The Milky Way Galactic Disk – Forever Blowing Bubbles

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Score another one for citizen science! In a study released just days ago, a new catalog containing over five thousand infrared bubble entries was added through the “Milky Way Project” website. The work was done independently by at least five participants who measured parameters for position, radius, thickness, eccentricity and position angle. Not only did their work focus on these areas, but the non-professionals were responsible for recovering the locations of at least 86% of additional bubble and HII catalogs. Cool stuff? You bet. Almost one third of the Milky Way Project’s studied bubbles are located at the edge of an even larger bubble – or have more lodged inside. This opens the door to further understanding the dynamics of triggered star formation!

Just what is the Milky Way Project? Thanks to the Galaxy Zoo and Zooniverse, scientists have been able to enlist the help of an extensive community of volunteers able to tackle and analyze huge amounts of data – data that contains information which computer algorithms might miss. In this case it’s visually searching through the Galactic plane for whole or broken ring-shaped structures in images done by Spitzer’s Galactic Legacy Infrared Survey Extraordinaire (GLIMPSE) project. Here the bubbles overlap and the structures are so complex that only humans can sort them out for now.

Screenshot of the Milky Way Project user interface.
Screenshot of the Milky Way Project user interface.
“The MWP is the ninth online citizen science project created using the Zooniverse Application Programming Interface
(API) tool set. The Zooniverse API is the core software supporting the activities of all Zooniverse citizen science projects.” says R. J. Simpson (et al). “Built originally for Galaxy Zoo 2, the software is now being used by 11 different projects. The Zooniverse API is designed primarily as a tool for serving up a large collection of `assets’ (for example, images or video) to an interface, and collecting back user-generated interactions with these assets.”

Through the interface, users mark the location of bubbles and other areas of significance such as small bubbles, green knots, dark nebulae, star clusters, galaxies, fuzzy red objects or simply unknowns. During this phase, the citizen scientist can make as many annotations as he or she wants before they submit their findings and receive a new assignment. Each annotated image is then stored in a database as a classification and the user can access their image again in an area of the website known as “My Galaxy”. However, images may only be classified once.

Example of raw user drawings and reduced, cleaned result using a sample MWP image. A GLIMPSE-only colour sam- ple is included to illustrate the dierences in the appearance of images inspected by CP06 and the MWP users.
When identifying galactic bubbles, the user creates a circle around the area which can be scaled to size and stretched into an elliptical configuration. Initially as the object is identified and marked, the user can control the position and size of the bubble. Once annotated the parameters can be edited, such as the ellipticity, annular thickness and rotation. The program even allows for regions where no obvious emission is present, such as a broken or partial bubble. This allows the user to match the bubbles they find in individual images to achieve an accurate representation You can even mark a favorite or interesting configuration as well!

“In order to assist in the data-reduction process, users are given scores according to how experienced they are at drawing bubbles. We treat the first 10 bubbles a user draws as practice drawings and these are not included in the final reduction. Users begin with a score of 0 and are given scores according to the number of precision bubbles they have drawn.” explains the team. “Precision bubbles are those drawn using the full tool set, meaning they have to have adjusted the ellipticity, the thickness and the rotation. This is done to ensure that users’ scores reflect their ability to draw bubbles well. While only precision bubbles are used to score volunteers, all bubbles drawn as included in the data reduction. The scores are used as weights when averaging the bubble drawings to produce the catalogue.”

Now it’s time to combine all that data. As of October of last year, the program has created a database of 520,120 user-drawn bubbles. The information is then sorted out and processed – with many inclusions left for further investigation. However, not all bubbles make the cut. When it comes to this project, only bubbles that have been identified fifty times or more are included into the catalog. What remains is a “clean bubble” – one that has been verified by at least five users and picked out at least 10% of the time by the volunteers when displayed.

“It is not known how many bubbles exist in the Galaxy, hence it is impossible to quantify the completeness of the MWP catalogue. There will be bubbles that are either not visible in the data used on the MWP, or that are not seen as bubbles.” says the team. “Distant bubbles may be obscured by foreground extinction. Faint bubbles may be masked by bright Galactic background emission or confused with brighter nebular structures. Fragmented or highly distorted bubbles present at high inclination angles may not appear as bubbles to the observer.”

Error measurements for MWP bubble MWP1G309059+01661. This bubble has a hit rate of 0.437, and a dispersion of 1.61'. Top gures show reduced and raw bubble drawings. Bottom figures show dispersions in measurements of position and size.
But don’t let it burst your bubble. This citizen science approach is an excellent idea from the the standpoint of observer objectivity and the final, reduced catalogue contains 5,106 visually identified bubbles. Of these, they are divided into a catalogue of 3,744 large bubbles identified by users as ellipses, and a catalogue of 1,362 small bubbles annotated by users at the highest zoom level images in the MWP.

