El Gran Telescopio Milimétrico/ The Large Millimeter Telescope

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1 El Gran Telescopio Milimétrico/ The Large Millimeter Telescope E. Carrasco 1 and the GTM/LMT collaboration 2 1 Instituto Nacional de Astrofísica, Óptica y Electrónica, Luis Enrique Erro 1, Tonantzintla, Puebla, Mexico bec@inaoep.mx 2 INAOE, Luis Enrique Erro 1, Tonantzintla, Puebla, Mexico and the University of Massachusetts Amherst, 710 North Pleasant Street, Amherst Massachusetts Summary. The Large Millimeter Telescope is a single, high-precision mm-wavelength telescope, 50 meters in diameter located at an elevation of 4580 m on Tlilteptl, in central Mexico. It will operate with good efficiency at wavelengths as short as 1 mm and it will be capable of observations at 0.8 mm. The telescope will be the largest in the world operating at these wavelengths, equipped with innovative and extremely sensitive receivers which will function as cameras for both heterodyne and continuum observations. The main characteristics of the telescope and its instrumentation are presented. 1 Introduction The Large Millimeter Telescope (LMT) is a major new astronomical facility developed as a collaboration between the Instituto Nacional de Astrofísica, Óptica y Electrónica and the University of Massachusetts Amherst. Located at 18.9 N latitute, at 7 km from Citlaltepetl -the highest peak in Mexico- it has full access to the Galactic Center. LMT is due to enter comissioning and first light science phase in 2007 and will be the largest millimeter telescope worldwide. With an area of almost 2000 m 2 and its innovative instrumentation, it will allow sensitive measurements of all cosmic structures from dust grains to the Universe as a whole [4]. LMT will be able to penetrate the dust that obscures the process of star formation in distant galaxies to elucidate the history of star formation over time. It will allow to analize the environment of active galactic nuclei to probe the relation of supermassive black holes to their host galaxies and will quickly follow up detections of gamma-ray burst to increase our understanding of the death of massive stars and the origin of the heaviest chemical elements. Furthermore, it will complement GLAST observations [1] and contribute to disentangling the nature of the new and old unidentified gamma-ray sources. For the study of our galaxy and other galaxies in the local Universe the LMT will provide new insights into the nature and distribution of the inter-

2 2 E. Carrasco and the GTM/LMT collaboration Fig. 1. The Large Millimeter Telescope on November 22, [Photo G. Cerón]. stellar gas and dust from which starts forms. It will contribute to elucidate the process of star formation itself and to establish the existence and nature of the massive black hole at the center of the Milky Way by providing critical north-south coverage and unparalleled sensitivity to Very Long Baseline Interferometry observations. In addition, the LMT will be a powerful tool for astrobiology and planetary science by providing the sensitivity to allow searches for complex organic molecules in space. LMT will be able to detect and provide an initial characterization of the disks of gas and dust around starts from which planets form. It will analize with unprecedented sensitivity the chemistry and physics of comets. The telescope will carry out the first comprehensive survey of small bodies in the solar system, including near Earth objects, main belt asteroids, centaurs and Kuiper belt objects. More information about the project is available at the official LMT/GTM webpage: 2 The telescope The LMT is a 50 m diameter filled aperture mm-wave telescope. For the effective surface area figure of 75 µm rms, the primary reflector accounts for 55 µm rms. The innovative and unique telescope design includes an active surface formed by 180 segments. Each segment is supported by a reaction structure which is attached to the reflector backstructure by a subframe. Four

3 El Gran Telescopio Milimétrico/ The Large Millimeter Telescope 3 actuators can adjust each subframe in relation to the backstructure to correct the deformations due to gravity and thermal gradients. Temperature sensors on all relevant parts will report to the control system and the surface will be periodically measured by holographic techniques. The sensitive surface is composed by electroformed nickel panels. Simulations indicate that the LMT will be able to maintain surface accuracy in the presence of winds up to 10 m/s. The main telescope specifications are shown in table 1. Property Table 1. LMT technical specifications Specification Goal Effective surface area 75 µm rms 70 µm rms Pointing accuracy 1 arcsec 0.6 arcsec Aperture efficiency (3mm) Aperture efficiency (1.2mm) Sensitivity (3mm) 2.2 Jy/K 2.0 Jy/K Sensitivity (1.2mm) 3.5 Jy/K 3.1 Jy/K FHWM beam size (3mm) 15 arcsec FWHM beam size (1.2mm) 6 arcsec 3 Instrumentation AzTEC, the astronomical thermal emission camera, is a large format array made of 144 silicon nitride micromesh bolometers. The camera operates at 1.1, 1.4 and 2.1 mm although only one bandpass will be available per observing run. To optimize the instrument performance the warm readout electronics have been designed to minimize the analog input path, simplifying the electrical connections between the front-end and the back-end electronics and to eliminate signal ground connections between all computers and the radiometer. The system architecture utilizes fiber optic connections carrying the AES/EBU for commanding, clock distribution and signal transmission. AzTEC field of view on LMT will be about 2.4 arcmin 2. The instrument has been used in the James Clerk Maxwell Telescope (JCMT) already achieving sub-mjy flux limits [6] acomplish its expected performance. On LMT, with 10 times larger collecting area than the JCMT, AzTEC 1σ per pixel sentivity will reach <3mJyHz 1/2 and a mapping speed of 0.36 deg 2 /hr/mjy 2. SPEED, the spectral energy distribution camera, uses the new technology of frequency selective bolometers to perform multifrequency observations [5]. The camera is configured as a 2 x 2 array with each pixel housing a 2.1, 1.3, 1.1 and 0.85 mm detector. By sampling these wavebands it measures the millimetric spectral energy of a source in a single pointing eliminating the need for repeated observations of the same target. It is the ideal instrument

