Erini Lambrides

We uncover a population of heavily obscured growing central supermassive black holes (or active galactic nuclei --AGN) that were previously identified as lower-luminosity un-obscured AGN or non-active star forming galaxies using archival data from the deepest X-ray survey to date, the Chandra 7Ms GOODS-South survey. The discovery has important implications for understanding how supermassive black holes (SMBHs) grow and evolve over billions of years. Supermassive black holes are found in almost all massive galaxies. Some models of galaxy evolution explain the SMBH-Host Galaxy connection via a merging paradigm: A quiescent black hole becomes active during a period of gas in-fall as a consequence of a gas-rich merger (Magorian+98, Di Matteo+05, Hopkins+06, Blecha+2013). On slide 2, we show a schematic of the merger paradigm. A predicted epoch of an SMBH’s life is a stage where it is rapidly growing, but the accretion disk which surrounds the black hole is obscured by gas and dust. There is currently no consensus in the literature on the universality of the merger paradigm. A large sample of obscured AGN is needed. In general, AGN show clues of their existence in a multitude of wavelengths. In our work, we take advantage of the unprecedented wavelength coverage of the GOODS-S field of the sky and connect the X-ray information (commonly used to find obscured AGN as well as AGN power) to the infrared (another AGN power indicator). In slide 3, we show a figure where we compare these two different AGN power indicators. The X-ray AGN luminosity is derived from the Chandra 7Ms X-ray catalogue, and should account for any obscuring material. Thus, a well defined relationship between the X-ray and the IR is expected for AGN. We show this relationship in the figure (solid line) along with the 2 sigma errors (dashed lines). We find the X-ray luminosity from the literature is under predicted compared to the infrared for the lowest flux sources (blue points or points within the rectangle) by over an order of magnitude. There is additional obscuration that has not been taken into account in the X-ray literature for over 30% of the X-ray AGN sample. When we account for this obscuration assuming the X-ray sources with under-predicted X-ray values are obscured AGN we match predicted values inferred from the X-ray background. In the leftmost plot on slide 4, we show the X-ray luminosities where no obscuration correction is applied. The x-axis is the same as in figure 1. By using the expected relationships derived for unobscured sources, we can infer the level of obscuration that must exist. The shaded area encapsulated by the dashed lines is the region of the parameter space where heavily obscured AGN are predicted to exist. We then use the sources that fall within this region of the parameter space to calculate the space density of heavily obscured AGN within the region of the sky probed by the Chandra Deep Field-South. The right-most plot on slide 4 shows the space density of the obscured AGN in our sample as a function of redshift. We are able, for the first time, to constrain predictions of the space density of lower-luminosity obscured AGN (top-most solid line). This is particularly important for differentiating between different models of AGN-Galaxy evolution, and different models of the X-ray background. Our results are in agreement with the Gilli+07 X-ray background model, and we are currently in the process of submitting a series of papers understanding the morphological properties of our most heavily obscured sources in the context of the merger paradigm.