ALMA Takes Close Look at Drama of Starbirth

Starbirth Surprisingly Energetic: New Insights Into Protostars

Stunning ALMA and NTT image of Newborn Star

 Astronomers using the Atacama Large Millimeter/submillimeter Array (ALMA) have obtained a vivid close-up view of material streaming away from a newborn star. By looking at the glow coming from carbon monoxide molecules in an object called Herbig-Haro 46/47 they have discovered that its jets are even more energetic than previously thought. The very detailed new images have also revealed a previously unknown jet pointing in a totally different direction.

Young stars are violent objects that eject material at speeds as high as one million kilometres per hour. When this material crashes into the surrounding gas it glows, creating a Herbig-Haro object . A spectacular example is named Herbig-Haro 46/47 and is situated about 1400 light-years from Earth in the southern constellation of Vela (The Sails). This object was the target of a study using ALMA during the Early Science phase, whilst the telescope was still under construction and well before the array was completed.

ALMA’s view of the outflow associated
with the Herbig-Haro object HH 46/47

The new images reveal fine detail in two jets, one coming towards Earth and one moving away. The receding jet was almost invisible in earlier pictures made in visible light, due to obscuration by the dust clouds surrounding the new-born star. ALMA has not only provided much sharper images than earlier facilities but also allowed astronomers to measure how fast the glowing material is moving through space.

These new observations of Herbig-Haro 46/47 revealed that some of the ejected material had velocities much higher than had been measured before. This means the outflowing gas carries much more energy and momentum than previously thought.

The team leader and first author of the new study, Héctor Arce (Yale University, USA) explains that "ALMA's exquisite sensitivity allows the detection of previously unseen features in this source, like this very fast outflow. It also seems to be a textbook example of a simple model where the molecular outflow is generated by a wide-angle wind from the young star."

The Herbig-Haro object HH 46/47 seen
 with ESO’s New Technology Telescope

 The observations were obtained in just five hours of ALMA observation time – even though ALMA was still under construction at the time – similar quality observations with other telescopes would have taken ten times longer.

"The detail in the Herbig-Haro 46/47 images is stunning. Perhaps more stunning is the fact that, for these types of observations, we really are still in the early days. In the future ALMA will provide even better images than this in a fraction of the time," adds Stuartt Corder (Joint ALMA Observatory, Chile), a co-author on the new paper.

Diego Mardones (Universidad de Chile), another co-author, emphasises that "this system is similar to most isolated low mass stars during their formation and birth. But it is also unusual because the outflow impacts the cloud directly on one side of the young star and escapes out of the cloud on the other. This makes it an excellent system for studying the impact of the stellar winds on the parent cloud from which the young star is formed."

Wide-field view of the star-forming region around the Herbig-Haro object HH 46/47

The sharpness and sensitivity achieved by these ALMA observations also allowed the team to discover an unsuspected outflow component that seems to be coming from a lower mass companion to the young star. This secondary outflow is seen almost at right angles to the principal object and is apparently carving its own hole out of the surrounding cloud.

The Herbig-Haro object HH 46/47 in the constellation of Vela

Arce concludes that "ALMA has made it possible to detect features in the observed outflow much more clearly than previous studies. This shows that there will certainly be many surprises and fascinating discoveries to be made with the full array. ALMA will certainly revolutionise the field of star formation!"

Source : European Southern Observatory - ESO

By Science and universe

A Magnetar at the Heart of Our Milky Way

This is an artist's impression of PSR J1745-2900, a pulsar with a very high magnetic field ("magnetar") in direct vicinity of the central source of our Galaxy, a supermassive black hole of approximately 4 million times the mass of our sun. Measurements of the pulsar imply that a strong magnetic field exists in the vicinity around the black hole. (Credit: MPIfR/Ralph Eatough.)

Astronomers have discovered a magnetar at the centre of our Milky Way. This pulsar has an extremely strong magnetic field and enables researchers to investigate the direct vicinity of the black hole at the heart of the galaxy. An international team of scientists headed by the Max Planck Institute for Radio Astronomy in Bonn have, for the first time, measured the strength of the magnetic field around this central source and were able to show that the latter is fed by magnetic fields. These control the inflow of mass into the black hole, also explaining the x-ray emissions of this gravity trap.

The discovery of a pulsar closely orbiting the candidate supermassive black hole at the centre of the Milky Way (called Sagittarius A*, or Sgr A* in short) has been one of the main aims of pulsar astronomers for the last 20 years. Pulsars, those extremely precise cosmic clocks, could be used to measure the properties of space and time around this object, and to see if Einstein's theory of General Relativity could hold up to the strictest tests.

Shortly after the announcement of a flaring X-ray source in the direction of the Galactic centre by NASA's Swift telescope, and the subsequent discovery of pulsations with a period of 3.76 seconds by NASA's NuSTAR telescope, a radio follow-up program was started at the Effelsberg radio observatory of the Max Planck Institute for Radio Astronomy (MPIfR).

"As soon as we heard about the discovery of regular pulsations with the NuSTAR telescope we pointed the Effelsberg 100-m dish in the direction of the Galactic centre," says Ralph Eatough from MPIfR's Fundamental Physics Research department, the lead author of the study. "On our first attempt the pulsar was not clearly visible, but some pulsars are stubborn and require a few observations to be detected. The second time we looked, the pulsar had become very active in the radio band and was very bright. I could hardly believe that we had finally detected a pulsar in the Galactic centre!" Because this pulsar is so special, the research team spent a lot of effort to prove that it was a real object in deep space and not due to human-made radio interference created on Earth.

