The Peninsula

Submitted by Kevin Jardine on 10 July, 2014 - 10:06

Simple mathematical patterns are seductive. Our brains and in particular, our visual processing systems, are pre-programmed to see them everywhere, even if they do not exist.
Whirlpool galaxy from Hubble
Disk galaxies are a good example. Hydrogen clouds, star formation regions and dust in disk galaxies often seem concentrated in prominent arms that are typically mathematically modelled using logarithmic spirals. However, a simple mathematical model, while tempting, can be actively misleading. If we carefully examine even grand design spiral galaxies such as the Whirlpool galaxy, Messier 51, shown to the right, we can see deviations from a simple spiral pattern, including shorter non-spiral arms, changes in arm thickness, and small "feathers" that connect the larger arms together.

It is often the deviations from a spiral pattern that are the most interesting features of a disk galaxy like the Milky Way.

This is why the RMS data discussed in my last two blog posts is so important. By providing distance estimates for HII regions and massive young stellar objects throughout the galaxy and well beyond what has been mapped using parallax alone, the RMS data provides the beginning of a real portrait of our home galaxy, revealing some of the unique features that are distinct from a mere mathematical model.

In this blog post I discuss what appears to be one of the most prominent deviations from spiral structure in the Milky Way: a vast structure in the inner galaxy that I've called the Peninsula.

I initially suggested the existence of the Peninsula when examining the mystery of RCW 122. RCW 122 is an enormous star formation region in the inner galaxy, especially prominent in images from the Spitzer infrared space telescope. The published radio parallax-based distance estimate of 3380 (-270 / +330) parsecs does not place it near any of the spiral arms or in the 3 kpc arms or bar.

When I examined the atomic hydrogen velocity data more closely, I suggested that a "peninsula" of hydrogen gas extended from the Norma arm towards the bar and that RCW 122 was part of this structure.
Peninsula in RMS overlay
The RMS data suggests that the Peninsula is real and is larger and more prominent than I had imagined.

In the rough diagram at the end of my article on RCW 122, I had shown the star formation region at the end of the Peninsula. However, the RMS data suggests that the Peninsula is much larger than I had estimated and RCW 122 is located in about the middle of this vast structure extending from the Norma arm towards the galactic bar.

The illustration to the left is a detail of the larger overlay image from my first RMS blog post. The image shows the Milky Way model from this site overlaid by the Hurt illustration, which is overlaid again by the RMS data. The Norma arm is in yellow. The RMS data shows complexes of massive young stellar objects and HII regions (blue circles) and individual objects (red circles). You can view a larger and more detailed version by clicking on the image.

As I pointed out in my previous blog post, the Hurt illustration appears to be inaccurate in the fourth quadrant. I discussed the position of the Sagittarius arm in my last blog post. In the overlay illustration both the RMS data and my own Milky Way model suggest that the Norma arm is more substantial and further away from the galactic bar than shown in the Hurt illustration.

All of this is not to say that adding the Peninsula to a model of the Milky Way explains the positions of all the known star formation regions. The RMS data shows two twin "islands" made up of many star formation regions that appear to float well beyond the Peninsula between the Norma arm and the bar. More mysteries to explore!

Benjamin versus Benjamin

Submitted by Kevin Jardine on 28 December, 2013 - 11:37

Robert Benjamin is one of my favourite astronomers. Best known for his association with the Spitzer infrared space telescope, Benjamin has used the data from this telescope for a number of creative and influential projects.

One of these. the gigantic GLIMPSE/MIPSGAL infrared mosaic of the inner galactic plane, helped inspire the Milky Way Explorer, was used for the Milky Way Zooniverse citizen science project, and influenced the even larger Galactic Plane Atlas expected to be released in 2014.

Benjamin and his colleagues also commissioned Robert Hurt to create his widely used image of the Milky Way, which has been used in many recent scientific papers, even by researchers who disagree strongly with some of Benjamin's conclusions.

