If we are lucky to be able to look up on a clear summer night at a place with little light pollution, we would see Milky Way with its faint stripe of stars. If we are a little bit luckier that we can see infrared light with our naked eyes, we would see Milky Way in its hidden and more beautiful form. In infrared, our mother galaxy would reveal itself as a long and broader stripe full of gas and dust, with sometimes complicated structures like twisted filaments. Likely your eyes would be caught by the many spherical shapes with reddish hues surrounded by a round blue circle (here the “red” and “blue” are the two new colors received by your far/near-infrared color receptive cones). These are what astronomers (loosely) call “bubbles” or “shells,” which are indeed almost everywhere across our Milky Way. If you ever participate in the citizen science project, the “Milky Way Project,” you are probably familiar with these bubbles on the infrared maps that pop up on your screen.
Milky Way Project
Citizen science projects took off with the fast development of internet speed and home computers in 2000s and have since evolved from the rotating numbers and curves in SETI@home to the pretty maps which you can actually work on in Galaxy Zoo. The Milky Way Project is part of the Zooniverse program which first started its crowd sourcing project with Galaxy Zoo in 2007. It now includes two sub-projects: BUBBLES and CLOUDS, where different maps are used to identify, unsurprisingly, bubbles and the so-called infrared dark clouds. In both projects, images taken by NASA Spitzer Space Telescope are used as the maps where citizen scientists look for these structures just as they do when identifying, say, a park on Google Maps.
M20 is a typical bubble in our own galaxy, with a huge HII region related to it. This picture is taken by Spitzer Space Telescope. Image credit: NASA, JPL-Caltech, J. Rho (SSC/Caltech)
BUBBLES, serving as the trailblazer for the Milky Way Project, ask participants to describe the shape and the size of a bubble on Spitzer maps covering the inner part of our galaxy. On the interface, citizen scientists are able to draw a circle on what they think is a bubble and to subsequently change the thickness of the circle, the shape (or, the ellipticity) of the circle and, when needed, to report an incompleteness in the circle, to match the geometry of the bubble. Like most other citizen science projects, BUBBLES propel the participants by giving scores and letting them compete with each other on the quality and the number of bubbles they identify. Like non-citizen science, citizen scientists are encouraged to review their results and discuss with their fellow scientists. If you want to have a taste of being an astronomer (or you ARE an astronomer), BUBBLES also provide advanced tools for citizen scientists to search in astronomy catalogues such as SIMBAD to look for previously known astronomical objects on the maps.
A screen shot of the Milky Way Project: BUBBLES interface. On the left are tools to adjust the shape of the circle.
For Astronomers
In the Milky Way Project, both Spitzer and Herschel data are being shown to the citizen scientists. In order to achieve the best result from crowd sourcing, different combinations of wavelengths are used respectively in BUBBLES and CLOUDS. While CLOUDS set their goal on identifying infrared dark clouds and use longer-wavelength maps from Herschel, BUBBLES use 4.5/8.0/24-µm images from Spitzer on blue/green/red color maps. Data in BUBBLES are taken from the Galactic Legacy Infrared Mid-Plane Survey Extraordinaire (GLIMPSE) and the Multiband Imaging Photometer for Spizter Galactic Plane Survey (MIPSGAL), from Spitzer Space Telescope, with a coverage of ±65˚ in galactic longitudes and ±1˚ in galactic latitudes.
The first result of the Milky Way Project was released in 2012, with a catalogue of 5,106 bubbles after combing through a total of 520,120 circles drawn by participants. The combination process pushes the user-drawn circles through two sets of 2˚ by 2˚ grids, with one offset by 1˚ from the other. An automatic clustering algorithm would start to group user-drawn circles if there are more than five circles in a 2˚ by 2˚ box, and a “hit rate” is calculated from the number of user-drawn circles over the number of times the map is shown to participants. The offset grids prevent circles on the border of the first set of grids from being left out. The whole process automatically leaves out “lonely circles” which are less likely to be real bubbles.
