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The shape of our galaxy

Many of you have likely seen the beautiful grand spiral galaxies captured by the likes of the Hubble space telescope. Images such as those below of the Pinwheel and Whirlpool galaxies display long striking spiral arms that wind into their centres. These huge bodies represent a collection of many billions of stars rotating around the centre at hundreds of kilometers per second. Also contained within is a tremendous amount of gas and dust, not much different from that found here on Earth, seen as dark patches on the otherwise bright galactic disc.

Pinwheel and whirlpool spiral galaxies, a.k.a. M101 and M51:

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Messier 101. Photo by NASA, ESA, K. Kuntz (JHU), F. Bresolin (University of Hawaii), J. Trauger (Jet Propulsion Lab), J. Mould (NOAO), Y.-H. Chu (University of Illinois, Urbana), and STScI
Pinwheel and whirlpool spiral galaxies, a.k.a. M101 and M51.
M51. Photo by NASA, ESA, S. Beckwith (STScI), and The Hubble Heritage Team (STScI/AURA).

Yet, rather embarrassingly, whilst we have many remarkable images of a veritable zoo of galaxies from across the Universe, we have surprisingly little knowledge of the appearance and structure of our own galaxy (the Milky Way). We do not know with certainty for example how many spiral arms there are. Does it have two, four, or no clear structure? Is there an inner bar (a long thin concentration of stars and gas), and if so does it rotate with the arms, or faster than them? Unfortunately we cannot simply take a picture from outside the galaxy as we can with those above, even if we could travel at the speed of light it would take tens of thousands of years to get far away enough to get a good picture!

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The current standard artists impression of the Milky Way. (Churchwell E. et al., 2009, PASP, 121, 213)
The current standard artists impression of the Milky Way (Churchwell et al. 2009), and a diagram of the supposed arm and bar features.
A diagram of the supposed arm and bar features.

The main difficulty comes from that we are located inside the disc of our galaxy. Just as we cannot know what the exterior of a building looks like if we are stuck inside it, we cannot get a good picture of what our own galaxy looks like from the Earth’s position. To build a map of our galaxy we rely on measuring the speeds of stars and gas, which we then convert to distances by making some assumptions of the structure. However the uncertainty in these distances is high, and despite a multitude of measurements we have no resounding consensus on the exact shape of our galaxy.

Movie showing how spiral arms (left) appear in velocity space (right).
Movie showing how spiral arms (left) appear in velocity space (right).

There is, however, a way around this problem. Instead of trying to calculate distances, we can simply look at the speed of the observed material in the galaxy. The movie above shows the underlying concept. By measuring the speed of material along the line of sight from where the Earth is located in the galaxy, you built up a pseudo-map of the structure. In this example the grey disc is the structure you would see if the galaxy were a featureless disc. If we then superimpose some arm features, where the amount of stars and gas is greater than that in the rest of the galaxy, we see the arms clearly appear in our velocity map. Maps of this kind exist for our galaxy, with those for hydrogen and carbon monoxide (shown below) gas displaying the best arm features.

CO emission map in velocity-line of sight space, showing clear spiral arm features (labeled) from Dame et al. (2001).
CO emission map in velocity-line of sight space, showing clear spiral arm features (labeled) from Dame T. M., Hartmann D., Thaddeus P., 2001, ApJ, 547,792

This may appear the problem is solved; we can simply trace the arm features and map them back onto a top-down map. Unfortunately doing so introduces the problems as measuring distances in the first place, and there is no single solution for mapping material from velocity to position space.

A different approach is to try and reproduce the map shown above by making informed estimates of what we believe the galaxy may look like. If we choose some top-down structure that re-creates the velocity map shown above, that we have observed directly from here on Earth, then we can assume the top-down map is also a reasonable map of the Milky Way.

Our work then began on a large number of simulations investigating the many different possibilities for the shape of the galaxy, investigating such parameters as the number of arms and speed of the bar. Care had to be taken with creating the velocity map, as what is actually measured by observations is the emission of the gas (akin to temperature). This can be absorbed and re-emitted by any additional gas the emission may pass through en route to the Earth.

In the two videos below are our best-fitting maps found for a two armed and four-armed model. Two arms tend not to produce enough structure, while the four-armed models can reproduce many of the features. Unfortunately it is very difficult to match all the features at the same time. This suggests that the arms of the galaxy may be of some irregular shape, and are not well encompassed by some regular, symmetric spiral pattern. This still leaves the question somewhat open, but also informs us that we need to investigate more irregular shapes and perhaps more complex physical processes to finally build a perfect top-down map of our galaxy.

Two-armed galaxy:

Four-armed galaxy:

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