Figure 4, we compare the 1◦ convolved NVSS image with the
similar resolution, single dish 408 MHz image of Haslam et al. (1981). This 408 MHz all
sky map is dominated in most places by galactic emission, and was usedby Bennett et al.
(2003) as a template for estimating the synchrotron contribution in CMB observations.
On scales of 1◦, the fluctuations in brightness are a combination of galactic (diffuse) and
extragalactic (smeared small source) contributions. In the region of the cold spot, we can
see the extragalactic contribution at 408 MHz by comparison with the smoothed NVSS
1.4 GHz image. Note that although there is flux everywhere in the NVSS image, this is
the “confusion” from the smoothed contribution of multiple small extragalactic sources in
each beam, whereas the 408 MHz map has strong diffuse galactic emission as well. Strong
brightness dips are seen in both images in the region of the WMAP cold spot - with the
brightness dropping by as much as 62% in the smoothed NVSS; in the 408 MHz map, this
is diluted by galactic emission.
To look more quantitatively at the source density in the cold spot region, we measured
the density of NVSS sources (independent of their fluxes) as a function of distance from
the cold spot. Figure 5 shows the counts in equal area annuli around the WMAP cold spot
down to two different flux limits. With a limit of 5 mJy, there is a 45±8% decrease in counts
in the 1◦ radius circle around the WMAP cold spot centroid. At the survey flux limit of
2.5 mJy, the decrease is 23±3%. This reduction in number counts is what is measured by
McEwen et al. (2007) in their statistical all-sky analysis.
3. Foreground Corrections
Several studies have claimed that the properties of the cold spot are most likely an effect
of incorrect foreground subtraction (Chiang & Naselsky 2004; Coles 2005; Liu & Zhang
2005; Tojeiro et al. 2006). This possibility has been investigated in detail for both the
first year (Vielva et al. 2004; Cruz et al. 2005, 2006) and third (Cruz et al. 2007) year
WMAP data. The arguments against foreground subtraction errors can be summarized in
three main points – 1) The region of the spot shows no spectral dependence in the WMAP
data. This is consistent with the CMB and inconsistent with the known spectral behavior
of galactic emission (as well as the SZ effect). The flat (CMB-like) spectrum is found both
in temperature and kurtosis, as well as in real and wavelet space. 2) Foreground emission
is found to be low in the region of the spot, making it unlikely that an over-subtraction
could produce an apparent non-Gaussianity. 3) Similar results are found when using totally
independent methods to model and subtract out the foreground emission (Cruz et al. 2006),
namely the combined and foreground cleaned Q-V-W map (Bennett et al. 2003) and the
– 5 –
weighted internal linear combination analysis (Tegmark et al. 2003).
Now that we know that there is a reduction in the extragalactic radio source contribution
in the direction of the cold spot, we can re-examine this issue. We ask whether a 20-30%
decrement in the local brightness of the extragalactic synchrotron emission could translate
into a foreground subtraction problem that could generate the WMAP cold spot. We are
not re-examining the foreground question ab initio, simply examining the plausibility that
the deficit of NVSS sources could complicate the foreground calculations in this location.
The characteristic brightness in the 3.4◦ convolved NVSS image around the cold spot is ∼
51 mK at 1.4 GHz; the brightness of the cold spot is ∼ 37 mK. This difference of 14 mK
in brightness (4 mK at 10◦ resolution) represents the extragalactic population contribution
only, as the NVSS is not sensitive to the large scale galactic synchrotron emission. By
contrast, the single dish 1.4 GHz brightness within a few degrees of the cold spot is ∼ 3.4 K,
as measured using the Bonn Stockert 25 m telescope (Reich and Reich 1986). Therefore the
total synchrotron contribution at 1.4 GHz is ∼ 0.7 K above the CMB, 50 times larger than
the localized extragalactic deficit.
One way to create the cold spot would be if the universal spectral index used for the
normal galactic (plus small extragalactic) subtraction was incorrect for the extra brightness
temperature contribution of the NVSS dip, δT (-14 mK at L band, 1.4 GHz). We make an
order of magnitude estimate of this potential error. Following the first year data analysis
(Bennett et al. 2003), and the similar exercise performed by Cruz et al. (2006), we consider
fitting a synchrotron template map at some reference frequency νref , and extrapolating the
model, (F(ν)model), with a spectral index of β = −3 to the Q, V, and W bands. This
spectral index is consistent with those of Cruz et al. (2006) and the average spectral index
observed in the WMAP images (Bennett et al. 2003; Hinshaw et al. 2006). Under the null
hypothesis that the spectral index of δT is the same as that of the mean brightness T0, we
then calculate in the region of the deficit,
F (ν)model = [T0(νref) + δ T (νref)] (ν/νref)
. (1)
However, if the actual spectrum of δT is -α (from L band through W band) instead of
-3, then the true foreground subtraction, F(ν)true, should have been
F (ν)true = T0 (νref) (ν/νref)
+ δT (νref) (ν/νref)
. (2)
The foreground subtraction would then be in error as a function of frequency as follows,
expressed in terms of the L band temperatures :
– 6 –
δF (ν) ≡ F (ν)true − F (ν)model = δT (νL)(νL/ν)
1− (νref/ν)
. (3)
Three different reference frequencies have been used for synchrotron extrapolation – the
Haslam et al. (1981) 408 MHz map (Bennett et al. 2003), the Jonas, Baart & Nicolson
(1998) 2326 MHz Rhodes/HartRAO survey (Cruz et al. 2006), and the internal K and Ka
band WMAP images (Hinshaw et al. 2006). Since the foreground subtraction errors would
be worst extrapolating from the lowest frequency template, we start at 408 MHz and look
at the problems caused by a spectral index for δT that is different than the assumed -3.