An example of user drawings and reduced, cleaned circles. The opacities of the cleaned circles are 2 times their hit rates. Simpson et al. 2012
Bubbles
So what are these bubbles? To answer this question, we have to know what are giving light in these infrared wavelengths first. Now, imagine that you are holding a dust grain in your hand and shrink it tenfold. That tiny little “dust” is what floats between the cold and vacuous spaces between stars. Just as we see in infrared cameras things that are too cold to emit light in optical wavelengths, we “see” these cold dust grains in infrared. Bubbles we identify on infrared images such as those taken by Spitzer are composed of these dust grains. The shapes of bubbles are the result of “stellar winds,” which are often related to young massive stars that are blowing “winds” and shining strong radiation. The stellar winds push these dust grains outward until there are too many that the force of winds is balanced by the interstellar pressure. What is left behind would be a more vacuous interior with heated up dust and gas, surrounded by a thick cold “ring” or, here we are, “bubbles.”
The reason why we study these bubbles is that they are often related to star formation, of which we still do not have a full picture although a simple scheme can be derived from a few decades of observations. We know that the star formation tends to happen in clusters and that it is affected by forces and radiation from stars. As massive stars live shorter lives, those we see on the sky are young and thus close to where they are born. The stellar winds from these massive stars thus cast strong impacts on the star forming sites and can either trigger or disturb the next generation of star formation (or more often, trigger and disturb at the same time). This makes bubbles interesting places that astronomers want to study, and identifying them would be the first step.
For Astronomers
The massive stars that push the expansion of bubbles do not only drive momentum-loaded materials but strong radiation as well. All O-type stars and the most massive B-type stars emit photons strong enough to ionize the hydrogen atoms, dissociated by cosmic rays and photons from nearby stars at the surface of denser and thus heavily self-shielded molecular regions. Due to the complicated geometry of stellar clusters, shapes of HII regions are often irregular and can sometimes be filamentary. Bubbles in the Milky Way Projects are not necessarily related to HII regions, and less massive stars and young stellar objects (YSOs) can create bubble-like cavities as well.
When stellar winds from massive stars blow outward in the dust and gas medium, they often move in supersonic speed and thus create discontinuities of physical properties (temperatures, densities, pressures, etc.) at the front of expanding bubbles. These discontinuities are called shocks and provide a genuine environment for characteristic emissions and chemical reactions. Masers and molecular line transitions of certain species like SiO are observed in this environment. People use these emissions to trace strong interaction between stars/YSOs and the interstellar medium to find out what effects this feedback process can have on the subsequent star formation.
“Bubbles, Bubbles” in World Wide Telescope
This module allows you to play around bubbles identified in the Milky Way Project in World Wide Telescope. By fading between different all-sky maps, you will be able to explore the correlation of emissions at different wavelengths and the bubbles. The complete GLIMPSE+MIPSGAL survey map is readily imported in World Wide Telescope in the same set of color combination as that in the Milky Way Project, so you can get a sense of how people identify bubbles on the exact map shown to them. Other infrared surveys taken by various instruments such as Wide-Field Infrared Survey Explorer (WISE) and Infrared Astronomical Satellite (IRAS) that can be interesting to overlay on are also available in World Wide Telescope. You can also play with the dust map and the H-Alpha emission map, which are also closely related to star formation.
In this module, catalogues taken from HII Region Discovery Survey (HRDS) is used to show positions of galactic HII regions. Although the HRDS catalogue does not intend to be complete, covering only a sub-region of -17˚ to 67˚ in galactic longitudes within ±1˚ in galactic latitudes, it does give a sense of where typical HII regions are distributed. By exploring this catalogue in the module together with the data from Milky Way Project, you can see how these two types of objects are (not) correlated with each other.
Screen shot of the interactive map shown on the HRDS website. When this WWT module is finished, the WWT html5 implementation could provide a better visualization of the HRDS data.
