The Accelerator at the Galactic Centre

About 30 cosmic rays travel through your body every second. In a night of sleep, up to a million of them will have gone through you! What are they? Well, they are cosmic, but they are not ‘rays’ in reality. They are mainly protons, alpha particles (helium nuclei), some electrons and even some anti-matter. They have a variety of energies (107 – 1020 eV) and their origin, in general, is not well understood.

In fact the majority of the primary cosmic rays do not reach the ground – they interact with other particles in our atmosphere, and it is typically secondary cosmic rays – muons, neutrons, protons, alpha particles, electrons, etc. – that hit the Earth’s surface (and us).

Going upwards in energy, the probable sources of cosmic rays are from solar flares and coronal mass ejections from our Sun, shocks in supernova remnants, other high energy sources or events within the Galaxy, as well as high energy sources or events from outside of our Galaxy.

One of the observatories able to detect cosmic rays is HESS (High Energy Stereoscopic System). It has operated over the last decade or so. In theory, sources within our Galaxy should be able to produce particles up to at least one petaelectronvolt (PeV; 1015 electronvolts). This implies that our Galaxy contains petaelectronvolt accelerators, so called ‘PeVatrons’ – but their location has proved elusive. Not even the handful of shell-type supernova remnants, commonly believed to supply most (lower energy) Galactic cosmic rays, has shown the characteristic tracers of petaelectronvolt particles.

Using HESS, a system of imaging atmospheric Cherenkov telescopes, astronomers have detected a very powerful source of cosmic rays in our Galaxy.

hess_full

High Energy Stereoscopic System (HESS) is a system of Imaging Atmospheric Cherenkov Telescopes in Namibia. Phase 1-4 telescopes were officially inaugurated on 28 September 2004, and the much larger fifth telescope – HESS II – has been operational since July 2012. (Credit: HESS Collaboration)

Deep γ-ray observations by HESS of the region surrounding the Galactic Centre show the expected tracer of the presence of PeV protons within the central 10 parsecs of the Galaxy. The area imaged is mostly molecular gas and the γ-rays result from the interactions of relativistic protons with the ambient gas in this region.

Artist’s impression of the giant molecular clouds surrounding the Milky Way’s centre, bombarded by very high energy protons accelerated in the vicinity of Sagittarius A*. (Credit: Mark A. Garlick / HESS Collaboration)

The well-known supermassive black hole Sagittarius A* (Sgr A*) at the centre of our Galaxy may be linked to this PeVatron. Although its current rate of particle acceleration is not sufficient to provide a substantial contribution to Galactic cosmic rays, Sagittarius A* could have plausibly been more active over the last 106–107 years, and therefore should be considered as a viable alternative to supernova remnants as a source of PeV Galactic cosmic rays.

However there are potentially other sources near to the Galactic Centre that can not be completely ruled out as cosmic ray producers: these include the γ-ray source HESS J1745-290, the pulsar wind nebula G 359.95 – 0.04 and a source of anninihilating dark matter. Other sources like supernova remnants do not have the capacity to inject ultra-high energy particles over the required timescales of a few thousand years. A key finding is that the source location observed by HESS is within ~10pc of the Galactic Centre. This combined with timescale arguments tend to rule out supernova remnants, compact stellar clusters and radio filaments as the source of PeV cosmic rays.

Chandra X-ray image of Sgr A* and surrounds. Three X-ray energies are show: low in red, medium in green and high in blue. The image is about 12 arcmin or 91 light years across. ( Credit: NASA/CXC/Univ. of Wisconsin/Y.Bai. et al.)

The most plausible origin of ultra-relativistic protons is from the vicinity of the 4 x 106 solar mass central black hole. The exact origin is not known but it is most likely to be associated with processes near the black hole – i.e. accretion or jet-related events. The requirement centres on the central black hole being 10-100 times more luminous to accelerate protons over the last 106–107 years to produce the observed PeV events. The most energetic source of cosmic rays from inside our Galaxy may have been found.

For more information, see:

[Glen Mackie]

 

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10th anniversary of the Mars Reconnaissance Orbiter

This week marked the 10th anniversary of science operations of the NASA Mars Reconnaissance Orbiter (MRO). Launched 12 August 2005, MRO commenced orbit around Mars on 10 March 2006.