We obtain a rough measure of the spectral index of the dip by comparing the 1◦
resolution maps (Figure 4) at 408 MHz and 1400 MHz. At lII ,bII = 207
◦,-55◦ , we find
δT = 2.6±0.75 K (30±12 mK) at 408 (1400) MHz, yielding a spectral index of -3.6±0.5.
Using Equation (3), this would actually lead to a WMAP “hot spot” if a spectral index of
-3.0 had been assumed for the extrapolation. Within the errors, the worst foreground ex-
trapolation mistakes would then be hot spots that range from +0.5 to +4.6 µK at Q band,
1◦ resolution, (≈ +0.25 to +2.3 µK at 4◦ resolution). Our derived spectral index for the dip
is steeper than expected for extragalactic sources, so we also do the calculation assuming
the flattest reasonable extragalactic spectrum of -2.5 . This would lead to a spurious cold
spot of -2.9 µK at 4◦ resolution . In either case, this is far below the -73 µK observed at this
resolution in WMAP, and we thus conclude that the deficit of NVSS sources does not lead
to a significant foreground subtraction error of either sign.
4. Discussion
The WMAP cold spot could have three origins: a) at the last scattering surface (z ∼
1000), b) cosmologically local (z < 1), or c) galactic. Because the spot corresponds to a
significant deficit of flux (and source number counts) in the NVSS, we have argued here that
the spot is cosmologically local and hence, a localized manifestation of the late ISW effect.
Cruz et al. (2005, 2007) derived a temperature deviation for the cold spot of ∼ −20µK
and a diameter of 10◦ using the WMAP 3 year data; on scales of 4◦ the average temperature is
lower, ∼ −73 µK. Using these two data points we derive an approximate relation between the
temperature deviation and the corresponding size of the cold spot: θ(∆T/T ) ≈ 4.5× 10−5,
where θ is the radius of the cold spot in degrees. We now perform an order of magnitude
calculation to see if the late ISW can produce such a spot, assuming that the entire effect
comes from the ISW.
The contribution of the late ISW along a given line of sight is given by (∆ T/T )|ISW =
– 7 –
Φ̇ dη, where the dot represents differentiation with respect to the conformal time η,
dη = dt/a(t), and a is the scale factor. The integrand will be non-zero only at late times
(z<1) when the cosmological constant becomes dynamically dominant.
We start with the Newtonian potential given by
Φ = GM/r ≈
r2 ρb δ. (4)
The proper size r and the background density ρb scale as a and a
−3, respectively. The growth
of the fractional density excess, δ(a) in the linear regime is given by D(a) = δ(a)/δ(a0), and
D(a) is the linear growth factor. For redshifts below ∼ 1 in ΛCDM, this factor can be
approximated as δ(a) ≈ aδ(a0)(3−a)/2. Assuming that the region is spherical, its comoving
radius is rc = 0.5∆z(c/H), and ∆z is the line of sight diameter of the region. The change
in the potential over dη can be approximated by
r2c ρc δ (∆z), (5)
where subscript c refers to the average comoving size of the void and the comoving back-
ground density. In ΛCDM the Hubble parameter is roughly given by H2 = H2
(1 + 2z),
for redshifts below ∼ 1. Incorporating these approximations we get the following relation
between the size of the region, its redshift and the temperature deviation from the late ISW:
∆Φ ≈ −
(1 + 2z)1/2(1 + z)−2 δ ≈
We now ask under what conditions this expression is consistent with the observed rela-
tion between the size and temperature of the cold spot derived earlier, θ(∆T/T ) ≈ 4.5×10−5.
For Ωm ∼ 0.3 and δ = −1 (i.e. a completely empty region) this leads to the simplified re-
lation, θ(1 + z) ≈ 6, where θ is in degrees, as before. Since the spot’s association with the
NVSS places it at z ∼ 0.5− 1, this leads to a self-consistent value of the radius of ∼ 3− 4◦
for the observed spot. For c/H0 = 4000 Mpc, the comoving radius of the void region is
120-160 Mpc.
How likely is such a large underdense region in a concordance cosmology? Suppose there
is only one such large underdense region in the whole volume up to z=1. The correspond-
ing void frequency is then the ratio of the comoving volume of the void to the comoving
volume of the Universe to z=1, which is roughly 3 × 10−5. Is this consistent with ΛCDM?
Void statistics have been done for a number of optical galaxy surveys, as well as numerical
structure formation simulations. Taking the most optimistic void statistics (filled dots in
Fig. 9 of Hoyle & Vogeley, 2004) which can be approximated by logP = −(r/Mpc)/15, a
– 8 –
140 Mpc void would occur with a probability of 5× 10−10, considerably more rare than our
estimate for our Universe (3× 10−5) based on the existence of the cold spot. One must keep
in mind, however, that observational and numerical void probability studies are limited to
rc ∼ 30 Mpc; it is not yet clear how these should be extrapolated to rc > 100 Mpc.
We note that Inoue & Silk (2006a,b) had already suggested that anomalous tempera-
ture anisotropies in the CMB, such as the cold spot, may be explained by the ISW effect. In
contrast to our calculation described above, their analysis considers the linear ISW plus the
second order effects due to an expanding compensated void, partially filled with pressureless
dust, embedded in a standard CDM (Inoue and Silk 2006a) or ΛCDM (Inoue and Silk 2006b)
background. It is reassuring that the size of the void indicated by their analysis—about 200
Mpc if located at z ∼ 1—is roughly the same as what we get here using linear ISW.
The need for an extraordinarily large void to explain the cold spot would add to the
list of anomalies associated with the CMB. (See Holdman, Mersini-Houghton & Takahashi
(2006a,b) for a theory that predicts such large voids based on a particular landscape model.)
