Kevin Jardine's blog

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 catalog 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.

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 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!

A few days ago I announced the Velocity Explorer, an interactive tool for exploring the velocity of gas in the galactic plane. As part of that announcement I mentioned that I had used Velocity Explorer to create a model of the Milky Way:

http://galaxymap.org/drupal/node/202

and even a partial map:

http://galaxymap.org/drupal/node/204

Maps derived from velocity data can't be treated very seriously for many reasons that essentially boil down to the fact that there is no straightforward relationship between velocity and distance. A more reliable map will be possible only when many more maser parallax observations become available, especially from the southern hemisphere.

Nevertheless, I think I can derive several interesting conclusions from the exercise. Of course since these conclusions are based on my (unreliable) map, they may be incorrect. However, in most cases they don't depend upon a distance-velocity relationship and so may very well be real.

The spur system is elaborate

The map shows many short bridges or "feathers" between the spiral arms. This is not surprising as such structures are common in spiral galaxies. What is a bit more unusual is the elaborate system of large spurs on the near side of the galaxy. These include the Orion, Vela and Cygnet spurs as well as the Perseus bridge and at least four other structures. This means that as maser parallax data becomes available the process of mapping objects located between the spiral arms will not be simple as we cannot assume that the masers are associated with one or two large spurs. They will in fact be part of an elaborate hydrogen web.

The velocity data shows a double ring

Instead of a simple ring (sometimes called the "near and far 3kpc arms"), surrounding the bar, the LAB velocity data clearly shows a more complex double ring structure. It is not clear at this point how this maps into a physical structure but it seems unlikely that the bar is surrounded by a simple elliptical ring.

There is anomalous velocity in the anticentre too

The Velocity Explorer shows at least two distinct bands of clouds with strongly negative velocities towards the outer galaxy.

There are major kinks in the Perseus and Sagittarius arms

The velocity data as described in the section on the spiral arms suggests that the Perseus and Sagittarius arms are not simple logarithmic spirals but have more elaborate shapes. In particular, a major bend in the Perseus arm between about 220° - 235° means that we are looking down rather than across the arm in this direction and will likely see an overdensity of spiral tracers.

There appears to be a large complex of clouds in the outer first quadrant

There is a continuous velocity spread between the Norma and Centaurus arms from 37° to 47° as can be seen in this Velocity Explorer image. I have labelled this region the Centaurus confluence on the map. It appears to be a region where the Milky Way's spiral structure breaks down and the Norma and Centaurus arms merge into a large flocculent cloud complex.

Many galactic astronomers use the term "arm" in an inconsistent and confusing way. As a good example, this image of the Milky Way on the Chandra website shows eight arms. In a more recent example, this press release promotes the (in my view) ill-advised term "Local Arm" for the structure this website calls the "Orion Spur".

This confusion is becoming a greater problem as we get closer to the great dream of mapping the Milky Way. For example, it is increasingly the case that the question "How many spiral arms does the Milky Way have?" has no clear answer not because we lack data, but because astronomers disagree over what an arm is, or use the term imprecisely.

So today I'm proposing a definition:

A spiral arm is a large scale structure of atomic hydrogen that:

• lies outside the galactic centre region,
• is warmer and denser than its surroundings, and
• wraps around the galactic centre for more than 180°.

In an astronomical context we can use the word "arm" as a short form of "spiral arm" when no confusion will result. We should not use the word "arm" to refer to any other galactic structure than a "spiral arm".

The emphasis on atomic hydrogen makes sense to me because atomic hydrogen is the basic building block of all galaxies and including the reference to atomic hydrogen avoids arguments that a spiral arm is not an arm because it lacks other tracers like molecular gas, red giants or HII regions.

The definition precludes smaller structures that may exist in the galactic bar and the near and far 3kpc "arms" ("ring" or "ring structures" may be better terms for what has been called the 3kpc "arms").

By this definition, the evidence suggests that the Milky Way has four spiral arms.

Also by this definition the Orion Spur does not appear to be an arm. Although it is a large structure, it is found (so far as we currently know) entirely or almost entirely on the near side of the galaxy. The concept of spiral in a spiral arm implies that it wraps around the galactic centre and the Orion Spur does not appear to do this. I also think that it is confusing in an astronomical context to use the term "arm" when we do not mean "spiral arm".

(Whether "Orion Spur" is an appropriate name for this structure is a different question which I'll address in a future blog post.)