I'm working on a research project about the adoption of DWH systems and doing some literature review I find many documents that refer to Information System. I tried to find the right definition of information system and if a data warehouse can be considered an IS or not, or is just a subset of it. In the wider definition IS comprises also persons and policy and not only the technology (software or hardware). So it become complicated in my opinion to define it. From my personal work experience a DWH is an IS, the hearth of IS. But what about from the academic and computer science point of view? If any system with data can be an IS, the extent is that an spreadsheet might be considered as an IS.
Yes, I'd say that a data warehouse is an information system. It stores information, using information technology; it's an information system.
That said, I think focusing too much on trying to obtain a precise definition of "Information System" and "Data Warehouse" is not likely to be super-helpful. Those are just phrases. "Information system" is a broad phrase that doesn't have an especially precise definition -- it gets used in whatever way the author finds helpful. And that's OK. It doesn't need a precise, universally-agreed-upon definition. It's just an aid to communication. Ultimately, it just doesn't matter that much how we classify data warehouses; if I told you they are (or aren't) an information system, that wouldn't really change how you should think about them.
My advice would be not to worry too much about terminology and trying to classify things into some taxonomy. Instead, think about what are the relevant characteristics and properties of the kinds of data warehouse systems you are looking at. Figure out what is your actual goal, and then identify what characteristics are relevant to that goal, and use that to hone in on the aspects of the system that are relevant. Not all information systems are alike -- the range of issues that arise in information systems is incredibly broad.