New Clues in the CMB Cold (Spot) Case

Title: The CMB Cold Spot as predicted by foregrounds around nearby galaxies

Authors: Diego Garcia Lambas, Frode K. Hansen, Facundo Toscano, et al.

First Author’s Institution:  Instituto de Astronomía Teórica y Experimental (IATE), CONICET-UNC, Córdoba, Argentina; Observatorio Astronómico de Córdoba (OAC), UNC, Córdoba, Argentina; Comisión Nacional de Actividades Espaciales (CONAE), Córdoba, Argentina

Status: Published in A&A [open access]

Reviewing the Case

The cosmic microwave background (CMB) is the oldest observable light in the Universe, and precise measurements of this light serve as a valuable tool we can use to probe our understanding of cosmology. As a result of density fluctuations in the early Universe, the standard cosmological model predicts the CMB to be a 2.73 K blackbody that varies in temperature at a level of 1 part in 100,000, with the largest of these variations occurring at 1° angular scales. This is exactly what we see when we observe the CMB – a sky filled with hot and cold spots characterized by the standard model’s expectations. However, this is not all we see – we also see a region of the CMB that deviates from the standard model’s prediction (circled in red in Figure 1), which has been puzzling researchers since it was first discovered by WMAP 20 years ago.

A Planck CMB temperature map showing the typical 1 degree hot and cold spots, along with the anomalous 10 degree Cold Spot in the CMB's southern hemisphere.
Figure 1: Planck CMB temperature map with the Cold Spot circled in red. Image adapted from ESA and the Planck Collaboration.

Known as the Cold Spot, not only is this region anomalously cold, boasting temperature decrements up to 7 times greater than that of a typical cold spot, but it is also anomalously large, spanning about 10° on the sky. Although it is possible that such an anomaly could arise given the conditions of the standard model, it has been shown to be statistically unlikely. So, we are left to solve a cosmological case of whodunit. One suspect is a cosmic void lurking in the CMB Cold Spot’s line of sight. This void could induce a gravitational redshift in the CMB photons passing through it via the integrated Sachs-Wolfe (ISW) effect, causing that region of the CMB to appear colder. Although the Dark Energy Survey (DES) has revealed that such a void does exist, this lead may be a red herring, as it has been shown that this void cannot be fully responsible for the Cold Spot signal. Another suspect that has recently been brought forward is the Eridanus supergroup, a clump of three nearby galaxy groups residing in the Cold Spot region: NGC 1407, NGC 1332, and the Eridanus group. It is theorized that the Cold Spot signal could be attributed to the foregrounds of galaxies in and around this supergroup. In today’s paper, the authors investigate this theory in an effort to come closer to cracking the case!

Gathering Evidence

The authors began their investigation by modeling the foregrounds of the galaxies of interest based on data from three galaxy survey catalogs: 2MRS, 6dF, and HIPASS. They compared their foreground model to a CMB temperature map generated from Planck observations and found that their foreground model produced a temperature decrement resembling the CMB Cold Spot in both shape and intensity (shown in panels a, b, and c of Figure 2). They went on to subtract this foreground temperature model from the Planck CMB temperature map and produced a standard-looking CMB map (shown panel d of Figure 2), which supports the notion that the Cold Spot could be attributed to these foregrounds.

Four panels displaying a Planck CMB temperature map of the Cold Spot, a temperature map of the authors' foreground model in the Cold Spot region, a galaxy gensity map of the authors' foreground model in the Cold Spot region, and the Planck CMB temperature map with the foreground temperature model subtracted from it. There is a remarkable similarity in shape and intensity of the CMB Cold Spot to the temperature decrement produced by the foreground model.
Figure 2: a) Planck CMB temperature map of Cold Spot region. b) Temperature map of foreground model in Cold Spot region. c) Galaxy density map of foreground model in Cold Spot region. d) Result of subtracting foreground temperature model from Planck CMB temperature map in Cold Spot region. The gray contours in panels a, b, and c indicate the [-20, -30] μK contours of the foreground temperature model to guide by-eye comparison. Image adapted from Figures 2 and 3 in today’s paper.

