<p>Large stars emit much of the there energy in the visible and near-ultraviolet wavelengths, but a galaxy at z=10 will be observed most brightly in the infra-red. Moreover, lots of the interesting spectral lines (such as Hydrogen alpha) will be shifted to the infra-red.</p>
<p>But there are other reasons for wanting a space infra-red telescope.  Infrared is less absorbed by dust, allowing us to see through dust clouds.  And several interesting objects glow more brightly in IR:  brown dwarfs and large exoplanets, for example.  But IR is absorbed by the atmosphere, so land based telescopes are inferior.</p>
<p>Now very distant galaxies are in a universe with a lot of hydrogen <em>atoms</em>. (in the universe now, most of the hydrogen is ionised forming an intersellar plasma) And hydrogen atoms absorb photons at wavelengths shorter than 91.2 nm (in the ultraviolet). For a galaxy at z=9, this will be redshifted to 912nm (in the near infrared). The galaxy will be very dim at optical or ultraviolet wavelengths.</p>
<p>So, the reason for using this band is: Distant galaxies are brighter in this band, there is lots of other science that can be done, and that science can't be done with land based telescopes.</p>

CC BY-SA 4.0
2022-08-10T15:00:41.270Z
I'd add that the spectra of high redshift galaxies is completely cut off for energies higher than the Lyman break, which means that they can be completely invisible to optical telescopes
Prallax
CC BY-SA 4.0
2022-08-10T15:24:01.833Z
So other band redshifts aren't as important because a majority of galactic emissions are initially in the UV/blue-light range, which then redshift into the infrared at great-enough distances? In other words, would it be useful to look for redshifted x-rays in the UV bands, or redshifted infrared-waves in submillimeter/microwave band?
Rich McDaniel
CC BY-SA 4.0
2022-08-10T16:42:34.063Z
Prallax's comment about the Lyman break is important, and I've incorporated it.  Sure it would be great to have an X-ray telescope with the sensitivity and resolution of the JWST,   but money is limited. Due to the Lyman break, one might not see much at extreme redshifts with a UV telescope. See https://en.wikipedia.org/wiki/GALEX
James K
CC BY-SA 4.0
2022-08-09T20:46:30.277Z
2022-08-10T16:28:48.913Z
2022-08-10T16:28:48.913Z
James K
<p>Was infrared chosen for the James Webb Space Telescope's mission to detect the most-redshifted galaxies and stars in the universe because most detectable galactic radiation emits most strongly in the UV/visible bands, and so are most easily detected in the infrared regions/bands their radiation appears to us in?</p>
<p>I'm trying to understand the &quot;why&quot; behind the decision to use infrared over other electromagnetic bands for this mission?</p>

CC BY-SA 4.0
2022-08-10T17:58:18.343Z
Also relevant: https://astronomy.stackexchange.com/questions/19008/why-are-wavelengths-shorter-than-visible-light-neglected-by-new-telescopes https://astronomy.stackexchange.com/questions/14043/how-do-we-know-galaxy-gn-z11-is-as-far-away-as-it-is
ProfRob
CC BY-SA 4.0
2022-08-10T20:40:46.817Z
@ProfRob Yes, [this question] (https://astronomy.stackexchange.com/questions/19008/why-are-wavelengths-shorter-than-visible-light-neglected-by-new-telescopes) was what I wasn't able to formulate. The prioritization of IR redshifts over other redshifts was exactly what I was curious about. Thanks so much.
Rich McDaniel
<cosmology><spectroscopy><space-telescope><james-webb-space-telescope><infrared>
Why is infrared the ideal band to detect the earliest and most-redshifted galaxies?
Rich McDaniel
# Why is infrared the ideal band to detect the earliest and most-redshifted galaxies?

(Asked by: username_0 on 2022-08-09)

Was infrared chosen for the James Webb Space Telescope's mission to detect the most-redshifted galaxies and stars in the universe because most detectable galactic radiation emits most strongly in the UV/visible bands, and so are most easily detected in the infrared regions/bands their radiation appears to us in?

I'm trying to understand the "why" behind the decision to use infrared over other electromagnetic bands for this mission?

**Tags:** cosmology, spectroscopy, space-telescope, james-webb-space-telescope, infrared
> **Comments:**
> **username_1**: Also relevant: https://astronomy.stackexchange.com/questions/19008/why-are-wavelengths-shorter-than-visible-light-neglected-by-new-telescopes https://astronomy.stackexchange.com/questions/14043/how-do-we-know-galaxy-gn-z11-is-as-far-away-as-it-is  
> **username_0**: @ProfRob Yes, [this question] (https://astronomy.stackexchange.com/questions/19008/why-are-wavelengths-shorter-than-visible-light-neglected-by-new-telescopes) was what I wasn't able to formulate. The prioritization of IR redshifts over other redshifts was exactly what I was curious about. Thanks so much.  

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## 1 answers

### Answer by username_2 on 2022-08-09. Score: 4 | Accepted answer

Large stars emit much of the there energy in the visible and near-ultraviolet wavelengths, but a galaxy at z=10 will be observed most brightly in the infra-red. Moreover, lots of the interesting spectral lines (such as Hydrogen alpha) will be shifted to the infra-red.

But there are other reasons for wanting a space infra-red telescope. Infrared is less absorbed by dust, allowing us to see through dust clouds. And several interesting objects glow more brightly in IR: brown dwarfs and large exoplanets, for example. But IR is absorbed by the atmosphere, so land based telescopes are inferior.

Now very distant galaxies are in a universe with a lot of hydrogen *atoms*. (in the universe now, most of the hydrogen is ionised forming an intersellar plasma) And hydrogen atoms absorb photons at wavelengths shorter than 91.2 nm (in the ultraviolet). For a galaxy at z=9, this will be redshifted to 912nm (in the near infrared). The galaxy will be very dim at optical or ultraviolet wavelengths.

So, the reason for using this band is: Distant galaxies are brighter in this band, there is lots of other science that can be done, and that science can't be done with land based telescopes.

> **Comments:**
> **username_3**: I'd add that the spectra of high redshift galaxies is completely cut off for energies higher than the Lyman break, which means that they can be completely invisible to optical telescopes  
> **username_0**: So other band redshifts aren't as important because a majority of galactic emissions are initially in the UV/blue-light range, which then redshift into the infrared at great-enough distances? In other words, would it be useful to look for redshifted x-rays in the UV bands, or redshifted infrared-waves in submillimeter/microwave band?  
> **username_2**: Prallax's comment about the Lyman break is important, and I've incorporated it.  Sure it would be great to have an X-ray telescope with the sensitivity and resolution of the JWST,   but money is limited. Due to the Lyman break, one might not see much at extreme redshifts with a UV telescope. See https://en.wikipedia.org/wiki/GALEX  

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