PI: Fausnaugh, Michael, Texas Tech University, Lubbock
Wide-Field Science – Regular
The Roman High Latitude Time Domain Survey (HLTDS) will cover a wide field (24 square degrees) with a uniform cadence of 5 days. Beyond supernova science, the HLTDS is an incredible opportunity for extragalactic timing experiments. We propose to investigate Active Galactic Nuclei (AGN) structure through reverberation mapping of targets in the HLTDS.
Reverberation mapping is a technique that uses light echoes at different wavelengths to reconstruct the geometry of structures near a supermassive black hole. For the Roman cadence and wavelength coverage, reverberation signals will probe obscuring structures on distance scales of about 0.01 to 0.5 pc, the so-called “dusty torus.” The dusty torus is a fundamental component of the standard AGN unification paradigm.
Population studies of AGN have been able to constrain the average properties of the dusty torus. However, the properties of individual objects have been elusive, and are subject to degeneracies when employing SED fitting and other indirect modeling techniques. By using timing information from reverberation, many of these degeneracies can be broken.
The Roman HLTDS therefore presents a unique opportunity to measure dusty torus geometric parameters for individual AGN. The key aspect of Roman AGN data that enables this investigation is the combination of long baseline, high cadence (with no seasonal gaps), and high signal to noise ratio light curves. These data will make it possible to model aspects of the reverberation signal that are tied to the torus geometry, including line-of-sight inclination angle, surface curvature, and covering factor. This level of detailed modeling is not possible with ground-based monitoring campaigns.
Furthermore, the large footprint and deep limiting magnitudes of the HLTDS ensures that an unprecedented number of AGN will have the requisite data to measure dusty torii parameters. We expect over 50,000 AGN to be amenable to the reverberation analysis, which is two orders of magnitude larger than previous studies. This will make it possible to construct distributions of AGN torus inclinations and covering factors. The Roman data will also be able to probe the geometry of additional AGN components that are poorly understood, such as the hot dust in poloidal winds that has been observed by IR interferometry.
The implications of measuring the distribution of dusty torii covering factors are far ranging: (1) The torus is likely shaped by radiation pressure, and detailed geometric parameters will allow us to investigate this process. (2) The covering factor distribution of dusty torii also enters directly into AGN unification models, which have complications at high luminosities that are not fully understood. (3) The structure of the torus has implications for its formation mechanism and feeding of the AGN, which in turn has implications for AGN feedback on galaxy evolution. (4) The geometry of the torus influences the fraction of obscured and Compton Thick AGN in the universe, which in turn influences the AGN luminosity function, the X-ray background spectrum, and the history of black hole accretion in the universe. (5) Lastly, the large sample means that it may be possible to calibrate the torus properties against features in the SED, which will ensure that torus properties can be reliably measured without expensive monitoring campaigns.