During its 10 years of orbital reconnaissance and exploration of Mars, MRO has travelled over 1.5 billion km, orbited Mars 45,000 times, returned 264 terabytes of data, taken over 216,000 images and been a scout for the landing site of 7 Mars missions, including Curiosity and the Phoenix Lander.

Some of Mars Reconnaissance Orbiter’s activities over the past decade by the numbers (Credit: NASA/JPL-Caltech)

The key science goal of MRO is to understand the history of water on Mars. Previous Mars missions have provided an abundance of evidence that water once flows on the surface of Mars, but exactly when and for how long surface water existed on Mars and whether it could provide a habit for life is what MRO was to investigate.

To date, MRO has shown us that Mars hosted a wide variety of wet environments several billion years ago and that more recently water has cycled between the gas phase and polar ice deposition and lower-latitude ice and snow. The most spectacular finding to date, announced 28 September 2015, was the evidence that liquid water flows on the surface of Mars today.

Since September 2006, MRO has been flying on nearly circular orbits above Mars at altitudes between 255 and 320 km. Each orbits take about 2 hours, allowing MRO to complete 12 orbits per day.  (You can follow where MRO is right now on its current orbit via this link.) The patch of ground directly below MRO is known as the “ground track”. After 359 days of orbits, the ground tracks cover the surface of Mars with less than 5 km separation.

There are 6 science instruments on MRO, which inlude 3 cameras, a spectrometer, a radiometer and a radar.

Imaging by the Context Camera (CXT) has mapped out almost 95% of the martian surface to a resolution of 6 meters per pixel in its greyscale images that cover a swath 30 km wide. These wide-field images provide the context for the high-resolution HiRISE (High Resolution Imaging Science Experiment) camera, which takes all the gorgeous surface images, and CRISM (Compact Reconnaissance Imaging Spectrometer) which provides information on the composition of Mars.

From Mars with love. MRO images from 14 February 2012 (left), 2011 (middle) and 2009 (right). (Credit: NASA/JPL-Caltech/U.Arizona)

From Mars with love: MRO CTX images from 14 February 2012 (left), 2011 (middle) and 2009 (right). (Credit: NASA/JPL-Caltech/MSSS)

HIRISE operates at optical and near-infrared wavelengths and can distinguish objects 1 metre in size, providing unprecedented resolution in planetary exploration missions. It studies a wide range of surface features to help us understand the dynamic environment of Mars.

Mars up close: a snapshot of some of the HIRISE surface images. (Credit: NASA/JPL-Caltech/U.Arizona)

Mars up close: a snapshot of some of the HIRISE surface images. (Credit: NASA/JPL-Caltech/U.Arizona)

CRISMs main science goal is to hunt for minerals that form in water, that might point to ancient hot springs, thermal vents, lakes or ponds that once existed on the surface of Mars. From an altitude of 300 km, CRISM can map features as small as 18 m wide.  CRISM’s wavelength coverage in from the optical at 360 nm through to the mid-infrared (3.92 microns) The optical traces iron minerals while the infrared traces sulfate, carbonate, hydroxyl and water in mineral crystals.

The CRISM instrument provided the long-awaited evidence that liquid water flows on the surface of Mars today. MRO has imaged dark streaks on the steep sides of some sun-facing craters that vary over time, showing darker features that appear to flow down the slopes during warm weather and fade as the temperatures cool. These features, called recurring slope lineae (RSL) have long been through to be related to liquid water.  The CRISM findings confirmed that the RSL are related to hydrated salts, which lower the freezing point of salty brines.  The spectral signature is that of hydrated minerals called perchlorates, which on Earth have been known to keeps liquids from freezing down temperatures of -70C.

Dark narrow streaks called Recurring Slope Lineae (RSL) emanating out of the walls of Garni crater on Mars. There RSL are up to few hundred meters in length, thought to be formed by the flow of briny liquid water on Mars today. (Credit: NASA/JPL/University of Arizona)

As well as exploring the surface composition of Mars, CRISM is also investigating the martian atmosphere, trying to understand  how atmospheric dust affects the climate, how non-condensing gases like carbon monoxide and nitrogen in the atmosphere vary by seasons, and what process occur during the formation and sublimation of the seasonal polar ice caps.

