Overview Of Our Current Understanding Of Galactic Spiral Arms In The Milky Way

How far are we from the Center of the Galaxy?

How far is the Sun from the center of the Milky Way (Galactic Center, GC)? Quite a few recent publications have appeared in the last 3 years on this hot topic.

Many recent determinations of the distance from the Sun to the Galactic Center hover around 8.0 kpc (about 25 000 lightyears), and not 8.3 or 8.5 kpc as before.

In the galactic map below, the Sun is often replaced by the Local Standard of Rest (LSR, shown by a star), and its moving direction (arrow) and velocity (230 km/s) are indicated. Also, the observed spiral arms are shown (each arm with a different color). The boxy bulge bar (thick dark gray) around the GC and the thin long bar (light gray) are shown. Some tangents to the observed spiral arms are shown (long dashes).

A recent statistical analysis pointed to an angular velocity of the Sun and stars (matter) going around the GC at a speed of about 29 km/s/kpc, while the slower spiral pattern itself to be going around at a speed of about 23 km/s/kpc. This difference in speed ensures that matter reaches the slower arms, is shocked upon entrance by the gravitational potential difference, proceeds to form new stars in the inner arm edge, then exits the arm on the outer arm edge.

Spiral arms are seen double – the CO spine differs from the dust lane by a thousand lightyears!

Also, a catalog of tangents from the Sun to the spiral arms was made, each tangent using a different chemical tracer (CO molecule, dust, HI neutral hydrogen atom, number of masers in a given line-of-sight, synchrotron intensity peak, cold dust, hot dust, etc). It was found that the dust tracer is always separated from the CO tracer, by about 310 parsecs (almost 1000 lightyears), the CO tracer following the middle of the spiral arm, while the dust tracer following the inner side of the arm closest to the Galactic Center. It thus appears that a typical spiral arm can be separated into ‘parallel lanes’ inside the arm – see Vallée 2016, Astrophys. J., 821, 53.

A recent review about spiral arms, such as kinematics, arm widths, arm cross-sections, confirmed the parallel but separated lanes (for dust, CO, other chemical tracers) in each spiral arm, in the Milky Way.

In the diagram below, for each arm along the x-axis, we showed the arm tangents seen in CO gas (in blue), and the arm tangents seen in dust (in red). For simplicity, other arm tangents in between are not shown. By convention among astronomers, the galactic longitude decreases from left to right.

Where does the Norma spiral arm start, near the Galactic Center?

Older spiral arm models are fitted to nearby observations of stars and gas and dust. Then, to go further down in the inner Galaxy, we statistically found the mean pitch angle of the recently mapped Norma arm in two galactic quadrants (observed tangentially at galactic longitudes near l=328^o and near l=20^o), using the twin-tangent method, and obtained -13.7^o ±1.4^o. We compared with other measurements in the literature. That process yielded a very accurate pitch angle.

Recently, going still further down this path, we found that the Norma arm starts around 2.2 kiloparsecs (about 7 000 lightyears) from the Galactic Center (GC), as evidenced by recent determinations of arm tangents in various tracers there. Our new model differs from all the older models as older models started their arms in the range of 3.1 to 4.0 kpc from the Galactic Center (lacking recent observations of arm tracers there).

A sketch of the face-on view of the spiral arms (and the Sun) is presented below. The new model (2017, continuous red curve) of the Norma arm is shown, along with the previous presentation (2008, curve with red diamonds).

Why are there so many “three-kiloparsecs” arms?

Many features near the Galactic Center (GC) have been called “3-kiloparsec arms”. Despite their naming as ‘Near 3-kiloparsec arms’ or ‘Far 3-kiloparsec arms’, these features are not major arms.

Also, radial velocity data on the so-called ‘3-kpc arms’ do not coincide with radial velocities of major spiral arms.

What are they, then?

We reached a point of having too many divergent data, making it difficult to be constrained by a single physical model. Their differing characteristics suggest different physical and dynamical objects.

