<p>The main reason is that the intrinsic spectra of galaxies are complex and therefore a redshift of their spectrum, whilst leading to a redder spectrum overall, does not necessarily lead to reddening in all colours.</p>

<p>For instance if there is apeak in the intrinsic spectrum, then as that peak moves redward, then colours formed from bands on the same side or straddling the peak in wavelength would behave differently.</p>

<p>The dramatic redward turn up at high redshifts (note, a large colour is redder) is caused by the "Lyman break" moving through the colour bands. Basically, the intrinsic spectrum is self-absorbed at wavelengths shortward of 91.2 nm by neutral hydrogen. This absorption edge is redshifts into the visible region at redshifts bigger than 3, causing the bluer bands to essentially disappear and any colour formed with them to become very red.</p>

CC BY-SA 4.0
2019-03-27T23:56:16.140Z
Thanks, @Rob Jeffries. I've got a follow-up question, I'll post it as a new one.
Jim421616
CC BY-SA 4.0
2019-03-28T00:10:24.827Z
https://astronomy.stackexchange.com/questions/30148/relationship-between-photometric-colour-and-redshift-part-two
Jim421616
CC BY-SA 4.0
2019-03-27T23:44:24.307Z
2019-03-27T23:44:24.307Z
ProfRob
<p>In <a href="https://pdfs.semanticscholar.org/b678/e64654eb1a430c7373b2d04f0621019aa3e1.pdf" rel="nofollow noreferrer">this paper</a>, the authors describe the theoretical relationship between photometric colours, particularly in Figures 1 and 6:</p>

<p><a href="https://i.sstatic.net/KnaBE.png" rel="nofollow noreferrer"><img src="https://i.sstatic.net/KnaBE.png" alt="enter image description here"></a>
<a href="https://i.sstatic.net/DOKWh.png" rel="nofollow noreferrer"><img src="https://i.sstatic.net/DOKWh.png" alt="enter image description here"></a></p>

<p>Since the colour is the difference between two magnitudes, and the redshift is defined as
<span class="math-container">$z=\frac{\lambda'-\lambda_0}{\lambda_0}$</span></p>

<p>I would have thought that the colour-z relationships would simply be a downward curve, since the light gets redshifted (goes down in frequency), and the ratio of two magnitudes is constant with redshift.</p>

<p>Why do the colour-z relationships in this paper (and in others I've seen) show such a complex relationship?</p>

<redshift><photometry>
Relationship between photometric colour and redshift
Jim421616
# Relationship between photometric colour and redshift

(Asked by: username_0 on 2019-03-27)

In [this paper](https://pdfs.semanticscholar.org/b678/e64654eb1a430c7373b2d04f0621019aa3e1.pdf), the authors describe the theoretical relationship between photometric colours, particularly in Figures 1 and 6:

[![enter image description here](https://i.sstatic.net/KnaBE.png)](https://i.sstatic.net/KnaBE.png)
[![enter image description here](https://i.sstatic.net/DOKWh.png)](https://i.sstatic.net/DOKWh.png)

Since the colour is the difference between two magnitudes, and the redshift is defined as
$z=\frac{\lambda'-\lambda\_0}{\lambda\_0}$

I would have thought that the colour-z relationships would simply be a downward curve, since the light gets redshifted (goes down in frequency), and the ratio of two magnitudes is constant with redshift.

Why do the colour-z relationships in this paper (and in others I've seen) show such a complex relationship?

**Tags:** redshift, photometry

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## 1 answers

### Answer by username_1 on 2019-03-27. Score: 4 | Accepted answer

The main reason is that the intrinsic spectra of galaxies are complex and therefore a redshift of their spectrum, whilst leading to a redder spectrum overall, does not necessarily lead to reddening in all colours.

For instance if there is apeak in the intrinsic spectrum, then as that peak moves redward, then colours formed from bands on the same side or straddling the peak in wavelength would behave differently.

The dramatic redward turn up at high redshifts (note, a large colour is redder) is caused by the "Lyman break" moving through the colour bands. Basically, the intrinsic spectrum is self-absorbed at wavelengths shortward of 91.2 nm by neutral hydrogen. This absorption edge is redshifts into the visible region at redshifts bigger than 3, causing the bluer bands to essentially disappear and any colour formed with them to become very red.

> **Comments:**
> **username_0**: Thanks, @Rob Jeffries. I've got a follow-up question, I'll post it as a new one.  
> **username_0**: https://astronomy.stackexchange.com/questions/30148/relationship-between-photometric-colour-and-redshift-part-two  

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