Interesting merger


Although maybe it’s a chance visual alignment? I am a total noob on this, so please forgive me, but I am not sure because the photometric redshifts of the two galaxies are pretty close but still outside of each other’s error margins. Adding to that, the redshifts 0.026 vs 0.031 seem quite different from each other to my mind or does it not matter in that sense?

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There even is a Hubble image of this pair:

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That is a fairly close value of redshift. For reference, the highest redshift galaxy discovered is 13.2 so a 0.005. A higher value of redshift means the galaxy is further away. This was discover by Hubble and immortalised in Hubble’s Law which (roughly) states that all sufficiently distant light sources show redshift for a velocity proportionate to their distance from Earth. In other words, redshift is (partially) caused by a galaxy which is moving away from us. This velocity (known as the recessional velocity) is proportional to distance.

More info
More info but minus pompous words

Notes on how redshift is caused

On how the redshift is actually caused (and why I said partially above), there are 3 reasons for redshift. The first is due to light travelling between objects moving apart. This is analogous to the Doppler Effect often seen in High School/GCSE Physics although it is known as the Relativistic Doppler Effect in this case as you have to take special relativity into account.

The second way is Cosmological Redshift. This is the redshift cause by the space itself the light is travelling through expanding. Put simply, a photon is just a wave/disturbance in a field. Given that the field itself is expanding (ish), your wavelength will expand. An increase in apparent wavelength is just redshift.

The final cause is Gravitational Redshift; this is due to the fact that photons travelling out of a gravitational well (i.e. towards an object in a weaker gravitational potential) seem to lose energy. From the energy for a photon equation again seen in high school/A-level physics (E=hf), we see that the frequency has decreased and so for the (unchanged) speed of light we have a longer wavelength i.e. redshift. It is important to note that energy is not actually lost. However, this is a messy and touchy topic and this article attempts to explain it though it might be out of date. In short, the work done to expand the universe is coming from the energy seemingly lost by photons (according to that 2019 article).

There is a bunch of physics to unpack for the last point but a crash course is as follows:

  • In mathematical physics, Noether’s theorem states that every differentiable symmetry of the action of a physical system with conservative forces has a corresponding conservation law. In short, if a system that follows some rules doesn’t change (i.e. is invariant) under a change you apply to it (a transformation or operation) then there is a law for that system that states that something is conserved.
  • One particularly important symmetry is that of time. If I conduct an experiment today and then replicate it exactly tomorrow, I should get the same results. One definition of energy (what is energy, anyway?) says if a theory does not change under time, then the quantity that is conserved is energy. The ill-defined notion of energy is what creates the possible energy-loss mess with universal expansion anyway - general relativity defines a bunch of stuff well but energy is not one of them.
  • I am not about to launch into a maths description of this, but (in a hand wavy way) as the velocity of an observer changes with time, the time symmetry is broken and thus the energy conservation is broken. (NB: The result depends on how you define energy. For the most part, in the definition of energy you need a time-like Killing vector field to have a constant energy. However, the FLRW metric tensor doesn’t allow for such a vector field. Please do not google the above words. I went down a wormhole after seeing them and nothing makes sense.)

Big NB: I might very well have made mistakes in the above. If I have, please correct me.

Apologies for the essay but redshift and this stuff is very cool to me :slight_smile:


Nice writeup, thanks!

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