Signatures of Galactic Expansion in Gaia DR3: Implications for the JWST Early-Galaxy Puzzle

TL;DR

Using Gaia DR3, the study tests whether the Milky Way exhibits central radial expansion by computing velocities in 27 sectors within $r_{gc}\le 5$ kpc and binning $v_{radial}$ vs $r_{gc}$. They report outward motions in 21 sectors and small outward expansion in disk and halo populations, with global averages of a few km s$^{-1}$, suggesting a measurable central expansion. If real, this expansion would imply a dynamically active Galactic nucleus and could offer a local analogue to AGN-driven growth proposed to explain JWST's early massive galaxies. The findings are preliminary, subject to projection effects and systematics, and require confirmation with future Gaia releases and complementary spectroscopy.

Abstract

Recent observations with the James Webb Space Telescope (JWST) of massive galaxies at ages below 1 Gyr pose a challenge to standard models of galaxy formation, which predict significantly longer assembly timescales. One possible explanation is that active galactic nuclei (AGN) drive large scale outflows that accelerate galaxy growth. To test this scenario in the local Universe, we analyzed Gaia DR3 data for stars within 5 kpc of the Galactic center, computing galactocentric radial velocities (v_radial_gc) in 27 spatial sectors covering the entire Galaxy, with radial binning of 0.25 kpc. Coordinate transformations and velocity calculations were performed using the Astropy library. We find that 21 of 27 sectors exhibit statistically significant outward motions of 3-50 km/s, while one quadrant shows negative velocities, likely related to the configuration of an activity zone and/or the Galactic bar. Both disk and halo populations also display small but significant mean expansion of 3-9 km/s (p<0.01). These results are consistent with our previous studies, where globular clusters showed outward velocities of 17-31 km/s up to 12 kpc, and axisymmetric analyses of Gaia DR3 stars indicated expansion of ~19 km/s to 5 kpc. Taken together, the evidence suggests that the Milky Way exhibits measurable central expansion, potentially reflecting AGN-driven feedback. This interpretation departs from standard theory and should be regarded as preliminary, requiring further study. However, if confirmed, such expansion could provide a natural explanation for the rapid appearance of massive galaxies observed by JWST.

Figures (3)

• Figure 1: Plots of the mean galactocentric radial velocity ($v\_{\mathrm{radial} \mathrm{\_gc}}$) with error bars in 0.25 kpc bins, shown as a function of galactocentric distance ($r\_gc\_kpc$) for stars in different Galactic sectors.
• Figure 2: Fourth quadrant displays negative velocities that transition to positive beyond $\sim$ 6 kpc
• Figure :