These include the systematically higher strength of the late ISW correlation measured for a
variety of mass tracers, compared to theWMAP predictions (see Fig. 11 of Giannantonio et al.
2006), and the alignment and planarity of the quadrupole and the octopole (de Oliveira-Costa et al.
2004; Land & Magueijo 2005). We can, however, conclude that models linking the cold spot
with the larger scale anomalies, such as the anisotropic Bianchi Type VIIh model of Jaffe et
al. (2005), are no longer necessary. While we suggest that the cold spot is a local effect, low
order global anisotropic models (e.g., Gumrukcuoglu, Contaldi & Peloso 2006) may still be
needed for the low−ℓ anomalies.
5. Concluding Remarks
We have detected a significant dip in the average surface brightness and number counts of
radio sources from the NVSS survey at 1.4 GHz in the direction of the WMAP cold spot. The
deficit of extragalactic sources is also seen in a single dish image at 408 MHz. Together with
previous work, we rule out instrumental artifacts in WMAP due to foreground subtraction.
A fuller examination of the statistical uncertainties associated with our combination of the
McEwen et al. (2007) wavelet results and our own a posteriori analysis should be performed.
With this caveat, we conclude that the cold spot arises from effects along the line of sight,
and not at the last scattering surface itself. Any non-gaussianity of the WMAP cold spot
therefore would then have a local origin.
A 140 Mpc radius, completely empty void at z≤1 is sufficient to create the magnitude
– 9 –
and angular size of the cold spot through the late integrated Sachs-Wolfe effect. Voids this
large currently seem improbable in the concordance cosmology, adding to the anomalies
associated with the CMB.
We suggest that a closer investigation of all mass tracers would be useful to search
for significant contributions from isolated regions. Also, if our interpretation of the cold
spot is correct, it might be possible to detect it indirectly using Planck, through the lack of
lensing-induced polarization B modes (Zaldarriaga & Seljak 1997).
ACKNOWLEDGMENTS We thank Eric Greisen, NRAO, for improvements in the
AIPS FLATN routine, which allows us to easily stitch together many fields in a flexible
coordinate system. The 408 MHz maps were obtained through SkyView, operated under
the auspices of NASA’s Goddard Space Flight Center. We appreciate discussions with M.
Peloso, T. J. Jones and E. Greisen regarding this work, and useful criticisms from the
anonymous referee. LR acknowledges the inspiration from his thesis adviser, the late David
T. Wilkinson, who would have appreciated the notion of deriving information from a hole.
At the University of Minnesota, this work is supported in part, through National Science
Foundation grants AST 03-07604 and AST 06-07674 and STScI grant AR-10985.
REFERENCES
Bennett, C.L., et al. 2003, ApJS 148, 1
Cabre, A., Gaztanaga, E., Manera, M., Fosalba, P. Castander, F. 2006, MNRAS 372, 23
Cayon, L., Jin J., Treaster, A. 2005, MNRAS 362, 826
Chiang, L.-Y., Naselsky, P.D. 2006, IJMPD 15,1283C
Coles, P. 2005, Nature 433, 248
Condon, J. J., Cotton, W. D., Greisen, E. W., Yin, Q. F., Perley, R. A., Taylor, G. B.,
Broderick, J. J. 1998, AJ 115, 1693
Crittenden, R. G., Turok, N. 1996, PRL 76, 575
Cruz, M., Martinez-Gonzalez, E., Vielva, P., Cayon, L. 2005, MNRAS 356, 29
Cruz, M., Tucci, M., Martinez-Gonzalez, E., Vielva, P. 2006, MNRAS 369, 57
Cruz, M., Cayon, L., Martinez-Gonzalez, E., Vielva, P., Jin, J. 2007, ApJ 655, 11
– 10 –
de Oliveira-Costa A., et al. 2004, PhRevD 69, 3516
Giannantonio, T. et al. 2006, PhRvD 74, 352
Gumrukcuoglu, A. E., Contaldi, C. R. & Peloso, M. 2006, astro-ph/0608405
Haslam, C. G. T., Klein, U., Salter, C. J., Stoffel, H., Wilson, W.E., Cleary, M.N., Cooke,
D.J., & Thomasson, P. 1981, A&A, 100,209
Hinshaw, G., et al. 2006, ApJ, submitted (astro-ph/0603451)
Holman, R.; Mersini-Houghton, L.; Takahashi, Tomo, 2006, hep-th/0611223
Holman, R.; Mersini-Houghton, L.; Takahashi, Tomo, 2006, hep-th/0612142
Hoyle, F., Vogeley, M. S. 2004, ApJ 607, 751
Inoue, K. T., Silk, J. 2006, ApJ, 648, 23
Inoue, K. T., Silk, J. 2006, astro-ph/0612347
Jaffe, T. R., Banday A. J., Eriksen, H. K., Forski, K. M, Hansen, F. K. 2005, ApJ 629, L1
Jonas, J., Baart, E. E., Necolson, G. D. 1998, MNRAS, 297, 997
Land, K., Magueijo, J.,2005, PRL 95,071301
Liu, X., & Zhang, S. N. 2005, ApJ, 633,542
McEwen, J. D., Vielva, P., Hobson, M. P., Martinez-Gonzalez, E., & Lasenby, A. N. 2007,
MNRAS, in press
Pietrobon, D., Balbi, A., Marinucci, D. 2006, PhysRevD 74, 352
Reich, W. and Reich, P. 1986, A&A Suppl. 63, 205
Tegmark, M., de Oliveira-Costa, A., Hamilton A. 2003, Phys.Rev. D68, 123523.
