CenterOfMass class

Determines position and velocity of the COM for a galaxy/particle type combination.

class galaxy.centerofmass.CenterOfMass(gal, ptype=2)

Class to define COM position and velocity properties of a given galaxy and simulation snapshot

Args:

gal (Galaxy object):

The desired galaxy/snap to operate on

ptype (int):

for particles, 1=DM/halo, 2=disk, 3=bulge

Throws:

ValueError, if there are no particles of this type in this galaxy (typically, halo particles in M33)

com_define(xyz, m)

Function to compute the center of mass position or velocity generically

Args:

xyz (array with shape (3, N)):

(x, y, z) positions or velocities

m (1-D array):

particle masses

Returns:

3-element array, the center of mass coordinates

com_p(delta=0.1, vol_dec=2.0)

Function to specifically return the center of mass position and velocity .

Kwargs:

delta (tolerance)

Returns:

One 3-vector, coordinates of the center of mass position (kpc)

com_v(xyz_com)

Center of Mass velocity

Args: X, Y, Z positions of the COM (no units)

Returns: 3-Vector of COM velocities

center_com(com_p=None, com_v=None)

Positions and velocities of disk particles relative to the CoM

Returns : two (3, N) arrays

CoM-centric position and velocity

angular_momentum(com_p=None, com_v=None, r_lim=None)

Returns:

L : 3-vector as array

The (x,y,x) components of the angular momentum vector about the CoM, summed over all disk particles

pos, v : arrays with shape (3, N)

Position and velocity for each particle

r_lim : float

Radius to include in calculation (implicit kpc, no units)

rotate_frame(to_axis=None, com_p=None, com_v=None, r_lim=None)

Arg: to_axis (3-vector)

Angular momentum vector will be aligned to this (default z-hat)

Returns: (positions, velocities), two arrays of shape (3, N)

New values for every particle. self.data remains unchanged.

Based on Rodrigues’ rotation formula Ref: https://en.wikipedia.org/wiki/Rodrigues%27_rotation_formula

shell_h(radii, m, xyz, vxyz)

Calculate specific angular momentum of spherical shells.

Args:

radii ((M,0) array of float):

boundaries of shells (implicit kpc from center)

m ((N,) array of float):

masses (implicit Msun)

xyz, vxyz ((3,N) arrays of float):

positions and velocities (implicit kpc, km/s)

Returns:

rad ((M-1,) array):

midpoints of shells (implicit kpc)

L ((M-1,3) array of float):

total angular momentum

h ((M-1,3) array of float):

specific angular momentum, i.e. L/m

sphere_h(radii, m, xyz, vxyz)

Calculate specific angular momentum within spheres.

Args:

radii ((M,0) array of float):

boundaries of shells (implicit kpc from center)

m ((N,) array of float):

masses (implicit Msun)

xyz, vxyz ((3,N) arrays of float):

positions and velocities (implicit kpc, km/s)

Returns:

L ((M,3) array of float):

total angular momentum

h ((M,3) array of float):

specific angular momentum, i.e. L/m

disp_by_radius(x, vy, xbins, binwidth=None)

Calculate mean velocity and dispersion sigma for a set of radius bins

Args:

x (array of float):

lateral distance from CoM

vy (array of float):

radial velocity

xbins (array of float)

midpoints of equally-spaced bins

binwidth (float):

optional and probably useless