The Hurt image was based on available parallax and kinematic data but was created primarily to illustrate the Spitzer scientists' conclusions about the structure of the Milky Way, including the position of the bar and the existence of two major spiral arms. It is therefore ironic that a recent paper used the Hurt image to question one of Benjamin's major conclusions about the Milky Way.

Is the Sagittarius arm a major arm?

Based on the concentrations of red giant stars observed by the Spitzer infrared space telescope, Benjamin concludes that the Milky Way has only two major spiral arms: the Perseus and Centaurus arms. The Hurt illustration follows this conclusion by showing the Sagittarius and Norma arms as much smaller structures.

However, a 2013 preprint described in my previous blog post uses kinematic distance estimates and data from the earlier MSX infrared space telescope to challenge Benjamin's conclusion.

The red MSX sources (RMS) researchers overlay their results on the Robert Hurt image in their paper's Figure 6. A detail of this image showing the fourth quadrant is shown to the right. I have added labels showing the position of the Sagittarius and Centaurus arms in Robert Hurt's image.

As you can see, the RMS figure shows numerous complexes of massive young stellar objects and HII regions (blue circles) and individual objects (red circles) in the vicinity of the Sagittarius arm - more than can be seen even at the location of the Centaurus arm in Hurt's image. The RMS paper concludes that the Sagittarius arm is also a major arm.

But does Hurt's image show the correct locations of the spiral arms in the fourth quadrant?

I think not.

Improved alignment

In the image on the left, I have overlaid the RMS image on top of the model of the Milky Way presented on this site. The Sagittarius arm is shown in magenta, the Centaurus arm in green and the Norma arm in yellow,

As you can see, there are major differences between my model and the Hurt image in the fourth quadrant. In particular, using my model, the red MSX sources now appear in the Centaurus arm. The Sagittarius arm is largely devoid of young stellar objects and HII regions in the fourth quadrant. More generally there is a greater alignment between these sources and my model than the Hurt image.

Although my model was developed completely independently from the RMS research, the alignment is not that surprising because the technique to determine distances used by my model is almost exactly the same as the one used by the RMS paper. Both use the same rotation model for the Milky Way, taken from Reid 2009 as explained here. In the absence of parallax data, both use this rotation model to derive distance estimates for objects identified in atomic hydrogen surveys. My model is based upon the position of large hydrogen clouds in the LAB HI survey. The RMS paper uses the more detailed VLA Galactic Plane Survey and the Southern Galactic Plane Survey results where available.

The Reid 2009 rotation model was recently confirmed by the Reid and Honma 2013 paper I blogged about previously.

Benjamin versus Benjamin

So in summary, Robert Benjamin was right because he was wrong. Benjamin was right in concluding that the Sagittarius arm is a minor arm with only a few major star formation regions. The RMS paper appears to have concluded otherwise because of inaccuracies in the Hurt image commissioned by Benjamin and his colleagues.

This raises an interesting question: why is the Hurt image inaccurate in the fourth quadrant?

A question of tangents

I do not know the sources used to construct the Hurt image. (I've written to Robert Benjamin to ask about that.) However, I think that it is a good assumption that the image in the fourth quadrant used kinematic distance estimates based on velocity data from atomic hydrogen or molecular cloud surveys. This is because almost no parallax measurements for fourth quadrant objects at large distances from the sun have been published and photometric estimates are unlikely because of the large distances and heavy dust obscuration.

One of the important pieces of information available from velocity surveys is the tangents of spiral structures. This is the galactic longitude at which these structures first become visible.

The tangent value depends upon the temperature of the detected gas. If we use 70K, then the Sagittarius arm first appears in the fourth quadrant LAB HI survey around 281°, the Centaurus arm around 288° and the Norma arm around 312.5°. This information can be viewed in the Velocity Explorer; for example, here. For simplicity, I've shown the combined LAB HI data from the Velocity Explorer fourth quadrant at the end of this blog post with tabs that highlight the velocity associated with the Sagittarius, Centaurus and Norma arms.

