Using the sharp-eyed NASA Hubble Space Telescope, astronomers have for the first time precisely measured the rotation rate of a galaxy based on the clock-like movement of its stars.
Missing galaxy mass found
Nature 506, 7488 (2014). http://www.nature.com/doifinder/10.1038/506274a
Author: Eugenie Samuel Reich
Gravitational lensing solves puzzle from the Big Bang’s echo.
Gaia payload module integration
Gaia has been in its operational orbit around L2 for about a month now, where it is undergoing a very rigorous test programme before starting on its main science observations. Like many relocations, it can take some time to settle in, especially for a satellite that demands very precise and stable conditions for smooth operations.
Although we are a long way from completing the test programme or “commissioning”, we want to pass along an interim status report after one month.
We are pleased to note that all Gaia’s subsystems are working well, including the on-board atomic clock, the antenna to send the science data down to the Earth, and the micropropulsion system. The latter is an important part of maintaining the spacecraft’s current very precise and constant spin rate of 0.016656 degrees per second, or one full revolution every 6 hours and 14.23 seconds. This has allowed us to successfully enter the nominal operational scanning mode.
Gaia calibration image
Very importantly, Gaia’s very large CCD focal plane detector array is also working well. You will have seen one of our first test images publicised last week as part of our ongoing work to precisely focus Gaia’s two telescopes (read the full story here). While images like this are not part of the nominal Gaia mission, they play a key role in this phase in helping us learn about the behaviour and performance of the instruments in space.
One unexpected characteristic we have noticed during commissioning concerns stray light. In our test images, an excess of diffuse illumination is sometimes seen on some of the detectors, repeating in a cycle that relates to Gaia’s spin period of 6 hours.
The primary suspect is sunlight. Gaia’s design uses a 10-m diameter sunshield to hide the sensitive payload from the light and heat of the Sun. Our current assumption is that at a few points in the spin period, some sunlight is being diffracted round the edge of the sunshield in such a way that a very small fraction is entering the satellite’s protective thermal tent before somehow managing to find its way to the focal plane. If left as is, this stray light could affect Gaia’s capability to measure fainter stars in certain modes.
However, we have a plan to try and resolve the problem. Gaia’s sunshield is currently tilted at a 45 degree angle to the Sun, but by reducing this to 42 degrees, it should be harder for diffracted sunlight to reach the focal plane, thus hopefully eliminating the stray light.
We are now working towards reducing the tilt angle, but it will take several weeks to implement and test. Clearly, we cannot just change the orientation of the spacecraft overnight: we first must ensure that its attitude control system is reconfigured to cope with this without generating false software alarms. We also need to prepare and validate a revised scanning law that will enable Gaia to carry out its scientific job of fully surveying the heavens.
This work is currently ongoing, along a myriad of other tests and checks of the spacecraft. But this is exactly what the planned several-month long commissioning phase is for: identifying and solving any issues to make sure the spacecraft is ready to conduct science with a clean bill of health.
We will update you when we have any new developments to report.
Gaia calibration image
ESA’s billion-star surveyor Gaia is slowly being brought into focus. This test image shows a dense cluster of stars in the Large Magellanic Cloud, a satellite galaxy of our Milky Way.
Once Gaia starts making routine measurements, it will generate truly enormous amounts of data. To maximise the key science of the mission, only small ‘cut-outs’ centred on each of the stars it detects will be sent back to Earth for analysis.
This test picture, taken as part of commissioning the mission to ‘fine tune’ the behaviour of the instruments, is one of the first proper ‘images’ to be seen from Gaia, but ironically, it will also be one of the last, as Gaia's main scientific operational mode does not involve sending full images back to Earth.
Gaia was launched on 19 December 2013, and is orbiting around a virtual point in space called L2, 1.5 million kilometres from Earth.
Gaia’s goal is to create the most accurate map yet of the Milky Way. It will make precise measurements of the positions and motions of about 1% of the total population of roughly 100 billion stars in our home Galaxy to help answer questions about its origin and evolution.
Repeatedly scanning the sky, Gaia will observe each of its billion stars an average of 70 times each over five years. In addition to positions and motions, Gaia will also measure key physical properties of each star, including its brightness, temperature and chemical composition.
To achieve its goal, Gaia will spin slowly, sweeping its two telescopes across the entire sky and focusing the light from their separate fields simultaneously onto a single digital camera – the largest ever flown in space, with nearly a billion pixels.
But first, the telescopes must be aligned and focused, along with precise calibration of the instruments, a painstaking procedure that will take several months before Gaia is ready to enter its five-year operational phase.
