Can I take a heapshot of my app already running on my device?

Question

I really really want to view the heap associated with my app that has been running on my device for several days. I do NOT want to quit and relaunch the app from Xcode or Instruments. I have tried the "Attach to Process" and "Choose Target" menu options in Instruments after connecting my device, but the choices are all grayed out and nothing happens when I click on them. Clicking "Mark Heap" also does nothing.

Is there a way to view the heap of a currently running app on a device? I just want to see the relative proportions of object types currently "alive"; I don't need to see the stacktraces (I assume that might not be possible).