PR 08-1998: Hipparcos reveals that the Milky Way is changing shape
01 April 1998Data of exceptional accuracy, from ESA's star-mapping satellite Hipparcos, show that distant stars are moving in unexpected directions. Their strange behaviour could mean that the shape of the Milky Way Galaxy is changing. A team of astronomers from Turin Observatory and Oxford University announced the discovery in the 2 April issue of the London science journal Nature.
Our home Galaxy, the Milky Way, is roughly flat, with a bulge in the middle. As inhabitants of the disk we see it edge-on as the band of light across the night sky which gives the Galaxy its name, and which comes from billions of distant stars lying in the disk. Astronomers have known for many years that the disk is slightly warped. What surprises them now is that distant stars are travelling in directions that, if continued, will change the warped shape.
Richard Smart of Turin Observatory, who is the lead author of the Nature paper, recounted, "Our results surprised us, but the extraordinary accuracy of Hipparos convinces us that distant stars have altered course. If we knew why, we'd be a lot wiser about the unseen hand of gravity at work in our Galaxy and others."
Tilted orbits and contradictory tracks
The Hipparcos satellite measured the positions and motions of stars far more precisely than ever before. Even before ESA's publication last year of the Hipparcos and Tycho Catalogues, of 118 000 and a million stars respectively, the Turin-Oxford group of astronomers had privileged access to some of the more exact Hipparcos Catalogue data.
They obtained positions and motions of 2422 very luminous blue stars spread half-way around the sky, selecting stars that turned out to be lying more than 1600 light-years away, towards the outskirts of the Galaxy.
Like the billions of other stars inhabiting the disk of the Milky Way, the Sun slowly orbits around the centre of the Galaxy, taking 220 million years to make one circuit. Inside the Sun's orbit, astronomers see no warp in the disk of the Milky Way. But outlying stars in the direction of the Cygnus constellation lie north of, or above, the plane of the Sun's orbit. Those in the opposite direction, in the Vela constellation, are displaced southward, below their expected positions if the Milky Way were truly flat.
The first use made of the Hipparcos data by the Turin-Oxford group was to check the precise shape of the warped disk of the Galaxy. Before Hipparcos, observations of stellar positions indicated that the warp started outside Sun's orbit and had general upward and downward turns.
The very precise star-fixing by Hipparcos showed the warp starting inside the Sun's orbit, with the more distant outlying parts of the Galaxy slanting more than the nearer parts do. As a result, the disk has an elegantly curved shape, like the brim of a hat.
If this shape of the warped disk were long-lasting, astronomers would expect the stars to follow corresponding orbits. Thus outlying stars in the Taurus constellation, midway between Vela and Cygnus, should be climbing "uphill" if they are to replace the stars lying high in Cygnus at present. The appropriate track for each star can be calculated, on the assumption that the warp will persist.
Before they could accurately compare the calculated motions with those detected by Hipparcos, Richard Smart and his colleagues had to take into account the Sun's own vertical motion. Like many stars, the Sun jumps and swoops like a dolphin as it proceeds in its orbit around the centre of the Galaxy. Hipparcos data show that the Sun is at present rising at 7 kilometres per second, relative to the disk of the Milky Way.
Outlying stars also show dolphin-like behaviour, so a statistical approach is needed, to gauge their average vertical motion. At a distance of 6000 light-years, in the direction of Taurus, the stars should on average be climbing northwards, relative to the Sun's orbit, at about 8 kilometres per second. The amazing conclusion by the Turin-Oxford group is that stars at that distance are on average descending southwards at 7 kilometres per second.
They cannot replace the present stars in the Milky Way in Cygnus. Instead they will go to positions shifted southwards in relation to the disk of the Milky Way - unless some new disturbance makes the stars change course again.
What warps galaxies?
The Milky Way is not the only galaxy to show deformations of its disk. About half of all other disk galaxies are seen to be misshapen. This remarkably high proportion may mean that galaxies are so rigid that any warp, once established, lasts for billions of years. Alternatively, galaxies may be very floppy, with new warps being created all the time.
The Hipparcos result on the Milky Way may favour the latter, more dynamic interpretation. The riddle of what warps galaxies has puzzled astronomers for decades. Explanations on offer range from intergalactic winds to magnetic contortions. A popular theory blames the warp in the Milky Way on the gravitational pull of invisible dark matter in the halo of the Galaxy. This would imply that the present warp should be a long-lived phenomenon. As the warp may now be only temporary, other explanations will be favoured.
Mario Lattanzi, of the Turin group, puts it this way: "As is often the case in experimental science, better experimental data challenge our current understanding of how the Milky Way works."
Prominent among the rival proposals about the warping of galaxies is the gravitational (tidal) effect of other galaxies passing close by. In the case of the Milky Way, the Magellanic Clouds and the recently discovered Sagittarius Dwarf Galaxy are candidates as warping agents. But Smart and his colleagues confess themselves to be baffled. "We are obliged to conclude," they write, "that there is currently no convincing interpretation of the implications of Hipparcos data for the dynamics of the warp in the Galactic disk."
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