How to Use Data Tables in WWT
Here are the WWT tour and the excel files that are shown in the tour. To view the tour, simply download the file in the link below. You can open the tour file in both web client and Windows client by going to the pull-down menu under “Explore,” finding “Open…” and then clicking on “Tours.” You are also welcome to download the data tables and explore them on your own in WWT. To view these data tables in WWT, first you have to install Windows client of the latest WWT version on your machine, and go here for WWT Excel Add-in. When you finish the installation, you will see a WWT tab in Excel when you open these tables. Select the columns you want to import into WWT and click on “Visualize Selection” button. After setting the correct labels for each column, you can click on “View in WWT” and start your exploration!
- The WorldWide Telescope tour: “Bubbles, Bubbles”
“Bubbles, Bubbles” - Milky Way Project: Large Bubbles (columns presented in the tour: RA, Dec, Effective Radius (Arcmin) and Hit Rate)
Milky Way Project: Large Bubbles - HII Region Discovery Survey: GBT HII Region Survey, all (columns presented in the tour: RA and Dec)
HII Region Discovery Survey: GBT, all - HII Region Discovery Survey: GBT HII Region Survey, distance (columns presented in the tour: RA, Dec and “D” (heliocentric distances))
HII Region Discovery Survey: GBT, distances
WWT saved view of imported data: red circles indicate bubbles identified in the Milky Way Project, and purple dots indicate positions of HII region observed in HRDS.
For Astronomers
The HII Region Discovery Survey released the result in a series of five papers. The targets were selected based mainly on previous results from 21-cm HI emission and continuum observation at Very Large Array (VLA), including a piloting effort on radio survey of the galactic plane: the Multi-Array Galactic Plane Imaging Survey (MAGPIS). The 24-µm image from Spitzer MIPSGAL is used as a reference in selecting targets as well. Seven radio recombination lines (RRLs, from H87α to H93α) with frequencies falling in the GBT X-band receiver are observed toward each target. The velocity resolution of such observations can reach ~ 0.4 km/s per channel and thus allows us to derive kinematic distances for these objects.
Distribution of HII RRLs FWHM widths. Anderson et al. 2011
What You Can Do!
Pick your favorite project from the zoo of citizen science projects and take part! Zooniverse would be a good place to start, with projects in fields ranging from astronomy to meteorology to biology. One advantage of Zooniverse projects over other citizen science projects (besides the pretty pictures) is their well-documented background explanation. By reading through the project introduction, you can pick up what is really intriguing to scientists and understand why people want to study these objects. As the Galaxy Zoo co-founder Kevin Schawinski said, “we prefer to call this [Galaxy Zoo] citizen science because it’s a better description of what you’re doing; you’re a regular citizen but you’re doing science. …You’re pro-actively involved in the process of science by participating.” If you are interested in science but was daunted by equations, give these projects a try. Science is actually a few clicks away!
For Astronomers
Data sharing is the future of science, and luckily for those who are interested in bubbles, the data presented in this module are all publicly available. The Milky Way Project released its first result of BUBBLES in 2012, which includes one catalogue of “large bubbles” and one of “small bubbles.” The “large” and “small” here indicate whether or not the bubbles/”knots” are large enough to be simulated by at least the smallest possible circular shapes available on the identification interface. In the catalogues, columns of galactic coordinates and geometric parameters are included together with “hit rate” and “dispersion” to indicate the reliabilities of identified bubbles.
The HII Region Discovery Survey data are available via the project website. The Arecibo and GBT parts of the survey are separated in two independent files. Correlation can be found using the source id in the catalogues. Here in this module only the GBT part is presented as the derived kinematic distances are available in the GBT catalogue but not the Arecibo one, due to the fact that Arecibo did not bear the capability to resolve the confusion of overlapped velocity components toward inner Milky Way with the settings of this survey. Besides coordinates, velocities and velocity-based distances are available in the catalogues, and those who are interested can refer to their papers for details about the line surveys.
ISM and Star Formation 