Exploring Supermassive Black Hole Growth with ALMA

Massive tori with ≈108-109 M☉ are predicted to extend on a ~100 pc scale around the center of elliptical galaxy progenitors by a model of a supermassive black hole (SMBH) growth coeval to the spheroidal population of the host galaxy. Direct detection of such massive tori would cast light on a key physical condition that allows the rapid growth of SMBHs and the appearance of QSOs at high redshift. For this reason, we examine the detectability of such structures at substantial redshift with the Atacama Large Millimeter Array (ALMA). We propose that submillimeter galaxies (SMGs) are the best targets to test our predictions. In order to assess the observational feasibility, we estimate the expected number counts of SMGs with massive tori and check the detectability with the ALMA instrument, the unique facility which can resolve the central region of high-redshift objects. Our work shows that ALMA will be able to resolve and detect high-J (J > 4) CO emissions from ~100 pc scale extended massive tori up to z ≈ 2. Observations of lensed SMGs will yield excellent spatial resolution, allowing us to resolve their massive tori at even higher redshift. We further discuss the detectability of the HCN molecule as a better tracer of the high-density gas expected in such tori. The final goal of these kind of observations is to pinpoint possible physical mechanisms for storing very large amounts of gas in the very central galactic regions on timescales of several 108 yr.