Get ready for another fabulous semester of SAO astro news :-)
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.
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
- Quasar quartet embedded in giant nebula reveals rare massive structure in distant universe, Hennawi et al. 2015, Science, 348, 779 [Swinburne Library SIMS access to full paper]
- Astronomers Baffled by Discovery of Rare Quasar Quartet, Keck Observatory Press Release (14 May 2015)
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.
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
- 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.
- 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.
- 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.
- 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.
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
- Fire & Ice: A MESSENGER recap, Science@NASA, 30 April 2015
- MESSENGER mission, NASA/JHU/CIW
- MESSENGER mission, NASA
- Messenger mission ends with plunge into Mercury, Spaceflight Now
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.
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.
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.
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
- Enormous hole in the universe may not be the only one, Carole Mundell, The Conversation, 22 April 2015
- Detection of a supervoid aligned with the cold spot of the cosmic microwave background, Szapudi et al. 2015, MNRAS, 450, 288. [Swinburne Library SIMS access for the full paper]
- Detection of a Supervoid aligned with the Cold Slot of the CMB, Dark Light
- Cold cosmic mystery solved: Largest known structure in the universe leaves its imprint on CMB radiation, Phys.Org
Today marks the 25th anniversary of the Hubble Space Telescope, launched 24 April 1990. Operating in the optical, ultraviolet and near-infrared wavebands, HST’s low-earth orbit gives it unprecedented image quality outside of the “blurring” of the Earth’s turbulent atmosphere. HST has revolutionised our view of the universe and brought us some of the highest resolution and most spectacular images from our solar system, our galaxy and the distant universe. The real success of HST has been its amazing public reach, bringing astronomy to people across the globe.
HST got off to a rather shaky start, however. There were problems with the shape of Hubble’s 2.4-m primary lens and the first images sent back to earth with disappointingly blurry! The first HST serving mission in December 1993 installed lenses to correct the optical aberration.
A total of 5 servicing missions between 1993 and 2009 were carried out, replacing failed equipment and installing new instruments. With the end of the space shuttle program in 2011, further HST servicing missions are no longer possible, but NASA will continue to maintain HST for as long a they can.
Some of HST’s key science outcomes have included studying weather patterns on other planets in our Solar System, studying the atmospheres of exoplanets, peering into star forming regions, studying interacting and merging galaxies, measuring the expansion rate of the universe, and views of the most distance universe 13.3 billion years ago.
Each year HST releases a special birthday image. For its 25th anniversary, this year’s image is of the stellar cluster and star forming region Westerlund 2 in the Gum 19 nebula. The central star cluster contains about 3000 stars. The image combines visible light from an image taken by the Advanced Camera for Surveys with near-infrared images from the Wide Field Camera 3. The red colours represent regions dominated by hydrogen, while the bluish-green hues are predominantly oxygen. (You can also watch a 3D fly through here).
Three great articles were published in The Conversation this week which we encourage you to read:
- Why the Hubble Space Telescope has been such a stellar success, Micheal Brown, The Conversation, 22 April 2015
- Hubble Space Telescope’s chief scientist on what it took to get the project off the ground, Bob O’Dell, The Conversation, 22 April 2015
- Hubble in pictures: astronomers’ top picks, Tanya Hill, The Conversation, 22 April 2015
For more information about HST and its 25 year history, see:
- Celebrating 25 years of the NASA/ESA Hubble Space Telescope, HST website
- Hubble 25, HST/ESA’s 25 year history of HST
Using the Atacama Large Millimeter/submillimeter Array (ALMA) telescope, astronomers have for the first time detected an extremely powerful magnetic field near the event horizon of a supermassive black hole in the centre of an active galaxy. This discovery supports the idea that strong magnetic fields help collimate the high-speed plasma jets emanating from many active galaxies.
Active galaxies are characterised by a very bright central nucleus, with high and rapidly varying luminosities on timescales of hours or days, emission across a wide range of wavelengths (and are usually most luminous in a non-optical part of the electromagnetic spectrum, e.g. UV or radio), non-thermal spectra, and often have radio jets emanating from the central region of the galaxy.
Different types of activity galaxies include quasars, Seyferts, BL Lac/blazars and radio galaxies. The central region, or nucleus, of all active galaxies are thought to be similar and are explained by the “Unified Model of AGN” (where AGN stands for Active Galactic Nuclei). The variation in AGN properties is thought to be related to the line of sight we have into the central region of the AGN. In the Unified Model, AGN have a central supermassive black hole that is fed by an accretion disk that is a few light days across and surrounded by a thick dusty torus.
Radio synchrotron emission is produced in many AGN, collimated into jets that propagate outwards from the nucleus perpendicular to the plane of the accretion disc. These radio jets are prominent in many radio galaxies and can be as large as several Mpc in size. Radiation from the jet moves close to the speed of light and can be beamed, and can vary on periods from hours to days.
