# Leo II Dwarf Orbits Milky Way: Dark Matter or Emerging Gravity

In a prior blog, “The Curiously Tangential Dwarf Galaxies”, I reported on results from Cautun and Frenk that indicate that a set of 10 dwarf satellite galaxies near the Milky Way with measured proper motions have much more tangential velocity than expected by random. Formally, there is a 5 standard deviation negative velocity anisotropy with over 80% of the kinetic energy in tangential motion.

While in no way definitive, this result appears inconsistent with the canonical cold dark matter assumptions. So one speculation is that the tangential motions are reflective of the theory of emergent gravity, for which dark matter is not required, but for which the gravitational force changes (strengthens) at very low accelerations, of order $c \cdot H$, where H is the Hubble parameter, and the value at which the force begins to strengthen works out to be accelerations of only less than about 2 centimeters per second per year.

One of the 10 dwarf galaxies in the sample is Leo II. The study of its proper motion has been reported by Piatek, Pryor, and Olszewski. They find that the galactocentric radial and tangential velocity components are 22 and 127 kilometers per second, respectively. While there is a rather large uncertainty in the tangential component, for their measured values some 97% of the kinetic energy is in the tangential motion.

Artist’s rendering of the Local Group of galaxies. This representation is centered on the Milky Way, you can see a large number of dwarf galaxies near the Milky Way and many near the Andromeda Galaxy as well. Leo II is in the swarm around our Milky Way. Image credit: Antonio Ciccolella. This file is licensed under the Creative Commons Attribution-Share Alike 4.0 International license.

So let’s look at the implications for this dwarf galaxy, assuming that it is in a low-eccentricity, nearly circular orbit about the Milky Way, which seems possible. We can compare calculations for Newtonian gravity with the implications from Verlinde’s emergent gravity framework.

Under the assumption of a near circular orbit, either there is a lot of dark matter in the Milky Way explaining the high tangential orbital velocity of Leo II, or there is excess gravity. So what do the two alternatives look like?

Let’s look at the dark matter case first. The ordinary matter mass of the Milky Way is measured to be 60 billion solar masses, mostly in stars, but considering gas as well. The distance to the Leo II dwarf galaxy is 236 kiloparsecs (770,000 light-years), well beyond the Milky Way’s outer radius.

So to first order, for a roughly spherical Milky Way, including a dark matter halo, we can evaluate what the total mass including dark matter would be required to hold Leo II in a circular orbit. This is determined by equating the centripetal acceleration v²/R to the gravitational acceleration inward GM/R². So the gravitational mass under Newtonian physics required for velocity v at distance R for a circular orbit is M = R v² / G. Using the tangential velocity and the distance measures above yields a required mass of 870 billion solar masses.

This is 14 times larger than the Milky way’s known ordinary matter mass from stars and gas. Now there are some other dwarf galaxies such as the Magellanic Clouds within the sphere of influence, but they are very much smaller, so this estimate of the total mass required is reasonable to first order. The assumption of circularity is a larger uncertainty. But what this says is something like 13 times as much dark matter as ordinary matter would be required.

Now let’s look at the emergent gravity situation. In this case there is no dark matter, but there is extra acceleration over and above the acceleration due to Newtonian gravity.  To be clear, emergent gravity predicts both general relativity and an extra acceleration term. When the acceleration is modest general relativity reduces to Newtonian dynamics. And when it is very low the total acceleration in the emergent gravity model includes both a Newtonian term and an extra term related to the volume entropy contribution.

In other words, gT = gN + gE is the total acceleration, with gN = GM/R² the Newtonian term and gE the extra term in the emergent gravity formulation. The gN term is calculated using the ordinary mass of 60 billion solar masses, and one gets a tiny acceleration of gN = $1.5 \cdot 10^{-11}$ centimeters / second / second (cm/s/s).

The extra, or emergent gravity, acceleration is given by the formula gE = sqrt ($gN \cdot c \cdot H / 6$), where H is the Hubble parameter (here we use 70 kilometers/second/Megaparsec). The value of $c \cdot H / 6$ turns out to be $1.1 \cdot 10^{-8}$ cm/s/s. This is just a third of a centimeter per second per year.

The extra emergent gravity term from Verlinde’s paper is the square root of the product of $1.1 \cdot 10^{-8}$ and the Newtonian term amounting to $1.5 \cdot 10^{-11}$. Thus the extra gravity is $4.1 \cdot 10^{-10}$ cm/s/s, which is 27 times larger than the Newtonian acceleration. The total gravity is about 28 times that or $4.3 \cdot 10^{-10}$ cm/s/s. Now a 28 times larger gravitational acceleration leads to tangential orbital velocities over 5 times greater than expected in the Newtonian case.

Setting v²/R = $4.3 \cdot 10^{-10}$ cm/s/s and using the distance to Leo II results in an orbital velocity of 177 kilometers/second. With the Newtonian gravity and ordinary matter mass of the Milky Way, one would expect only 33 km/s, a velocity over 5 times lower.

Now the observed tangential velocity is 127 km/s, so the calculated number with emergent gravity is a bit high, but there is no guarantee of a circular orbit. Also, Verlinde’s model assumes quasi-static conditions, and this assumption may break down for a dynamically young system. The time to traverse the distance to Leo II using its radial velocity is of order 10 billion years, so the system may not have settled down sufficiently. There could also be tidal effects from neighbors, or possibly from Andromeda.

This is not a clear argument demonstrating that the Leo II dwarf galaxy’s observed tangential velocity is explained by emergent gravity. But it is a plausible alternative explanation, and made here to show how the calculations work out in this sample case.

So the main alternatives are a Milky Way dominated by dark matter and with a mass close to a trillion solar masses, or a Milky Way of ordinary matter only amounting to 60 billion solar masses. But in that latter case, the Milky Way exerts an extra gravitational force due to emergent gravity that only becomes apparent at very small accelerations less than about $10^{-8}$ cm/s/s.

Future work with the Hubble and future telescopes is expected to determine many more proper motions in the Local Group so that a fuller dynamical picture of the system can be developed. This will help to discriminate between the emergent gravity and dark matter alternatives.