J/MNRAS/498/385 Role of galactic dynamics in shaping GMCs (Jeffreson+, 2020)
The role of galactic dynamics in shaping the physical properties of giant
molecular clouds in Milky Way-like galaxies.
Jeffreson S.M.R., Kruijssen J.M.D., Keller B.W., Chevance M., Glover S.C.O.
<Mon. Not. R. Astron. Soc., 498, 385-429 (2020)>
=2020MNRAS.498..385J 2020MNRAS.498..385J (SIMBAD/NED BibCode)
ADC_Keywords: Molecular clouds ; Interstellar medium ; Velocity dispersion ;
Models ; Optical
Keywords: galaxies: star formation - ISM: clouds - ISM: evolution -
ISM: kinematics and dynamics - galaxies: evolution - galaxies: ISM
Abstract:
We examine the role of the large-scale galactic-dynamical environment
in setting the properties of giant molecular clouds in Milky Way-like
galaxies. We perform three high-resolution simulations of Milky
Way-like discs with the moving-mesh hydrodynamics code AREPO, yielding
a statistical sample of ∼80000 giant molecular clouds and ∼55000 HI
clouds. We account for the self-gravity of the gas, momentum, and
thermal energy injection from supernovae and HII regions, mass
injection from stellar winds, and the non-equilibrium chemistry of
hydrogen, carbon, and oxygen. By varying the external gravitational
potential, we probe galactic-dynamical environments spanning an order
of magnitude in the orbital angular velocity, gravitational stability,
mid-plane pressure, and the gradient of the galactic rotation curve.
The simulated molecular clouds are highly overdense (∼100x) and
overpressured (∼25x) relative to the ambient interstellar medium.
Their gravoturbulent and star-forming properties are decoupled from
the dynamics of the galactic mid-plane, so that the kpc-scale star
formation rate surface density is related only to the number of
molecular clouds per unit area of the galactic mid-plane. Despite
this, the clouds display clear, statistically significant correlations
of their rotational properties with the rates of galactic shearing and
gravitational free-fall. We find that galactic rotation and
gravitational instability can influence their elongation, angular
momenta, and tangential velocity dispersions. The lower pressures and
densities of the HI clouds allow for a greater range of significant
dynamical correlations, mirroring the rotational properties of the
molecular clouds, while also displaying a coupling of their
gravitational and turbulent properties to the galactic-dynamical
environment.
Description:
We consider three simulated galaxy discs, spanning a range of
galactic-dynamical environments at Milky Way ISM mid-plane pressures.
The simulations are set up as isolated gaseous discs in an external
gravitational potential that models the dark matter halo, the stellar
disc, and the stellar bulge. Subsequent star formation produces live
stellar particles.
We consider two different classes of external potential, which span
values of the galactic shear parameter β from the high-shear case
of β=0 for a flat rotation curve up to β=1 for solid-body
rotation. The FLAT (β∼0) and SLOPED (β~<0.5) initial
conditions follow a Milky Way-like external potential consisting of a
stellar bulge, a (thick) stellar disc, and a cusped dark matter halo
(e.g. Bland-Hawthorn & Gerhard 2016ARA&A..54..529B 2016ARA&A..54..529B). The CORED
(0~<β~<1) initial condition follows an M33-like potential profile
with a stellar disc, a cored dark matter halo, and no stellar bulge
(e.g. Corbelli 2003MNRAS.342..199C 2003MNRAS.342..199C).
File Summary:
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FileName Lrecl Records Explanations
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ReadMe 80 . This file
gmcs.dat 163 83153 Catalog of the physical properties and
galactic-dynamical parameters of all GMCs
presented in this paper
h1clouds.dat 154 55911 Catalog of the physical properties and
galactic-dynamical parameters of all HI clouds
presented in this paper
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Byte-by-byte Description of file: gmcs.dat
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Bytes Format Units Label Explanations
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1- 6 A6 --- Name Name of the galaxy model
(FLAT, SLOPED or CORED)
8- 15 E8.3 kpc R Galactocentic radius
17- 25 E9.3 --- beta Shear parameter (G1)
27- 34 E8.3 --- Q Toomre Q parameter (G2)
36- 43 E8.3 rad/Myr Omega Galactic orbital angular velocity
45- 52 E8.3 --- phiP Gas/stellar contribution to the
mid-plane hydrostatic pressure (G3)