Exactly what powers these jets is still somewhat of a mystery. It is believed that strong magnetic fields must exist close to the accretion disk, and the magnetic field lines must collimate and power the jets. Hence the recent results of Ivan Martí-Vidal and colleagues published in the journal Science are of great interest.
Using ALMA, Marti-Vidal et al. detected polarised light related to the strong magnetic field at the base of the jet emanating from the distant AGN, PKS 1830−211, which is at a redshift of z = 2.5. The high-resolution ALMA observations were at a wavelength of about 0.3 mm. The high-resolution was crucial to probe the region very close to the black hole, and only millimetre wavelength light can escape from the dusty region very close to the black hole, since longer wavelength radiation is absorbed.
Polarised light means that the electric field vectors of an electromagnetic wave have a preferred direction. As the polarised light propagates through a magnetised medium, the direction of polarisation can change – this phenomena is known Faraday rotation. The amount of Faraday rotation, determined by a quantity known as the rotation measure, is proportional to the magnetic field strength.
The high rotation measure derived in PKS 1830-211 suggest magnetic fields of at least tens of Gauss, and possibly considerably higher, on scales of the order of light-days (about 0.01 parsec) from the supermassive black hole.
“Our discovery is a giant leap in terms of observing frequency, thanks to the use of ALMA, and in terms of distance to the black hole where the magnetic field has been probed — of the order of only a few light-days from the event horizon. These results, and future studies, will help us understand what is really going on in the immediate vicinity of supermassive black holes.” Sebastien Muller, co-author of the Science paper.
Zooming in on the distant active galaxy PKS 1830-211 (Credit:ALMA (ESO/NAOJ/NRAO)/I. Martí-Vidal/Nick Risinger (skysurvey.org) & NASA/ESA)
For more information, see
- ALMA Reveals Intense Magnetic Field Close to Supermassive Black Hole, ESO Press Release [16 April 2015]
- A strong magnetic field in the jet base of a supermassive black hole, Marti-Vidal et al. (2015), Science, 348, 311 [Swinburne student login]
- Active Galactic Nuclei at the half-century mark, talk by Bradley Peterson, April 2011
- Probing the jet base of the blazar PKS 1830−211 from the chromatic variability of its lensed images, Marti-Vidal et al. (2013) A&A, 558, A123
[Glen Mackie & Sarah Maddison]
For the first time, astronomers have found complex organic molecules in a protoplanetary disk. Using the ALMA telescope, the discovery of methyl cyanide (CH3CN), cyanoacetylene (HC3N) and hydrogen cyanide (HCN) in the dusty, gas-rich disk surrounding the young star MWC 480, show that the building blocks of life are not unique to our Solar System.
These complex organic molecules were detected in the cold outer regions of the disk surrounding MWC 480, in the neighbourhood of its Kuiper Belt equivalent where we expect comets and icy planetesimals to reside. Water and complex organics have been found in both asteroids and comets in our Solar System, and organic molecules have been detected in giant molecular clouds where stars form. Simple molecules like water (H2O) and hydrogen cyanide (HCN) have also been found in protoplanetary disks, indicating that some volatile molecules can either survive during the formation of disks or that they quickly form in young disks. But what about more complex organics? It was not known whether complex organic molecules could survive the energetic shocks and intense radiation that result when molecular cloud cores gravitationally collapse to form young protostars and their surrounding protoplanetary disks.
This new discovery, lead by Karin Öberg of the Harvard-Smithsonian Center for Astrophysics, shows that the conditions that produce complex organic cyanide molecules must also be common around young planet-forming disks with a range of conditions. MWC 480 is a 1.8 solar mass star which hosts a massive disk (about 0.2 solar masses) in the constellation of Taurs. Being a massive Herbig Ae star, MWC 480 exposes the disk material to much higher levels of ultraviolet radiation compared to stars the mass of our Sun and the disk will also be much hotter, 2-3 times warmer at a given radius that our Solar System.
Öberg et al. not only found complex organics in the outer disk of MWC 480, but determined that their abundance ratios are similar to those found in the comets of our Solar System. This implies that rich organic chemistry of that existed in the solar nebula that formed our Solar System was not unique.
Laboratory experiments indicate that the chemistry which produces CH3CN also produces simple sugars and amino acids. If complex cyanides and other rich organics are common in icy bodies around young stars, and if as in our Solar System planetary migration brings this icy material to the surface of the inner rocky planets, then the conditions for life are not unique to our Solar System. The results are published in the April 9 issue of the journal Nature.
For more information, see
- Complex Organic Molecules Discovered in Infant Star System, ESO Science Release [8 April 2015]
- Complex Organic Molecules Discovered in Infant Star System: Hints that Prebiotic Chemistry Is Universal, ALMA Press Release [8 April 2015]
- The comet-like composition of a protoplanetary disk as revealed by complex cyanides, Öberg et al. (2015), Nature, 520, 198 [Swinburne student login]