MRO also provides crucial support for rover and  lander missions to Mars. As well as determining potential landing sites, MRO also helps rover teams choose both routes and destinations, as well as relay data from the surface missions back to the NASA Deep Space Network antennas on Earth.  MRO is also helping choose potential landing sites for future human missions in NASA’s Journey to Mars.

For more information, see

Next mission to Mars? The ESA + Roscosmos ExoMars programme, which aims to investigate whether life has ever existed on Mars.  The ExoMars programme includes two missions, the first to be launched this week!   Watch the ESA ExoMars launch on 14 March from 08:30 GMT, with the launch scheduled for 09:31 GMT.

[Sarah Maddison]

 

 

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The detection of gravitational waves

About 1 billion years ago, two black holes orbiting each other finished their mutual ‘death-spiral’ and coalesced. Prior to the final merger event, during their mutual orbit they lost rotational energy, which made them move closer together and also speed up their orbit, which in turn made them lose more energy, etc. … resulting in a runaway merger.

The merger was over in a fraction of a second and the result was a single black hole with a mass of about 60 solar masses. As the two black holes came together, they orbited at nearly the speed of light. Very massive objects moving extremely quickly tend to buckle or distort nearby spacetime. It was an incredibly energetic cosmic event in which about 3 solar masses were converted into energy in only 0.1 seconds. The final death-spiral and resultant black hole merger propagated energetic waves, gravitational waves, through space.

Gravitational waves are both similar and different to other more well-known waves, such as electromagnetic waves. Spacetime is the medium in which electromagnetic waves travel. For gravitational waves, however, spacetime itself constitutes the waves. Electromagnetic waves are created by accelerating or oscillating electric charges. Gravitational waves are created by accelerating or oscillating mass distributions (such as two merging black holes). Both types of waves travel at the speed of light.

Gravitational waves alternatively squeeze and stretch spacetime. In the plane perpendicular to the direction the wave is travelling, space is stretched along one axis and compressed along the orthogonal axis. One half-wave cycle later the opposite occurs. (Credit: M. Pössel/Einstein Online)

On the 14th of September, 2015, the Advanced Laser Interferometer Gravitational-Wave Observatory (aLIGO) laser interferometers in Louisiana and Washington state in the USA, which are separated by 3000 km, almost simultaneously detected a signal characteristic of a pair black holes merging into one. It was the event, described above, that occurred about 1 billion years ago.

The signals of gravitational waves detected by the twin LIGO observatories at Livingston, Louisiana, and Hanford, Washington. The signals came from two merging black holes, each about 30 solar masses. The top two plots show data received at Livingston and Hanford, along with predicted waveforms showing what two merging black holes should look like according to the General Theory of Relativity. Time is plotted on the X-axis and strain on the Y-axis. Strain represents the fractional amount by which distances are distorted. The bottom plot compares data from both detectors. The Hanford data have been inverted for comparison, due to the differences in orientation of the detectors at the two sites. The data were also shifted to correct for the travel time of the gravitational wave signals between Livingston and Hanford (the signal traveling at the speed of light, reached Hanford seven thousandths of a second later than at Livingston). (Credit: Caltech/MIT/LIGO Lab)

Gravitational waves are a direct consequence of Einstein’s General Theory of Relativity. However their strength or ‘amplitude’ was considered so small, many, including Einstein, doubted the waves would ever be detected.

Long baseline gravitational wave interferometers like aLIGO use laser beams in evacuated tubes reflected by mirrors to test the path length of the subsequent laser path. Two tubes at right angles are used.

The end mirrors follow the stretching and compressing of spacetime from a gravitational wave. As a wave passes through Earth, the distance between one pair of mirrors in one tube gets smaller, while the other gets larger. Yet even the strongest gravitational waves expected on Earth by various merger events stretch space by an extremely small amount: The strain (change in distance divided by the distance) between two objects is expected to be less than 10-18. To make the change in distance large enough for an interferometer to detect, designers must make the laser baseline kilometres in length. As well, in aLIGOs case the sensitivity to detect small values of strain had been vastly improved over earlier instruments.

