列出目录内容时/lib/modules，我看到目录列表generic-pae（Ubuntu 12.04）：
drwxr-xr-x 4 root root 4096 Apr 23 2012 3.2.0-23-generic-pae
drwxr-xr-x 5 root root 4096 Nov 20 2012 3.2.0-27-generic-pae
drwxr-xr-x 5 root root 4096 Nov 20 2012 3.2.0-29-generic-pae
drwxr-xr-x 5 root root 4096 Nov 20 2012 3.2.0-30-generic-pae
drwxr-xr-x 5 root root 4096 Nov 20 2012 3.2.0-31-generic-pae
drwxr-xr-x 5 root root 4096 May 16 2013 3.2.0-32-generic-pae
drwxr-xr-x 5 root root 4096 May 16 2013 3.2.0-33-generic-pae
drwxr-xr-x 5 root root 4096 May 16 2013 3.2.0-34-generic-pae
drwxr-xr-x 5 root root 4096 May 16 2013 3.2.0-35-generic-pae
drwxr-xr-x 5 root root 4096 May 16 2013 3.2.0-36-generic-pae
drwxr-xr-x 5 root root 4096 May 16 2013 3.2.0-37-generic-pae
drwxr-xr-x 5 root root 4096 May 16 2013 3.2.0-38-generic-pae
drwxr-xr-x 5 root root 4096 May 16 2013 3.2.0-39-generic-pae
drwxr-xr-x 5 root root 4096 May 16 2013 3.2.0-41-generic-pae
drwxr-xr-x 5 root root 4096 Jun 18 08:26 3.2.0-48-generic-pae
drwxr-xr-x 5 root root 4096 Oct 3 07:47 3.2.0-54-generic-pae
这些目录是什么？它们的内容是什么？
我可以删除数字较小的那些，因为它们占用相当多的空间吗？