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, http://adsabs.harvard.edu/abs/2009ApJ...692.1075G.

The default height of the Sun above the Galactic midplane is taken to be 27 pc, as measured by http://adsabs.harvard.edu/abs/2001ApJ...553..184C.

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

Parameters:

representation : BaseRepresentation or None

A representation object or None to have no data (or use the other keywords)

galcen_distance : Quantity, optional, must be keyword

The distance from the Sun to the Galactic center.

galcen_ra : Angle, optional, must be keyword

The Right Ascension (RA) of the Galactic center in the ICRS frame.

galcen_dec : Angle, optional, must be keyword

The Declination (Dec) of the Galactic center in the ICRS frame.

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!

copy : bool, optional

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

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_distance=8.3 kpc, galcen_ra=266d24m18.36s, galcen_dec=-28d56m10.23s, z_sun=27.0 pc, roll=0.0 deg): (x, y, z) in kpc
    [(-9.6083818980977, -9.400621883358546, 6.520560663896347),
     (-21.283023068029138, 18.763340128812384, 7.846938548636718)]>

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_distance=8.1 kpc, galcen_ra=266d24m18.36s, galcen_dec=-28d56m10.23s, z_sun=27.0 pc, roll=0.0 deg): (x, y, z) in kpc
    [(-9.407859235565343, -9.400621883358546, 6.520665737962164),
     (-21.08239383088295, 18.763340128812384, 7.84798134569032)]>

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)
    [(86.22349058727241, 28.8389413808627, 4.391577882957292e-05),
     (289.6680265194508, 49.88763881149547, 85.96407345372828)]>

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)
    [(86.25852490164378, 28.85773187391088, 2.7562547481200286e-05),
     (289.77285254989323, 50.062904565432014, 85.92160096237191)]>

Attributes Summary

`default_representation`
`frame_attributes`
`frame_specific_representation_info`
`galcen_dec`
`galcen_distance`
`galcen_ra`
`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_representation¶

frame_attributes = OrderedDict([('galcen_distance', <astropy.coordinates.baseframe.FrameAttribute object at 0x7f2fca9f2f10>), ('galcen_ra', <astropy.coordinates.baseframe.FrameAttribute object at 0x7f2fca9f2f50>), ('galcen_dec', <astropy.coordinates.baseframe.FrameAttribute object at 0x7f2fca9ff0d0>), ('z_sun', <astropy.coordinates.baseframe.FrameAttribute object at 0x7f2fca9ff210>), ('roll', <astropy.coordinates.baseframe.FrameAttribute object at 0x7f2fca9ff350>)])¶

frame_specific_representation_info¶

galcen_dec = <Angle -28.936175 deg>¶

galcen_distance = <Quantity 8.3 kpc>¶

galcen_ra = <Angle 266.4051 deg>¶

name = 'galactocentric'¶

roll = <Quantity 0.0 deg>¶

z_sun = <Quantity 27.0 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.

Navigation

Galactocentric¶