Apart from the minute strain measurement needed, binary merger events like black hole mergers that undergo an accelerated inspiral orbit produce a characteristic ‘chirp’ waveform whose amplitude and frequency both increase with time until eventually the two bodies merge. The merger results in a highly deformed single black hole which rids itself of its deformity by emitting gravitational radiation that is characteristic of the mass and spin of the final black hole. This is called the ring-down signal.

The waveform as predicted by analytical approach of Buonanno and Damour (1999) to the binary black hole problem. The chirp and ring-down is seen. (Credit: Cardiff University)

The aLIGO detectors were just firing up again after a five-year, $200-million upgrade, which equipped them with new noise-damping mirror suspensions and an active feedback system for canceling out extraneous vibrations in real time. The upgrades gave aLIGO a major sensitivity boost.

The detected signal of the September 14, 2015 event swept upwards in frequency from 35 to 250 Hz with a peak gravitational wave strain of 1.0×10−21. Yes, the detected change in distance was 1 part in 1021.

It matched the waveform predicted by the General Theory of Relativity for the inspiral and merger of a pair of black holes and the ringdown of the resulting single black hole.

This detection adds to a growing list of recent ‘major’ scientific breakthroughs that have attracted major media attention. Some add it to an increasing number of discoveries that may even cause the public, politicians and even funding bodies ‘breakthrough fatigue’. The detection of gravitational waves is however truly a major discovery.

The magnitude of such scientific discoveries are judged by the awards and prizes given by peers. It is very likely that the next Nobel Prize for Physics will be awarded to (up to three) researchers associated with aLIGO and gravitational wave research. Stay tuned for the October announcement from Stockholm.

For more information, see:

[Glen Mackie]

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S1-2016 begins!

The wheels fell off (again…) last semester, but we’re back! So get ready for another fabulous semester of SAO astro news:-)

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A very special quasar quartet

Quasars are galaxies with unusually large luminosities, much more than can be associated with the typical constituents of galaxies, i.e. their stars, gas and dust. Quasars are known to host supermassive black holes (SMBH) at their centres, which is thought to power the quasar. Their large luminosities are due to physical processes related to the accretion disk of very hot gas which surrounds the SMBH and the formation of extended jets of particles that are most likely collimated by a strong magnetic field. Quasars usually have large UV and X-ray luminosities due to very hot gas in the accretion disk. They can also have large infrared and radio luminosities due to synchrotron radiation associated with relativistic jets and their nuclei. The associated central region of a quasar is typically 100 times brighter than the total brightness of a normal galaxy.

The number of quasars in the Universe is a function of the age of the Universe. There seems to be a special time, often called the ‘quasar epoch’, at a redshift of z~2-3 that has a very high density of quasars. This special time could be related to when the SMBHs were being fed most efficiently by nearby sources of gas. A team of astronomers using the Keck Observatory conducted a survey of 29 quasars at redshift z~2. The survey identified quasars with Lyman-alpha (Lyα) emission, resulting from ionised hydrogen with a rest wavelength of 1216 Angstroms. The Lyα emission is generated by the quasar as it energises the surrounding gas.

However when follow-up spectroscopy was conducted on other objects near one of the quasars SDSSJ0841+3921 (z = 2.0412), three other quasars were detected at the same or similar redshift. The objects were observed with the Keck instrument Low Resolution Imaging Spectrometer (LRIS) for 3 hours in late 2012, using a custom-built narrow-band filter that was tuned to the wavelength of the redshifted Lyα hydrogen gas. This system is the only ‘quadruple quasar’ known, and the probability of finding such a system is very, very small, about ~ 10-7.

