Does a Galaxy Fly?
Yutaka Fujita(Osaka Univ., Japan)
2006, PASJ, 58, 809

Abstract
Disk galaxies in a cluster of galaxies move in hot gas filling the cluster. Generally, they move at transonic or supersonic velocities. If
ram-pressure stripping is insufficient to destroy the gas disk, the galaxies should be affected by the wind of the surrounding hot gas,similar to an airfoil. In this study, it is considered whether the aerodynamic interaction can be strong enough to force a disk galaxy todeviate from the orbit that it would have been in. It is shown that while the lift force is not effective, the drag force could affect face-on
disk galaxies in poor clusters on a long time scale.

1. Introduction

3. Results

Clusters of galaxies are filled with hot gas (intracluster mediumor ICM). Most galaxies move at transonic or supersonicvelocities in the ICM. So far, the interaction between disk
galaxies and the surrounding ICM has been considered mostly
in terms of “ram-pressure stripping” (Fig 1).

Case 1 (mainly lift force)
The lift force is represented bywhere ρICM is the ICM density, V is the galaxy velocity, S is the diskarea, α is the attack angle, and  is the Mach number (Fig.1; e.g.
Landau and Lifshitz 1987, Fluid Mechanics).
For typical values, it isFigure 1
What is ram-pressurestripping?

On the other hand, if ram-pressure stripping is insufficient todestroy the gas disk, the galaxies should be affected by the windof the surrounding hot gas, similar to an airfoil.

The effective disk radius r has been reduced from the originalvalue (r ~ 25 kpc) because of ram-pressure stripping. The galaxygains extra velocity perpendicular to its original direction of travel,∆V ~ (Flift /M) tcross~ 25 km s-1, where M (~ 1011 M) is the galaxy
mass, and tcross~ 5×108 yr is the time for the galaxy to cross thecentral region of the cluster. Since this is much smaller than thevelocity dispersions of galaxies in rich clusters (~ 1000 km s-1), thelift force can be ignored.

2. Estimation of Aerodynamic Forces
From now on, only a massive disk galaxy (M ~ 10 M) isconsidered. Fig.2 is the longitudinal section of the galaxy.
11Figure 2
Longitudinal section of thegalaxy. The galaxy is moving
leftward.

I considered two possible cases where ram-pressure stripping isineffective (not too strong ram-pressure) and aerodynamicalforces could not be ignored.

Case 2 (mainly drag force)
The drag force is represented bywhere CD is the drag coefficient. For a disk of α = 90°,
the coefficient is CD = 1.17, while for a sphere with the same radius,
the coefficient is CD = 0.2–0.47 depending on the Reynolds
number (White 1986, Fluid Mechanics). The values of CD are forincompressible fluids ( 0.3); they rise by a few tens percent at
 ~ 0.6–0.8 (Henderson 1976, AIAA J., 14, 707).
For typical values, it isCase 1 (mainly lift force)
A galaxy moving in a massive cluster with a non-zero but small
attack angle.
.
Case 2 (mainly drag force)
A face-on galaxy moving in a small cluster.

Considering weak ram-pressure pressure stripping, the disk radius
is assumed to be r = 10 kpc. After t = 10 Gyr, which is the typicalage of poor clusters, the galaxy velocity decreases by ∆V ~ (Fdrag/M)
× t ~ 90 km s-1 and the deviation from the orbit that the galaxy would
have been in if it were not for the ICM is D ~ (1/2)(Fdrag/M) t2 ~ 0.4
Mpc, which is comparable to the size of poor clusters (~ 1 Mpc).
Thus, the drag force could not be ignored.