Observational signature of co-rotating spiral arms

The spiral arm seen in disk galaxies have fascinated generations, and their nature and origin are still unknown. Since the 1960s, the most accepted explanation of spiral arms has been the density wave theory, in which spiral arms rotate around galaxy like a Mexican wave in a stadium crowd, passing through stars. The theory thus predicts long-lived spiral arms.

However even the most recent high-resolution numerical simulations cannot create the type of long-lived spiral arms expected from density-wave theory – instead transient and recurrent arms are found. Our recent studies showed that this happens because the spiral arms rotate at the same speed as the stars, i.e. co-rotating, at all radii.

The European Space Agency (ESA)’s billion-euro mission, Gaia (launched in Dec. 2013), will produce accurate measurements of the positions and velocities for about a half billions disk stars in the Milky Way. This will be our great opportunity to study the structures of Galactic disk, such as the nature of the spiral arm features. Using high-resolution numerical simulations of Milky Way-like disk galaxy, we are making prediction of observational signatures of such co-rotating spiral arms to test this new spiral arm theory with the upcoming Gaia data.

Figure 1: Snapshot of the simulated galaxy. Left (Right) panel shows the face-on view of the star (gas) particle distribution. The solid line indicates the position of the spiral arm identified. The observer is assumed to be located at (x,y)=(0,-8) kpc. Three line of sight directions are highlighted with the dotted lines. The galaxy is rotating clockwise.

Challenges

These studies use particle-based N-body simulations and also follow hydrodynamics and the physical process of galaxy evolution, such as radiative cooling, star formation, supernovae feedback and chemical evolution. Currently, we use about 106-7 particles, which requires parallel computing facilities.

Solution

We ran a disk galaxy simulation using our original parallel N-body/Smoothed Particle Hydrodynamics galactic chemodynamics code, GCD+. Fig. 1 shows a snapshot of the simulation. We observed the simulated galaxy in a similar way to how we can observe the Milky Way. We observed the rotation and radial velocities of the gas and stars as a function of the distance from our assumed location of the observer at the three lines of sight. We find that the stars around the spiral arm show a large variation in both radial and rotational velocities owing to the co-rotating spiral arm. If the spiral arm is indeed co-rotating spiral arm, we should observe a similar variation of the rotation and radial velocities around the spiral arm at every Galactocentric radius. An accurate measurement of the distance, proper motion and radial velocity of the stars is required, and the Gaia data will be a critical test for the co-rotating spiral arm.

Figure 2: Rotation velocity of stars (upper panels) and gas (lower panels) as a function of the distance from the observer. Left, middle and right panels show the sample of stars and gas particles at the different lines of sights indicated in Figure 1. The vertical dotted lines indicate the position of the spiral arm. Colours correspond to the gain (redder) and loss (bottom) of the angular momentum from 20 Myr ago and 20 Myr later. The star symbols in the lower panels are the rotation velocity of the observed maser sources in the Milky Way (Reid et al. 2014, ApJ, 783, 130).

We also find that the stars and gas behind the spiral arm always gain angular momentum, while the stars at the front of the spiral arm lose angular momentum. The stars and gas tend to rotate slower (faster) behind (at the front of) the spiral arm and move outward (inward), because of the radial migration (Fig. 2). The trend is much clear for the gas. We have compared the results with the observed data of the maser sources from Reid et al. (2014, ApJ, 783, 130).  Interestingly the data show similar trend to the simulation. More data from the accurate astrometric measurement of the maser sources will provide additional constraints on the nature of the spiral arm.

This study benefited from our access to IRIDIS. We acknowledge generous support provided by SES (Centre for Innovation).

Further Reading and References

Project Contact

Daisuke Kawata, Robert Grand, Jason Hunt (Mullard Space Science Laboratory, University College London)

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