Question: How to access the scaling factor for the spectra in the grid file?

[Nikhil0504]

Hello, I am doing a combined fit of photometry and spectrum for a galaxy. I set AUTO_SCALE to 1 and SAVE_CHI_GRID to 1 but I don't seem to find the scaling factor in FITS file after converting the grid to .FITS.

This was the output I got after using the convert script provided:

found 1 galaxies, with 42 properties, 6 grid parameters and 674016 models
grid names: z, Av, Av_bc, lage, metal, ltau
prop names: lscale, sscale, lmass, lsfr, lssfr, lldust, lldust_bc, llion, lmform, la2t, CN_1, CN_2, Ca4227, G4300, Fe4383, Ca4455, Fe4531, Fe4668, Hbeta, Fe5015, Mg_1, Mg_2, Mg_b, Fe5270, Fe5335, Fe5406, Fe5709, Fe5782, Na_D, TiO_1, TiO_2, Hdelta_A, Hgamma_A, Hdelta_F, Hgamma_F, Halpha, tsf, past_sfr, sfr10, sfr50, brate10, brate50
[--------------------------------] 28982688 100%, 104ms elapsed, 0ns left, 104ms total  

Can you please tell me where the scaling factor is located at? Or is it something we will need to extrapolate somehow?

[cschreib]

Hi, and thanks for the feedback. I wasn't sure I remembered, so I looked at the code to confirm; the spectrum rescaling should be saved alongside the other properties of the model (SFR, etc). It should be found in the column called sscale. Can you find it? If not, could you show me the list of columns you see in the FITS file please?

NB: The value found in sscale is actually the ratio L/S, where L and S are the model scaling factors for the photometey and the spectrum, respectively. So sscale can essentially be interpreted as the factor that would have to be applied to the spectrum for it to match the photometry. For a well calibrated spectrum, it should be equal to one.

[Nikhil0504]

Sure, here are the columns for the source:

ColDefs(
    name = 'CHI2'; format = '1E'; dim = '(1)'
    name = 'MODEL'; format = '1K'; dim = '(1)'
    name = 'LSCALE'; format = '1E'; dim = '(1)'
    name = 'SSCALE'; format = '1E'; dim = '(1)'
    name = 'LMASS'; format = '1E'; dim = '(1)'
    name = 'LSFR'; format = '1E'; dim = '(1)'
    name = 'LSSFR'; format = '1E'; dim = '(1)'
    name = 'LLDUST'; format = '1E'; dim = '(1)'
    name = 'LLDUST_BC'; format = '1E'; dim = '(1)'
    name = 'LLION'; format = '1E'; dim = '(1)'
    name = 'LMFORM'; format = '1E'; dim = '(1)'
    name = 'LA2T'; format = '1E'; dim = '(1)'
    name = 'CN_1'; format = '1E'; dim = '(1)'
    name = 'CN_2'; format = '1E'; dim = '(1)'
    name = 'CA4227'; format = '1E'; dim = '(1)'
    name = 'G4300'; format = '1E'; dim = '(1)'
    name = 'FE4383'; format = '1E'; dim = '(1)'
    name = 'CA4455'; format = '1E'; dim = '(1)'
    name = 'FE4531'; format = '1E'; dim = '(1)'
    name = 'FE4668'; format = '1E'; dim = '(1)'
    name = 'HBETA'; format = '1E'; dim = '(1)'
    name = 'FE5015'; format = '1E'; dim = '(1)'
    name = 'MG_1'; format = '1E'; dim = '(1)'
    name = 'MG_2'; format = '1E'; dim = '(1)'
    name = 'MG_B'; format = '1E'; dim = '(1)'
    name = 'FE5270'; format = '1E'; dim = '(1)'
    name = 'FE5335'; format = '1E'; dim = '(1)'
    name = 'FE5406'; format = '1E'; dim = '(1)'
    name = 'FE5709'; format = '1E'; dim = '(1)'
    name = 'FE5782'; format = '1E'; dim = '(1)'
    name = 'NA_D'; format = '1E'; dim = '(1)'
    name = 'TIO_1'; format = '1E'; dim = '(1)'
    name = 'TIO_2'; format = '1E'; dim = '(1)'
    name = 'HDELTA_A'; format = '1E'; dim = '(1)'
    name = 'HGAMMA_A'; format = '1E'; dim = '(1)'
    name = 'HDELTA_F'; format = '1E'; dim = '(1)'
    name = 'HGAMMA_F'; format = '1E'; dim = '(1)'
    name = 'HALPHA'; format = '1E'; dim = '(1)'
    name = 'TSF'; format = '1E'; dim = '(1)'
    name = 'PAST_SFR'; format = '1E'; dim = '(1)'
    name = 'SFR10'; format = '1E'; dim = '(1)'
    name = 'SFR50'; format = '1E'; dim = '(1)'
    name = 'BRATE10'; format = '1E'; dim = '(1)'
    name = 'BRATE50'; format = '1E'; dim = '(1)'
    name = 'Z'; format = '1E'; dim = '(1)'
    name = 'AV'; format = '1E'; dim = '(1)'
    name = 'AV_BC'; format = '1E'; dim = '(1)'
    name = 'LAGE'; format = '1E'; dim = '(1)'
    name = 'METAL'; format = '1E'; dim = '(1)'
    name = 'LTAU'; format = '1E'; dim = '(1)'
)

[cschreib]

That's it: name = 'SSCALE'; format = '1E'; dim = '(1)'. Are you able to read the data from that column?

[Nikhil0504]

Yes, but is it supposed to be below 1?

[cschreib]

That's a different question. Theoretically yes, it can be below one; there is nothing in the code stopping that if that is what the data and model prefer.

In practice, I would expect it to be exclusively close to or above one only if all the conditions below are met:

• The spectrum has been flux-calibrated using a point source as reference (e.g. typically a star), as is most common.
• The absolute flux calibration of that reference source is the same as the photometry used in the fit, or very close to it. Typically this is achieved if the reference source photometry is taken from the same catalog.
• The spectrum has no data issues (I.e. bad data point driving the fit) and no unexpected features that FAST++ cannot account for (e.g., significant emission lines).
• The photometry has no data issues (i.e., bad data point driving the fit) and no unexpected features that FAST++ cannot account for (e.g., AGN, dust emission, high EW emission lines, etc.).
• The spectrum and photometry are given in the right units.
• The best fit model is a good fit to the data (reduced chi2 close to 1).

If all the above is true, then:

• The rescaling factor should be above one if the source is extended and no slit loss correction for extended sources has been applied, or if the source is compact but has been misplaced within the slit.
• Otherwise it should be close to one, within the uncertainties on the flux calibrations.

[Nikhil0504]

Thank you so much for your help and inputs! I'm going to close this issue as it's resolved.

[cschreib]

Glad I could help. Good luck!