And that’s not all… “In addition to the reduced bubble catalogue, a crowd sourced `heat map’ of bubble drawings has also been produced. The MWP `heat maps’ allow the bubble drawings to be explored without them needing to be reduced to elliptical annuli. Rather, the `heat maps’ allow contours of overlapping classifications to be drawn over regions of the Galactic plane reflecting levels of agreement between independent classifiers. In most cases the structures outlined in these maps are photo-dissociation regions traced by 8 um emission, but more fundamentally they are regions that multiple volunteers agree reflect the rims of bubbles.”

Yep. They are bubbles alright. Bubble produced around huge stars when an HII region is hollowed out by thermal overpressure, stellar winds, radiation pressure or a combination of them all. This impacts the surrounding, cold interstellar medium and creates a visible shell – or bubble. These regions serve as perfect observation points “to test theories of sequential, massive star formation triggered by massive star winds and radiation pressure” and to keep us forever fascinated…

And forever studying bubbles.

Original Story Source: The Milky Way Project First Data Release: A Bubblier Galactic Disk. For Further Reading: The Milky Way Project Zooniverse Blog.

A New Look at the Milky Way’s Central Bar

[/caption]You may have heard about the restaurant at the end of the Universe, but have you heard of the bar in the middle of the Milky Way?

Nearly 80 years ago, astronomers determined that our home, the Milky Way Galaxy, is a large spiral galaxy. Despite being stuck inside and not being able to see what the entire the structure looks like — as we can with the Pinwheel Galaxy, or our nearest neighbor, the Andromeda Galaxy — researchers have suspected our galaxy is actually a “barred” spiral galaxy. Barred spiral galaxies feature an elongated stellar structure , or bar, in the middle which in our case is hidden by dust and gas. There are many galaxies in the Universe that are barred spirals, and yet, there are numerous galaxies which do not feature a central bar.

How do these central bars form, and why are they only present in some, but not all spiral galaxies?

A research team led by Dr. R. Michael Rich (UCLA), dubbed BRAVA (Bulge Radial Velocity Assay), measured the velocity of many old, red stars near the center of our galaxy. By studying the spectra (combined light) of the M class giant stars, the team was able to calculate the velocity of each star along our line of sight. During a four-year time span, the spectra for nearly 10,000 stars was acquired with the CTIO Blanco 4-meter telescope located in Chile’s Atacama desert.

Analyzing the velocities of stars in their study, the team was able to confirm that the Milky Way’s central bulge does contain a massive bar, with one end nearly pointed right at our solar system. One other discovery made by the team is that while our galaxy rotates like a wheel, the BRAVA study found that the rotation of the central bar is more like that of a roll of paper towels in a dispenser. The team’s discoveries provide vital clues to help explain the formation of the Milky Way’s central region.

BRAVA data. Image Credit: D. Talent, K. Don, P. Marenfeld & NOAO/AURA/NSF and the BRAVA Project

The spectra data set was compared to a computer simulation created by Dr. Juntai Shen (Shanghai Observatory) showing how the bar formed from a pre-existing disk of stars. The team’s data fits the model quite well, suggesting that before the central bar existed, there was a massive disk of stars. The conclusion reached by the team is in stark contrast to the commonly accepted model of formation of our galaxy’s central region – a model that predicts the Milky Way’s central region formed from an early chaotic merger of gas clouds. The “take-away” point from the team’s conclusions is that gas did play some role in the formation of our galaxy’s central region, which organized into a massive rotating disk, and then turned into a bar due to the gravitational interactions of the stars.

One other benefit to the team’s research is that stellar spectra data will allow the team to analyze the chemical composition of the stars. All stars are composed of mostly hydrogen and helium, but the tiny amounts of other elements (astronomers refer to anything past helium as “metals”) provides insight into the conditions present during a star’s formation.

The BRAVA team found that stars closest to the plane of the Milky Way Galaxy have fewer “metals” than stars further from its galactic plane. The team’s conclusion does confirm standard views of stellar formation, yet the BRAVA data covers a significant area of the galactic bulge that can be chemically analyzed. If researchers map the metal content of stars throughout the Milky Way, a clear picture of stellar formation and evolution emerges, similar to how mapping CO2 concentrations in the Antarctic ice shelf can reveal the past weather patterns here on Earth.

