usb - How do I install Ubuntu on an Intel iMac with no OS nor CD drive

What I want:

• Linux on an Intel iMac (Core Duo).

What I have:

• An empty PC (17" Intel iMac) with broken CD drive. It's model A1173.


• This PC with Ubuntu 12.04 and an old Windows Vista partition.


• a USB flash drive and an Ubuntu ISO.

Problems:

• No CD means the only boot drive I could use is USB.


• There is no BIOS on Macs, so I can't set boot settings or even see if it detects my USB drive. When I start the machine and press Option, the first and only thing I see is an old corrupted Windows XP partition and not a single option or additional information.


• So assuming blindly that the Mac hardware/firmware works normally, I don't have any Mac OS to use any of the tools that I found on different tutorials for building a bootable drive for Macs.


• I can't find much software on Linux/Windows to substitute to those tools, for example among others converting an .iso file (Win/Linux) to .img (Mac I guess). Which makes me think that the scenario where someone like me has Mac hardware but no Mac OS is extremely rare.


• So other than finding someone that has a Mac I have no solution. So I ask, what would you do? The only thing is it should not involve any money (I know Mac software is rarely free) which also excludes getting any Mac OS unless I can use a free macos.img for VM or restore the original Mac for free.