Given Earth's current level of technology, space- and ground- based astronomical tools, etc., to what extent would it be possible to locate the Milky Way from a somewhat distant galaxy, say, in another nearby supercluster?

I know we have a reasonable mapping of our Local Group of galaxies, the Laniakea Supercluster and the rough large scale structure of the universe. But would it be possible for a group of people using current technology to locate themselves in the universe over the course of ~100 years and to locate the Milky Way specifically? Or would it be more likely they would have no idea where in the universe they found themselves and would not be likely to find out for a very long time?


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  • $\begingroup$ Using current technology and knowledge? They'd have Hubble and all that? I'll bet they'd find out where they were at pretty quickly once they realized they weren't in the Milky Way any more. $\endgroup$ – Samuel Apr 17 '15 at 23:30

Detecting Galaxies

Let's look at the example of UDF 423, one of the brightest galaxies in the Hubble Ultra Deep Field. It has an apparent magnitude of around 20, and an estimated distance of 7.7 billion light years (well outside of our supercluster). This gives it an absolute magnitude of somewhere around -21.8, making it only around three times brighter than the Milky Way (at magnitude -20.9). Since it's clearly resolved in the UDF, we can treat that as a good estimate for the level of detail we'd need to achieve in our observations to see the Milky Way directly.

The instrument that Hubble used to capture the UDF is the ACS (Advanced Camera for Surveys), with a resolution of around 0.05 arcseconds. The main instrument on JWST is NIRCAM, with a similar resolution and sensitivity.

The exposure for which the sensitivities are calculated is around 10,000 seconds, and the field of view is 4.4 arcseconds square. This means that a single JWST would take around 3000 years to scan the whole sky at that resolution.

Speeding it Up

Of course, we only need to find a couple galaxies that we recognize. Due to the huge number of them, the probability that we see a familiar one is actually pretty high. Once we are confidant of the location of one or more, we can start concentrating our search on a specific sector of sky, and the speed at which we start locating galaxies will be superexponential.

Recognizing Galaxies

As "BrettFromLA" says, we likely wouldn't recognize galaxies by visual comparison, especially since many would appear in different orientations (one which was once edge-on could now appear face-on). Galaxies would instead be matched by their brightness and spectra. This only gives us a fuzzy match, but

Other Sources

Although pulsars make good candidates for locating ourselves (since they are uniquely identifiable by their period) they are generally too dim to be visible billions of light years away. We can look at quasars, which do not have as clear identifying features, but are significantly brighter.

The Sloan Digital Sky Survey identified most known quasars over around 35% of the sky, and took just ten years to complete. A network of telescopes could cover the whole sky in just a few years. Again, the quasars would be matched by their spectra. After our location is pinned down by the quasar survey, we can use spaceborne telescopes to locate the Milky Way.

  • $\begingroup$ If the quasars are directional, would a displaced point of view see the same ones? Pulsars even more so, but you have a chance of being in the beam's sweep cone of at least some. $\endgroup$ – JDługosz Apr 18 '15 at 14:02
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    $\begingroup$ @JD We would see the ones close to the line between our old and new locations. "Close" depends on how tight the beaming is. $\endgroup$ – 2012rcampion Apr 18 '15 at 15:11
  • $\begingroup$ Ah, yes. Even if it takes a complete survey to look through, this will tell you the line on which you moved. $\endgroup$ – JDługosz Apr 19 '15 at 0:01

Identifying the Milky Way by its appearance alone would be tricky. It's not an exceptional galaxy. Also, as far as I know, we haven't mapped a lot of it because we simply can't see through the dust and the dense galactic center to see what's on the other side.

However, if our travelers can rely on our current technology AND on our current knowledge of the mapping of the known universe, it should be doable. They first map the large celestial bodies around them. Then they "fit" that map into their existing map of the known universe. Based on that "fit", they will know where they are, and of course they'll know where the Milky Way galaxy is.


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