As part of that process, the Gaia team have been using a test mode to download sections of data from the camera, including this image of NGC1818, a young star cluster in the Large Magellanic Cloud. The image covers an area less than 1% of the full Gaia field of view.
The team is making good progress, but there is still work to be done to understand the full behaviour and performance of the instruments.
While all one billion of Gaia’s target stars will have been observed during the first six months of operations, repeated observations over five years will be needed to measure their tiny movements to allow astronomers to determine their distances and motions through space.
As a result, Gaia’s final catalogue will not be released until three years after the end of the nominal five-year mission. Intermediate data releases will be made, however, and if rapidly changing objects such as supernovae are detected, alerts will be released within hours of data processing.
Eventually, the Gaia data archive will exceed a million Gigabytes, equivalent to about 200 000 DVDs of data. The task of producing this colossal treasure trove of data for the scientific community lies with the Gaia Data Processing and Analysis Consortium, comprising more than 400 individuals at institutes across Europe.
Change in the chemical composition of infalling gas forming a disk around a protostar
Nature 507, 7490 (2014). doi:10.1038/nature13000
Authors: Nami Sakai, Takeshi Sakai, Tomoya Hirota, Yoshimasa Watanabe, Cecilia Ceccarelli, Claudine Kahane, Sandrine Bottinelli, Emmanuel Caux, Karine Demyk, Charlotte Vastel, Audrey Coutens, Vianney Taquet, Nagayoshi Ohashi, Shigehisa Takakuwa, Hsi-Wei Yen, Yuri Aikawa & Satoshi Yamamoto
IRAS 04368+2557 is a solar-type (low-mass) protostar embedded in a protostellar core (L1527) in the Taurus molecular cloud, which is only 140 parsecs away from Earth, making it the closest large star-forming region. The protostellar envelope has a flattened shape with a diameter of a thousand astronomical units (1 au is the distance from Earth to the Sun), and is infalling and rotating. It also has a protostellar disk with a radius of 90 au (ref. 6), from which a planetary system is expected to form. The interstellar gas, mainly consisting of hydrogen molecules, undergoes a change in density of about three orders of magnitude as it collapses from the envelope into the disk, while being heated from 10 kelvin to over 100 kelvin in the mid-plane, but it has hitherto not been possible to explore changes in chemical composition associated with this collapse. Here we report that the unsaturated hydrocarbon molecule cyclic-C3H2 resides in the infalling rotating envelope, whereas sulphur monoxide (SO) is enhanced in the transition zone at the radius of the centrifugal barrier (100 ± 20 au), which is the radius at which the kinetic energy of the infalling gas is converted to rotational energy. Such a drastic change in chemistry at the centrifugal barrier was not anticipated, but is probably caused by the discontinuous infalling motion at the centrifugal barrier and local heating processes there.
Cold dark matter heats up
Nature 506, 7487 (2014). doi:10.1038/nature12953
Authors: Andrew Pontzen & Fabio Governato
A principal discovery in modern cosmology is that standard model particles comprise only 5 per cent of the mass-energy budget of the Universe. In the ΛCDM paradigm, the remaining 95 per cent consists of dark energy (Λ) and cold dark matter. ΛCDM is being challenged by its apparent inability to explain the low-density ‘cores’ of dark matter measured at the centre of galaxies, where centrally concentrated high-density ‘cusps’ were predicted. But before drawing conclusions, it is necessary to include the effect of gas and stars, historically seen as passive components of galaxies. We now understand that these can inject heat energy into the cold dark matter through a coupling based on rapid gravitational potential fluctuations, explaining the observed low central densities.
An international team led by astronomers from the Instituto de Astrofisica de Canarias (IAC) and La Laguna University (ULL) has just released the first analysis of the observations of the Abell 2744 cluster of galaxies, a coordinated program of the Hubble and Spitzer space telescopes. They have discovered one of the most distant galaxies known to date, which clearly shows the potential of the multi-year Frontier Fields project. The project uses a phenomenon called "gravitational lensing" where select foreground galaxy clusters amplify the faint light from far-more-distant background objects. By combining Hubble and Spitzer data, these astrophysicists have determined the properties of this young galaxy with a better precision than previous studies of other samples at similar cosmic epochs. This galaxy, named Abell2744_Y1, is about 30 times smaller than our galaxy, the Milky Way, but is producing at least 10 times more stars. From Earth, this galaxy is seen as it was 650 million years after the big bang. It is one of the brightest galaxies discovered at such a lookback time, say researchers. This study provides new constraints on the density and properties of the galaxies in the early universe. These results are accepted for publication in the scientific journal Astronomy and Astrophysics Letters.