Image of the rare quasar quartet from the Keck Observatory. The four quasars are indicated by arrows. The quasars are embedded in a giant nebula of cool dense gas visible in the image as a blue haze. (Credit: Hennawi & Arrigoni-Battaia, MPIA)

Whilst the probability of finding such a unique system of quasars is very small, the system can tell us much about the role of quasars in dense environments. Quasars at high redshift should reside in the most massive galaxies. These massive galaxies should be indicators of the highest density regions of the distant universe, which at redshifts of z~2-3 in theory should indicate regions in the early stages of cluster formation. The light from the quadruple quasar took 10.5 billion years to reach us, and we therefore get a unique view of the universe about 3.3 billion years after the big bang. However quasar surveys do not always find quasars in the highest density areas, so the relationship between quasars and protoclusters, the progenitors of rich clusters we see in the nearby universe, is unclear.

What is clear however is that about 10% of distant quasars and protoclusters do seem to have a common environment – being in or associated with very large, (hundreds of kiloparsecs across) Lyα emission nebula. The nebula associated with SDSSJ0841+3921 has a size of 310 kpc and the 4 quasars appear to be roughly oriented along a line similar to the major axis of the nebula. The protocluster appears special even amongst other protoclusters – as it appears to be about 20 times as rich in Lyα within a radius of 200 kpc. In the case of the SDSSJ0841+3921 quadruple system, there is a physical connection between the nebula, the 4 quasars and a probable (but extremely rare) protocluster. The protocluster has a size of several hundred thousand light years, and within this region there is a galaxy overdensity of ~100 above that expected at an “average” place in the distant universe. However the gas in the SDSSJ0841+3921 system is colder (and denser) than expected. It may be a case of catching a cluster in the earliest stages of formation at a very, special time. For more information, see

[Glen Mackie]

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Thanks MESSENGER!

After more than 10 years in space and over 4 years orbiting Mercury, the MESSENGER mission ended last week when the spacecraft crashed into the surface of the planet on April 30th. While the impact was not visible from Earth, MESSENGER managed to send back 5 final images at an altitude of about 40 km from the surface. Travelling at a speed of 8,750 mph (or 14,000 km/hr) the impact would have produced a new crater about 15 m wide on the Mercury’s surface.

One of the final images sent back by MESSENGER from an altitude of about 40 km before colliding with the surface of Mercury of 30 April 2015. (Credit: NASA/JHU Applied Physics Laboratory/Carnegie Institution of Washington)

MESSENGER was launched 3 August 2004 and travelled for  more than six and a half years going into orbit around Mercury on 18 March 2011, becoming the first spacecraft to orbit  the innermost planet in our Solar System. During its journey to Mercury, MESSENGER completed a flyby of Earth (August 2005), two flybys of Venus (October 2006 and June 2007) and three flybys of Mercury (January 2008, October 2008 and September 2009).

The primary mission was successfully completed in March 2012 and MESSENGER continued to complete two additional extended missions, XM1 and XM2. Finally on 24 April 2015 MESSENGER ran out of propellant, which prevented it from maintaining its altitude and so it finally succumbed to gravity and crashed into the surface of Mercury on 30 April 2015.  Unfortunately the impact was not visible as the collision was on the side of the planet facing away Earth and space telescopes cannot look at Mercury due to its proximity to the Sun, which would damage sensitive telescope optics.

During its mission, MESSENGER provided an enormous amount of data and has greatly changed our understanding of Mercury. The primary goals of the mission were to study Mercury’s chemical composition, geology and magnetic field to better understand the formation and evolution of the planet, as well as its interaction with the Sun. Some of the key science highlights of the MESSENGER mission include:

  • Being the first satellite to map the entire surface of Mercury. The surface is dominated by impact craters, but also be volcanism. This was clearly demonstrated by MESSENGER when it imaged volcanic vents near the rim oftheCaloris Basin, one of the largest and youngest impact crater in the solar system

    False-colour image of the 1,500 km-wide Caloris impact basin. The orange areas are lava that flooded the original basin, and subsequent impacts are shown in blue, revealing original basin floor material. (Credit: NASA, JHU APL, Arizona State U., CIW)

 

  • MESSENGER confirmed that water ice exists in the polar regions of Mercury.  The day side of Mercury canexceed300C, but due to the lack of obliquity (or axial tilt) in Mercury’s orbit, the floor of solar polar craters never receive any sunlight and temperatures are kept at achilly-170C.  MESSENGER detected water ice in the polar regions covered with an as-yet mysterious dark organic material.

