New Galaxy Zoo Project: Does AGN Type Relate to Host Galaxy Inclination?

We’re beginning on a new project utiltizing Galaxy Zoo data.

Some background: Active Galactic Nuclei (AGN) are accreting supermassive black holes at the centers of some galaxies. In some of these systems, we observe broad and narrow emission lines in the optical spectra whereas other systems only have narrow emission lines. (In the spectra of normal galaxies that don’t have an active nucleus, we generally don’t see these prominent emission lines. In AGN, photons from the accretion disk photoionize the surrounding gas, causing the emission lines we observe.)

The unified model for AGN explains these observational differences by invoking an obscuring torus (or doughnut) of dust and gas that surrounds the accretion disk feeding the black hole. For a helpful visualization, look here. (Keep in mind that this picture is “zoomed-in” to the center of the galaxy.) If this configuration is aligned so that we’re looking through the opening of the torus, we can see the emission from the accretion disk and gas that’s moving rapidly due to its proximity to the black hole (which causes the broad emission lines in the optical spectra, and the gas from this region is subsequently referred to as the “Broad Line Region”). These sources are classified as Type 1 AGN. If, however, the system is aligned such that the torus is edge-on, that accretion disk and fast moving gas is blocked from our view. We will therefore see emission from gas that is further away from the black hole but close enough to be photionized by accretion disk photons, producing narrow emission lines (and no surprise, this region is called the “Narrow Line Region”). These AGN are Type 2 AGN, sometimes referred to as “obscured AGN” since the central region is hidden from our view.

The hypothesis: These AGN live in galaxies. Are our optical classifications of Type1/Type2 related to the orientation of the host galaxy? Do Type 1 (face-on) AGN preferentially live in face-on galaxies? Do Type 2 (edge-on) AGN tend to inhabit edge-on galaxies? The issue of the alignment of the torus with host galaxy inclination has been studied quite a bit in the past. In local AGN (called Seyfert galaxies), there is a lack of Type 1 systems in edge-on galaxies, but Type 2 AGN seem live in galaxies with any orientation (Keel 1980, Schmitt et al. 1997, Simco et al. 1997, Kinney et al. 2000).

The project: These past studies have been limited to relatively small sample sizes (under 100 galaxies). The Sloan Digital Sky Survey has spectra for hundreds of thousands of objects and Galaxy Zoo has classified thousands of galaxies. Combining these two rich data sets, we can test the above hypothesis with a sample size of several thousand galaxies.

I will blog updates as the project proceeds, so stay tuned to see where this journey takes us!


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