CT-3: Plotting in Python.¶

Step 1: Complete this tutorial on plotting in Python.

Step 2: Manipulate an astronomical dataset with Python.

Part A. Download this data table [GlobularClusters_clean.tab] to your class folder. This table is a cleaned up version of the data table at http://spider.seds.org/spider/MWGC/mwgc.html.

If you right-click on the *.tab link and choose ‘Download Linked File As’, you can save it directly to the work folder you created. Or, you can download it to your Desktop or Downloads folder and use your knowledge of Unix to move it into your work folder.

Open GlobularClusters_clean.tab in your text editor. For a reminder on how to do this, reread CT-1a.

The table contains the RA and DEC locations, distance from our sun and from the galactic center in kilolightyears (kly), apparent magnitude in the V-band, and angular size of the Globular Clusters in our Milky Way galaxy.

Import python libraries¶

In [12]:

#Set up astropy 
from astropy.table import Table,Column
from astropy.coordinates import SkyCoord, Distance
from astropy import units as u
from astropy.io import ascii
#Set up plotting libraries
%config InlineBackend.rc = {}
import matplotlib
import matplotlib.pyplot as plt
%matplotlib inline
import seaborn

Read datafile¶

In [6]:

Globulars = ascii.read('data/GlobularClusters_clean.tab')
Globulars

Out[6]:

name	RA	DEC	distSun(kly)	distGalCenter(kly)	m_v	angular_diameter
Tuc	6.02362	-72.0813	14.7	24.1	3.95	50.0
Scl	13.1885	-26.5826	29.0	39.1	8.09	13.0
Tuc	15.8094	-70.8488	28.0	30.6	6.4	14.0
Cet	30.7375	-3.25278	98.1	112.5	15.03	1.2
Hor	48.0675	-55.2162	53.1	59.0	8.29	6.8
Cep	53.3335	79.5811	36.2	56.1	13.18	2.8
Hor	58.7596	-49.6153	2.0	406.2	15.72	0.5
Eri	66.1854	-21.1869	93.7	309.7	14.7	1.0
Aur	71.5246	31.3815	88.7	114.1	13.04	2.2
Col	78.5282	-40.0466	39.4	54.1	7.14	12.0
Lep	81.0462	-24.5247	42.1	61.3	7.73	9.6
...	...	...	...	...	...	...
Sge	298.444	18.7792	13.0	21.8	8.19	7.2
Sgr	301.52	-21.9212	68.1	47.9	8.52	6.8
Del	308.547	7.40447	50.9	41.7	8.83	7.1
Aqr	313.365	-12.5373	55.4	42.1	9.27	6.6
Del	315.372	16.1873	34.3	125.5	10.56	3.6
Peg	322.493	12.167	33.9	33.9	6.2	18.0
Aqr	323.363	-0.82325	37.5	33.9	6.47	16.0
Cap	325.092	-23.1799	26.4	23.1	7.19	12.0
Cap	326.662	-21.2526	61.9	51.5	11.99	2.9
Peg	346.685	12.772	84.8	87.7	13.47	0.7
Aqr	347.111	-15.6115	85.7	82.5	11.29	4.2

Create and Astropy Coordinate Object¶

To locate an object in 3D space we need three numbers. RA and dec to tell us an objects location on the sky and a distance

In [13]:

GlobularCoords=SkyCoord(Globulars['RA'],Globulars['DEC'],unit=(u.degree, u.degree),\
                   distance=Distance(Globulars['distSun(kly)']/3.26,u.kpc),frame='icrs')

Plot the Globular Clusters on the Sky¶

Globular clusters are plotted in a Mollweide projection. Points are scalled according to the angular diameter and colored according to distance (blue close - pink far)

In [33]:

fig = plt.figure (figsize=(13,7))
ax = fig.add_subplot(111,projection="mollweide")
ax.grid(True)
plt.title("Globular Clusters",fontsize=24)
ax.scatter(GlobularCoords.ra.wrap_at(180.*u.degree).radian,GlobularCoords.dec.radian,c=Globulars['distSun(kly)'],\
           cmap='cool',s=4*Globulars['angular_diameter'])
ax.set_xlabel("RA",fontsize=16)
ax.set_ylabel("dec",fontsize=16)

Out[33]:

<matplotlib.text.Text at 0x1f750a20>

Questions:

Why are the biggest points mostly light blue and the pink points all small?
Why are the globular clusters centered/clumped around that RA/DEC? Hint: what's the RA/DEC of the center of our galaxy?

Plot the globular clusters in Galactic Coordinates¶

Here we'll make the same plot but ransformed to Galactic Coordinates (l,b). In Glaactic coordinates the center of the Galaxy is at (0.0,0.0)

In [30]:

fig = plt.figure (figsize=(13,7))
ax = fig.add_subplot(111,projection="mollweide")
ax.grid(True)
plt.title("Globular Clusters",fontsize=24)
ax.scatter(GlobularCoords.galactic.l.wrap_at(180.*u.degree).radian,GlobularCoords.galactic.b.radian,c=Globulars['distSun(kly)'],\
           cmap='cool',s=4*Globulars['angular_diameter'])
ax.set_xlabel("l",fontsize=16)
ax.set_ylabel("b",fontsize=16)

Out[30]:

<matplotlib.text.Text at 0x19cd9940>

Congratulations! You've completed Computational Project #3 (CT-3)¶

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