It appears that the Hurt image places the tangent of the Sagittarius arm around 288° and the tangent for the Centaurus arm at around 312°. As a result, the Hurt image appears to assign the tangent my model uses for the Centaurus arm to the Sagittarius arm, and the tangent my model uses for the Norma arm to the Centaurus arm.

The Hurt image makes the fourth quadrant Norma arm a minor structure running close to the galactic bar, something I would argue is incompatible with the velocity data. In fact, as I'll discuss in a future blog post, both hydrogen and RMS data suggest that the Norma arm is much more substantial than the Sagittarius arm and perhaps could be ranked with the Perseus and Centaurus arms as a major Milky Way structure.

But wait, there's more!

There are many other interesting features visible in the RMS data, some of which I'll discuss in my next blog post.

Red MSX sources

Submitted by Kevin Jardine on 23 December, 2013 - 11:23

After I posted my two part blog series comparing the latest parallax-based distance estimates with my model of the Milky Way for the inner galaxy and the Perseus arm, Winchell Chung drew my attention to news coverage of an Urquhart, Figura arXiv preprint analysing red MSX sources (RMS) - HII regions and massive young stellar objects visible in the MSX infrared survey.

Like the recent parallax results, the RMS paper displays distance estimates overlaid on Robert Hurt's image of the Milky Way. There are two major differences from the parallax results, however:

  • the RMS results cover almost the entire galaxy with the exception of the bar and ring (3 Kpc arms), and
  • most of the distances were estimated from kinematic and spectrophotometric data rather than using the more reliable parallax method.

As the RMS paper points out, parallax-based estimates are not possible for most of the galaxy because the data is not yet available.

I have taken the RMS/Hurt image from the paper (Figure 6) and overlaid it on my model. The colours in figure 6 are inverted in this illustration, and hence in this version, complexes of HII regions and massive young stellars objects are represented by blue circles and individual sources by red circles.

A larger 2324x2168 version of this image is available here.

There are many interesting features about this image. I'll highlight some in the fourth quadrant in my next blog post.

New parallax estimates part 2: the third quadrant Perseus arm

Submitted by Kevin Jardine on 15 December, 2013 - 13:35

This blog post is a continuation of the discussion started in New parallax estimates part 1: the inner galaxy, but this time for the Perseus arm in the outer galaxy. Please read the first blog post in this series for important background information.

The Perseus arm is highly visible in the velocity data for the second galactic quadrant.

There is some confusion / merging with the Norma arm between 90° and 102°, something I attribute to a structure called the Cygnet spur connecting the Perseus and Norma arms. However, after this point, the Perseus arm is clearly visible as a distinct structure of warm atomic hydrogen until it is compressed with the other velocity data in the anticentre (180°) direction. (Rotation models compress all velocity in the 0° and 180° directions to zero.)

The Perseus arm re-emerges from the anticentre compression in the third quadrant, although it is not quite as prominent as the local or Norma emission. However, after 217°, the velocity data shows complex changes to the Perseus arm structure and location.

You can see some of this complex structure in the image below.

This image is taken from the Velocity Explorer, which represents velocity data from atomic and molecular surveys in polar coordinates.

The highlighted part at the top shows the atomic hydrogen velocity associated with the Perseus arm in this direction. The part at the bottom shows the same data for molecular clouds.

After the large cloud at 217°, the atomic hydrogen gas splits into three narrow filaments, with an outer filament bending toward the Norma arm and an inner filament bending towards the local emission. All three filaments are reunited by about 235° but then the entire arm appears to bend towards the outer galaxy.

There is an interesting double bridge of hydrogen clouds linking the Perseus arm with local emission around 232°. However, after this point an enormous empty gulf opens up between the Perseus arm and the local emission as the arm bends towards the outer galaxy.

This gulf, which contains only very cold hydrogen gas, is a distinctive feature of the third quadrant. There is no such gulf in the second quadrant. The region between the Perseus arm and more local emission in the second quadrant is crisscrossed by several warm bridges and even the interarm space between the Perseus arm and local emission is relatively warm in the second quadrant.

