Hubble finding overturns theory of how globular clusters of stars are formed

A new fining from ESA/Hubble:

The riddle of the missing stars

Thanks to the NASA/ESA Hubble Space Telescope, some of the most mysterious cosmic residents have just become even more puzzling. New observations of globular clusters in a small galaxy show they are very similar to those found in the Milky Way, and so must have formed in a similar way. One of the leading theories on how these clusters form predicts that globular clusters should only be found nestled in among large quantities of old stars. But these old stars, though rife in the Milky Way, are not present in this small galaxy, and so, the mystery deepens.

Four globular clusters in Fornax

Four globular clusters in the dwarf galaxy Fornax.
Left to right: Fornax 1, Fornax 2, Fornax 3 and Fornax 5.
Their positions within the galaxy are shown below.

New observations of the clusters — large balls of stars that orbit the centres of galaxies — show they are very similar to those found in our galaxy, the Milky Way. The finding is at odds with leading theories on how these clusters form — in these theories, globular clusters should be nestled among large quantities of old stars — and so the mystery of how these objects came to exist deepens.

Globular clusters — large balls of stars that orbit the centres of galaxies, but can lie very far from them — remain one of the biggest cosmic mysteries. They were once thought to consist of a single population of stars that all formed together. However, research has since shown that many of the Milky Way’s globular clusters had far more complex formation histories and are made up of at least two distinct populations of stars.

Of these populations, around half the stars are a single generation of normal stars that were thought to form first, and the other half form a second generation of stars, which are polluted with different chemical elements. In particular, the polluted stars contain up to 50-100 times more nitrogen than the first generation of stars.

The proportion of polluted stars found in the Milky Way’s globular clusters is much higher than astronomers expected, suggesting that a large chunk of the first generation star population is missing. A leading explanation for this is that the clusters once contained many more stars but a large fraction of the first generation stars were ejected from the cluster at some time in its past.

This explanation makes sense for globular clusters in the Milky Way, where the ejected stars could easily hide among the many similar, old stars in the vast halo, but the new observations, which look at this type of cluster in a much smaller galaxy, call this theory into question.

Astronomers used Hubble’s Wide Field Camera 3 (WFC3) to observe four globular clusters in a small nearby galaxy known as the Fornax Dwarf Spheroidal galaxy [1].

Fornax galaxy with four globular clusters marked Digitized Sky Survey 2 image of the dwarf galaxy Fornax. Highlighted here are four
globular clusters found in the galaxy called Fornax 1, 2, 3 and 5. (Large image)

“We knew that the Milky Way’s clusters were more complex than was originally thought, and there are theories to explain why. But to really test our theories about how these clusters form we needed to know what happened in other environments,”says Søren Larsen of Radboud University in Nijmegen, the Netherlands, lead author of the new paper. “Before now we didn’t know whether globular clusters in smaller galaxies had multiple generations or not, but our observations show clearly that they do!”

The astronomers’ detailed observations of the four Fornax clusters show that they also contain a second polluted population of stars [2] and indicate that not only did they form in a similar way to one another, their formation process is also similar to clusters in the Milky Way. Specifically, the astronomers used the Hubble observations to measure the amount of nitrogen in the cluster stars, and found that about half of the stars in each cluster are polluted at the same level that is seen in Milky Way’s globular clusters.

This high proportion of polluted second generation stars means that the Fornax globular clusters’ formation should be covered by the same theory as those in the Milky Way.

Based on the number of polluted stars in these clusters they would have to have been up to ten times more massive in the past, before kicking out huge numbers of their first generation stars and reducing to their current size. But, unlike the Milky Way, the galaxy that hosts these clusters doesn’t have enough old stars to account for the huge number that were supposedly banished from the clusters.

“If these kicked-out stars were there, we would see them — but we don’t!” explains Frank Grundahl of Aarhus University in Denmark, co-author on the paper. “Our leading formation theory just can’t be right. There’s nowhere that Fornax could have hidden these ejected stars, so it appears that the clusters couldn’t have been so much larger in the past.”

This finding means that a leading theory on how these mixed generation globular clusters formed cannot be correct and astronomers will have to think once more about how these mysterious objects, in the Milky Way and further afield, came to exist.

The new work is detailed in a paper published today, 20 November 2014, in The Astrophysical Journal.

—-

Video about the findings:

Loading player…

ESO: Spooky alignment of quasars spans billions of light-years

A report from the European Southern Observatory (ESO):

Spooky Alignment of Quasars Across Billions of Light-years

New observations with ESO’s Very Large Telescope (VLT) in Chile have revealed alignments over the largest structures ever discovered in the Universe. A European research team has found that the rotation axes of the central supermassive black holes in a sample of quasars are parallel to each other over distances of billions of light-years. The team has also found that the rotation axes of these quasars tend to be aligned with the vast structures in the cosmic web in which they reside.

