Translating from sky coorindates (angles) to a position in a galaxy’s disk requires doing some clever three-dimensional geometry. This geometry is easy to get wrong, especially for galaxies that are large on the sky so that the curvature of the celestial sphere starts to become problematic. To this end, I’ve started work on a python package that provides methods to make this more tractable. Ultimately the galaxies package will give lots of galatic properties, but for now it just calculates position in the galaxy’s disk given inputs.

First, you’ll need to intall the package. Clone it off of the github page using the command line. This will create a directory called galaxies wherever you created it. Enter the directory and run the python setup script:

python setup.py install

This will install the package plus any other dependencies that are required, such as astroquery. At least that’s the theory.

Now, you should have the galaxies package in your python distribution. Now, you can run, from within python,

from galaxies import Galaxy
mygalaxy = Galaxy("M101")

You can then print(mygalaxy) and see some basic information about it. Of note, the mygalaxy object contains positional information about the galaxy expressed in terms of celestial coordinates: right ascension and declination. These are embodied in the highly abstract but powerful SkyCoord class from astropy. For our purposes, we should just know that these are the longitude and latitude coordinates for a spherical coordinate system. Right ascension (RA) is the equivalent of longitude and declination (DEC) is the equivalent of latitude.

What is cool is that the cloud properties also identify the centre positions of these clouds in RA/DEC coordinates. Thus, we can just directly relate these coordinates to that of the galaxy. There are deep subtleties about different RA and DEC systems, but you should postpone caring about these for as long as possible.

The mygalaxy object created above has a method called radius. For the table, you can calculate the distance from the galactic centre as

cpropstable = Table.read('myfitstable.fits')
rgal=mygalaxy.radius(ra = cpropstable['XPOS'], dec = cpropstable['YPOS'])

Then, you can see the distance from the centre if you print rgal. Notice the results are given in astropy units. To get out ordinary numbers, you can always just call rgal.value, which returns a numpy array. However the units framework provides some awesome capabilities. For example, conversions where you can run

import astropy.units as u
print rgal.to(u.kpc)  # get values in kpc
print rgal.to(u.m) # get values in metres

It also does excellent conversions when combined with the astropy.constants package, which we will deal with later.