54- 61 E8.3 k.K/cm3 Pmp Mid-plane pressure
63- 70 E8.3 Msun Mass Cloud mass
72- 79 E8.3 pc leff Cloud size (diameter)
81- 88 E8.3 Msun/pc2 Sigma Cloud surface density (G4)
90- 97 E8.3 km/s sigma Cloud velocity dispersion (G5)
99- 106 E8.3 --- alphavir Cloud virial parameter (G6)
108- 115 E8.3 10+5.k.K/cm3 Pturb Cloud turbulent pressure (G7)
117- 125 E9.3 km/s vdiv Cloud velocity divergence (G8)
127- 134 E8.3 --- epsilon Cloud aspect ratio (G9)
136- 144 E9.3 pc.km/s L Cloud specific angular momentum (G10)
146- 154 E9.3 --- Bsigma Cloud velocity anisotropy (G11)
156- 163 E8.3 Msun/kpc2/yr SigmaSFR Per-cloud star formation rate surface
density, averaged over the last 5Myr
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Byte-by-byte Description of file: h1clouds.dat
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Bytes Format Units Label Explanations
--------------------------------------------------------------------------------
1- 6 A6 --- Name Name of the galaxy model
(FLAT, SLOPED or CORED)
8- 15 E8.3 kpc R Galactocentic radius
17- 25 E9.3 --- beta Shear parameter (G1)
27- 34 E8.3 --- Q Toomre Q parameter (G2)
36- 43 E8.3 rad/Myr Omega Galactic orbital angular velocity
45- 52 E8.3 --- phiP Gas/stellar contribution to the
mid-plane hydrostatic pressure (G3)
54- 61 E8.3 k.K/cm3 Pmp Mid-plane pressure
63- 70 E8.3 Msun Mass Cloud mass
72- 79 E8.3 pc leff Cloud size (diameter)
81- 88 E8.3 Msun/pc2 Sigma Cloud surface density (G4)
90- 97 E8.3 km/s sigma Cloud velocity dispersion (G5)
99- 106 E8.3 --- alphavir Cloud virial parameter (G6)
108- 115 E8.3 10+5.k.K/cm3 Pturb Cloud turbulent pressure (G7)
117- 125 E9.3 km/s vdiv Cloud velocity divergence (G8)
127- 134 E8.3 --- epsilon Cloud aspect ratio (G9)
136- 144 E9.3 pc.km/s L Cloud specific angular momentum (G10)
146- 154 E9.3 --- Bsigma Cloud velocity anisotropy (G11)
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Global Notes:
Note (G1): The galactic shear parameter is defined by β=dlnvc/dlnR,
for a circular velocity vc(R) at galactocentric radius R
Note (G2): The Toomre (1964ApJ...139.1217T 1964ApJ...139.1217T) Q parameter for the gravitational
stability of the mid-plane gas is given by
Q=κsqrt(σ2g+c2s)/πGΣg,
with an epicyclic frequency κ, a mid-plane gas velocity
dispersion σg, a mid-plane sound speed cs, and a mid-plane
gas surface density Σg.
Note (G3): The variable ΦP quantifies the relative gas and stellar
contributions to the mid-plane hydrostatic pressure
Pmp=πGΦPΣ2g/2, defined in Elmegreen
(1989ApJ...338..178E 1989ApJ...338..178E) as:
ΦP=1+Σsσg/Σgσs
Note (G4): The cloud surface density is defined as:
Σx=ΣNimi,x/Ax, where x={H2,HI}. That is,
{mi,x} are the masses of H2 or HI in the gas cells i=1...N of
each cloud and Ax is the pixel-by-pixel area of the cloud's
footprint on the galactic mid-plane
Note (G5): The cloud line-of-sight velocity dispersion is defined as:
σlos,x=σx/sqrt(3)=sqrt(<|vi-<vi>x|2>i,x/3),
where x={H2,HI}, {vi} are the velocities of the gas cell
centroids, and <...>x denotes a mass-weighted average
Note (G6): The virial parameter is defined as:
αvir=5σ2los/Gsqrt(πMΣ)
Note (G7): The turbulent pressure is defined as:
Pturb~ρσ2los~Σσ2los/L
Note (G8): The degree to which a cloud is collapsing globally towards its centre
of mass can be quantified by the magnitude of its internal radial
velocity streaming, as Dx=<vr,i>x, with x∈{H2,HI},
where {vr,i} are the radial velocities of the gas cells in the
cloud, with respect to the velocity of its centre of mass
Note (G9): The elongation of each GMC and HI cloud within the galactic mid-plane
can be quantified by the aspect ratio, εx, as:
εx=Δlmaj,x/Δlmin,x, with x∈{H2,HI},
where Δlmaj and Δlmin are the major and minor axes,
respectively, of an ellipse fitted to the footprint of each cloud in
the galactic mid-plane
Note (G10): The specific angular momentum vector for a given cloud is defined as
Lx=Lz,xz+Lθ,xθ+LR,xR=<rivi>x,
with x∈{H2,HI}, where {ri} are the positions of the gas cell
centroids relative to the cloud centre of mass, and {R,θ,z}
are the galactic unit vectors in cylindrical polar coordinates
Note (G11): The velocity anisotropy is defined as:
Bσ,x=1-σt,x/2σr,x, with x∈{H2,HI},
whereσt and σr are the tangential and radial
components of the cloud velocity dispersion respectively.
See equations 52 and 53 of the article for more details.
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History:
From electronic version of the journal
(End) Ana Fiallos [CDS] 01-Aug-2023