Wednesday, March 2, 2016

The Milky Way: A pre-Gaia map of our home galaxy

Today's post will be about the galaxy we live in: the Milky Way. It will not be about exoplanets. However, I will come back to this topic in one of my next blogs because I will try to show where the exoplanets we know are located in this 'map' of the Milky Way.

First of all: If somebody shows you a map of the Milky Way, you should immediately be aware of the fact that this cannot be a real map in the sense that everything that is shown represents a real object with a measured position. There is no real map of the galaxy we live in, simply because (a) we cannot travel out and make a picture from above or below, and (b) we can only see 'far' enough to observe a tiny fraction of the stars in the Milky Way. The latter will hopefully improve soon because Gaia is already operating and observes more stars of the Milky Way than was ever possible before. With a little bit of luck we will get a much better impression of how our galaxy looks like this year.

What you see in the picture is my own little artist impression of how the Milky Way might look like. Again I emphasize that this might be completely wrong. Every other picture, even the I guess most famous one by NASA (R. Hurt), probably is pretty much wrong too. This 'map' is just an illustration which is supposed to show four things we believe to know about our galaxy: (a) It is a (flat) spiral galaxy. (b) It has a bright center with a bar-like structure. (c) It has four spiral arms at roughly about these locations. (d) It has a diameter of roughly about 100000 light years (ly).

Additionally, I tried to incorporate some real astrophysical data into this map. The white circles are measurements of embedded clusters (by Camargo) using the WISE telescope. The circles in cyan present the positions of molecular clouds coming from a catalogue by Ellsworth-Bowers. The only stellar data in the map are galactic cepheids (by Berdnikov, shown in magenta), which are variable stars used for distance measurements. There certainly are other dataset that should be in there to have a more complete picture, but I think these three are good enough to get the general picture. These data points give you an idea of what is actually measured and used to conclude that the Milky Way looks like what I drew.

There is a fourth dataset in the picture which cannot be seen. This dataset is the one with the best distance measurements for stars we have, at least until the first Gaia results get published. It is the Hipparcos data. However, all the data is located close to the black cross which marks the position of our Sun in the Milky Way. Our position in the Milky Way is about 8200 parsec or roughly 27000 ly away from the galactic center.

On the left side you see a blowup of the region of the Sun, where I used the new Hipparcos catalogue (van Leeuwen, 2007) to draw the positions of more than 100000 stars (white dots). Hipparcos measured the positions of stars better than one milli-arcsecond, which means the most distant stars in this catalogue are more than 3000 ly away. Of course, most of the observed stars are much closer to the Sun; 90 % of the stars with measure distances are within a radius of 1660 ly.
Although the Hipparcos map consists of a huge number of measurements, the distances from the Sun are not nearly far enough to tell us something about the large-scale structure of the Milky Way. Gaia will hopefully be about a factor 100 better than this, which will do the trick and give us a pretty good picture about a large part of the Milky Way covering maybe even 1 % of all the stars in our galaxy. Still, Gaia will not be able to see everything; some parts will be blocked from view, and some stars are just too faint or too far away to be seen.