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I'm looking through the data in SDSS DR 12:

_RAJ2000,_DEJ2000,RA_ICRS,DE_ICRS,mode,q_mode,class,SDSS12,m_SDSS12,ObsDate,Q,umag,e_umag,gmag,e_gmag,rmag,e_rmag,imag,e_imag,zmag,e_zmag,zsp,zph,e_zph,<zph>
02 59 43.85208,+00 14 50.0532,44.932717,0.247237,1,+,6,J025943.85+001450.0,*,2004.7267,3,25.252,0.709,24.427,0.377,22.923,0.186,22.844,0.255,22.173,0.505,,,,
02 59 43.18896,+00 14 49.9776,44.929954,0.247216,1,+,3,J025943.18+001449.9,,2004.7267,3,25.77,0.713,23.41,0.267,22.467,0.183,21.612,0.132,20.905,0.301,,0.5976,0.1462,0.5771
02 59 45.92184,+00 15 09.6372,44.941341,0.252677,1,+,3,J025945.92+001509.6,,2004.7267,3,23.707,1.067,22.886,0.219,21.501,0.105,21.026,0.104,20.984,0.423,,0.423,0.0924,0.4151
02 59 44.95968,+00 15 11.7252,44.937332,0.253257,1,+,3,J025944.95+001511.7,,2004.7267,3,22.729,0.546,24.475,0.791,21.847,0.152,21.244,0.137,21.931,0.96,,0.4073,0.1245,0.4036

particularly the relationship between the photometric zph and spectroscopic zsp redshifts. The data contains two photometric estimates: zph is "estimated by robust fit to nearest neighbors in a reference set", and <zph> is the average redshift of the nearest neighbours. The relationship between the photometric and spectroscopic redshifts is not great:

enter image description here

so I wonder:

  • how do they estimate the photometric redshifts?
  • why do they top out at z~1?
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  • $\begingroup$ For question 1, see here sdss.org/dr12/algorithms/photo-z For question 2, have you tried to make your plot but including the error bars on photoz? Im not sure why they don't go above 1, Ive used DR12 in the past and they went above 1. Are you getting data from the full DR12 catalogue, or only some subset of it that was preprocessed? $\endgroup$
    – Daddy Kropotkin
    Commented Jun 8, 2022 at 15:08

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