Estimating the Fuel Supply Rate on the Galactic Disk from High Velocity Clouds (HVCs) Infall

TL;DR

This study revises the estimated fuel supply rate from high velocity clouds to the Galactic disk by incorporating hydrodynamic interactions, revealing that traditional estimates are significantly overestimated.

Contribution

It introduces new simulation models that account for hydrodynamic effects, providing a more accurate estimate of the HVC infall rate.

Findings

Traditional infall rate overestimated by a factor of ~14.

Hydrodynamic interactions reduce the estimated fuel supply rate.

Simulations show the true rate can be as low as 0.072 times the traditional estimate.

Abstract

Previous studies suggest that the estimated maximum accretion rate from approaching high velocity clouds (HVCs) on the Galactic disk can be up to ~ 0.4 solar mass per year. In this study, we point out that the hydrodynamic interaction between the HVCs and the Galactic disk is not considered in the traditional method of estimating the infall rate and therefore the true supply rate of fuel from HVCs can be different from the suggested value depending on the physical configurations of HVCs including density, velocity, and distance. We choose 11 HVC complexes and construct 4 different infall models in our simulations to give an idea of how the fuel supply rate could be different from the traditional infall rate. Our simulation results show that the fuel supply rate from HVC infall is overestimated in the traditional method and can be lowered by a factor of ~ 0.072 when the hydrodynamic interaction of the HVC complexes and the disk is considered.

Tables

Table 1. Table 1: HVC Infall Parameters

Name	$b$aafootnotemark:	$v_{dev}$bbfootnotemark:	$\Omega$ccfootnotemark:	$D$ddfootnotemark:	$M$eefootnotemark:
	[∘]	[$km{s}^{-1}$]	[∘2]	[kpc]	[${10}^6{M}_{☉}$]
A	+40	141	288	8.0	1.0
ACHV	-30	100	397	10.0	1.0
ACVHV	-30	221	338	10.0	1.0
C	+58	122	1546	10.0	5.0
GCN	-31	243	130	20.0	0.22
M	+62	95	174	4.0	1.0
Smith	+13.4	73**footnotemark:	58	12.4	1.0
WA	+32	126	102	8.0	0.13
WB	+32	52	289	8.0	0.70
WD	+32	106	253	4.4	1.1
WE	-20	106	51	9.4	0.12

Equations

$$\dot{M}=\sum \frac{M_i{v}_i}{D_i},$$

$$v_{sd}=\frac{4}{3}\frac{1}{1+\sqrt{{\rho }_d/{\rho }_i}}{v}_i,$$

Full text

Galaxy: disk — ISM: clouds — hydrodynamics

1 Introduction

First discovered by Muller, Oort, and Raimond in 1963, High Velocity Clouds (HVCs) are known as neutral atomic hydrogen clouds moving with a velocity that deviates from the galactic rotation by up to $\sim 300~{}km~{}s^{-1}$. The typical size of an HVC complex can be a few to 15 kpc across with the HI mass in the range of $10^{5}\sim 5\times 10^{6}M_{\sun}$. While the exact distances to the HVC complexes are yet questionable and should be further constrained, it is widely accepted at the current stage that a majority of the complexes exist at a distance $\leq$ 10 kpc (Thom et al., 2008; Wakker et al., 2007, 2008; Putman et al., 2012). Suggested in recent studies is that HVCs have multiple origins including stripped gas material of nearby (dwarf) galaxies like the Magellanic Stream, Galactic fountains, and inflowing intergalactic gas (Wakker & van Woerden, 1997; Blitz et al., 1999; Wakker & van Woerden, 2013). Assuming that the HVCs are inflowing intergalactic gas is often favored in theoretical Galactic Chemical Evolution (GCE) models, especially when it is metal-poor material that is supplied into the galaxy from HVCs. For example, in the GCE model with two main infall episodes, the present infall rate of primordial material is predicted as $\sim 0.4~{}M_{\sun}~{}yr^{-1}$ (Chiappini et al., 2001; Chiappini, 2008)111This two episode model with the infalling low-metal material was able to explain observational constraints in the solar vicinity including the “G-dwarf problem”.. Note that the continual and at the same time occasional infall of HVCs can be a source of metal-poor gas with the mass accretion rate being up to $\sim 0.4~{}M_{\sun}~{}yr^{-1}$ although only about half of the currently infalling material has low-metallicity (Putman et al., 2012).

The infall rate is derived from the physical properties such as mass, velocity, and distance of the HVCs and traditionally given in the form of an equation given as,

[TABLE]

where $M_{i}$ is the mass of an HVC, $v_{i}$ is the observed radial velocity, and $D_{i}$ is the distance (Wakker et al., 2007; Thom et al., 2008; Putman et al., 2012). The advantage of estimating the gas infall rate from the equation above is the simplicity that allows us to directly utilize the observed physical properties of HVC complexes. However, there are some obvious limitations of this approach due to the fact that $D_{i}$ will not be the same distance as the distance from the HVC to the point where it will collide with the galactic disk. Furthermore, $v_{i}$ will also be different from the true radial velocity with the possibility of acceleration/deceleration. But more significantly, we believe there is a different part in this method that can be reconditioned to better estimate the supply rate of material into the galactic disk from such gas inflow. The essence of the traditional method of infall rate estimation is the assumption of steady and full accretion of HVC mass until “time to impact (=$v_{i}/D_{i}$)”. Kinematic consequences that occur from the hydrodynamic interaction between the disk and HVCs are neglected in the traditional approach and therefore, in this study, we make an attempt to qualitatively show that the true fuel supply rate can change depending on the physical properties of the HVC regardless of the total infall rate itself. Further, we select 11 different HVC complexes and set up four different infall cases in our numerical simulations to give an idea how the fuel supply rate could be different from the traditional infall rate depending on the physical configuration of each HVC complex.

2 Simulation Methods

2.1 Simulation Setup

We use FLASH 2.5 (Fryxell et al., 2000) for our simulations, which is a modular, adaptive-mesh, parallel simulation code capable of handling general compressible flow problems. Message-Passing Interface (MPI) library is used for parallelization and the PARAMESH library manages Adaptive Mesh Refinement (AMR). The computation domain is configured in a 2-D cylindrical geometry that can extend up to 10 kpc in the horizontal (i.e., r-) direction and 40 kpc in the vertical (i.e., z-) direction depending on the distance and radius of the infalling HVC. As the HVC complex approaches and interacts with the galactic disk, the refinement level gradually increases to its maximum which corresponds to 9.7 pc $\times$ 9.7 pc spatial resolution for each cell. For the boundary conditions, it is reflecting at the left vertical axis (r = 0) and outflowing at the rest of the three remaining boundaries. The thickness of the gaseous disk is 250 pc (Rougoor, 1964) with the HI volume number density of 0.1 $cm^{-3}$ (Kalberla & Dedes, 2008). The HI volume number density of the ISM is $1.0\times 10^{-4}~{}cm^{-3}$.