How galaxies make their rotation curves flat and what about dark matter?

The rotation curves of disc galaxies are flat and dark matter is speculated as explanation. Alternatively, the gravity of material disk could explain the flat curves. Using the gravitational force that a disk exerts on a body in the disk, we have computed the the rotation curves of disc galaxies and the curve of their mass densities. The numerical result fits the flat curves and the observed mass densities of galaxies. This theory gives a new way to measure the masses of galaxies using their rotation velocities and shape.

Rotation curve

In a disc galaxy stars orbit the center of the galaxy at velocities that depend on the radial distance from the centre. The measured orbital velocities of typical galaxies are plotted versus radial distance in Figure 1, which are the rotation curves of these disk galaxies. Figure 1 shows that the observed rotation curves are flat for large radial distance, which means that these velocities are roughly constant with respect to radial distance [1].

The flat aspect of the rotation curves is puzzling because the Newtonian theory of gravity predicts that, like the velocities of the planets in the solar system, the velocity of an orbiting object decreases as the distance from the attracting body increases. Since the centers of galaxies are thought to contain most of their masses, the orbital velocities at large radial distance should be smaller than those near the center. But the observed the rotation curves clearly show the contrary and the observed orbital velocities are bigger than expected.

The masses of galaxies estimated using the luminosity of visible stars are too low to maintain the stars to move at such high speed. So, large amount of matter is needed to explain the observed velocity, but we do not see this matter. The missing matter should act gravitational force because it should hold the flying stars, but should not radiate light because invisible. So, it is dubbed as dark matter. However, dark matter has not been observed directly despite the numerous actively undertaken experiments to detect it. Several alternatives to dark matter exist to explain the rotation curve.

We propose to model galaxy as regular matter disk. Because of its shape, the resultant gravitational force of material disk within the plane of a galaxy is different from that of the galaxy taken as concentrated masses. But, is this resultant force bigger?

Gravity of material disk

Galaxies are made of stars that move in circular orbits. Let us take out one circular orbit with all its stars and put them in free space without an attracting body at the center. These stars form a circle and attract each other gravitationally, which would make them to fall toward the center if they were stationary, see Figure 2.

Figure 2 shows a simplified image of stars in a circle with F_A being the combined gravitational force that the other stars act on the star A. For the star A not to fall out of the circle, it must rotate at a nonzero velocity which is labeled as v_A. By symmetry, all the stars feel the same gravitational force and must rotate at the same velocity to stay in the circle. So, this circle of stars should rotate although no attracting body is at the center. It is the proper mass of all the stars of the circle that maintains the stars rotating.

In the explanation, we will often use the case of a single object in a circular orbit around an attracting body. For referring to this case, we give it the following name.

Definition: The Newtonian orbital acceleration and velocity of a single object in a circular orbit around an attracting body are named Single-Orbit acceleration and velocity.

If a star orbits in circle around a central mass M, the gravitational force on it is from M only and is labeled as F_M. The star would move at the Single-Orbit velocity which is v_s, see equation (1).

Now, let us add the mass M at the center of the circle of stars, see Figure 3. The gravitational force acted on the star A by the other stars of the circle is still F_A. But in addition it feels the force F_M from the central mass M. So, the total force on the star A is F_A+F_M which is bigger than F_M. In consequence, to stay in the circle the star A should orbit at a velocity bigger than v_s, say at v_s+∆v_A with ∆v_A being the contribution of the force F_A. So, if a circle of stars orbit an attracting body at the center, their orbital velocity should be bigger than the Single-Orbit velocity of one star around the same body, which is kind of like the case of the bigger than expected orbital velocity in galaxies.

Now, let us smear the stars of the circle into a disk of dust around the central mass M which is not held by cohesion but by gravitational force, see Figure 4. Like the stars of the circle, the gravitational force on a dust is from the mass M and the proper mass of the disk. So, the orbital velocity of the dust, v_d in Figure 4, will be bigger than the Single-Orbit velocity around the mass M like the stars in the disk of a galaxy.

This is the working principle of our model that explains the faster than expected orbital velocity in galaxies. Now, let us see if the gravity of material disk could make the orbital velocity constant for large radial distance.

Force in a disk

For computing the gravitational force that the entire disk exerts on a chunk of material of mass m_1, we use the Newtonian law of gravitation which expresses the gravitational force that an elementary mass dm exerts on the chunk of material, see equation (2) where dF_d is the gravitational force, G the universal gravitational constant, R_3 the distance between m_1 and dm, e_3 the unit vector pointing from m_1 to dm, see Figure 5.