Tojeiro, R., Castro, P.G., Heavens, A.G., Gupta, S. 2006, MNRAS,365,265
Vielva, P., Martinez-Gonzalez, E., Varreiro, R. B., Sanz, J. L., Cayon, L. 2004, ApJ 609, 22
York, D. G. et al. 2000, AJ 120, 1579
Zaldarriaga, M. & Seljak, U. 1997, PhRvD 55, 1830
This preprint was prepared with the AAS LATEX macros v5.2.
http://arxiv.org/abs/astro-ph/0608405
http://arxiv.org/abs/astro-ph/0603451
http://arxiv.org/abs/hep-th/0611223
http://arxiv.org/abs/hep-th/0612142
http://arxiv.org/abs/astro-ph/0612347
– 11 –
Fig. 1.— 50◦ field from smoothed NVSS survey at 3.4◦ resolution, centered at lII , bII
= 209◦, -57◦. Values range from black: 9.3 mJy/beam to white: 21.5 mJy/beam. A 10◦
diameter circle indicates the position and size of the WMAP cold spot.
– 12 –
Fig. 2.— Aitoff projection of NVSS survey, centered at lII , bII = 180
◦, 0◦, showing the
median brightness in sliding boxes of 3.4◦. The WMAP cold spot is indicated by the black
box. Closer to the plane, large dark patches arise from sidelobes around strong NVSS sources.
Fig. 3.— The cumulative distribution, normalized to 1000, of median brightness levels (mK)
in 3.4◦ sliding boxes of the NVSS images in two strips above |bII | > 30
◦ (see text). The
minimum brightness (which is from the cold spot region) is indicated by a vertical line.
– 13 –
Fig. 4.— 18◦ fields, with 1◦ resolution, centered at lII , bII = 209
◦, -57◦. Left: 408 MHz
(Haslam et al. 1981). Right: 1.4 GHz (Condon et al. 1998). A 10◦ diameter circle indicates
the position and size of the WMAP cold spot.
– 14 –
Fig. 5.— Number of NVSS sources in 3.14 square degree annuli as a function of distance
from the cold spot. The counts axis refers to the results for counts of sources with S>5mJy;
the grey line refers to counts for S>2.5mJy with those counts multiplied by 0.56. Each bin
is independent.
	Introduction
	Analysis and Characterization of the NVSS ``dip''
	Foreground Corrections
	Discussion
	Concluding Remarks

  We detect a dip of 20-45% in the surface brightness and number counts of NVSS
sources smoothed to a few degrees at the location of the WMAP cold spot. The
dip has structure on scales of approximately 1-10 degrees. Together with
independent all-sky wavelet analyses, our results suggest that the dip in
extragalactic brightness and number counts and the WMAP cold spot are
physically related, i.e., that the coincidence is neither a statistical anomaly
nor a WMAP foreground correction problem. If the cold spot does originate from
structures at modest redshifts, as we suggest, then there is no remaining need
for non-Gaussian processes at the last scattering surface of the CMB to explain
the cold spot. The late integrated Sachs-Wolfe effect, already seen
statistically for NVSS source counts, can now be seen to operate on a single
region. To create the magnitude and angular size of the WMAP cold spot requires
a ~140 Mpc radius completely empty void at z<=1 along this line of sight. This
is far outside the current expectations of the concordance cosmology, and adds
to the anomalies seen in the CMB.

Introduction
The detection of an extreme “cold spot” (Vielva et al. 2004) in the foreground-corrected
WMAP images was an exciting but unexpected finding. At 4◦ resolution, Cruz et al. (2005)
determine an amplitude of -73 µK, which reduces to -20 µK at ∼10◦ scales (Cruz et al.
1larry@astro.umn.edu
2brown@astro.umn.edu
3llrw@astro.umn.edu
http://arxiv.org/abs/0704.0908v2
– 2 –
2007). The non-gaussianity of this extreme region has been scrutinized, (Cayon, Jun &
Treaster 2005; Cruz et al. 2005, 2006, 2007) concluding that it cannot be explained by either
foreground correction problems or the normal Gaussian fluctuations of the CMB. Thus, the
cold spot seems to require a distinct origin – either primordial or local. Across the whole sky,
local mass tracers such as the optical Sloan Digital Sky Survey (SDSS, York et al. 2000) and
the radio NRAO VLA Sky Survey (NVSS, Condon et al. 1998) are seen to correlate with
the WMAP images of the CMB (Pietrobon, Balbi & Marinucci 2006; Cabre et al. 2006),
probably through the late integrated Sachs-Wolfe effect (ISW, Crittenden & Turok 1996).
McEwen et al. (2007) extended the study of radio source/CMB correlations by per-
forming a steerable wavelet analysis of NVSS source counts and WMAP images. They
isolated 18 regions that, as a group, contributed a significant fraction of the total NVSS-
ISW signal. Three of those 18 regions were additionally robust to the choice of wavelet form.
The centroid of one of those three robust correlated regions, (#16), is inside the 10◦ cold
spot derived from WMAP data alone (Cruz et al. 2007), although McEwen et al. (2007)
did not point out this association.
The investigations reported here were conducted independently and originally without
knowledge of the McEwen et al. (2007) analysis. However, our work is a posteriori in
nature, because we were specifically looking for the properties in the direction of the cold
spot. These results thus support and quantify the NVSS properties in the specific direction
of the cold spot, but should be considered along with the McEwen et al. (2007) analysis for
the purposes of an unbiased proof of a WMAP association.
