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GRAVITY


GRAVITY
 

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GRAVITY

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Introduction

GRAVITY is an adaptive optics assisted, near-infrared VLTI instrument for precision narrow-angle astrometry and interferometric phase referenced imaging of faint objects.

GRAVITY will make fundamental measurements over a wide range of astronomy and astrophysics

Precision astrometry and phase-referenced interferometric imaging will bring to fruition the most advanced vision of optical/infrared interferometry with the VLT. Much like in the case of long baseline radio interferometry (e.g. super-luminal motions in jets, the maser disk around the black hole in NGC 4258, supernova shell expansions etc.) infrared astrometry with 10 micro-arcsecond accuracy and phase referenced imaging with 4 mas resolution will make possible a number of key advances.

With an accuracy of 10 micro-arcseconds, GRAVITY will be able to study motions to within a few times the event horizon size of the Galactic Center massive black hole and potentially test General Relativity in its strong field limit. GRAVITY will be able to detect intermediate mass black holes throughout the Galaxy by their gravitational action on surrounding stars. GRAVITY will directly determine the masses of exo-planets and brown dwarfs, and trace the origin of protostellar jets. GRAVITY will be capable of spatially resolving coherent gas motions in the broad line regions of active galactic nuclei in external galaxies.

Through its high performance infrared wavefront sensing system, GRAVITY will open up deep interferometric imaging studies of stellar and gas components in dusty, obscured regions, such as obscured active galactic nuclei, dust-embedded star forming regions, and protoplanetary disks.

By virtue of its phase referencing concept GRAVITY will make possible interferometric imaging for objects as faint as mK = 20, thereby opening up precision interferometry for a wide range of astronomical objects in both Galactic and extragalactic targets. GRAVITY thus will carry out a number of fundamental experiments, as well as increase substantially the range and number of astronomical objects that can be studied with the VLTI. GRAVITY will bring the tool of precision interferometry to a wide range of users.

The combination of GRAVITY and the VLTI are unique

The VLTI is the only array of 8m class telescopes that explicitly included interferometry in its design and implementation. No other array is equipped with a comparable infrastructure. The VLTI, with its four 8m telescopes, is the only interferometer to allow direct imaging at high sensitivity and high image quality. The two 10m Keck telescopes can only perform astrometry and phase referenced imaging on a single baseline. The VLTI is also the only array of its class offering a large 2'' field of view (40 times the diffraction limit of the 8m Unit Telescopes in K-band). By combining GRAVITY with the VLTI, it will be possible to use to full capacity all these basic unique features. VLTI+GRAVITY will thus be a unique facility worldwide for many years to come.

GRAVITY will for the first time utilize the unique 2'' field of view of the VLTI concept. GRAVITY will take advantage of exploiting this capability to push the astrometric accuracy for narrow angle astrometry with the VLTI by a factor 10 beyond the present goal for PRIMA. There is no competing facility worldwide that will even get close to the combination of sensitivity and accuracy of GRAVITY.

The second major new element of GRAVITY is the use of infrared wavefront sensors to open a new window for astronomy. GRAVITY provides four infrared wavefront sensors for the 1 - 2.5 micron wavelength range, which can also be used with other instruments. Installed in the VLTI laboratory, GRAVITY will not only sense and correct the atmospheric turbulence, but also wavefront errors and seeing introduced in the VLTI beam relay. None of the competitive interferometric arrays has presently planned or will be able to upgrade their adaptive optics for infrared wavefront sensing.

The beam combiner provides simultaneous interferometry of two objects in a 2'' field of view for up to four telescopes. This allows narrow angle astrometry with an accuracy of < 10 micro-arcseconds, a factor ten better than PRIMA when operated with the Unit Telescopes. For interferometric imaging of faint objects, GRAVITY provides internal fringe tracking on one of the objects. The application of phase referenced imaging - instead of closure phases - is a major advantage in terms of model-independence and fiducial quality of interferometric maps with a sparse array such as the VLTI. The whole instrument will be enclosed in a cryostat for optimum sensitivity and stability. GRAVITY covers instantaneously the wavelength range from 1.9 - 2.5 micron at a spectral resolution of 30 or 500. The limiting magnitude in a few hours is mK = 20.

GRAVITY will be developed as an end-to-end general use facility at the VLTI, and will be operated within the ESO data flow system.

GRAVITY is an integral part of a larger coherent effort of the participating institutes to further the VLTI for the observation of faint objects

This effort involves - among others - the financing of the 3rd Auxiliary Telescope and the funding for two star-separator-systems for the 8m Unit Telescopes. Our proposed instrument is the logical continuation of this effort in the scientific exploration of the VLTI, driven by several of the key science goals of the PRIMA-VLTI reference study. Following a detailed analysis of the available VLTI infrastructure and instruments, we have identified the need for an infrared adaptive-optics assisted, throughput-optimized, imaging and narrow angle astrometry instrument. The GRAVITY instrument is to a large extent funded through the consortium.

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