Is the sersic-index value (n), 0, more of a placeholder for 'PSF' morphologies?

Question

Hiya again,

As a follow-up question to https://help.desi.lbl.gov/index.php?qa=141&qa_1=what-the-allowable-range-sersic-index-for-the-ser-type-objects , would you say that Sersic_index = '0' is more of a placeholder value to accompany 'PSF' morphology types?

(i) To help understand my train of thought, I note that the typical response function of a telescope to a point-like source (i.e. delta function) is a Gaussian function. I think (but please correct me!) that n = 0.5 allows a Sersic-profile (~ exp[r^-1/n], where n = Sersic index) to be 'refactored' into a Gaussian function (~exp[r^2], with mean-value = 0). Therefore, has 'Sersic index' been set to 0.0 (rather than 0.5) to help simplify catalogue creation, and identify 'PSF' sources more easily?

(ii) Relatedly, in order to truly identify 'all' of the point-like sources in the LS DR10 catalogue, should I combine filters for "Sersic index = 0" and "Sersic index = 0.5"?  

Best wishes,

Sarah

Answer 1

Hi,

As you guessed, all sources with TYPE='PSF' get SERSIC=0.

It's important to understand that the catalogs contain "above-the-atmosphere" shape estimates for galaxies -- we're reporting what we think you would see in perfect seeing, not what we actually observed in a variety of conditions in many different images.

As such, sources identified with PSF are the ones we really think are point-like.  We do a model-selection test, first PSF again REX -- categorizing objects as point-like or maybe resolved -- and then for resolved things, we fit DEV and EXP, and if one of those is accepted as a better fit, we try SERsic.

(Also: for Sersic, large n are the sharper profiles.)

cheers,

dustin

Comments

Hiya Dustin,

Thank you, the "above-the-atmosphere" interpretation makes sense.

Am just a bit puzzled about the "large n are the sharper profiles" comment, as I thought it was the other way round? E.g. see https://en.wikipedia.org/wiki/S%C3%A9rsic_profile  i.e. are you describing the innermost region has being "sharp", or the bend/knee in the profile as being "sharp"?

Thanks,
Sarah

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Yes, I meant the innermost region.  On the wikipedia page, you can see that at small radii, the n=1 profile is almost flat - constant brightness toward the core - while for n=10, the surface brightness continues rising at small radii.

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Ah yes. Thanks again!