Galactocentric

class astropy.coordinates.Galactocentric(*args, **kwargs)[source] [edit on github]

Bases: astropy.coordinates.BaseCoordinateFrame

A coordinate or frame in the Galactocentric system. This frame requires specifying the Sun-Galactic center distance, and optionally the height of the Sun above the Galactic midplane.

The position of the Sun is assumed to be on the x axis of the final, right-handed system. That is, the x axis points from the position of the Sun projected to the Galactic midplane to the Galactic center – roughly towards \((l,b) = (0^\circ,0^\circ)\). For the default transformation (\({\rm roll}=0^\circ\)), the y axis points roughly towards Galactic longitude \(l=90^\circ\), and the z axis points roughly towards the North Galactic Pole (\(b=90^\circ\)).

The default position of the Galactic Center in ICRS coordinates is taken from Reid et al. 2004, http://adsabs.harvard.edu/abs/2004ApJ…616..872R.

\[\begin{split}{\rm RA} = 17:45:37.224~{\rm hr}\\ {\rm Dec} = -28:56:10.23~{\rm deg}\end{split}\]

The default distance to the Galactic Center is 8.3 kpc, e.g., Gillessen et al. (2009), https://ui.adsabs.harvard.edu/#abs/2009ApJ…692.1075G/abstract

The default height of the Sun above the Galactic midplane is taken to be 27 pc, as measured by Chen et al. (2001), https://ui.adsabs.harvard.edu/#abs/2001ApJ…553..184C/abstract

The default solar motion relative to the Galactic center is taken from a combination of Schönrich et al. (2010) [for the peculiar velocity] and Bovy (2015) [for the circular velocity at the solar radius], https://ui.adsabs.harvard.edu/#abs/2010MNRAS.403.1829S/abstract https://ui.adsabs.harvard.edu/#abs/2015ApJS..216…29B/abstract

For a more detailed look at the math behind this transformation, see the document Description of Galactocentric coordinates transformation.

The frame attributes are listed under Other Parameters.

Parameters:
data : BaseRepresentation subclass instance

A representation object or None to have no data (or use the coordinate component arguments, see below).

x : Quantity, optional

Cartesian, Galactocentric \(x\) position component.

y : Quantity, optional

Cartesian, Galactocentric \(y\) position component.

z : Quantity, optional

Cartesian, Galactocentric \(z\) position component.

v_x : Quantity, optional

Cartesian, Galactocentric \(v_x\) velocity component.

v_y : Quantity, optional

Cartesian, Galactocentric \(v_y\) velocity component.

v_z : Quantity, optional

Cartesian, Galactocentric \(v_z\) velocity component.

representation_type : BaseRepresentation subclass, str, optional

A representation class or string name of a representation class. This sets the expected input representation class, thereby changing the expected keyword arguments for the data passed in. For example, passing representation_type='cartesian' will make the classes expect position data with cartesian names, i.e. x, y, z in most cases.

differential_type : BaseDifferential subclass, str, dict, optional

A differential class or dictionary of differential classes (currently only a velocity differential with key ‘s’ is supported). This sets the expected input differential class, thereby changing the expected keyword arguments of the data passed in. For example, passing differential_type='cartesian' will make the classes expect velocity data with the argument names v_x, v_y, v_z.

copy : bool, optional

If True (default), make copies of the input coordinate arrays. Can only be passed in as a keyword argument.

Other Parameters:
 
galcen_coord : ICRS, optional, must be keyword

The ICRS coordinates of the Galactic center.

galcen_distance : Quantity, optional, must be keyword

The distance from the sun to the Galactic center.

galcen_v_sun : CartesianDifferential, optional, must be keyword

The velocity of the sun in the Galactocentric frame as Cartesian velocity components.

z_sun : Quantity, optional, must be keyword

The distance from the sun to the Galactic midplane.

roll : Angle, optional, must be keyword

The angle to rotate about the final x-axis, relative to the orientation for Galactic. For example, if this roll angle is 0, the final x-z plane will align with the Galactic coordinates x-z plane. Unless you really know what this means, you probably should not change this!

