NuLANDS: The NuSTAR Local AGN N(H) Distribution Survey

Supermassive black holes tend to be shy eaters. While growing, usually hide behind thick veils of gas and dust. These veils provide the fuel to make the black holes grow, but also mean that we must use special techniques to peer beneath them if we are to learn about growth. I am leading a large survey of growing supermassive black holes in the nearby universe using infrared and X-ray telescopes to overcome the handicap caused by gas and dust. NuLANDS (the NuSTAR Local Agn N_H Distribution Survey) aims to robustly quantify the number of heavily obscured actively growing supermassive black holes and to measure their growth rates.

Details on the newly approved NuSTAR time are here.

Read some recent related press releases (including some NuLANDS targets) in which I was involved:
(1) Local extreme sources.
(2) A rare unveiling event.
(3) Revealing hidden obscured quasars.

Starting sample: Keel et al. (1994) AGN sample based upon: (i) IRAS 60-micron all-sky flux limit ensuring IR detection; (ii) IRAS 25--60 micron colour to select sources with hot dust (i.e. AGN dominated); (iii) followed up by optical spectroscopy to classify the AGN as Seyfert 1s and or 2s. (iv) Finally, we limit the initial sample to 75 AGN below z=0.04.

1 Megasecond of NuSTAR observing time has been allocated to NuLANDS. In addition, all sources will have either Swift or XMM follow-up.

X-ray selection is incomplete when it comes to Compton-thick AGN where the obscuring torus can suppress even the highest energy X-rays. Instead, IR selection is key for us because the IR probes reprocessed emission from hot dust heated by the AGN, and this has been shown to be largely isotropic. In other words, we may not be complete, but we can be sure that NuLANDS will be representative of the underlying AGN population.
     The plot below shows no difference between obscured and unobscured AGN (red and black) when comparing two isotropic quantities ([OIII] and IR luminosities). On the other hand, when using X-ray luminosities, a huge fraction of the type 2 (obscured) objects are not detected. This is why the IR is so much better for isotrtopic AGN selection.

Left: Distribution of the [OIII] to 60-micron (optical to IR) flux ratio (taken to be an isotropic indicator) for all Seyfert 1s and 2s of the redshift cut Keel sample of 75 AGN below z=0.04. The two distributions return a K-S statistic of 0.70 - fully representative of the global N(H) distribution. Right: Distribution of the BAT to 60-micron (hard X-ray to IR) flux ratio. Upper limits are shown as horizontal arrows. This plot shows a bias towards detecting sources with considerable hard X-ray emission, i.e. less obscured.

NuLANDS has begun! In its very first observation of Seyfert 2 galaxy (2MAS J2139), we have identified a previously unknown Compton-thick AGN. The source lies at redshift z=0.036 and is one of the faintest sources expected in NuLANDS. Despite this, it is detected well beyond 10 keV and the strong Fe fluorescence emission line (EW of about 1.5 keV) combined with the rising spectral shape is unambiguous evidence of a Compton-thick AGN.

The intrinsic power (3e43 erg/s) is several hundred times higher than the observed luminosity, and matches perfectly with expectation from the IR torus power. This augers well for observations to come.

Watch this space for more information.

2MJ2139 (Sey 2)
ESO344 (Sey 1)
U09944 (Sey 2): another extreme CT candidate. Follow-up observations requested.
IC1198 (Sey 1).