Moment maps and statistics¶
Moment maps¶
Producing moment maps from a
SpectralCube
instance is
straightforward:
>>> moment_0 = cube.moment(order=0)
>>> moment_1 = cube.moment(order=1)
>>> moment_2 = cube.moment(order=2)
By default, moments are computed along the spectral dimension, but it is also
possible to pass the axis
argument to compute them along a different
axis:
>>> moment_0_along_x = cube.moment(order=0, axis=2)
Note
these follow the mathematical definition of moments, so the second moment is computed as the variance. For linewidth maps, see the Linewidth maps section.
The moment maps returned are Projection
instances,
which act like Quantity
objects, and also have
convenience methods for writing to a file:
>>> moment_0.write('moment0.fits')
and converting the data and WCS to a FITS HDU:
>>> moment_0.hdu
<astropy.io.fits.hdu.image.PrimaryHDU at 0x10d6ec510>
The conversion to HDU objects makes it very easy to use the moment map with plotting packages such as APLpy:
>>> import aplpy
>>> f = aplpy.FITSFigure(moment_0.hdu)
>>> f.show_colorscale()
>>> f.save('moment_0.png')
Linewidth maps¶
Making linewidth maps (sometimes refered to as second moment maps in radio astronomy), you can use:
>>> sigma_map = cube.linewidth_sigma()
>>> fwhm_map = cube.linewidth_fwhm()
These also return Projection
instances as for the
Moment maps.