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Petite AGNs Reveal New Secrets

July 6, 2012

Figure 1. Michelle and T-ReCS mid-infrared images of some of the low-luminosity AGN in this study. Some of the galaxies, such as NGC 1052, have strong, compact nuclei reminiscent of higher-luminosity Seyfert galaxies or quasars. Others, like NGC 3169, show extended emission that could be due to stars forming around the active nucleus. To the right of each of the mid-infrared images is a Hubble Space Telescope optical image of the same region.

Figure 2. The strength of the silicate dust emission feature in many of the low-luminosity AGN (denoted S10, the black circles), is unusually large compared to the amount of gas in their nucleus (measured by log NH). One possible explanation is that these galaxies harbour just a small amount of optically-thin dust, which is expected according to some models that predict the disappearance of the dusty torus in low-luminosity AGN.

High-resolution, mid-infrared observations at Gemini North and South have revealed a wide range of morphologies for low-luminosity active galactic nuclei (AGN). While the data present a broad characterization of these objects' properties in this spectral region, they also present an interesting puzzle to ponder.

Active galactic nuclei (AGN), the supermassive black holes that feed on gas, dust, and stars at the centers of galaxies, spend most of their existence in a near dormant state. Until recently, astronomers had observed only a handful of low-luminosity AGN in the infrared at high resolution. Therefore, we didn't have a good, general overview of their properties in this potentially revealing spectral region. Our observations of 22 low-luminosity AGN, taken with both of Gemini's mid-infrared instruments (Michelle and T-ReCS), have changed this situation.

The images reveal a wide range of morphologies, from galaxies dominated by a central, compact source (much like images of higher-luminosity Seyferts and quasars) to those with weak nuclei embedded in large amounts of extended, mid-infrared emitting material that could signal star formation around the nucleus (Figure 1). To complement these observations, we combed the literature for other high-resolution measurements that reveal the emission of the nucleus from radio to X-ray frequencies. We also took advantage of low-resolution but exquisitely sensitive spectroscopy from the Spitzer Space Telescope archive.

A rather complex picture emerged from the data. In some of the most weakly-accreting AGN, even Gemini's resolution doesn’t separate the infrared emission of the nucleus from that of the surrounding galaxy. However, we do find some cases where the infrared emission comes not from dust or the outer regions of the accretion disk, but from synchrotron radiation –– fast-moving electrons spiraling round magnetic field lines in the galaxy’s core. In a couple of those galaxies, the evidence suggests that the dusty torus is indeed absent (see sidebar). This is predicted by some models describing the nature and origin of the torus.

The more strongly-accreting AGN (but still weaker than most of those studied to date), look in many ways a lot like "conventional" Seyfert galaxies in the infrared. It's possible, then, that these low-luminosity AGN aren't as different as we had thought. However, the data do present some tantalizing hints that in these AGN, too, the dusty torus no longer exists. When we compare the dust emission features in their Spitzer spectra with the amount of gas around the nucleus (determined from published X-ray observations), it appears that there is an unusually small amount of dust compared to gas (Figure 2). This, again, is expected from some models that attempt to explain the origin of the torus.

If the torus doesn't exist in these objects, then we will need to find another way of explaining their Seyfert-like infrared emission. To better understand the observations, we have started to compare detailed models of the accretion disk, dust and synchrotron emission to the data. But right now we are simply happy to have high-quality observations to puzzle over in the months to come.

The article about this research has been published by the Astronomical Journal.