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I am trying to do kinematical analysis of spectra using ppxf-algorithm from Capellari. The velocity dispersion I get seems to be to high and I am not sure what the reason could be. On the website of the ppxf algorithm ( https://pypi.org/project/ppxf/#problems-with-your-first-fit), he suggests to plot galaxy spectra and template convolved with gaussian and try to make the parameters roughly fit by eye, which I tried:

   sigma = 6  # Velocity dispersion in pixels
shift = 40  # Velocity shift in pixels
flux_temp = np.roll(ndimage.gaussian_filter1d(flux_temp, sigma), shift)
plt.plot(loglam_gal,flux_gal, 'k', label ='galaxy')
plt.plot(loglam_temp, flux_temp * np.median(flux_gal) / np.median(flux_temp), 'r', label='template convolved with gaussian')
plt.legend()
plt.show()

Resulting in the plot:

enter image description here

In the middle arround the Mg-triplet this is roughly a good match I would say, on the outer parts not so much. Just looking at the plot, are there any concerns about the applicability of the ppxf-algorithm ?

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  • $\begingroup$ Trivial question: have you tried to add polynomials to the fit? What is the exact range being fit (e.g. the blue end of the spectrumis clearly unphysical, some masking may be necessary there) $\endgroup$
    – astabada
    Nov 4, 2022 at 9:10

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