Free Astronomy Magazine May-June 2015

SPACE CHRONICLES the cloud was compact both before and after its closest approach, as it swung around the black hole. As well as providing very sharp im- ages, the SINFONI instrument on the VLT also splits the light into its component infrared colours and hence allows the velocity of the cloud to be estimated. Before clos- est approach, the cloud was found to be travelling away from the Earth at about ten million kilome- tres/hour and, after swinging around the black hole, it was mea- sured to be approaching the Earth at about twelve million kilome- tres/hour. (Because the dusty cloud is moving relative to Earth — away from Earth before closest approach to the black hole and towards Earth afterwards — the Doppler shift changes the ob- served wave- length of light. These changes in wavelength can be measured us- ing a sensitive s p e c t r og r a ph such as the SIN- FONI instrument on the VLT. It can also be used to measure the spread of ve- locities of the material, which would be ex- pected if the cloud was ex- tended along its orbit to a sig- nificant extent.) Florian Peissker, a PhD student at the University of Cologne, who did much of the observing, says: “Being at the telescope and seeing the data arriving in real time was a fascinating experience,” and Mo- nica Valencia-S., a post-doctoral re- searcher also at the University of Cologne, who then worked on the challenging data processing adds: “It was amazing to see that the glow from the dusty cloud stayed compact before and after the close approach to the black hole.” Although earlier observations had suggested that the G2 object was being stretched, the new observa- tions did not show evidence that the cloud had become significantly smeared out, either by becoming visibly extended, or by showing a larger spread of velocities. In addi- tion to the observations with the SINFONI instrument the team has also made a long series of measure- ments of the polarisation of the light coming from the supermassive black hole region using the NACO n instrument on the VLT. These, the best such observations so far, reveal that the behaviour of the material being accreted onto the black hole is very stable, and — so far — has not been disrupted by the arrival of material from the G2 cloud. The resilience of the dusty cloud to the extreme gravitational tidal ef- fects so close to the black hole strong- ly suggest that it surrounds a dense object with a massive core, rather than being a free-floating cloud. This is also supported by the lack, so far, of evidence that the central monster is being fed with material, which would lead to flaring and in- creased activity. Andreas Eckart sums up the new re- sults: “We looked at all the recent data and in particular the period in 2014 when the closest approach to the black hole took place. We can- not confirm any significant stretch- ing of the source. It certainly does not behave like a coreless dust cloud. We think it must be a dust- shrouded young star.” Supermassive black hole Sept. 2014 T his video sequence shows the motion of the dusty cloud G2 as it closes in on, and then passes, the supermassive black hole at the centre of the Milky Way. In this sequence observations of the cloud during the period from 2006 and 2014 are shown. The final two images are from February and Septem- ber 2014, before and after the object passed closest to the black hole. The cross marks the position of the supermassive black hole. [ESO/A. Eckart]