For some years, astronomers have known that the gaseous outer edges of the Milky Way galaxy flare out away from the neat, flat disc that contains the spiral arms. However, this gas is hard to see and harder to study, and nobody knew for sure where it came from. But a few months ago, a team of astronomers based at the South African Astronomical Observatory announced their discovery of a small number of stars within these flares. These stars are of a very special type that could not only answer the question of the origin of the gas, but could also help us to better understand the nature of the halo of dark matter surrounding the galaxy.
The Milky Way galaxy, like all spiral galaxies, consists of a supermassive black hole surrounded by a vast, dense globe of old stars. This core region, roughly a thousand light years across, sits in the middle of a broad, flat disc containing a number of stars arranged in spiral arms, and large quantities of gas and dust. The entire structure is embedded in a halo of thinner material and dark matter, dotted throughout by hundreds of globular clusters. But it was only in this century that radio astronomers noticed that the outer gaseous edges of the disk spread up and down away from the disk in an enormous galactic flare. Unfortunately, they could not tell much about this gas beyond its composition.
Earlier this year, Prof Michael Feast (University of Cape Town – UCT, SAAO), Dr John Menzies (SAAO), Dr Noriyuki Matsunaga (the University of Tokyo, Japan) and Prof Patricia Whitelock (SAAO, UCT) published a paper in Nature detailing their discovery of five Cepheid variable stars within this flared region. This type of star is so special because they are extremely useful to astronomers. Cepheid variables, named for the constellation Cepheus (where they were first discovered), are young and hot, and pulsate in a very predictable way. They grow and shrink like clockwork, causing them to become brighter and dimmer. The entire process takes only a few days, and the exact timing depends on the size of the star.
This means that if you see a distant variable star, and it’s light curve (which is literally the graph astronomers draw to show its brightness over time) matches the classical Cepheid pattern, then all you have to do is time the pulsations as accurately as you can to know exactly how big it is. Once you know its size, you know exactly how much light it puts out and by comparing the apparent brightness to the actual brightness, astronomers can calculate the distance of the star very precisely. This method is used as the benchmark by which almost all other distance measures are calibrated, which explains why the discovery of Cepheid in an unexplored area is so valuable.
However, in this case, the astronomers weren’t trying to use the Cepheid to measure the distance of the flare – they already knew that. Rather, they wanted to be sure that the stars which a previous project had identified as being within the flare were actually at the right distance. Because they are at the opposite end of the galaxy, we’re looking at them though almost a hundred thousand light years of dust and gas. That complicates the distance calculations because all that stuff adds to the dimming of light caused by distance – a phenomenon called “Reddening”, since the dust filters out less red light than other, shorter wavelengths. Since we can’t be sure exactly how much we’re looking through, we can’t easily compensate for the reddening, so Feast and his colleagues had to find other methods.
To deal with this problem, they imaged the stars in visual light, as well as in several different wavelengths in the near-infrared band. By repeating standard photometric techniques across the different wavelengths, and testing them against different theoretical models, they managed to settle on a moderately accurate value for the reddening. The resulting distance measurements were rough at best, but still accurate enough to reliably place the stars within the flared gaseous region.
But further confirmation was needed, so they made a second set of observations. This time they observed the stars through a spectrograph fitted to the South African Large Telescope (SALT), a 10 meter monster located at the observatory in Sutherland. SALT is one of the largest telescopes on Earth, and with its precision spectroscopic instruments, is the perfect instrument for this sort of work. By identifying known spectral features and measuring how far they were shifted from their standard position, the astronomers could see the blue- or red-shift and calculate how fast the star is moving towards or away from us. Result: They’re moving away from us at a high speed, but no more than any object orbiting the center of the Milky Way would at that distance, which further confirmed that these stars are indeed a part of the flared gaseous outer region of the galactic disk.
So how is this useful knowledge? Well it turns out that astronomers aren’t entirely sure where the gas in the outer regions of the galaxy comes from. Was it always part of the galaxy, or is it the stripped out remains of a dwarf galaxy that drifted too close and got cannibalized? And then there’s the reason that it flares out in the first place: Dark Matter. Through most of the galaxy, the bulk of the gravity that directs everything in its orbit around the center comes from the stars. Hundreds of billions of stars, all very heavy and all adding their gravitational tug which adds up to draw everything into the disk and into an orbit around the center. But out near the edge, where the density of stars gets lower, this source of gravity gets weaker. Go far enough out and the gravity of the dark matter halo begins to matter more than the gravity of distant stars, and material stops being shaped into a thin pancake and begins to spread out.
Seeing how the gas is arranged gives some clues about how much dark matter there is and how it is arranged, but without seeing things actually moving we couldn’t really pin anything down. But these stars can be easily seen, and their movements calculated. Five stars isn’t enough to work with, but now that the method for identifying them has been nailed down, we can find plenty more. And once we open that floodgate of data, astronomers will be able to better understand the history of our galaxy, build better maps of previously unexplored regions of the galaxy, and significantly improve our knowledge of how Dark Matter is distributed around the galaxy and how it affects us.
The discovery and its implications were described in a paper published in Nature in May 2014. The paper, titled Cepheid Variables in the Flared Outer Disk of our Galaxy, was authored by Michael W. Feast, John W. Menzies, Noriyuki Matsunaga, and Patricia A. Whitelock. It can be read on arxiv here: arXiv:1406.7660v1 [astro-ph.GA]