2. Analysis and Characterization of the NVSS “dip”
We examined both the number counts of NVSS sources in the direction of the WMAP
cold spot and their smoothed brightness distribution. The NVSS 21 cm survey covers the
sky above a declination of -40◦ at a resolution of 45”. It has an rms noise of 0.45 mJy/beam
and is accompanied by a catalog of sources stronger than 2.5 mJy/beam. Because of the
short interferometric observations that went into its construction, the survey is insensitive to
diffuse sources greater than ≈ 15’ in extent. Convolution of the NVSS images to larger beam
sizes, as done here, shows the integrated surface brightness of small extragalactic sources;
this is very different than what would be observed by a single dish of equivalent resolution. In
the latter case, the diffuse structure of the Milky Way Galaxy dominates (e.g, Haslam et al.
(1981)), although it is largely invisible to the interferometer.
To explore the extragalactic radio source population in the direction of the WMAP
– 3 –
cold spot, we first show in Figure 1 the 50◦×50◦ region around the cold spot convolved to a
resolution of 3.4◦. Here, the region of the cold spot is seen to be the faintest region on the
image (minimum at lII ,bII = 207.8
◦, -56.3◦). At minimum, its brightness is 14 mK below
the mean, with an extent of ≈5◦. The WMAP cold spot thus picks out a special region in
the NVSS – at least within this 2500� region.
We examined the smoothed brightness distribution across the whole NVSS survey using
another averaging technique that reduces the confusion from the brightest sources. We first
pre-convolved the images to 800”, which fills all the gaps between neighboring sources, and
then calculated the median brightness in sliding boxes 3.4◦ on a side. The resulting image
is shown in Figure 2 which is in galactic coordinates centered at lII=180
◦. The dark regions
near the galactic plane are regions of the NVSS survey that were perturbed by the presence
of very strong sources. Note that the galactic plane itself, which dominates single dish maps,
is only detectable here between -20◦ < lII < 90
◦, where there is a local increase in the number
of small sources detectable by the interferometer.
To evaluate the NVSS brightness properties of the cold spot, we compared it with the
distribution of median brightnesses in two strips from this all sky map. The first strip was
in the north, taking everything above a nominal galactic latitude of 30◦ (More precisely,
we used the horizontal line in the Aitoff projection tangent to the 30◦ line at lII=180
The second strip was in the south, taking everything below a nominal galactic latitude of
-30◦, but only from 10◦ < lII <180
◦, to avoid regions near the survey limit of δ=-40◦. The
minimum brightness in the cold spot region (≈ 20 mK) is equal to the lowest values seen in
the 16,800 square degree area of the two strips, and is ≈ 30% below the mean (Figure 3).
Formally, the probability of finding this weakest NVSS spot within the ≈10◦ (diameter)
region of the WMAP cold spot is 0.6%. This a posteriori analysis thus is in agreement with
the statistical conclusions of McEwen et al. (2007) that the NVSS properties in this region
are linked to those of WMAP. We also note that the magnitude of the NVSS dip is at the
extreme, but not an outlier of the overall brightness distribution. We thus expect that less
extreme NVSS dips would also individually correlate with WMAP cold regions, although it
may be more difficult to separate those from the primordial fluctuations.
The NVSS brightness dip can be seen at a number of resolutions, and there is probably
more than one scale size present. At resolutions of 1◦, 3.4◦, and 10◦, we find that the dip is
≈ 60%, 30% and 10% of the respective mean brightness. At 10◦ resolution, the NVSS deficit
overlaps with another faint region about 10◦ to the west, while the average dip in brightness
then decreases from -14 mK (at 3.4◦) to -4 mK.
The dip in NVSS brightness in the WMAP cold spot region is not due to some peculiarity
– 4 –
of the NVSS survey itself. In Figure 4, we compare the 1◦ convolved NVSS image with the
similar resolution, single dish 408 MHz image of Haslam et al. (1981). This 408 MHz all
sky map is dominated in most places by galactic emission, and was usedby Bennett et al.
(2003) as a template for estimating the synchrotron contribution in CMB observations.
On scales of 1◦, the fluctuations in brightness are a combination of galactic (diffuse) and
extragalactic (smeared small source) contributions. In the region of the cold spot, we can
see the extragalactic contribution at 408 MHz by comparison with the smoothed NVSS
1.4 GHz image. Note that although there is flux everywhere in the NVSS image, this is
the “confusion” from the smoothed contribution of multiple small extragalactic sources in
each beam, whereas the 408 MHz map has strong diffuse galactic emission as well. Strong
brightness dips are seen in both images in the region of the WMAP cold spot - with the
brightness dropping by as much as 62% in the smoothed NVSS; in the 408 MHz map, this
is diluted by galactic emission.
To look more quantitatively at the source density in the cold spot region, we measured
the density of NVSS sources (independent of their fluxes) as a function of distance from
the cold spot. Figure 5 shows the counts in equal area annuli around the WMAP cold spot
down to two different flux limits. With a limit of 5 mJy, there is a 45±8% decrease in counts
in the 1◦ radius circle around the WMAP cold spot centroid. At the survey flux limit of
2.5 mJy, the decrease is 23±3%. This reduction in number counts is what is measured by
McEwen et al. (2007) in their statistical all-sky analysis.
3. Foreground Corrections
Several studies have claimed that the properties of the cold spot are most likely an effect
of incorrect foreground subtraction (Chiang & Naselsky 2004; Coles 2005; Liu & Zhang
2005; Tojeiro et al. 2006). This possibility has been investigated in detail for both the
first year (Vielva et al. 2004; Cruz et al. 2005, 2006) and third (Cruz et al. 2007) year
WMAP data. The arguments against foreground subtraction errors can be summarized in
three main points – 1) The region of the spot shows no spectral dependence in the WMAP
data. This is consistent with the CMB and inconsistent with the known spectral behavior
of galactic emission (as well as the SZ effect). The flat (CMB-like) spectrum is found both
in temperature and kurtosis, as well as in real and wavelet space. 2) Foreground emission
is found to be low in the region of the spot, making it unlikely that an over-subtraction
could produce an apparent non-Gaussianity. 3) Similar results are found when using totally
independent methods to model and subtract out the foreground emission (Cruz et al. 2006),
namely the combined and foreground cleaned Q-V-W map (Bennett et al. 2003) and the
– 5 –
weighted internal linear combination analysis (Tegmark et al. 2003).
