Gaia Radial Velocities

The release of Gaia radial (line-of-sight) velocities in DR2 represents for me the first fruits of 17 years of work in the Gaia project. Such is the timescale of a mission as demanding, and different, as Gaia. I knew from the start, at mission adoption in 2001, that this would be a remarkable endeavour, not only from the scientific and technical perspective, but also from the long term working relationships and comradery that engaging deeply with it would entail.

This image shows the pattern of more than 7 million line-of-sight velocities in the Milky Way, as seen from the position of the Sun. The centre of the Milky Way is at the centre of the map, and its disk runs horizontally from left to right. This is the first line-of-sight velocity map of the entire sky from a single instrument. The particular velocities of the two Milky Way satellite galaxies, the Magellanic Clouds, can be seen in the lower right-hand quadrant. (Image credit: D. Katz et al./DPAC/ESA.)

It was almost a whimsy that led to my attendance at the first Gaia community meeting after mission adoption that July. Hearing the presentations, my interests were pricked to such an extent that while travelling home I wrote out some ideas which I considered might be useful and sent them to the Gaia Project Scientist at the time, Michael Perryman. From the distance of the present, that appears presumptious, but they were taken seriously. I judged that the greatest opportunity would be to contribute in the area of the Radial Velocity Spectrometer, which seemed the least developed component of the payload, despite its great challenges. During this time, I met David Katz, the young lead of the RVS Working Group (we were all younger then), and together we made an unsolicited bid to ESA to reconsider and develop the instrument concept. This was successful, and with other early core members, we were contracted over four years to work with ESA and the two industry bidders for prime contractor (we also had support from some national agencies).

Having previously led our own instruments for ESA, working on Gaia, which was built entirely in industry, was a different experience for a scientist. The relationships which developed were almost universally positive and productive, and it was a pleasure to work with these accomplished teams in the subsequent years. I learned much from this experience, and hope that I contributed to them too. In any case, this was a prime example of “knowledge transfer” between academia and industry, so sought after by some governments.

From 2006, the Working Groups mostly morphed into the Data Processing and Analysis Consortium (Gaia-DPAC), a large team of scientists and software engineers distributed over many European countries. It has been said that distributing a software project over several countries, each with their own funding cycles, is almost the worst way to organise the thing, but my experience is of a high level of communication and general sign-up to this complex and multi-faceted project, and it has delivered on time for the Data Releases. DR2 is the first in which the radial velocities are being released, and it is a particular pleasure to be one of those engaged in producing these for anyone who might want to use them.

Mark Cropper

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