Additional observations were performed in parallel and subsequently with other radio telescopes around the world (Jodrell Bank, Very Large Array, Nançay). "We were too excited to sleep in between observations! We were calculating flux densities at 6am on Saturday morning and we could not believe that this magnetar had just turned on so bright." says Evan Keane from the Jodrell Bank Observatory. Other collaborations worked at different telescopes (Australia Telescope/ATCA, Parkes and Green Bank Telescope). A research paper on the ATCA results by Shannon & Johnston appears in this week's issue of the British journal MNRAS.

"The Effelsberg radio telescope was built such that it could observe the Galactic centre. And 40 years later it detects the first radio pulsar there," explains Heino Falcke, professor at Radboud Universiteit Nijmegen. "Sometimes we have to be patient. It was a laborious effort, but finally we succeeded."

The newly found pulsar, labelled PSR J1745-2900, belongs to a specific subgroup of pulsars, the so-called magnetars. Magnetars are pulsars with extremely high magnetic fields of the order of 100 million (108) Tesla, about 1000 times stronger than the magnetic fields of ordinary neutron stars, or 100,000 billion times Earth's magnetic field. The emission from these objects is also known to be highly polarized. Measurements of the rotation of the plane of polarization caused by an external magnetic field (the so-called Faraday effect) can be used to infer the strength of the magnetic field along the line-of-sight to the pulsar.

The magnetic field strength in the vicinity of the black hole at the centre of the Galaxy is an important property. The black hole is gradually swallowing its surroundings (mainly hot ionized gas) in a process of accretion. Magnetic fields caused by this in-falling gas can influence the structure and dynamics of the accretion flow, helping or even hindering the process. The new pulsar has allowed measurements of the strength of the magnetic field at the beginning of the accretion flow to the central black hole, indicating there is indeed a large-scale and strong magnetic field.

"In order to understand the properties of Sgr A*, we need to comprehend the accretion of gas into the black hole," says Michael Kramer, director at MPIfR and head of its Fundamental Physics research department. "However, up to now, the magnetization of the gas, which is a crucial parameter determining the structure of the accretion flow, remains unknown. Our study changes that by using the discovered pulsar to probe the strength of the magnetic field at the start of this accretion flow of gas into the central object."

Source : Max-Planck-Gesellschaft.

By Science and Universe

NASA'S NuSTAR Helps Solve Riddle of Black Hole Spin

Rapidly rotating black hole accreting matter

Two X-ray space observatories, NASA's Nuclear Spectroscopic Telescope Array (NuSTAR) and the European Space Agency's XMM-Newton, have teamed up to measure definitively, for the first time, the spin rate of a black hole with a mass 2 million times that of our sun. 

A rapidly rotating supermassive black hole has been found in the heart of a spiral galaxy by ESA’s XMM-Newton and NASA’s NuSTAR space observatories, opening a new window into how galaxies grow.

Supermassive black holes are thought to lurk in the centre of almost all large galaxies, and scientists believe that the evolution of a galaxy is inextricably linked with the evolution of its black hole. 

How fast a black hole spins is thought to reflect the history of its formation. In this picture, a black hole that grows steadily, fed by a uniform flow of matter spiralling in, should end up spinning rapidly. Rapid rotation could also be the result of two smaller black holes merging. 

On the other hand, a black hole buffeted by small clumps of material hitting from all directions will end up rotating relatively slowly.

These scenarios mirror the formation of the galaxy itself, since a fraction of all the matter drawn into the galaxy finds its way into the black hole. Because of this, astronomers are keen to measure the spin rates of black holes in the hearts of galaxies.

One way of doing so is to observe X-rays emitted just outside the ‘event horizon’, the boundary surrounding a black hole beyond which nothing, including light, can escape.

In particular, hot iron atoms produce a strong signature of X-rays at a specific energy, which is smeared out by the rotation of the black hole. The nature of this smearing can then be used to infer the spin rate.

Using this technique, previous observations have suggested there are extremely rapidly spinning black holes in some galaxies. However, confirming the spin rate has been very difficult, because the X-ray spectrum can also be smeared out by absorbing clouds of gas lying close to the disc. Until now, telling the two scenarios apart has been impossible.

For roughly 36 hours in July 2012, ESA’s XMM-Newton and NASA’s NuSTAR – the Nuclear Spectroscopic Telescope Array – simultaneously observed the spiral galaxy NGC 1365. XMM-Newton captured the lower energy X-rays, NuSTAR the higher energy data.

The combined data proved to be key to unlocking the puzzle. A spinning black hole model makes a clear prediction for the ratio of high-energy to low-energy X-rays. The same is true for an absorbing cloud of gas.

But importantly, the predictions are different and the new data agree only with a rapidly spinning black hole. This suggests that the galaxy has grown steadily with time, with material streaming uniformly into the central black hole.

However, astronomers cannot yet rule out a single large event where two galaxies and their black holes subsequently merged, producing a sudden acceleration of the resulting supermassive black hole.

“But we can completely rule out the absorption model,” says Guido Risaliti, INAF – Osservatorio Astrofisico di Arcetri, Italy, who led the investigation.

“Now that we know how to measure black hole spin rates for certain, we can more confidently use them to infer the evolution of their host galaxies.”

Credit : ESA/NASA