# How do I plot galactic coordinates using matplotlib and astropy in python?

I want to make a projection of millisecond pulsars on the galactic plane, much like this one from Sala et al. 2004:

I have tried several methods, and have not gotten anywhere. This is my current code, along with what it is producing:

import math as m
import numpy as np
import csv
import matplotlib.pyplot as plt
import pandas as pd
import astropy.coordinates as coord
import astropy.units as u
from astropy.io import ascii
from astropy.coordinates import SkyCoord

xarr = np.array(data.iloc[:,0])
yarr = np.array(data.iloc[:,1])
eq = SkyCoord(xarr[:], yarr[:], unit=u.deg)
gal = eq.galactic

#print(xarr)
#xarr = np.array(df.iloc[:,0])
#yarr = np.array(df.iloc[:,1])
#zarr = np.array(df.iloc[:,2])

#ra = coord.Angle(xarr[:], unit=u.hour)
#ra.degree
#ra = ra.wrap_at(180*u.degree)
#dec = coord.Angle(yarr[:], unit=u.deg)
#print(ra)

plt.figure(figsize=(6,5))
fig = plt.figure()
plt.plot(gal.l.wrap_at(180*u.deg), gal.b.wrap_at(180*u.deg), linestyle='None')
ax.scatter(gal.l, gal.b, linestyle='None')

#ax.set_facecolor('xkcd:battleship grey')
#fig.patch.set_facecolor('white')
#ax.tick_params(axis='both', which='major', labelsize=10)
#ax.grid(color='b', linestyle='solid')
fig.show()
#plt.savefig('millisecondcoloraitoff.png', dpi=600)



Here are a few lines of the input file 'galacticwperiod.csv':

Gl,Gb
111.383,-54.849
305.898,-44.888
305.913,-44.877


This is the image it produces:

I am almost certain this is wrong because they are not distributed along the galactic plane, which they should be. The data I am using is from the ATNF Pulsar Catalog.

These are the sites I have already looked at, for reference:

Any help with this would be greatly appreciated.

• Can you provide a few sample lines of galacticwperiod.csv? What are its fields? – Mike G Jul 10 '19 at 23:12
• 111.383,-54.849 305.898,-44.888 305.913,-44.877 They are from the ATNF Pulsar Catalog, using a search criteria of P0<0.03. This comment doesn't show it but they are in columns, with Gl (111,305,etc) in the first column and Gb (-54,-44,etc) in the second. – Maria Jul 16 '19 at 16:20

This code reads coordinates as equatorial (ra, dec) and transforms them to galactic (l, b):

eq = SkyCoord(xarr[:], yarr[:], unit=u.deg)
gal = eq.galactic


The contents of 'galacticwperiod.csv' are already in galactic coordinates and should not be transformed. Something like this may give better results:

gal = SkyCoord(xarr[:], yarr[:], frame='galactic', unit=u.deg)


The other issue is that pyplot's geographic projections seem to expect angles in radians. This code:

plt.subplot(111, projection='aitoff')
plt.grid(True)


produces this plot:

• I agree that I should not be transforming my coordinates. However, this did not fix the issue. – Maria Jul 16 '19 at 16:39
• @Maria Updated. – Mike G Jul 17 '19 at 7:40
• This worked. Thanks so much! – Maria Jul 17 '19 at 16:25

Looking at the plot you have, I notice that there is a concentration of dots at the "poles". The concentration should be along the equator, and particularly towards the centre of the galaxy.

I think you have your axes switched, so points along the galactic equator are being drawn along the vertical "prime meridian".

from astropy.io import fits
from astropy import wcs
import matplotlib.pyplot as plt

x = fits.open('f160w_noopt_drz.fits')['SCI']