Extragalactic Astronomy

The scientific potential of this interferometric array will provide a counterpart (at similar resolutions) to VLBI (and VSOP - VLBI Space Observatory Program) at near IR wavelengths (see for example, Rantakyrö et al, 1998 [13], observations of AGNS with 50 $\mu$as resolutions at $\lambda$=3mm or Lobanov et al, 1998 [8] for quasar jet instabilities with resolutions of 0.91$\times$0.31 mas at 6cm). It will also allow to refine, extend and complement models that will have been developed for KeckI and VLTI observations. For instance, in the case of Mauna Kea, three different orders of magnitudes of resolutions will be available:

$$\longrightarrow$$ 8 - 10 meters 50 mas

$$\longrightarrow$$ KeckI 5 mas^*

$$\longrightarrow$$ `OHANA 0.5 mas

^* The KeckI baseline with the two Kecks only will be 85 meters.

In the case of Ultra Luminous InfraRed Galaxies (ULIRGs), this translates to the following characteristic scales: adaptive optics allows to probe the Narrow Line Region (NLR) and the close environment of the nucleus. KeckI and VLTI will allow to resolve the molecular torus and explore the Broad Line Region (BLR), while `Ohana will allow to start exploring the environment of the accretion disk and may be resolve the sources that are responsible for the broad lines. In practice this means that a few visibility points may help to differentiate between simple yet fundamentally different models and geometries (such as disk, jets, point like or extended nucleus). It is only with many baselines (i.e. a large u-v plane coverage, using supersynthesis effects) that one could hope to reconstruct complex objects, as shown in figure 1.

  

Figure 1: Left: uv plane coverage for the `Ohana array for a source at $50^{\circ}$ declination for 8 hours of tracking (super-synthesis). Right: Associated PSF at 2.0 $\mu$m; note that the scale on the axes is in milliarcseconds. For reference, the declination of Markarian 231, one of the brightest Ultra Luminous Infrared galaxies is $+56^{\circ}52'28''$.