server hardening: whose keys are this in my apt keyring?

[sneakyimp]

I'm in the process of setting up and hardening a new server. I'm new to the concept of apt-secure, but am trying to get my footing here. Having just finished Ubuntu setup, I see that there appear to be four keys in my apt keyring:

prompt%:~/biz/myplan$ apt-key finger
/etc/apt/trusted.gpg
--------------------
pub   1024D/437D05B5 2004-09-12
      Key fingerprint = 6302 39CC 130E 1A7F D81A  27B1 4097 6EAF 437D 05B5
uid                  Ubuntu Archive Automatic Signing Key <[e-mail]>
sub   2048g/79164387 2004-09-12

pub   1024D/FBB75451 2004-12-30
      Key fingerprint = C598 6B4F 1257 FFA8 6632  CBA7 4618 1433 FBB7 5451
uid                  Ubuntu CD Image Automatic Signing Key <[e-mail]>

pub   1024D/3E5C1192 2010-09-20
      Key fingerprint = C474 15DF F48C 0964 5B78  6094 1612 6D3A 3E5C 1192
uid                  Ubuntu Extras Archive Automatic Signing Key <[e-mail]>

Whose keys are these? How can I be sure that I can trust them?