# Relationship between photometric colour and redshift, part two

In the graphs in my previous question here, 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:

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?

• 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. Mar 29 '19 at 12:30
• Color indices VS redshift can also reflect i) reddening, and ii) stellar population, assuming that the effect of cosmological redshift is constant. Mar 29 '19 at 12:37
• 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? Mar 31 '19 at 21:30
• 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. Apr 2 '19 at 11:46