The Most Distant Bubble?

A little while ago Sarah Fitzmaurice, a work experience student at Zooniverse Oxford, spent a week working with the Milky Way Project database. She did some fun things with the data, including plotting the locations of many of the bubbles according to their distance from us. For many, the current canonical view of our own Galaxy comes from a combination of data sources, compiled by Robert Hurt, working at NASA JPL. The image is shown below, and you may recognise it: we use it as our Twitter/Facebook avatar. It is an artist’s impression based on several data sources and guided by astronomers.

The Milky Way may be our home in the Universe but we know startlingly little about it. On key missing piece of information for many objects in our Galaxy is their distance from us. From the Spitzer data alone, we do not know the distance to the bubbles in the MWP. For our first Data Release paper, we compared the MWP Bubble catalogue to known objects, some with distances, and this allowed us to find  out how far way some of the bubbles are. This enables us to investigate how large and sometimes how massive they may be.

During her work experience week, Sarah plotted the bubbles with known distances onto Robert Hurt’s map of the Milky Way. The result is shown below. The bubbles are marked with crosses, and the size of the cross shows the relative size of the bubble. The distances to these bubbles were derived by comparing them to a known set of radio sources that are expected to look like bubbles in Spitzer data.

You can see that the bubbles generally follow the distribution of spiral arms and that it is easier to see the bubbles nearby than those farther away. This is good because it is roughly what we expect. This map also allows us to easily spot the isolated, nearby or most-distant bubbles in the project. Much of Sarah’s week was spent looking at each of the interesting bubbles and finding out some more about them.

Although there may well be more distant bubbles in the catalogue, Sarah’s map provides a candidate for ‘most distant bubble’ in the MWP. It is one of a pair of bubbles located on the far side of the Perseus arm, almost 45,000 light years away from the Sun – in the top part of the above image.

Using the new MWP coordinates tool we can take a look at this distant object, and two nice images of it are shown below. Our ‘most distant bubble’ is actually located within another larger, clearer bubble, the image of this is also given. This is a line-of-sight effect and they are not necessarily near each other.

This bubble is located literally on the other side of our Galaxy and is roughly 15 light years across. The fact that the two bubbles are positioned on top of each other makes it hard to decide which one is farther away. There are many more instances where bubbles lie on top of each other where it would be impossible to decide which is actually on top of which. The nebulous material of which these objects are made makes them hard to disentangle. In this case there are stars and IR objects on top of the smaller bubble that make it easier to pick out the nearer and farther bubble.

In this case, the distance value is derived from a radio source that we expect to be associated with a bubble. Both of these bubbles lie at roughly the correct position to be associated with the radio source. Since we know the radio source is very far away, we can say that the smaller bubble is most likely the object associated with the radio source.

These kinds of confusing caveats are one of the things that make Galactic astronomy difficult and challenging. For these reasons, this might be the most distant bubble we know of in the MWP – or it might not. Either way, this awesome little bubble has provided the opportunity to discuss the ways that we determine the distances to objects in the MWP catalogue, and how doing astronomy in our cosmic backyard is tricky territory indeed.