    Water ice in the northern polar region of Mercury, seen in yellow inside craters that are in constant darkness. (Credit: NASA/JHU Applied Physics Laboratory/Carnegie Institution of Washington)

  • A detailed understanding of Mercury’s global cooling, which produced huge cliffs known as lobate scarps.  These form when the giant core of Mercury cools and effectively causes the entire planet to shrink.  The core comprises about 65% of Mercury by mass, and as it cools it contracts (and it is thought that the cooling of Mercury’s core caused it to shrink by 1-2 km in radius), causing the overlying layers to similarly contract. This results in wrinkly lobate scarps on the surface.

Y-shaped lobate scarp in a large old crater on Mercury. The right-hand side of the Y shape crosses the crater floor and the crater rim is a classic lobate scarp seen in almost all areas of Mercury. (Credit: NASA/JHU Applied Physics Laboratory/Carnegie Institution of Washington)

  • Amazing new details have also been uncovered about Mercury’s incredibly thin atmosphere, known as an “exosphere”.  The exosphere is so thin that atoms and molecules in the this atmosphere are actually more likely to collide with the surface than other particles in the exosphere!  The material found in the exosphere, which comprise volatiles like hydrogen, helium, sodium, potassium and calcium, are through to result from sputtering of the surface of Mercury, kicked up by solar radiation and solar wind, as well as by meteorite impacts.  Due to interactions with the strong solar wind, the exosphere stretches out into an amazing 2 million km tail away from the Sun.

    The giant sodium exosphere tail of Mercury comparison during the second and third Mercury flybys (compared with models). The changes in The sodium derives from material being sputtered from the surface at high latitudes on the day side of Mercury. Interactions with the solar wind carry the sodium atoms “downstream” of the solar wind. The changes seen are through to result from variations in Mercury’s exospheric “seasons.” (Credit: NASA/JHU Applied Physics Laboratory/Carnegie Institution of Washington)

  • A further mystery of Mercury which MESSENGER has helped shed light on in the global magnetic field of the planet. The Mariner 10 mission detected Mercury’s magnetic field in the 1970s, which was puzzling to astronomers as the huge iron core of Mercury was through to have cooled long ago, preventing a global magnetic field.  While only about 1% the strength of the Earth’s magnetic field, a global magnetic field is difficult to understand. The field was thought to a ‘relic field’, frozen into the rocks of the outer surface of the planet when the core cooled and the global magnetic field presumably died away.  However, MESSENGER data confirms the global magnetic field of Mercury and there is now consensus that Mercury indeed hosts a global active magnetic dynamo in the core similar to the Earth.  Mercury’s magnetic field interacts with the interplanetary magnetic field and charged particles from the solar wind, both of which distort the shape of the field like a windsock.

    Mercury’ global magnetic field interacts with the solar wind, resulting in a windsock type shape, similar to other global planetary magnetic fields in the solar system (Credit: NASA/JHU Applied Physics Lab/Carnegie Institute of Washington)

While the MESSENGER mission has ended, there is still a wealth of data for astronomers to work through and we can expect more exciting results from the mission in the coming years.

For more information, see

[Sarah Maddison]

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The CMB cold spot – explained?

The cosmic microwave background (CMB) is the afterglow of the Big Bang, cooled to the microwave region of the electromagnetic spectrum by the expansion of the Universe for ~14 billion years. Across the entire sky radiation at 2.7 K is evident once local sources are removed. It is not completely uniform in temperature however, with deviations to 1 part in 100, 000.

According to standard cosmology, the CMB gives a snapshot of the Universe around 380,000 years after the Big Bang. The temperature of the expanding Universe cooled until it reached about 3000 K, at which point electrons and protons combined to form hydrogen atoms. Radiation was scattering in the free electrons and protons of the younger, hotter Universe, but once hydrogen formed radiation could travel unimpeded.  The Universe therefore became ‘transparent’ to radiation, which we see as the CMB. This time is generally known as the ‘time of last scattering’ or the epoch of recombination.

