These are NGC5426 and NGC5427, collectively known as Arp271, a pair of interacting spiral galaxies located about 130 million lightyears away.

I took this image back in 2018 with VIMOS, an instrument at ESO's Very Large Telescope in Chile. I was the Instrument Scientist, and that night we were decommissioning the instrument to make room for a new one, so I decided to take one last farewell image to celebrate all the cool science done with this instrument over the years.

If you've ever wondered how big professional astronomical instruments are, well, they're definitely larger than your DSLR camera! 😅 Here's VIMOS attached to the 8.2 metre telescope UT3, also known as 'Melipal' ('Southern Cross' in Mapudungun).


While VIMOS has taken really some cool images, it was mainly a spectrograph: it took the light of hundreds of objects at once and decomposed it into its constituent colours – its spectrum. From an object's spectrum we can get lots of physical properties. For instance, a galaxy's spectrum tells us how far away it is, and VIMOS main goal was to survey the 3D layout of galaxies in the universe.

For instance, in the plot below, each point is a galaxy whose distance was measured with VIMOS. We would be to the left, and distance increases to the right, all the way up to when the universe was roughly half of its present age. As you can see, galaxies aren't uniformly scattered across the universe, but they form filamentary structures with voids in between.

📷 ESO/L. Guzzo/VIPERS survey

@astro_jcm Perhaps there are fewer galaxies farther out, but the mass to create more of them exists anyway? Can we tell?

@tasket It's actually the other way around: the farther away we look, the earlier in time we're seeing, and we know that the frequency of galaxy mergers was higher in the past. When we look at distant galaxies, we see a higher fraction of them interacting with each other.

It also depends on the environment: it's easier to find galaxy mergers in regions with a high density of galaxies.

@astro_jcm So maybe "galaxy" was a red herring for me. More interested in the distribution of mass, out into the distance (past). I don't recall CMB map addressing that. #amatuerquestions

@tasket The small variations of temperature in the CMB correspond to slight differences in the density of the gas back then, which acted like seeds around which galaxies later formed. The early universe was quite chaotic in that sense, and galaxy mergers were fairly common back then.

@astro_jcm And to think that these are small compared to how large the ELT instruments will be!

@yosata Indeed! I remember a talk about HARMONI a while back where its size was measured in units of giraffes 😂

@astro_jcm A worthy farewell for sure! What a beautiful image of a beautiful pair.

@thriveth Indeed! Not gonna lie, I felt a bit emotional when I took that image, knowing it would be the very last thing VIMOS would ever see.

@astro_jcm Yes I can definitely imagine I would too. But it's a worthy goodbye!

@platypus @astro_jcm Absolutely agree. Thank you so much for sharing this thread, Juan Carlos!

@astro_jcm Como interactúan entre ellas? Se intercambian cuerpos celestes?

@jesusmargar La interacción gravitatoria entre ambas galaxias distorsiona la distribución de estrellas, gas y polvo en ellas. No se producen colisiones de estrellas, ya que la separación entre las estrellas de una galaxia es enorme; es como si dos enjambres de mosquitos se cruzaran.

@astro_jcm This is so cool! Thanks for sharing! I remember seeing this when it dropped and being fascinated then too. I heard something like this will happen with Andromeda and our Milky Way

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