Now that we know that there is a reduction in the extragalactic radio source contribution
in the direction of the cold spot, we can re-examine this issue. We ask whether a 20-30%
decrement in the local brightness of the extragalactic synchrotron emission could translate
into a foreground subtraction problem that could generate the WMAP cold spot. We are
not re-examining the foreground question ab initio, simply examining the plausibility that
the deficit of NVSS sources could complicate the foreground calculations in this location.
The characteristic brightness in the 3.4◦ convolved NVSS image around the cold spot is ∼
51 mK at 1.4 GHz; the brightness of the cold spot is ∼ 37 mK. This difference of 14 mK
in brightness (4 mK at 10◦ resolution) represents the extragalactic population contribution
only, as the NVSS is not sensitive to the large scale galactic synchrotron emission. By
contrast, the single dish 1.4 GHz brightness within a few degrees of the cold spot is ∼ 3.4 K,
as measured using the Bonn Stockert 25 m telescope (Reich and Reich 1986). Therefore the
total synchrotron contribution at 1.4 GHz is ∼ 0.7 K above the CMB, 50 times larger than
the localized extragalactic deficit.
One way to create the cold spot would be if the universal spectral index used for the
normal galactic (plus small extragalactic) subtraction was incorrect for the extra brightness
temperature contribution of the NVSS dip, δT (-14 mK at L band, 1.4 GHz). We make an
order of magnitude estimate of this potential error. Following the first year data analysis
(Bennett et al. 2003), and the similar exercise performed by Cruz et al. (2006), we consider
fitting a synchrotron template map at some reference frequency νref , and extrapolating the
model, (F(ν)model), with a spectral index of β = −3 to the Q, V, and W bands. This
spectral index is consistent with those of Cruz et al. (2006) and the average spectral index
observed in the WMAP images (Bennett et al. 2003; Hinshaw et al. 2006). Under the null
hypothesis that the spectral index of δT is the same as that of the mean brightness T0, we
then calculate in the region of the deficit,
F (ν)model = [T0(νref) + δ T (νref)] (ν/νref)
. (1)
However, if the actual spectrum of δT is -α (from L band through W band) instead of
-3, then the true foreground subtraction, F(ν)true, should have been
F (ν)true = T0 (νref) (ν/νref)
+ δT (νref) (ν/νref)
. (2)
The foreground subtraction would then be in error as a function of frequency as follows,
expressed in terms of the L band temperatures :
– 6 –
δF (ν) ≡ F (ν)true − F (ν)model = δT (νL)(νL/ν)
1− (νref/ν)
. (3)
Three different reference frequencies have been used for synchrotron extrapolation – the
Haslam et al. (1981) 408 MHz map (Bennett et al. 2003), the Jonas, Baart & Nicolson
(1998) 2326 MHz Rhodes/HartRAO survey (Cruz et al. 2006), and the internal K and Ka
band WMAP images (Hinshaw et al. 2006). Since the foreground subtraction errors would
be worst extrapolating from the lowest frequency template, we start at 408 MHz and look
at the problems caused by a spectral index for δT that is different than the assumed -3.
We obtain a rough measure of the spectral index of the dip by comparing the 1◦
resolution maps (Figure 4) at 408 MHz and 1400 MHz. At lII ,bII = 207
◦,-55◦ , we find
δT = 2.6±0.75 K (30±12 mK) at 408 (1400) MHz, yielding a spectral index of -3.6±0.5.
Using Equation (3), this would actually lead to a WMAP “hot spot” if a spectral index of
-3.0 had been assumed for the extrapolation. Within the errors, the worst foreground ex-
trapolation mistakes would then be hot spots that range from +0.5 to +4.6 µK at Q band,
1◦ resolution, (≈ +0.25 to +2.3 µK at 4◦ resolution). Our derived spectral index for the dip
is steeper than expected for extragalactic sources, so we also do the calculation assuming
the flattest reasonable extragalactic spectrum of -2.5 . This would lead to a spurious cold
spot of -2.9 µK at 4◦ resolution . In either case, this is far below the -73 µK observed at this
resolution in WMAP, and we thus conclude that the deficit of NVSS sources does not lead
to a significant foreground subtraction error of either sign.
4. Discussion
The WMAP cold spot could have three origins: a) at the last scattering surface (z ∼
1000), b) cosmologically local (z < 1), or c) galactic. Because the spot corresponds to a
significant deficit of flux (and source number counts) in the NVSS, we have argued here that
the spot is cosmologically local and hence, a localized manifestation of the late ISW effect.
Cruz et al. (2005, 2007) derived a temperature deviation for the cold spot of ∼ −20µK
and a diameter of 10◦ using the WMAP 3 year data; on scales of 4◦ the average temperature is
lower, ∼ −73 µK. Using these two data points we derive an approximate relation between the
temperature deviation and the corresponding size of the cold spot: θ(∆T/T ) ≈ 4.5× 10−5,
where θ is the radius of the cold spot in degrees. We now perform an order of magnitude
calculation to see if the late ISW can produce such a spot, assuming that the entire effect
comes from the ISW.
The contribution of the late ISW along a given line of sight is given by (∆ T/T )|ISW =
– 7 –
Φ̇ dη, where the dot represents differentiation with respect to the conformal time η,
dη = dt/a(t), and a is the scale factor. The integrand will be non-zero only at late times
(z<1) when the cosmological constant becomes dynamically dominant.