Recently, we were able to link some ‘3-kpc arms’ far from the GC (outside 13 degrees in galactic longitude) as an extension to some of the long spiral arms (Norma, etc). Some of these features are associated with the observed major spiral arms: the inner Perseus arm, the inner Sagittarius arm, the inner Norma arm, and the inner Scutum arm. Their slight velocity differences may be due to turbulence around a shock in a Galactic density wave between 2 and 4 kpc from the Galactic Center.

But those ‘3-kpc arms’ features very near the Galactic Center (within 13 degrees of Galactic longitude) may be different than those farther out (having different properties). These closer features may be due to a nuclear rotation between 0 and 2 kpc from the Galactic Center region, or a putative radial expansion between 0 and 4 kpc from the Galactic Center. Some of these features may thus be associated with the observed central bars. More details in Vallee, J.P., 2017, Astrophys. Space Sci, 362, 84.

The large angular scale distribution of spiral arms – better arm pitch

The Galaxy has been divided into four Galactic Quadrants. Looking inward toward the Galactic Center, there are two quadrants: ‘IV’ from longitude 270 degrees to 360 degrees (GC), and then ‘I’ from the 0 degrees (GC) up to 90 degrees of longitude. Similarly, looking outward away from the GC toward the Anti-Galactic Center, there are two quadrants: ‘II’ from longitude 90 degrees up to 180 degrees (Anti-GC), and ‘III’ from the Anti-GC up to 270 degrees.

Which methods are best for measuring the pitch angle of a very long spiral arm?

From the Sun, one could look down to the right of the Galactic Meridian (longitude zero) and reach a spiral arm (Sagittarius, say, at longitude l_I). Then we can look down to the left of the Galactic Meridian and reach the same spiral arm (Carina, say, at longitude l_IV). In both cases, we have reached the same spiral arm (the Carina-Sagittarius arm). As seen in other galaxies, spiral arms are shaped logarithmically (natural logarithm, ln), so one can use trigonometric functions like sinusoids (sin) and tangents (tan) and pi angles and insert these two ‘arm-tangent galactic longitudes’ l inside a wonderful equation, in order to obtain mathematically the unknown pitch angle p. That wonderful equation is shown in the figure here.

One can employ this wonderful equation independently, either for CO, or else for hot dust, or else for radio masers, or else for HII regions, … In the Milky Way, using many spiral arms, and many chemical arm tracers, then this equation yields the pitch p, at about 13 degrees.

A new fit to the large-scale dynamics of spiral arms – higher speeds now

Having obtained the best arm pitch (13 degrees), and the best origin of each spiral arm (2.2 kpc) from the Galactic Center (GC), and the best distance of the Sun to the Galactic Center (8.0 kpc), and the best orbital circular speed around the GC (230 km/s), then what? We can now provide the best dynamical model fit to the spiral arms!

With such a closer start for each spiral arm and the usual assumptions of symmetry and arm logarithmic shape, we upgraded our dynamical modeling and came out with the new kinematics below. A view toward the GC is shown first. Each arm has been drawn with a different color. The galactic longitude runs on the horizontal scale, from 90 degrees to 0 degrees (GC) up to -90 degrees (a.k.a. 270 degrees). Speeds (radial velocities as seen from an astronomer on Earth) of up to 160 km/s (going away from us) and down to -160 km/s (coming towards us) are seen.

Another view, toward the Anti-GC, is shown. Each arm has been drawn with a different color. The galactic longitude runs on the horizontal scale, from 90 degrees to 180 degrees (Anti-GC) up to 270 degrees. Speeds (radial velocities as seen from an astronomer on Earth) of up to 160 km/s (going away from us) and down to -160 km/s (coming towards us) are seen.

These findings are described in the following articles:

• Recent advances in the determination of some galactic constants [Astrophysics and Space Science, vol.362, art.79]
• On the many 3-kiloparsecs arms – shock wave and nuclear rotation [Astrophysics and Space Science, vol.362, art.84]
• The Norma spiral arm: large-scale pitch angle [Astrophysics and Space Science, vol.362, art.175]
• Constraining the pitch angle of the galactic spiral arms [New Astronomy Review, vol.79, p.49]
• A guided map to the spiral arms in the galactic disk [Astronomical Review, vol.13, p.113)

This work was led by Jacques Vallée from the National Research Council of Canada.