Yesterday, the Space Shuttle Atlantis docked with the Hubble Space Telescope, and now the removal and replacement of WFPC2 has commenced.
As you probably know, I’m going to miss that camera. It’s been unveiling the secrets of the Universe for the last 16 years, and in a way that no other camera ever has before. So, you can check out parts one, two, and three in my series of ways that this camera has changed the Universe, and then look below for today’s edition of saying goodbye to Hubble’s grand old camera, WFPC2.
In theory, you’re supposed to get arcs of the lensed images that are magnified and either stretched or present in multiple images. In practice, this is very difficult to do, because of how faint these distant objects are and how susceptible they are to atmospheric distortion. Here’s what “gravitational lensing” looked like before the Hubble Space Telescope.
Not so impressive, is it? What’s astounding is that those four images are four separate images of the same, distant galaxy! It shows up as multiple images because of the fact that the light is bent into four separate paths by the intervening lens. Well, this object, known as an Einstein Cross, was imaged by Hubble before WFPC2 was installed. The results are hugely disappointing, and shown below.
So, what? Why would I show you this disappointment? Because I want you to appreciate what WFPC2 has done. Take a look at this image from 1996, taken of Cluster 0024+1654.
When you look at a cluster, sometimes you get lucky, and there are galaxies (or even other clusters) directly behind it. These background galaxies can show up as lensed images. You see those blue arcs, that look like they trace out part of a circle? Those are the same few galaxies, stretched and shown multiple times. Because of the high resolution of Hubble with WFPC2, they were able to pull out which images were of the same galaxy, and reconstruct resolutions down to less than one arc-second, or 1/12960000 of a square degree!
(And click to enlarge.) Amazing! Simply amazing. What can you learn from this? Well, other than all sorts of things about the lensed galaxies, you can learn about dark matter! You see, gravitational lensing only cares about mass, and so we can figure out where — in a cluster like this — the mass is distributed. The results are breathtaking.
What this shows you is that yes, there are spikes where the individual galaxies are. But the cluster is dominated by this giant spherically-distributed mass that’s present everywhere, both where there are galaxies and where there aren’t. And that has got to be dark matter.
And just so you don’t think this is an isolated incident, here are a couple of other clusters imaged with WFPC2, for your lensing perusal.
So we’ve seen how WFPC2 has taught us about the large-scale Universe, about planets in our Solar System, about individual galaxies, and — as you’ve seen today — about clusters and dark matter. Is there anything missing? Is there anything left? And perhaps more importantly, is there anything this camera couldn’t do? Check back tomorrow, for part 5!