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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 et.al. 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.

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.

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.

Zhang, Reid, et.al. 2013 contributed two more accurate radio parallax distance measurements for star formation regions in the Perseus arm and then made the very interesting comment:

We have found almost no H2O maser sources in the Perseus arm for 50° < l < 80°, suggesting that this ≈6 kpc section of the arm has little massive star formation activity.

This attracted my attention because as you can see from the image below, there are also gaps in atomic hydrogen and molecular clouds in this direction:

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.

As you can see, there are similar results for both atomic and molecular data and they show a major gap between the warm clouds associated with the Perseus arm in the outer galaxy and the warm clouds associated with the Perseus arm in the inner galaxy. Within this gap there is one isolated warm cloud and a bit of emission associated with the direction in which the Perseus arm appears to cross the solar circle.

We should keep in mind that velocity data near the solar circle may be associated with the local movement of hydrogen clouds near the sun rather than galactic rotation, so the solar crossing emission shown above may not be associated with the Perseus arm.

The wide gap in star formation regions, atomic hydrogen and molecular clouds raises the question of whether the inner and outer Perseus arms are perhaps separate structures.

This question becomes even more interesting when we consider the Cygnet spur, a bridge between the Norma and Perseus arms in the outer galaxy described here. Instead of considering the Cygnet velocity structure as a spur, it may actually reveal that the outer Perseus arm branches off the Norma arm.

I show the two distinct parts of the Perseus arm in the face-on image I described in my previous blog post.

Earlier this year, I published a face-on map of the Milky Way in atomic hydrogen and added a large new section to this site explaining how it was derived from radio parallax and atomic hydrogen surveys.

As that section explains, visual and atomic hydrogen maps of galaxies are related but different. Atomic hydrogen surveys reveal complex structures that are sometimes visually obscured by dust in visual images.

I'm often asked for a face-on image of the Milky Way as it would appear from a spacecraft hovering far above the galactic centre. We don't have enough data yet to construct such an image in full detail, but NASA illustrator Robert Hurt has produced a schematic that does a good job combining a lot of available data. I've mentioned before that there are some inaccuracies in the Hurt image, and now that I have an atomic hydrogen map, I decided to produce a revised version of Hurt's image.

I started with Nick Risinger's version of the Hurt image, which adds more detail from real galaxies to make the Hurt image appear more realistic. I then warped the arm locations to conform to my atomic hydrogen map and added more details such as the complex network of spurs and bridges in our region of the galaxy. I've also split the Perseus arm into two distinct segments based on the evidence I presented in this blog post.

The result is below. You can download a full resolution (2528x2360) image here.