When a Bar Tricks the Eye: How Streaming Gas Motions Imitate a Bulge in the Milky Way
Understanding the mass and structure of the Milky Way is a cornerstone of modern astronomy. Junichi Baba investigates how the non-circular motions of gas, caused by the Galaxy’s central bar, can make it look like there is more mass at the center than actually exists. This study combines advanced simulations with observations to show that gas motions in the inner few kiloparsecs (roughly the inner 10,000 light-years) can mislead astronomers into overestimating the mass of the bulge and dark matter at the Galaxy’s center.
Introduction
The paper opens by explaining why the circular speed curve (or “rotation curve”), the graph of how fast stars and gas orbit at different distances from the Galactic center, is so important for measuring the Galaxy’s mass. The authors note that in the inner Milky Way, the motion of gas and stars is influenced by a non-axisymmetric structure called the bar, a cigar-shaped group of stars stretching from the center. Traditionally, astronomers use the “terminal velocity” method to measure the speed of gas close to the center, assuming it moves in perfect circles. However, past studies have suggested that this assumption is flawed because the bar creates non-circular, streaming motions. Baba’s goal is not to propose a new idea but to demonstrate, with a realistic Milky Way model, that the bar alone can explain the steep rise in the observed rotation curve without requiring a massive bulge.
Galaxy Models and Numerical Methods
To test this idea, Baba performed three-dimensional computer simulations of gas in a Milky Way-like galaxy. The simulation includes cooling, heating, star formation, and feedback from young stars, and it uses a realistic gravitational potential informed by stellar dynamical models. This potential incorporates the stellar bar, thin and thick stellar disks, dark matter halo, nuclear stellar disk, nuclear star cluster, and even the central supermassive black hole. The gas is modeled using a method called smoothed particle hydrodynamics, which allows it to behave realistically. The gas starts off in a smooth, rotating disk and responds to the bar’s gravitational influence as the simulation evolves.
Bias in Gas-based Circular Speed Curves due to Bar-induced Streaming
Baba next examines what happens to the gas as it settles into the barred potential. The gas forms distinctive structures: a dense nuclear disk at the center, offset dust lanes along the leading edges of the bar, and spiral arms further out. These patterns closely resemble observations of the Milky Way. The key result here is that gas moving along elongated orbits in the bar can reach much higher line-of-sight velocities than expected for circular motion. When these velocities are analyzed using the terminal-velocity method, they mimic the presence of a much larger central mass.
Overestimation of Circular Speed and Enclosed Mass Profiles
By comparing the “true” circular speed from the simulated gravitational potential with the “pseudo” circular speed inferred from the simulated gas, Baba finds that the pseudo curve rises much more steeply at small radii. At a radius of about 0.4 kiloparsecs, the inferred circular speed is nearly twice the true value, and the enclosed mass is overestimated by up to a factor of four. This effect is strongest in the inner few kiloparsecs, exactly where the bar dominates the dynamics. The simulation also reproduces the observed longitude–velocity diagrams, plots showing gas speed against direction, which further supports the idea that the bar explains the steep inner rise seen in real data.
Biases in Terminal Velocities Induced by Bar-driven Orbits
To better understand these biases, Baba examines the orbits gas follows in the barred potential. The gas tends to move along “x1 orbits,” which are elongated and aligned with the bar. Depending on how the bar is oriented relative to the observer, these orbits can cause the gas to appear much faster or slower than it really is. When the bar is viewed end-on (as is roughly the case for the Milky Way), the line of sight passes through parts of the orbit where the gas is moving quickly toward or away from us, exaggerating the apparent mass at the center. Simulated rotation curves at different viewing angles confirm that the steep inner rise is strongly tied to the bar’s orientation and not necessarily to any real mass concentration.
Discussion and Summary
In the final section, Baba emphasizes that the steep rise in the inner rotation curve, which has previously been attributed to a massive classical bulge or exotic forms of dark matter, can be explained entirely by the bar’s influence on gas motions. This finding challenges interpretations that assume all the inner gas velocities are due to circular motion. Correcting for this effect suggests that the true mass of the inner Milky Way is much lower than some earlier estimates. The paper also notes that this has implications for testing dark matter models, such as ultralight dark matter, which predict distinct density bumps at the center. Since the gas motions are misleading, stellar kinematic data, less affected by non-circular motion, offer a more reliable way to measure the inner mass distribution.
Source: Baba
