I am a statistician with love and passion for astronomy. I try to explain some outliers I found in the values of quasars - color indexes. I found four (4) values close to twenty (20). Is that scientifically possible or there is an error in the CSV file? What values can a color index get?

Ty in advance.

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    $\begingroup$ What colors are we talking about? In principle something line $U-V$ could get high if all UV light is absorbed (by neutral hydrogen), but that would mean an unrealistically large range in observational thresholds. Are these actual observations, or simulated quasars? Also, are we talking AB or Vega magnitudes? $\endgroup$
    – pela
    Commented Apr 2, 2019 at 12:20
  • $\begingroup$ These are actual observations. We are talking about SDSS ugriz magnitude system. The outliers are detected on the u_g, g_r and i_z color indexes. $\endgroup$
    – crystal
    Commented Apr 2, 2019 at 14:02
  • $\begingroup$ Does the catalog you are looking at have quality or "trustworthiness" flags for either the individual magnitudes and/or the color indices ? These could be a way of weeding out bad values or non-detections in one band $\endgroup$ Commented Apr 2, 2019 at 18:08

1 Answer 1


The only physical mechanism I can think of for quasars to have such a large difference in two bands would be at very high redshift ($z\gtrsim6$) where you have the Gunn-Peterson trough completely erasing everything blueward of 912 Å (restframe). But with typical magnitudes of $\sim20$ or so at these redshifts, a color of $20$ that would then mean that the magnitude in the bluest band would be of the order $\sim 40$, which no instrument is able to detect.

So effectively, there would be no detection on the bluest band, in which case the color would be infinite, or rather undefined.

In other words, I think those values must be errors.


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