Milky Way evolution on a human timescale

Abstract

How do galaxies form and evolve? This is one of the most puzzling questions in astronomy. Galaxy assembly takes place throughout the entire history of the Universe, but our understanding of it is hampered by the unfortunate fact that we can only observe galaxies at a single moment in time. Here, we use archival data of decades-long monitoring of the Milky Way to examine some of its key characteristics, namely the mass of its central black hole, the pattern speed of the bar, and the distance from the Sun to the Galactic centre. We find a surprisingly fast evolution of these three properties on a timescale of only a few decades, and speculate that it might be driven by shared physical processes.

Figures (4)

• Figure 2: Compilation of bar pattern speed measurements (an extended version of Figure 10 from Hunt2025). An unmistakable downward trend is quantified using the same approach as in Section \ref{['sec:mbh']}, and a diagonal corner plot of the two most important parameters -- current pattern speed $\Omega_{2026}$ and slowdown rate $\eta$ -- is tucked into the left corner.
• Figure 3: Evolution of the Milky Way bar. The two images, taken 15 years apart, are on the same scale; the apparent mismatch in size is due to difference in exposure time. More important is the change in the bar angle, from $\sim 35^\circ$ on the left to $20^\circ$ on the right, amounting to the CCW rotation rate of $1^\circ\mathrm{yr}^{-1}$ in the opposite sense to the general CW Galactic rotation.
• Figure 4: Compilation of measurements of the distance from the Sun to the Galactic centre over the last four decades. A broken-linear trend line is fitted using the same likelihood approach as in Section \ref{['sec:mbh']}, demonstrating that the earlier inward motion of the Sun has been replaced by an outward migration since late 1990s.
• Figure :