<p>In the graphs in my previous question <a href="https://astronomy.stackexchange.com/questions/30146/relationship-between-photometric-colour-and-redshift/30147#30147">here</a>, there are negative-sloped regions as the colour-z relationship oscillates. So if I look at how the I-W2 and W3-U colours change with redshift, as I've done here:</p>

<p><a href="https://i.sstatic.net/3bcxP.png" rel="nofollow noreferrer"><img src="https://i.sstatic.net/3bcxP.png" alt="enter image description here"></a></p>

<p>with I-W2 and W3-U both getting lower (bluer, is that right?) does that mean that my graphs indicate one of the upward-sloping regions?</p>

<p>What else does the redshift evolution of I-W2 against W3-U indicate?</p>

CC BY-SA 4.0
2019-03-29T12:30:37.870Z
From the plot, increasing I-W2 index is bluer. 

I have never seen W3bandpass before but I assume it is just another UV bandpass and has effective central wavelength bluter than U. So, decreasing W3-U index is bluer.

Therefore, top-left = blue, bottom-right = red. You mentioned " I-W2 and W3-U both getting lower (bluer, is that right?)," I think this is not right.
Kornpob Bhirombhakdi
CC BY-SA 4.0
2019-03-29T12:37:25.640Z
Color indices VS redshift can also reflect i) reddening, and ii) stellar population, assuming that the effect of cosmological redshift is constant.
Kornpob Bhirombhakdi
CC BY-SA 4.0
2019-03-31T21:30:25.783Z
You've said that my "I-W2 and W3-U both getting lower (bluer, is that right?)" comment is wrong. It's my understanding that higher colour values are redder. So if the colours are decreasing, doesn't that mean they're getting bluer?
Jim421616
CC BY-SA 4.0
2019-04-02T11:46:36.197Z
For I-W2, I is redder compared to W2. I-W2 is defined by $M_I$ - $M_{W2}$. So, $M_I$ < $M_{W2}$ implies that $M_I$ is brighter.
Kornpob Bhirombhakdi
<redshift><photometry>
Relationship between photometric colour and redshift, part two
Jim421616
# Relationship between photometric colour and redshift, part two

(Asked by: username_0 on 2019-03-28)

In the graphs in my previous question [here](https://astronomy.stackexchange.com/questions/30146/relationship-between-photometric-colour-and-redshift/30147#30147), there are negative-sloped regions as the colour-z relationship oscillates. So if I look at how the I-W2 and W3-U colours change with redshift, as I've done here:

[![enter image description here](https://i.sstatic.net/3bcxP.png)](https://i.sstatic.net/3bcxP.png)

with I-W2 and W3-U both getting lower (bluer, is that right?) does that mean that my graphs indicate one of the upward-sloping regions?

What else does the redshift evolution of I-W2 against W3-U indicate?

**Tags:** redshift, photometry

> **Comments:**
> **username_1**: From the plot, increasing I-W2 index is bluer.   I have never seen W3bandpass before but I assume it is just another UV bandpass and has effective central wavelength bluter than U. So, decreasing W3-U index is bluer.  Therefore, top-left = blue, bottom-right = red. You mentioned " I-W2 and W3-U both getting lower (bluer, is that right?)," I think this is not right.  
> **username_1**: Color indices VS redshift can also reflect i) reddening, and ii) stellar population, assuming that the effect of cosmological redshift is constant.  
> **username_0**: You've said that my "I-W2 and W3-U both getting lower (bluer, is that right?)" comment is wrong. It's my understanding that higher colour values are redder. So if the colours are decreasing, doesn't that mean they're getting bluer?  
> **username_1**: For I-W2, I is redder compared to W2. I-W2 is defined by $M_I$ - $M_{W2}$. So, $M_I$ < $M_{W2}$ implies that $M_I$ is brighter.  

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## 0 answers