Cosmologists have realised that measuring the angular variations, or anisotropy, in this radiation would provide important clues to why matter (like galaxies and clusters) is distributed the way it is today. The regions that are slightly more dense than others gravitationally attract photons, causing them to lose some energy in transition. These regions thus appear to be at a lower CMB temperature. Regions that are less dense suffer less from this effect and the radiation appears to be at a relatively higher CMB temperature.

In 2004 while examining a map of the CMB obtained by the WMAP satellite, astronomers discovered the Cold Spot, a larger-than-expected unusually cold area of the sky. The physics surrounding the Big Bang predicts warmer and cooler spots of various sizes in the infant Universe, but a spot this large and this cold was unexpected.

An all-sky picture of the cosmic microwave background (CMB) created from nine years of WMAP data. The image reveals temperature fluctuations, shown as colour differences, that correspond to the ‘seeds’ that eventually became galaxies. This image shows a temperature range of ± 200 µK . The dark blue Cold Spot is at bottom right of the image. (Credit: NASA / WMAP Science Team)

The Planck satellite also detected the CMB Cold Spot (as well as other suggested anomalies initially seen in the WMAP data.). The Cold Spot deviates by about -70 µK from the average CMB temperature and is centred on (l, b) ~ (209°, -57°) in Galactic coordinates. The Cold Spot is perhaps the most significant among the ‘anomalies’ in the CMB that are potential departures from isotropic or Gaussian statistics.

Explanations of the Cold Spot range from a statistical fluke through to unknown physics, an imprint of a parallel universe, to the Integrated Sachs-Wolfe (ISW) effect from a ~200 h−1 Mpc supervoid, which is an area lacking in galaxies, centred on the Cold Spot.

In the ISW effect photons from the CMB can be gravitationally redshifted or blueshifted due to intervening gravitational fields. If photons have to travel through a dense cluster of galaxies they gain energy by “falling” into the cluster’s gravitational potential. As the photons “climb” out of the cluster potential they lose energy, but not as much as they have gained since the Universe expanded in that intervening time. The net effect is that these photons are slightly warmer. For photons travelling through a void the effect is the opposite. The photons exit the void with less energy and therefore at a longer wavelength, which corresponds to a colder temperature.

Hence the ISW effect is driven by the intervening galaxy distribution and can slightly alter the temperature of the CMB photons that we detect.  If correct, the void that could cause a Cold Spot in the CMB would be readily detectable in large-scale structure surveys of galaxies.

An all sky map of the CMB separated into the two hemispheres of the sky, indicated by the curved white line. The Cold Spot that extends over a patch of sky that is much larger than expected is circled in the bottom left of the map. (Credit: ESA and the Planck Collaboration)

So what is the origin of this CMB Cold Spot? A recent study by István Szapudi of the University of Hawaii and collaborators of galaxy properties in and around the Cold Slot has recently been published in the Monthly Notices of the Royal Astronomical Society. This new work uses the extensive WISE-2MASS all-sky infrared databases and Pan-STARRS1 (PS1) data set of galaxies that have a mean z ~ 0.14.

The Cold Spot area resides in the constellation Eridanus in the southern galactic hemisphere. The insets show the environment of this anomalous patch of the sky as mapped using PS1 and WISE data and as observed in the CMB temperature data taken by the Planck satellite. The angular diameter of the vast supervoid aligned with the Cold Spot, which exceeds 30 degrees, is marked by the white circles. (Credit: graphic by Gergő Kránicz, image by ESA Planck Collaboration)

They find a very large void – a large under dense region of galaxies – in the constellation of Eridanus with a radius Rvoid ~ 220 Mpc centred at z = 0.22. This equates to a void that is 1.8 billion light-years across, in which the density of galaxies is much lower than usual in the known Universe. This corresponds to a 3.3σ fluctuation in a CMB expected for a Λ Cold Dark Matter model for the Universe.

Overall the detection of a very large void in the galaxy distribution is a viable cause for the CMB Cold Spot. However the current estimates of the ISW effect are smaller than the observed -70 µK deviation. Could the void in Eridanus actually be much larger than ~220 Mpc? This is possible. The team that studied the Cold Spot in Eridanus is now taking a look at another CMB cold spot near the constellation of Draco. These further studies should give us better insight into the physical properties of the CMB.

For more information, see

[Glen Mackie]

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