Prof. Niranjan Thatte

Abstract:

SINFONI, the SINgle Faint Object Near-infrared Investigation, is an instrument for the Very Large Telescope (VLT), designed to provide spectroscopy at the telescope diffraction limit in the near-infrared. This unique capability is achieved by combining two state-of-the-art developments, an integral field spectrometer (SPIFFI) and a curvature sensor based adaptive optics system (MACAO). SINFONI is a collaborative effort by the Max-Planck-Institut fur extraterrestrische Physik (MPE) and the European Southern Observatory (ESO).SINFONI will operate at the Cassegrain focus of Unit Telescope 1 (UT1) of the VLT, in conjunction with a Laser Guide Star (LGS) for almost complete sky coverage. It will provide integral field data cubes, with a hexagonal field of view ranging from similar to 1 " to 8 ", with corresponding pixel sizes of 0." 03 to 0." 25. The field of view contains 1024 spatial pixels, with similar to 100% filling factor in the focal plane. Spectra are obtained for each of the 1024 pixels. Spectral resolutions of R=2000 to R=4500 will be available, covering the J, H and K spectral windows. The high spectral resolution made will allow software OH suppression in the J and H bands. The detector is a 1024(2) HgCdTe HAWAII array from Rockwell. Spectroscopy of faint objects (m(K) < 21 and m(H) < 22) will be easily feasible.

Abstract:

The SINFONI1 project combines the MPE cryogenic near-infrared imaging spectrometer SPIFFI2 with an ESO adaptive-optics system on the ESO-VLT to perform high spatial and spectral resolution studies of compact objects. This paper describes the optical design of SPIFFI and the novel techniques used in building its integral-field unit.The image slicer comprises of a bundle of 1024 silica/silica fibers, where each fiber tip is flared to increase the core diameter by a factor of 15. The tapered end is polished to form a spherical microlens with a hexagonal cross-section to couple Light into the optical fiber. This not only yields a high light-coupling efficiency and a high geometrical filling factor but also allows us to use the fiber bundle at a working temperature of 77 K without losing positioning accuracy.

Abstract:

SPIFFI (SPectrometer for Infrared Fibre-fed Field Imaging) is an integral field spectrograph with an HAWAII array that enables us to simultaneously take near infrared spectra of 1024 spatial pixels in an hexagonal field of view on the sky. It can be used on 4 to 8 meter class telescopes with a maximum pixel scale of 0.5 arcsec and with adaptive optics pixel scales, Nyquist sampling the point spread function of the telescope.A fiber bundle of 1024 silica/silica fibers rearranges the two-dimensional field of view into the one-dimensional entrance slit of the spectrometer. A novel technique involving flared fibers is used to achieve a high filling factor and coupling efficiency. Each fiber tip in the bundle is flared to increase the fiber core diameter by a factor of 15. The tapered end is polished to form a spherical micro-lens with a hexagonal cross-section to couple light into the fiber core. Apart from yielding a high coupling efficiency and a high geometrical filling factor, the monolithic micro-lens/fiber system can be used at a working temperature of 77 K without loosing positioning accuracy.The spectrometer optics is achromatic from 1.1 to 2.5 microns and uses four reflection gratings on a wheel as dispersing elements with a resolving power from 2000 to 4500. The fore-optics includes the filter wheel, the cold pupil stop and a scale changing mechanism to switch between three different image scales according to observing and seeing conditions. The spectrometer collimator is a f/4.3 three lens achromat, the spectrometer camera is a f/1.2 folded Schmidt camera. The optical design of the spectrometer is distortion free to get straight, equidistant spectra that match the columns of the detector, thus minimizing cross-talk from adjacent spectra to less than 5%.

Abstract:

The powerful tools of integral field spectroscopy and adaptive optics have made great contributions to the progress in astronomy in recent years. The combined use of these techniques now enables spectroscopy in the near infrared close to the telescope diffraction limit. This will provide new and interesting insights into a variety of objects such as AGNs, QSOs, circumstellar disks around highly extincted YSOs, etc.Spectroscopy at or close to the telescope diffraction limit has some caveats which one has to be aware of when designing the instruments so as to maintain the maximum possible throughput and to optimize spectral resolution. Astronomical campaigns with our H - and K - bend Integral Field Array Spectrograph 3D ( Weitzel ct al.(1)) in combination with the Laser Guide Star Adaptive Optics System ALFA ( Hippler et al.(2), Quirrenbach et al.(3) Glindemann et al.(4)) at the 3.5-m telescope at Calar Alto require special observational techniques in order to make the most efficient use of the observing time available. Chopping by moving the telescope to do background subtraction makes it necessary to relock the A.O. system on the guide star after moving the telescope back to source. This procedure is usually rather time consuming. The Aperture Interchange Module ( AIM), which we present here, enables us to perform chopping between source and blank sky while keeping the telescope fixed at a certain point in the sky. For this purpose AIM uses two different optical channels. The ON channel always paints to the center of the 3'ALFA FOV, picking off a FOV of roughly 4" x 4''. With the OFF channel one can choose any offcenter position within the ALFA. FOV execpt a central obscuration of 38"diameter. The AIM optics are designed in such a way that the optical pathlengths for the on and off- axis positions are kept equal. AIM also includes a scale changer which magnifies the scale from 0."25 / pix to 0."07 / pix. The 3D spectrometer itself is equipped with two interchangeable grisms, so that one can choose between H- and K- bands and between spectral resolutions of 1100 and 2100. The commissioning run of AIM together with 3D and ALFA took place in July 1997 at the 3.5m Calar Alto telescope.

Abstract:

We present an ISO SWS and ISOPHOT-S, mid-infrared spectroscopic survey of 15 ultra-luminous IRAS galaxies. We combine the survey results with a detailed case study, based on near-IR and mm imaging spectroscopy, of one of the sample galaxies (UGC 5101). We compare the near- and mid-IR characteristics of these ultra-luminous galaxies to ISO and literature data of thirty starburst and active galactic nuclei (AGN), template galaxies. We find that 1) 70-80% of the ultra-luminous IRAS galaxies in our sample are predominantly powered by recently formed massive stars. 20-30% are powered by a central AGN. These conclusions are based on a new infrared 'diagnostic diagram' involving the ratio of high to low excitation mid-IR emission lines on the one hand, and on the strength of the 7.7um PAH feature on the other hand. 2) at least half of the sources probably have simultaneously an active nucleus and starburst activity in a 1-2 kpc diameter circum-nuclear disk/ring. 3) the mid-infrared emitting regions are highly obscured. After correction for these extinctions, we estimate that the star forming regions in ULIRGs have ages between 10^7 and 10^8 years, similar to but somewhat larger than those found in lower luminosity starburst galaxies. 4) in the sample we have studied there is no obvious trend for the AGN component to dominate in the most compact, and thus most advanced mergers. Instead, at any given time during the merger evolution, the time dependent compression of the circum-nuclear interstellar gas, the accretion rate onto the central black hole and the associated radiation efficiency may determine whether star formation or AGN activity dominates the luminosity of the system.