<p>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 (<span class="math-container">$z\gtrsim6$</span>) where you have the <a href="https://en.wikipedia.org/wiki/Gunn%E2%80%93Peterson_trough" rel="nofollow noreferrer">Gunn-Peterson trough</a> completely erasing everything blueward of 912 Å (restframe). But with typical magnitudes of <span class="math-container">$\sim20$</span> or so at these redshifts, a color of <span class="math-container">$20$</span> that would then mean that the magnitude in the bluest band would be of the order <span class="math-container">$\sim 40$</span>, which no instrument is able to detect.</p>

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

<p>In other words, I think those values must be errors.</p>

CC BY-SA 4.0
2019-04-02T14:23:11.947Z
2019-04-03T11:20:01.700Z
2019-04-03T11:20:01.700Z
pela
<p>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?</p>

<p>Ty in advance.</p>

CC BY-SA 4.0
2019-04-02T12:20:21.040Z
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?
pela
CC BY-SA 4.0
2019-04-02T14:02:39.177Z
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.
crystal
CC BY-SA 4.0
2019-04-02T18:08:35.327Z
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
astrosnapper
<observational-astronomy><quasars>
What values can a color index variable get?
crystal
# What values can a color index variable get?

(Asked by: username_0 on 2019-04-02)

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.

**Tags:** observational-astronomy, quasars
> **Comments:**
> **username_1**: 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?  
> **username_0**: 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.  
> **username_2**: 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  

---

## 1 answers

### Answer by username_1 on 2019-04-02. Score: 3 | Accepted answer

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](https://en.wikipedia.org/wiki/Gunn%E2%80%93Peterson_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.


---

