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All the nearby galaxies we see come in visible range, many are visible from ground based telescopes. And the farthest galaxy that hubble discovered recently, 13.4 billion light years away is in infra red. Was this Galaxy ever in visible part of spectrum? How long will it take galaxies like andromeda or whirlpool to go infrared so that they won't be visible to us if we are not using in infraRed lens? What's the specific limit where the light makes transition from visible to infrared part of spectrum?

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  • $\begingroup$ Andromeda and the entire local group will never go infra-red because Andromeda is moving towards us and is due to collide with the milky way in 4 billion years and the entire local group is gravitationally bound. I'm not sure at what distance galaxies are no longer bound to us and likely to move away via dark-energy. $\endgroup$
    – userLTK
    Mar 25, 2018 at 12:11
  • $\begingroup$ Actually you pose the question how big is the visibly (wavelength in Vis) observable universe. I think that because of what @James K said, it coincides with the observable universe, ie till cosmic background. $\endgroup$
    – Alchimista
    Mar 26, 2018 at 9:38

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Galaxies give off a range of frequencies of light: Visible, but also infrared, microwave, and radiowaves. at longer wavelengths, ultraviolet, Xrays and gamma rays at short wavelengths. But stars give off quite a lot of their radiation in visible light, so nearby galaxies are quite bright in visible light and rather less bright in Ultraviolet. The cutoff between the types of light is somewhat arbitrary, but usually a wavelength of longer than 700nm is considered to be infrared, and shorter than 400nm is ultraviolet.

Very distant galaxies are moving away from us due to the general expansion of the universe. This means that the light is stretched. Visible light gets stretched to infra-red, ultraviolet gets stretched to visible light. The distant galaxy is very very far away, and so very dim. It is a little bit brighter in infrared, although there is also a little visible light (and other wavelengths too)

Galaxies never go completely invisible in visible light, even if they are red-shifted, because there is always ultraviolet light to red-shift to visible.

However, ultraviolet is an ionising radiation, and the early universe contained a lot of neutral Hydrogen. This Hydrogen can absorb ultraviolet light. This would make very early galaxies appear dark in the ultraviolet. Except, due to redshift, that ultraviolet would have been stretched to visible light by the time it reaches us. This makes them dark in visible light. This is called the Lyman break. Lyman-break galaxies are more distant than about z=3 (21 billion light years), but Lyman break galaxies with z>7 (30 billion light years) are mostly redshifted to infra-red, and are very dim in visible light.

So the galaxy did emit light in the visible part of the spectrum. The fact it is now only visible in the infrared is a result of redshift and the Lyman break, not because they only emit infrared light.

Galaxies like Andromeda are so close that they aren't part of the general expansion of the universe, so they will never be redshifted to invisibility.

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    $\begingroup$ I wanna know about the distance limit from earth not particularly about the wavelengths. $\endgroup$
    – Roxy
    Mar 25, 2018 at 8:51

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