Examples

To transform to the Galactocentric frame with the default frame attributes, pass the uninstantiated class name to the transform_to() method of a coordinate frame or SkyCoord object:

>>> import astropy.units as u
>>> import astropy.coordinates as coord
>>> c = coord.ICRS(ra=[158.3122, 24.5] * u.degree,
...                dec=[-17.3, 81.52] * u.degree,
...                distance=[11.5, 24.12] * u.kpc)
>>> c.transform_to(coord.Galactocentric) 
<Galactocentric Coordinate (galcen_coord=<ICRS Coordinate: (ra, dec) in deg
    ( 266.4051, -28.936175)>, galcen_distance=8.3 kpc, galcen_v_sun=( 11.1,  232.24,  7.25) km / s, z_sun=27.0 pc, roll=0.0 deg): (x, y, z) in kpc
    [( -9.6083819 ,  -9.40062188,  6.52056066),
     (-21.28302307,  18.76334013,  7.84693855)]>

To specify a custom set of parameters, you have to include extra keyword arguments when initializing the Galactocentric frame object:

>>> c.transform_to(coord.Galactocentric(galcen_distance=8.1*u.kpc)) 
<Galactocentric Coordinate (galcen_coord=<ICRS Coordinate: (ra, dec) in deg
    ( 266.4051, -28.936175)>, galcen_distance=8.1 kpc, galcen_v_sun=( 11.1,  232.24,  7.25) km / s, z_sun=27.0 pc, roll=0.0 deg): (x, y, z) in kpc
    [( -9.40785924,  -9.40062188,  6.52066574),
     (-21.08239383,  18.76334013,  7.84798135)]>

Similarly, transforming from the Galactocentric frame to another coordinate frame:

>>> c = coord.Galactocentric(x=[-8.3, 4.5] * u.kpc,
...                          y=[0., 81.52] * u.kpc,
...                          z=[0.027, 24.12] * u.kpc)
>>> c.transform_to(coord.ICRS) 
<ICRS Coordinate: (ra, dec, distance) in (deg, deg, kpc)
    [(  86.22349059, 28.83894138,  4.39157788e-05),
     ( 289.66802652, 49.88763881,  8.59640735e+01)]>

Or, with custom specification of the Galactic center:

>>> c = coord.Galactocentric(x=[-8.0, 4.5] * u.kpc,
...                          y=[0., 81.52] * u.kpc,
...                          z=[21.0, 24120.0] * u.pc,
...                          z_sun=21 * u.pc, galcen_distance=8. * u.kpc)
>>> c.transform_to(coord.ICRS) 
<ICRS Coordinate: (ra, dec, distance) in (deg, deg, kpc)
    [(  86.2585249 ,  28.85773187,  2.75625475e-05),
     ( 289.77285255,  50.06290457,  8.59216010e+01)]>

Attributes Summary

default_differential
default_representation
frame_attributes
frame_specific_representation_info
galcen_coord
galcen_dec
galcen_distance
galcen_ra
galcen_v_sun
name
roll
z_sun

Methods Summary

get_roll0() The additional roll angle (about the final x axis) necessary to align the final z axis to match the Galactic yz-plane.

Attributes Documentation

default_differential
default_representation
frame_attributes = {'galcen_coord': <astropy.coordinates.attributes.CoordinateAttribute object at 0x7f7ffef8ac50>, 'galcen_distance': <astropy.coordinates.attributes.QuantityAttribute object at 0x7f7ffef8ac18>, 'galcen_v_sun': <astropy.coordinates.attributes.DifferentialAttribute object at 0x7f7ffef23f28>, 'roll': <astropy.coordinates.attributes.QuantityAttribute object at 0x7f7ffef23f98>, 'z_sun': <astropy.coordinates.attributes.QuantityAttribute object at 0x7f7ffef23f60>}
frame_specific_representation_info
galcen_coord = <ICRS Coordinate: (ra, dec) in deg (266.4051, -28.936175)>
galcen_dec
galcen_distance = <Quantity 8.3 kpc>
galcen_ra
galcen_v_sun = <CartesianDifferential (d_x, d_y, d_z) in km / s (11.1, 232.24, 7.25)>
name = 'galactocentric'
roll = <Quantity 0. deg>
z_sun = <Quantity 27. pc>

Methods Documentation

classmethod get_roll0()[source] [edit on github]

The additional roll angle (about the final x axis) necessary to align the final z axis to match the Galactic yz-plane. Setting the roll frame attribute to -this method’s return value removes this rotation, allowing the use of the Galactocentric frame in more general contexts.