This documentation is for astroML version 0.2

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SDSS Line-ratio Diagrams

This shows how to plot line-ratio diagrams for the SDSS spectra. These diagrams are often called BPT plots [1], Osterbrock diagrams [2], or Kewley diagrams [3]. The location of the dividing line is taken from from Kewley et al 2001.


[1]Baldwin, J. A.; Phillips, M. M.; Terlevich, R. (1981)
[2]Osterbrock, D. E.; De Robertis, M. M. (1985)
[3]Kewley, L. J. et al. (2001)
# Author: Jake VanderPlas <>
# License: BSD
#   The figure is an example from astroML: see
import numpy as np
from matplotlib import pyplot as plt

from astroML.datasets import fetch_sdss_corrected_spectra
from import log_OIII_Hb_NII

data = fetch_sdss_corrected_spectra()

i = np.where((data['lineindex_cln'] == 4) | (data['lineindex_cln'] == 5))

plt.scatter(data['log_NII_Ha'][i], data['log_OIII_Hb'][i],
            c=data['lineindex_cln'][i], s=9, lw=0)

NII = np.linspace(-2.0, 0.35)
plt.plot(NII, log_OIII_Hb_NII(NII), '-k')
plt.plot(NII, log_OIII_Hb_NII(NII, 0.1), '--k')
plt.plot(NII, log_OIII_Hb_NII(NII, -0.1), '--k')
plt.xlim(-2.0, 1.0)
plt.ylim(-1.2, 1.5)

plt.xlabel(r'$\mathrm{log([NII]/H\alpha)}$', fontsize='large')
plt.ylabel(r'$\mathrm{log([OIII]/H\beta)}$', fontsize='large')