If you’d like to read the full paper, a pre-print version is available at: http://arxiv.org/abs/1112.1955

Source: National Optical Astronomy Observatory press release

The Hercules Satellite – A Galactic Transitional Fossil

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On Friday, I wrote about the population of the thick disk and how surveys are revealing that this portion of our galaxy is largely made of stars stolen from cannibalized dwarf galaxies. This fits in well with many other pieces of evidence to build up the general picture of galactic formation that suggests galaxies form through the combination of many small additions as opposed to a single, gigantic collapse. While many streams of what is, presumably, tidally shredded galaxies span the outskirts of the Milky Way, and other objects exist that are still fully formed galaxies, few objects have yet been identified as a satellite that is undergoing the process of tidal disruption.

A new study, to be published in the October issue of the Astrophysical Journal suggests that the Hercules satellite galaxy may be one of the first of this intermediary forms discovered.

In the past decade, numerous minor stellar systems have been discovered in the halo of our Milky Way galaxy. The properties of these systems have suggested to astronomers that they are faint galaxies in their own right. Although many have elongated and elliptical shapes (averaging an ellipticity of 0.47; 0.15 higher than that of brighter dwarf galaxies that orbit further out), simulations have suggested that even these stretched dwarfs are still able to remain largely cohesive. In general, the galaxy will remain intact until it is stretched to an ellipticity of 0.7.  At this point, a minor galaxy will lose ~90% of its member stars and dissolve into a stellar stream.

In 2008, Munoz et al. reported the first Milky Way satellite that was clearly over this limit. The Ursa Major I satellite was shown to have an ellipticity of 0.8. Munoz suggested that this, as well as the Hercules and Ursa Major II dwarfs were undergoing tidal break up.

The new paper, by Nicolas Martin and Shoko Jin, further analyzes this proposition for the Hercules satellite by going further and examining the orbital characteristics to ensure that their passage would continue to distort the galaxy sufficiently. The system already contains an ellipticity of 0.68, which puts it just under the theoretical limit.

The team looked to see just how closely the satellite would pass to our own galactic center. The closer it passed, the more disruption it would feel. By projecting the orbit, they estimated the galaxy would come within ~6 kiloparsecs of the galactic center which is about 40% of the radius of the galaxy overall. While this may not seem especially close Martin and Jin report that they cannot conclude that it will be insufficient. They state that disruption would be dependent on “the properties of the stellar system at that time of its journey in the Milky Way potential and, as such, out of reach to the current observer.”

However, there were some telling signs that the dwarf may already be shedding stars. Along the major axis of the galaxy, deep imaging has revealed a smaller number of stars that does not appear to be bound to the galaxy itself. Photometry of these stars has shown that their distribution on a color-magnitude diagram is strikingly similar to that of the Hercules galaxy itself.

At this point, we cannot fully determine if the Hercules galaxy is doomed to become another stellar stream around the Milky Way, but if it is not truly in the process of breaking up, it seems to be on the very edge.

What Galaxy is the Earth In?

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Were you wondering what galaxy is the Earth in? You’ll probably recognize the answer: it’s the Milky Way Galaxy.

If you go to a dark spot, away from the bright city lights, and look up, you should be able to see the Milky Way as a cloudy band stretching across the sky. It really does look like spilt milk spread across the sky. But if you take a telescope and examine it more closely, you’ll see that the clouds are actually the collective light from thousands of stars.

Since we’re embedded inside the Milky Way, we’re seeing our home galaxy edge-on, from the inside. To get a better idea, grab a dinner plate and take a look at it edge on, so you can’t see the circular shape of the galaxy. You can only see the edge of the plate.

The Milky Way is an example of a barred spiral galaxy. It measures approximately 100,000 light years across and it’s only 1,000 light years thick; although, it’s more thick at the core where the galaxy bulges out. If you could fly out of the Milky Way in a rocket and then look back, you’d see a huge spiral shaped galaxy with a bar at the center. At the ends of this bar, there are two spiral arms which twist out forming the structure of the Milky Way.

The Earth is located in the Solar System, and the Solar System is located about 25,000 light-years away from the core of the galaxy. This also means that we’re about 25,000 light-years away from the outer edge of the Milky Way. We’re located in the Orion Spur, which is a minor arm located in between the two major galactic arms.

If you’d like more information on the Milky Way, check out NASA’s Starchild info on the Milky Way, and here’s more info from the WMAP mission.

We’ve written many articles about the Milky Way for Universe Today. Here’s an article with facts about the Milky Way, and here is a map of the Milky Way.

We’ve also recorded several episodes of Astronomy Cast about the Milky Way. Listen here, Episode 99: The Milky Way.