Artist’s impression of mysterious alignment of quasar rotationArtist’s impression of mysterious alignment of quasar rotation axes.

Quasars are galaxies with very active supermassive black holes at their centres. These black holes are surrounded by spinning discs of extremely hot material that is often spewed out in long jets along their axes of rotation. Quasars can shine more brightly than all the stars in the rest of their host galaxies put together.

A team led by Damien Hutsemékers from the University of Liège in Belgium used the FORS instrument on the VLT to study 93 quasars that were known to form huge groupings spread over billions of light-years, seen at a time when the Universe was about one third of its current age.

The first odd thing we noticed was that some of the quasars’ rotation axes were aligned with each other — despite the fact that these quasars are separated by billions of light-years,” said Hutsemékers.

The team then went further and looked to see if the rotation axes were linked, not just to each other, but also to the structure of the Universe on large scales at that time.

When astronomers look at the distribution of galaxies on scales of billions of light-years they find that they are not evenly distributed. They form a cosmic web of filaments and clumps around huge voids where galaxies are scarce. This intriguing and beautiful arrangement of material is known as large-scale structure.

The new VLT results indicate that the rotation axes of the quasars tend to be parallel to the large-scale structures in which they find themselves. So, if the quasars are in a long filament then the spins of the central black holes will point along the filament. The researchers estimate that the probability that these alignments are simply the result of chance is less than 1%.

A correlation between the orientation of quasars and the structure they belong to is an important prediction of numerical models of evolution of our Universe. Our data provide the first observational confirmation of this effect, on scales much larger that what had been observed to date for normal galaxies,” adds Dominique Sluse of the Argelander-Institut für Astronomie in Bonn, Germany and University of Liège.

The team could not see the rotation axes or the jets of the quasars directly. Instead they measured the polarisation of the light from each quasar and, for 19 of them, found a significantly polarised signal. The direction of this polarisation, combined with other information, could be used to deduce the angle of the accretion disc and hence the direction of the spin axis of the quasar.

The alignments in the new data, on scales even bigger than current predictions from simulations, may be a hint that there is a missing ingredient in our current models of the cosmos,” concludes Dominique Sluse.

Simulating the bouncing of the Philae lander

I recently posted about SpaceTraveller, ”a solar system simulator and space mission visualizer program”  under development by BINARY SPACE (see Simulating Rosetta and Philae with ‘SpaceTraveller’). Here is an animation created with SpaceTraveller showing the Philae lander as it bounces twice on Comet 67P/C-G. The parameters for the first bounce were derived from the Rosetta images released yesterday (see previous posting). The second bounce uses a guess for the recoil velocity. Eventually ESA will find the lander and it will be interesting to see how close this simulation came to predicting where Philae settled on the comet.

For further info on SpaceTraveller, contact  info@binary-space.com.

Rosetta camera captures images of Philae before and after 1st bounce

The Rosetta mission releases some great photos showing the Philae lander as it approached the comet and then bounced away from it:

OSIRIS spots Philae drifting across the comet 

OSIRIS_spots_Philae_drifting_across_the_comet_node_full_image_2[1]Rosetta ‘s OSIRIS camera  shows the  lander Philae coming down onto  the surface
of the comet, leaving a spot where it hit, and then bouncing away
. (High-Res image)

The mosaic comprises a series of images captured by Rosetta’s OSIRIS camera over a 30 minute period spanning the first touchdown. The time of each of image is marked on the corresponding insets and is in GMT. A comparison of the touchdown area shortly before and after first contact with the surface is also provided.

The images were taken with Rosetta’s OSIRIS narrow-angle camera when the spacecraft was 17.5 km from the comet centre, or roughly 15.5 km from the surface. They have a resolution of 28 cm/pixel and the enlarged insets are 17 x 17 m.

From left to right, the images show Philae descending towards and across the comet before touchdown. The image taken after touchdown, at 15:43 GMT, confirms that the lander was moving east, as first suggested by the data returned by the CONSERT experiment, and at a speed of about 0.5 m/s.

The final location of Philae is still not known, but after touching down and bouncing again at 17:25 GMT, it reached there at 17:32 GMT. The imaging team is confident that combining the CONSERT ranging data with OSIRIS and navcam images from the orbiter and images from near the surface and on it from Philae’s ROLIS and CIVA cameras will soon reveal the lander’s whereabouts.