Identifying Individual Suspects

As the next step of their investigation, the authors attempted to identify particular galaxy candidates whose foregrounds could be contributing to the Cold Spot signal. They compared the Cold Spot region of a Planck CMB temperature map to maps of the galaxies examined from each catalog in that region, color-coded by redshift or size (shown in Figure 3). They found that although all the galaxy groups comprising the Eridanus supergroup overlap with the Cold Spot, only NGC1332 coincides with one of the coldest regions, or subspots, of the Cold Spot (subspot 1). However, there are larger galaxies in the 2MRS catalog that are within or close to subspots 1, 2 and 3 that could be responsible for these temperature decrements. Despite there not being a clear 2MRS galaxy candidate for subspot 4, the 6dF and HIPASS catalogs show that there are several galaxies nearby, so this subspot could be caused by one of these galaxies or a superposition of many of these galaxies, though it is also possible that subspot 4 could just be one of the CMB’s standard 1° cold spots.

Four panels showing a Planck temperature map of the CMB Cold Spot region and three maps of the 6dF, 2MRS, and HIPASS galaxies examined in this paper. The panels are superimposed with circles indicating the locations of the three galaxy groups within the Eridanus supergroup and circles indicating the Cold Spot's coldest regions, or subspots.
Figure 3: a) Planck CMB temperature map in Cold Spot region. b) Galaxies in 6dF catalog in Cold Spot region, color-coded according to redshift. c) Galaxies in 2MRS catalog in Cold Spot region, color-coded according to size. d) Galaxies in HIPASS catalog in Cold Spot region, color-coded according to redshift. The gray circles in panels a and b indicate the location of the galaxy groups within the Eridanus supergroup, with the groups labeled in panel a. The red circles in panel a and gray circles in panels c and d indicate the locations of the largest temperature decrements (subspots) within the Cold Spot, which are labeled 1-4 in panel c. Image adapted from Figure 6 in today’s paper.

To evaluate these galaxies from another angle, the authors also considered their atomic hydrogen (HI) content. This was quantified by the hydrogen deficiency parameter, which is the fraction of neutral hydrogen that is stripped from the galaxies by tidal interactions. They found that weighting their galaxy foreground map by the most HI deficient galaxies produced a more prominent Cold Spot compared to the unweighted map (shown in Figure 4). This suggests that the stripping of material from these galaxies could play a key role in generating the Cold Spot-inducing foreground.

A comparison of the authors' foreground temperature model weighted and unweighted by the most HI deficient galaxies. The foreground model weighted by the most HI deficient galaxies produces a much more pronounced temperature decrement in the Cold Spot region.
Figure 4: Left: Temperature map of galaxy foregrounds. Right: Temperature map of galaxy foregrounds weighted by the most HI deficient galaxies. Image adapted from Figure 8 in today’s paper.

One Step Closer to Cracking the Case

Although today’s authors have not definitively closed the case of the CMB Cold Spot, they have presented compelling evidence that points to the Eridanus supergroup being the culprit. Their foreground model of the galaxies in and around the Eridanus supergroup produces a cold region that is strikingly similar to the CMB Cold Spot. They were also able to pinpoint particular galaxies that could be responsible for the Cold Spot signal and demonstrated that the tidal stripping of these galaxies could be the mechanism behind its existence.

Astrobite edited by Abbe Whitford and Yoni Brande

Featured image credit: adapted from ESA and the Planck Collaboration

About Cesiley King

I'm currently a 4th year PhD candidate at Case Western Reserve University. I work on instrumentation for CMB-S4, a next generation ground-based cosmic microwave background (CMB) experiment. I am also working on analyzing data from Spider's (a balloon-borne CMB experiment) second flight.

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