Ack/Ag does not return search result without *

I am trying to search text in a directory and it turned out that the following syntaxes do not return any result ack -i "0xabcdef" ./ ack -i "0xabcdef" ack -i "0xabcdef" . while the following command works ack -i "0xabcdef" * Can someone explain why that is the case? What is the significance of *? I also noticed that the directory has symbolic links.