Insights into the structure and kinematics of a Milky Way-like galaxy

TL;DR

The paper investigates how Milky Way–like large-scale kinematics shape the ISM using a controlled isothermal, MW-analogue simulation with a live bar, bulge, and dark halo. It employs Fourier analysis and spiral-arm ridge tracking to show that the stellar spiral pattern is weak and multi-segmented, while gaseous spirals are sharper and more numerous, indicating a transient, non-grand-design disc response. Strong bar-driven radial motions yield substantial velocity residuals up to $\pm 50$ km s$^{-1}$ and drive rapid evolution of spiral structures on timescales of $10$–$20$ Myr, aligning with Gaia DR3 observations and suggesting a dynamic rather than density-wave–driven MW spiral framework. The results highlight a complex ISM–star-formation relationship in MW-like systems and provide a robust MW analogue to interpret kinematic data, while outlining future work to incorporate self-gravity, chemistry, and feedback.

Abstract

Understanding how the large-scale kinematics of the MW shape the formation and evolution of the interstellar medium remains challenging from an observational perspective, and numerical models that can reproduce the observed structure and kinematics of the MW are much needed in order to infer how the MW might work as a star formation engine. This work aims to use a numerical framework that is a close match to the observed large-scale distribution of stars and gas in the MW to isolate and understand the impact of galaxy-driven flows on the formation, agglomeration, and longevity of spiral patterns, prior to the inclusion of chemistry, star formation, and feedback. We use an isothermal simulation of a MW-like galaxy, found to closely match the longitude-velocity observational features of the MW in previous work, that includes the coupled evolution of gas, stars, and dark matter under purely gravitational and hydrodynamical processes. We characterise the morphology and kinematics of the stars and gas in the disc, quantify velocity residuals and their association with spiral features, and analyse the time-evolution of individual spiral-ridge segments. Our results demonstrate that our model reproduces many observed MW structural and kinematic signatures, from the inner Galaxy to the Solar neighbourhood, supporting its suitability as an analogue of the MW. The stellar spiral pattern in our model is relatively weak and shows lower multiplicity relative to the sharper gaseous arms, offering an explanation for discrepancies in observational determinations of the number and location of MW spiral arms. Both gas and stellar spiral arms are highly segmented, without a single coherent spiral pattern as expected from a grand-design type of galaxy. We find strong radial motions linked to the non-circular motions driven by the presence of a bar, and which extend well into the disc. The gas radial and...

Figures (20)

• Figure 1: Left: Column density maps of the stellar distribution within our model, projected in the $xy$, $xz$ and $yz$ planes. Right: Residual column density maps of the stellar disc distribution within our model compared to S22, with contours representing cumulative fractions of the total stellar mass our model: $10\%, 25\%, 50\%, 75\%,$ and $90\%$. This figure uses colour coding where red indicates a lack and blue denotes an excess of mass in our model, compared to the S22 analytic distribution.
• Figure 2: Stellar surface density profiles along the $x$ (left), $y$ (centre), and $z$ (right) axes of our model (red-dotted line) compared to the analytic sormani2022stellar model (yellow-crossed line) including the overall best fit of an S22-type profile to our model (shown as a black solid line), and its various components represented by a distinct colour-coded dashed line: disc in dark blue, bar$_{1}$ in pink, bar$_{2}$ in green, and bar$_{3}$ (i.e. long bar) in light blue.
• Figure 3: Top-view representations of the tangential ($-V_\phi$, left panel) and radial ($V_R$, right panel) velocity fields for the stellar component of our model. Note that $V_\phi$ is negative due to the clockwise rotation of the galaxy. The Sun is located at a viewing angle of $\phi_{\mathrm{obs}}=20^{\circ}$ with respect to the bar. The original Sun's position as per the best viewing angle from DuranCamacho2024 is marked as a black circle, and the Sun's reflection point (Sun$_{\rm RP}$) at $180^{\circ}$ offset is marked with a cross.
• Figure 4: Spherical enclosed mass as a function of Galocentric radius for the analytical model from sormani2022stellar (blue-dashed line), and our model at initial (orange-dotted line), optimal (green-dotted line), and late (red-dotted line) times. The position of the two ILRs in our model are shown as green vertical dashed lines.
• Figure 5: Density plots of the Fourier amplitudes ($C_n$) for each mode $n$ as a function of galactocentric radius, for stellar particles (top) and gas cells (bottom). The colour scale represents the amplitude, and therefore the strength of different spiral modes, with strong peaks indicating dominant spiral arms.