Welcome to Galaxy Map!

The purpose of this site is to provide maps of the Milky Way and to track the scientific research that makes these maps possible. You can find

  • an introduction to this site here,
  • the beginning of a book on Milky Way cartography,
  • a blog where I describe interesting research and occasionally mention changes I've made or plan to make to this site,
  • and a twitter feed where I post random thoughts and links.

You can email me here:

kevinjardine at yahoo.com

Benjamin versus Benjamin


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 (red circles) and individual objects (blue 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

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.

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

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.

New parallax estimates part 1: the inner galaxy

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.

Is the Perseus Arm a single structure?

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.

New face-on image

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.

An integrated nebula catalog

In my last blog post, I announced that I had completed my commentaries on the Sharpless, RCW and Gum nebulae and pointed out that there is considerable overlap between the three catalogs.

It was a logical next step to create an integrated catalog combining all three catalogs, removing duplicates and adding cross references.

I've gone further than that, however. The new integrated catalog attempts to identify all the extended areas of nebulosity visible in Douglas Finkbeiner's full sky hydrogen-alpha map. This is not a complete hydrogen-alpha nebula catalog, however, for three main reasons:

  • the Finkbeiner map combines three very sensitive but low resolution hydrogen-alpha surveys and so smaller nebulae are often not visible unless they are very bright,
  • there is at least some hydrogen-alpha emission visible in every region of the sky and the cut-off point for inclusion in a catalog is arbitrary, and
  • these are clouds, after all, with indistinct boundaries and complex internal structures - where one nebula stops and another begins is unclear, especially in the absence of detailed distance data.

Neverthless, I was able to expand considerably beyond the 483 distinct objects listed in the Sharpless, Gum and RCW catalogs. There are 733 nebulae listed in the full integrated catalog.

The catalog includes the object name, catalog name and galactic coordinates (l and b) for a central point for each object. Tn order to make the catalog more useful for astrophotographers, it also gives the central point in equatorial coordinates (right ascension and declination) as well as a radius in arcminutes and the constellation within which it is located.

I have provided cross reference numbers for the nebulae in the Sharpless, Gum and RCW catalogs. For the primary name I have preferred the Sharpless designation followed by the Gum designation and then finally RCW.

In addition to the Sharpless, Gum and RCW catalogs, the integrated catalog also includes the BFS nebulae and a large number of the nebulae listed in the 1976 paper by Dubout-Crillon as well as a small number of other sources.

There are 78 nebulae that I could not find in any catalog in SIMBAD and for convenience I have designated these GMN 1 to GMN 78 (Galaxy Map Nebula catalog). In some cases these are faint nebulae and in others, nebular regions that encompass a number of the nebulae in the other catalogs. Inclusion in the GMN does not mean that the object is a new discovery as many catalogs still are not available from SIMBAD and many individual studied nebulae have not been gathered into a catalog. At some point I'll write a commentary on the GMN objects and describe what information is available on them.

You can download the integrated catalog in Excel format here.

You can also view the integrated catalog data overlaid on a false colour version of the Finkbeiner map in the Milky Way Explorer. The circles surrounding the nebulae are colour coded:

  • yellow marks a nebula in the Sharpless, Gum, RCW and BFS catalogs,
  • orange marks an HII region in another published catalog,
  • green marks an unknown nebula listed in the Galaxy Map Nebula (GMN) catalog, and
  • cyan (blue-green) marks other prominent objects not in the integrated catalog but visible in the Finkbeiner map: stars, planetary nebulae or galaxies.

Nebula commentaries completed

Eight years ago I started work on a commentary on the Sharpless nebula catalog and eventually expanded to the Gum and RCW catalogs as well. Together these catalogs cover almost all of the prominent emission nebulae of the Milky Way visible in hydrogen-alpha. (These catalogs overlook a few fainter large objects and miss many smaller objects. There are other catalogs describing smaller emission nebulae such as the BFS and Bran catalogs. At some point I will look at these.)

It took me longer than expected but today the Sharpless, Gum and RCW commentaries are complete. Over the years the database and Python code I was using to present the commentaries became bitrotted so now I am using a new Haskell-based system to generate static pages from an off-line database. The resulting pages display faster and more reliably.

I have used the new Haskell-based system to improve the format of the commentaries. There are now proper Wikipedia-style footnotes, links to each nebula in the Milky Way Explorer, and a selection of distance estimates from the scientific literature instead of a single estimate. I have updated the commentaries to use the latest research and to improve the images. Of course, updating the commentaries will be an ongoing task.

There are a total of 313 Sharpless objects, 209 RCW objects and 97 Gum objects. There are more objects in the RCW and Gum catalogs than catalog numbers because both of these catalogs describe subnebulae (for RCW in the notes and for Gum in the main catalog). Sometimes these subnebulae identify the brightest parts of a larger object, but often they identify separate objects.

There is considerable overlap between the three catalogs as this Venn diagram shows:

Because of the overlaps, there are 483 distinct objects in the three catalogs. There are actually fewer nebulae than this, because some catalog entries simply designate nebulous regions that contain separate objects described in other catalogs, and in the case of the RCW catalog, there appear to be a number of objects that are unidentifiable or simply do not exist. More details can be found in the commentaries.

Stewart Lane Sharpless (1926 - 2013)

Stewart Sharpless, the creator of the Sharpless nebula catalog, died on January 19, 2013 at the age of 86. A bit sadly, I have seen no obituaries in scientific publications yet for Sharpless. I found out about his death recently by accident while searching for information on Sharpless and encountered this guestbook page with the details of his death.

His page at the University of Rochester describing him as a "professor emeritus" has been removed, but I see that he is still listed as of today at the International Astronomical Union and is still described as alive on his English Wikipedia page (but the German version correctly reports his death).

I find it sad when scientists die or retire unacknowledged. I notice that Veta Avedisova has been removed from the staff listing at the Institute of Astronomy, Russian Academy of Sciences. I hope in her case that she has a new job or is perhaps beginning a long and happy retirement!

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