We start with the Newtonian potential given by
Φ = GM/r ≈
r2 ρb δ. (4)
The proper size r and the background density ρb scale as a and a
−3, respectively. The growth
of the fractional density excess, δ(a) in the linear regime is given by D(a) = δ(a)/δ(a0), and
D(a) is the linear growth factor. For redshifts below ∼ 1 in ΛCDM, this factor can be
approximated as δ(a) ≈ aδ(a0)(3−a)/2. Assuming that the region is spherical, its comoving
radius is rc = 0.5∆z(c/H), and ∆z is the line of sight diameter of the region. The change
in the potential over dη can be approximated by
r2c ρc δ (∆z), (5)
where subscript c refers to the average comoving size of the void and the comoving back-
ground density. In ΛCDM the Hubble parameter is roughly given by H2 = H2
(1 + 2z),
for redshifts below ∼ 1. Incorporating these approximations we get the following relation
between the size of the region, its redshift and the temperature deviation from the late ISW:
∆Φ ≈ −
(1 + 2z)1/2(1 + z)−2 δ ≈
We now ask under what conditions this expression is consistent with the observed rela-
tion between the size and temperature of the cold spot derived earlier, θ(∆T/T ) ≈ 4.5×10−5.
For Ωm ∼ 0.3 and δ = −1 (i.e. a completely empty region) this leads to the simplified re-
lation, θ(1 + z) ≈ 6, where θ is in degrees, as before. Since the spot’s association with the
NVSS places it at z ∼ 0.5− 1, this leads to a self-consistent value of the radius of ∼ 3− 4◦
for the observed spot. For c/H0 = 4000 Mpc, the comoving radius of the void region is
120-160 Mpc.
How likely is such a large underdense region in a concordance cosmology? Suppose there
is only one such large underdense region in the whole volume up to z=1. The correspond-
ing void frequency is then the ratio of the comoving volume of the void to the comoving
volume of the Universe to z=1, which is roughly 3 × 10−5. Is this consistent with ΛCDM?
Void statistics have been done for a number of optical galaxy surveys, as well as numerical
structure formation simulations. Taking the most optimistic void statistics (filled dots in
Fig. 9 of Hoyle & Vogeley, 2004) which can be approximated by logP = −(r/Mpc)/15, a
– 8 –
140 Mpc void would occur with a probability of 5× 10−10, considerably more rare than our
estimate for our Universe (3× 10−5) based on the existence of the cold spot. One must keep
in mind, however, that observational and numerical void probability studies are limited to
rc ∼ 30 Mpc; it is not yet clear how these should be extrapolated to rc > 100 Mpc.
We note that Inoue & Silk (2006a,b) had already suggested that anomalous tempera-
ture anisotropies in the CMB, such as the cold spot, may be explained by the ISW effect. In
contrast to our calculation described above, their analysis considers the linear ISW plus the
second order effects due to an expanding compensated void, partially filled with pressureless
dust, embedded in a standard CDM (Inoue and Silk 2006a) or ΛCDM (Inoue and Silk 2006b)
background. It is reassuring that the size of the void indicated by their analysis—about 200
Mpc if located at z ∼ 1—is roughly the same as what we get here using linear ISW.
The need for an extraordinarily large void to explain the cold spot would add to the
list of anomalies associated with the CMB. (See Holdman, Mersini-Houghton & Takahashi
(2006a,b) for a theory that predicts such large voids based on a particular landscape model.)
These include the systematically higher strength of the late ISW correlation measured for a
variety of mass tracers, compared to theWMAP predictions (see Fig. 11 of Giannantonio et al.
2006), and the alignment and planarity of the quadrupole and the octopole (de Oliveira-Costa et al.
2004; Land & Magueijo 2005). We can, however, conclude that models linking the cold spot
with the larger scale anomalies, such as the anisotropic Bianchi Type VIIh model of Jaffe et
al. (2005), are no longer necessary. While we suggest that the cold spot is a local effect, low
order global anisotropic models (e.g., Gumrukcuoglu, Contaldi & Peloso 2006) may still be
needed for the low−ℓ anomalies.
5. Concluding Remarks
We have detected a significant dip in the average surface brightness and number counts of
radio sources from the NVSS survey at 1.4 GHz in the direction of the WMAP cold spot. The
deficit of extragalactic sources is also seen in a single dish image at 408 MHz. Together with
previous work, we rule out instrumental artifacts in WMAP due to foreground subtraction.
A fuller examination of the statistical uncertainties associated with our combination of the
McEwen et al. (2007) wavelet results and our own a posteriori analysis should be performed.
With this caveat, we conclude that the cold spot arises from effects along the line of sight,
and not at the last scattering surface itself. Any non-gaussianity of the WMAP cold spot
therefore would then have a local origin.
A 140 Mpc radius, completely empty void at z≤1 is sufficient to create the magnitude
– 9 –
and angular size of the cold spot through the late integrated Sachs-Wolfe effect. Voids this
large currently seem improbable in the concordance cosmology, adding to the anomalies
associated with the CMB.
We suggest that a closer investigation of all mass tracers would be useful to search
for significant contributions from isolated regions. Also, if our interpretation of the cold
spot is correct, it might be possible to detect it indirectly using Planck, through the lack of
lensing-induced polarization B modes (Zaldarriaga & Seljak 1997).
ACKNOWLEDGMENTS We thank Eric Greisen, NRAO, for improvements in the
AIPS FLATN routine, which allows us to easily stitch together many fields in a flexible
coordinate system. The 408 MHz maps were obtained through SkyView, operated under
the auspices of NASA’s Goddard Space Flight Center. We appreciate discussions with M.
