Modeling the Accretion of High-Velocity Clouds from a Rotating Halo

Abstract

High-Velocity Clouds (HVCs) are a major fuel reservoir for star formation in the Galactic disk. Determining their origin and kinematics is thus crucial for understanding Galactic evolution. In this paper, we employ simple test-particle simulations to model HVC kinematics, generating line-of-sight velocity maps and probability density functions (PDFs) for comparison with observational results. We find that models assuming low angular momentum and an initial scale of tens of kiloparsecs (kpc) successfully reproduce the observed kinematic trends for both blue-shifted and red-shifted components. This consistency may support the dominance of intermediate-halo dynamics (tens of kpc scale) in regulating Galactic evolution, consistent with HVC formation via thermal instability in metal-polluted gas in the halo. Furthermore, by considering the entire bulk mass involved in the continuous accretion process -- including diffuse or ionized components that often escape direct observation -- our theoretical estimates yield a total mass accretion rate of several solar masses per year. This indicates that HVC accretion has the potential to supply a sufficient amount of gas to the Galactic disk to sustain ongoing star formation over several Gyr. Our findings suggest that the Galactic baryon cycle and disk evolution are governed by dynamics within the intermediate halo, providing key kinematic constraints for future magnetohydrodynamical simulations that resolve spatial structures of high velocity clouds.

Figures (3)

• Figure 1: $V_{\rm LSR}$ map in Galactic coordinates, where $l$ and $b$ are the Galactic longitude and latitude, respectively. The color shows $V_{\rm LSR}$. This figure compares the observation (top panel) and four distinct models (bottom panels). Top panel: Observational all-sky map constructed from the catalog Westmeier2018, based on the HI4PI Survey HI4PI+2016. Bottom panels: Comparison of four distinct models: (top-left) the Outflow model, and three inflow models with varying rotational ratios: (top-right) sub-rotating inflow ('z20R0.1'), (bottom-left) co-rotating inflow ('z20R1.0'), and (bottom-right) super-rotating inflow ('z20R2.0').
• Figure 2: The probability density function (PDF) of $V_{\rm LSR}$ weighted by the inverse of the distance from the observer, as indicated in Equation \ref{['eq:Results:pdf']}. The red histogram is derived from our calculation, while the blue- and black-histograms are derived from the HVCs catalogs in Wakker2001 and Moss+2013, respectively. This figure compares three distinct sub-rotation models ($f_{\rm rot} = 0.1$): (left) inner-halo ('z3R0.1'), (middle) intermediate-halo ('z20R0.1'), and (right) whole-halo ('z300R0.1').
• Figure 3: Estimated physical sizes (top panel) and masses (bottom panel) of HVCs as a function of the line-of-sight velocity ($V_{\rm LSR}$). Grey dots represent the observational data from the catalog of Moss+2013, where distances were assigned via a nearest-neighbor search in our simulation's phase space. The grey solid lines indicate the mean values of the sizes and masses. Red dots denote HVCs with distance constraints from absorption-line studies using Gaia distances Lehner+2022.