The third quadrant gulf can also be detected by the complete absence of molecular gas in this direction and velocity range. In fact, the Perseus arm appears to end as a continuous structure in molecular gas by about 222.5°, with only a few isolated molecular clouds visible after that point.

The structure of the Perseus arm is quite visible in the atomic hydrogen velocity data. Moreover, the data in this direction and velocity range is not affected by the compression and ambiguity of the velocity data in the inner galaxy and so is more likely to reflect real physical structures in the Perseus arm. I was therefore quite interested to see that the new parallax distance estimates include several locations in the third quadrant Perseus arm.

You can see these in the following image. The Perseus arm location used by my model is shown in red, Norma in yellow and the Orion spur / Vela molecular ridge in orange.

The white circles show the parallax distance estimates I used to derive my model. Where parallax-based distance estimates were not available (as in almost all of the third quadrant), I used a simple rotation model and atomic hydrogen velocity data as described here.

The coloured dots show the BeSSeL estimates. New estimates (not available when I created my model) are the coloured dots not surrounded by a white circle.

Although the Perseus arm location I used in my model is clearly an oversimplification of the complex structure revealed in the atomic hydrogen velocity data, I was interested to see that the new star formation region locations appear to be consistent with my model, and in particular, a significant bend in the Perseus arm towards the outer galaxy. Of course, currently I'm only working with an image from a preprint. I'm looking forward to seeing the actual parallax data and analysis when the appropriate BeSSeL paper is published.

Addition in July 2014: The BeSSel paper for the Perseus arm that contains the details for this data is now available on arXiv.

New parallax estimates part 1: the inner galaxy

Submitted by Kevin Jardine on 13 December, 2013 - 09:54

Reid and Honma gave out an early Christmas present this week by submitting a paper to arXiv, Micro-Arcsecond Radio Astrometry, that includes a map of the latest BeSSeL distance estimates (see Figure 2 in the paper).

To my surprise (and a mixture of both delight and disappointment) the new estimates are largely consistent with the model of the Milky Way I released earlier this year. As a result I'll need to make minor adjustments to my model at most.

It appears that major progress on mapping the Milky Way may come from information on where major star formation regions are not located as much as where they are.

In this blog post I'll look at the new results for the near inner galaxy in the first quadrant. In my next blog post I'll look at the new results for the Perseus arm in the third quadrant.

The Reid and Honma map overlays the latest BeSSeL results on an image of the Milky Way created by Robert Hurt. The Hurt image is implicitly a model of the Milky Way that differs in significant detail from the one presented on this site. In the inner galaxy the Hurt model places the Norma and Centaurus arms much closer to the galactic centre.

I've created an image below that labels the RH (Robert Hurt) arm positions and also includes the location of the Centaurus (green), Norma (yellow) and Sagittarius (magenta) arms as well as the Orion spur (orange) and bar (cyan) from my model.

Where parallax measurements from BeSSeL or other sources were not available, I used a simple rotational model and the LAB HI velocity survey to position the arms and spurs as described here. Such kinematic distance estimates are a poor substitute for parallax-based estimates, especially given the compressed and ambiguous velocity data for the inner galaxy, which is why I was surprised to see that the latest data remains consistent with my model.

The white circles show the parallax distance estimates I used to derive my model. The coloured dots show the BeSSeL estimates. New estimates are the coloured dots not surrounded by a white circle.

Other than the previously known estimate for RCW 122, which I have analysed in detail here, all the existing and new parallax estimates line up with either structures in my model or the intense region at the near end of the galactic bar.

This is especially surprising given the large differences between my model and the Robert Hurt model in this region. Currently the data remain consistent with either model. In order to compare the model accuracy, we would need to know if there are actually star formation regions in the arm locations identified by the Hurt model. If there are no or very few star formation regions in these locations, it would provide evidence that the Hurt model may not be accurate for the near inner galaxy. On the other hand, if there are star formation regions at these locations, this would provide evidence for the accuracy of the Hurt model and suggest that my model was inaccurate in this region.


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