4 4 E. Carrasco and the GTM/LMT collaboration to follow up AzTEC sources. The sensitivies of these instruments on LMT are shown in table 2. Table 2. Estimated sensitivities for LMT continuum instruments AzTEC SPEED Channel Center frequency Beam size [arcsec] NEP [aw/ Hz] NET RJ [mk/ Hz] NEFD [mjy/ Hz] Mapping speed [deg 2 /hr/mjy 2 ] Fig. 2. AzTEC cryostat and readout electronics in the UMass Amherst Cryogenic Device Laboratory [4]. [Photo: J. Austermann]. SEQUOIA, the second Quabbin optical imaging array [2], is a 3mm heterodyne focal plane array tunable in the GHz range. It has 32 pixels arranged in dual polarized 4 x 4 arrays. Two dewars contain 16 pixels each, the beams from which are combined using a wire grid. The array uses indium phosphide monolitic microwave integrated circuit (MMIC) preamplifiers followed by subharmonic mixers with an intermediate frequency (IF) band of

5 El Gran Telescopio Milimétrico/ The Large Millimeter Telescope GHz. SEQUOIA was in operation at the Five College Radio Astronomy Observatory (FCRAO) 14 m telescope, mostly used for extended mapping of molecular clouds regions [3]. When mounted on LMT, SEQUOIA will be able to cover similar extensions to those achieve on the 14 m with improved signal to noise ratio and angular and velocity resolutions. The instruments characteritcs are shown in table 3. Table 3. Specifications of SEQUOIA Num of pixels 32 (2 de 4 x4) Beam size [arcsec] 15 Space between beams [arcsec] 30 Polarizations 2 Instantaneous RF bandwidth [GHz] Instantaneous FI bandwidth [GHz] 5-20 T sys (one pixel) < 60 K, GHz < 90 K, GHz T sys (sky) K A 1 mm commisssioning receiver is being built to operate in the GHz atmospheric window providing the state of the art sensitivity using a side band separation squeme to separate the upper and lower sidebands. This receiver will employ detectors based on the superconductor-insulator-superconductor technology. The redshift receiver (RR) is a ultra wide band spectrograph to cover the spectral range of GHz. It uses the new MMIC amplifiers technology that achieve noise temperature below 50 K over the same frequency range. It has a very low loss waveguide polarization combiner that covers the full band and a new fast electrical beam switching based on a Faraday rotation polarization switch. The receiver will have two dual polarized interchangeable beams so that one beam remains on the source at all times. The four receivers, making up the autocorrelation spectrometer, have a combined IF bandwidth of 144 GHz and the entire band needs to be spectrally processed simultaneously. Recently the RR saw first light with the acquisition of a 6 GHz wide spectrum of M82 on the 14 m FRCAO telescope, that used only 1/4 of the full coverage. Designed to determine the redshift of dusty galaxies via the frequency measurement of contiguous CO lines, the RR can also be used as a low dispersion spectrograph for identifying Galactic sources. The wide band spectrometer is the spectroscopic capability for the heterodyne receivers as SEQUOIA, the 1mm commissing receiver and future focal plane arrays. It computes the autocorrelation function (ACF) of the input signal and the Fourier transform of the ACF then gives the spectrum. The correlators will support all the envisioned data collection modes including positioning (< 1 Hz), beam ( 1 Hz) and frequency (> 1 Hz) switching and

6 6 E. Carrasco and the GTM/LMT collaboration on the fly mapping. The bandwidth (BW) and resolution requirements are imposed by the scientific goals as indicated in table 4. The spectrometer will be optimally configured for broadband coverage or high spectral resolution or some combination of both extremes. Table 4. Wideband spectrometer bandwidth (BW) and resolution (δν) requirements, in km/s, for some of the main scientific projects. Scientific goals BW δν Identification of unknown redshifts of primeval galaxies > Extragalactic imaging Galactic surveys and high velocity sources Giant molecular clouds Dark clouds Spectral line searches 1 GHz Conclusions The Large Millimeter Telescope with a 50 m diameter dish will be the largest single, high precision antena. The telescope commissioning will start en 2007 with initial science late the same year. As most of the first generation instruments have been commissioned -and the rest will be- on other telescopes, it is foreseen that when LMT enters full operation in 2008, it will produce scientific results inmediatly. References 1. Carramiñana, A., Astrop. Spa. Sci. proc. of The multimessenger approach to unidentified gamma-ray sources eds. J. Paredes, O. Reimer, D. Torres (2006) 2. N.R. Erickson, R.M. Grosslein, R.B. Erickson et al. IEEE Trans. Microwave Theory Techniques 47, 2212 (1999) 3. J. Jackson, et al: ApJS 163, 145 (2006) 4. W.M. Irvine, E. Carrasco & I. Aretxaga: The Large Millimeter Telescop - Neighbors explore the cosmos. ed by W.M. Irvine, E. Carrasco & I. Aretxaga (Creative Services, University of Massachusetts Amherst, 2005) 5. G.W. Wilson, J. Austermann, D.W. Logan, M. Yun: SPIE 5498, 246 (2004) 6. G.W. Wilson, et al: AAS 208, 6606 (2006)