Peloso, T. J. Jones and E. Greisen regarding this work, and useful criticisms from the
anonymous referee. LR acknowledges the inspiration from his thesis adviser, the late David
T. Wilkinson, who would have appreciated the notion of deriving information from a hole.
At the University of Minnesota, this work is supported in part, through National Science
Foundation grants AST 03-07604 and AST 06-07674 and STScI grant AR-10985.
REFERENCES
Bennett, C.L., et al. 2003, ApJS 148, 1
Cabre, A., Gaztanaga, E., Manera, M., Fosalba, P. Castander, F. 2006, MNRAS 372, 23
Cayon, L., Jin J., Treaster, A. 2005, MNRAS 362, 826
Chiang, L.-Y., Naselsky, P.D. 2006, IJMPD 15,1283C
Coles, P. 2005, Nature 433, 248
Condon, J. J., Cotton, W. D., Greisen, E. W., Yin, Q. F., Perley, R. A., Taylor, G. B.,
Broderick, J. J. 1998, AJ 115, 1693
Crittenden, R. G., Turok, N. 1996, PRL 76, 575
Cruz, M., Martinez-Gonzalez, E., Vielva, P., Cayon, L. 2005, MNRAS 356, 29
Cruz, M., Tucci, M., Martinez-Gonzalez, E., Vielva, P. 2006, MNRAS 369, 57
Cruz, M., Cayon, L., Martinez-Gonzalez, E., Vielva, P., Jin, J. 2007, ApJ 655, 11
– 10 –
de Oliveira-Costa A., et al. 2004, PhRevD 69, 3516
Giannantonio, T. et al. 2006, PhRvD 74, 352
Gumrukcuoglu, A. E., Contaldi, C. R. & Peloso, M. 2006, astro-ph/0608405
Haslam, C. G. T., Klein, U., Salter, C. J., Stoffel, H., Wilson, W.E., Cleary, M.N., Cooke,
D.J., & Thomasson, P. 1981, A&A, 100,209
Hinshaw, G., et al. 2006, ApJ, submitted (astro-ph/0603451)
Holman, R.; Mersini-Houghton, L.; Takahashi, Tomo, 2006, hep-th/0611223
Holman, R.; Mersini-Houghton, L.; Takahashi, Tomo, 2006, hep-th/0612142
Hoyle, F., Vogeley, M. S. 2004, ApJ 607, 751
Inoue, K. T., Silk, J. 2006, ApJ, 648, 23
Inoue, K. T., Silk, J. 2006, astro-ph/0612347
Jaffe, T. R., Banday A. J., Eriksen, H. K., Forski, K. M, Hansen, F. K. 2005, ApJ 629, L1
Jonas, J., Baart, E. E., Necolson, G. D. 1998, MNRAS, 297, 997
Land, K., Magueijo, J.,2005, PRL 95,071301
Liu, X., & Zhang, S. N. 2005, ApJ, 633,542
McEwen, J. D., Vielva, P., Hobson, M. P., Martinez-Gonzalez, E., & Lasenby, A. N. 2007,
MNRAS, in press
Pietrobon, D., Balbi, A., Marinucci, D. 2006, PhysRevD 74, 352
Reich, W. and Reich, P. 1986, A&A Suppl. 63, 205
Tegmark, M., de Oliveira-Costa, A., Hamilton A. 2003, Phys.Rev. D68, 123523.
Tojeiro, R., Castro, P.G., Heavens, A.G., Gupta, S. 2006, MNRAS,365,265
Vielva, P., Martinez-Gonzalez, E., Varreiro, R. B., Sanz, J. L., Cayon, L. 2004, ApJ 609, 22
York, D. G. et al. 2000, AJ 120, 1579
Zaldarriaga, M. & Seljak, U. 1997, PhRvD 55, 1830
This preprint was prepared with the AAS LATEX macros v5.2.
http://arxiv.org/abs/astro-ph/0608405
http://arxiv.org/abs/astro-ph/0603451
http://arxiv.org/abs/hep-th/0611223
http://arxiv.org/abs/hep-th/0612142
http://arxiv.org/abs/astro-ph/0612347
– 11 –
Fig. 1.— 50◦ field from smoothed NVSS survey at 3.4◦ resolution, centered at lII , bII
= 209◦, -57◦. Values range from black: 9.3 mJy/beam to white: 21.5 mJy/beam. A 10◦
diameter circle indicates the position and size of the WMAP cold spot.
– 12 –
Fig. 2.— Aitoff projection of NVSS survey, centered at lII , bII = 180
◦, 0◦, showing the
median brightness in sliding boxes of 3.4◦. The WMAP cold spot is indicated by the black
box. Closer to the plane, large dark patches arise from sidelobes around strong NVSS sources.
Fig. 3.— The cumulative distribution, normalized to 1000, of median brightness levels (mK)
in 3.4◦ sliding boxes of the NVSS images in two strips above |bII | > 30
◦ (see text). The
minimum brightness (which is from the cold spot region) is indicated by a vertical line.
– 13 –
Fig. 4.— 18◦ fields, with 1◦ resolution, centered at lII , bII = 209
◦, -57◦. Left: 408 MHz
(Haslam et al. 1981). Right: 1.4 GHz (Condon et al. 1998). A 10◦ diameter circle indicates
the position and size of the WMAP cold spot.
– 14 –
Fig. 5.— Number of NVSS sources in 3.14 square degree annuli as a function of distance
from the cold spot. The counts axis refers to the results for counts of sources with S>5mJy;
the grey line refers to counts for S>2.5mJy with those counts multiplied by 0.56. Each bin
is independent.
	Introduction
	Analysis and Characterization of the NVSS ``dip''
	Foreground Corrections
	Discussion
	Concluding Remarks
