grammar - Is 'subject' in 'is subject to considerable debate' a verb or a noun?

Every once in a while I stumble upon this phrase:

... is subject to considerable debate

Examples are easily found on the web, for instance:

In the context of suspected cognitive disorders,
the validity of memory complaints is subject to considerable debate

However, this phrase does not seem grammatically correct to me. I could expect it to be:

... is a subject for considerable debate

in which case 'subject' is a noun, or:

... is subjected to considerable debate

where 'subject' is a verb in past participle.

Still, both alternatives are much less common, judging by the number of occurrences in Google, than the original phrase. So here are the questions:

1. Is this phrase appropriate for formal writing?


2. Is it appropriate for less formal conversation?


3. Finally, is 'subject' in this phrase a noun or verb?
   
   
   
   
   1. Where should I put the stress when I read the phrase aloud?
   
   

Answer

Using the comments posted to the original question, I can assume the whole answer, which is:

1. Yes, it is appropriate for formal writing.


2. No, it probably won't fit in a casual informal conversation.


3. 'subject' is an adjective, therefore its form is perfectly correct.
   
   
   
   
   1. The stress should be on the first syllable as ['sʌbʤekt].