A sensitive new radiomenter, excellent weather, and a newly adjusted dish combined to make the first FANATIC observing run on the JCMT a smashing scientific success. FANATIC replaces the Schottky RxG, which had been in operation at the JCMT for the past several years. This new system saw its first light on the JCMT during semester 94A in March 1994. FANATIC's mixer is an SIS tunnel junction which was fabricated at the Institut de Radio Astronomie Millimemetrique (IRAM) in Grenoble, France (Schuster etal 1993). The local oscillator is a solid state Gunn oscillator followed by a doubler and a tripler. The LO is tunable with sufficient power to drive the mixer over a range from 660-695 GHz. The DSB receiver temperature is about 800 K over most of the band, factors of 3-5 improvement over the Schottky system(Figure 1). The backend for the system is the RxG 1.1 GHz acousto-optical spectrometer, with 1.2 MHz channel spacing. A complete description of the system is given in Harris etal (1994).

To determine the beam distribution of the system on the sky we made a series of five-point maps at a range of spacings from the center of Jupiter. Assuming that, as in the past, the distribution was best fit by a composition of 2 Gaussians, we fit convolutions of Gaussians with a disk to the five-point data. The result of these fits shows that 40% of the power that couples to Jupiter is in a 7 arcsec beam and the remaining 60% is in a 20 arcsec error beam component (Harris etal 1994). The absolute coupling to Jupiter is 0.20.

We observed several sources which were previously observed with the Schottky system to cross-check the intensity scales and found agreement to within 10%. This agreement in relative calibration indicates that the system is well behaved and that we have correctly determined the coupling efficiency.

Here we present a short summary of two new results from this run. Other results from FANATIC are also presented in this Newsletter in articles by Matthews and Thum, and Macleod etal.

Mapping and a Spectral Line Survey of the Orion Core

Wideband spectral line observations are complementary to spatial mapping. While spatial mapping yields information on the extent and excitation centers of sources, line observations yield information on the dynamics, energetics and chemistry within a beam. For example, Blake etal (1987) used information from over 800 lines in the 1.3 mm spectral region to identify distinct sources with izes smaller than the beam and to calculate the physical conditions in these sources. In order to get both spectral and spatial information about the excited gas in the Orion hot core and plateau areas, Harris etal have spatially mapped the region in the H13CN J=8-7 line and have measured the spectrum from 685-692 GHz with FANATIC.

Figure 1. DSB receiver noise temperature vs. tuning frequency, averaged over a 500 Mhz IF bandwidth.

For the mapping part of the program, we chose the J=8-7 of H13CN to point out the regions with dense, highly excited gas. The map was made on a 4 arcsec grid. Figure 2 shows the spectra from the map. The emission in this line is very compact, suggestive of acentrally condensed density distribution in this core region. The size of the H13CN core emitting region is 4-6 arcsec, similar to that of the NH3 and millimeter continuum regions ( e.g. Genzel etal 1982; Wright and Vogel, 1985; Schilke etal 1992).

The position of the H13CN peak was chosen to be targeted in the spectral survey. The frequency range from 685.3 to 692.1 GHz was chosen for the scan because of the plethora of strong lines present in this band. The observations were made in steps of 300 MHz, such that each line was observed twice in the 700 MHz bandwidth of FANATIC. This procedure has allowed us to separate unambiguously lines from each sideband.

Based on their linewidths two types of lines are seen in the scan: broad lines coming from the plateau region, and strong, narrow lines presumably arising from the compact ridge. Interestingly, lines with widths characteristic of the hot core region itself are not seen.

Lines of SO are especially prominent and broad, exhibiting widths which indicate that they arise from the plateau. Also seen are narrow lines of CH3OH which are attributable to the compact ridge. The sensitivity limit of the survey is a few degrees, and it is somewhat surprising that the forest of lines which appear in Orion surveys at lower frequencies is not evident in our survey. In particular, no lines from heavy species such as ethyl cyanide, methyl formate, or dimethyl ether are detected, nor are any species with line temperatures of 5-10 K. This may be in part due to the fact that at these high frequencies we are beyond the peaks of the excitation curves for the heavy molecules, which contribute so many lines in lower frequency surveys.

A full description of this work will be found in Harris etal (1994).

Extended Warm Dense Gas in M82:

The mid-J lines of CO can be used as a very sensitive diagnostic of the temperature and excitation properties of the molecular gas present in the nuclei of starburst galaxies. In nearly all nuclei where multiple lines of 12CO have been observed the emission is found to be optically thick. The distribution of CO intensity with J is rather flat, and then there is a decrease in intensity as the CO becomes subthermally excited. The transition at which this `rollover'' occurs is very sensitive to the pressure of the molecular gas. For those galaxies with large UV radiation fields and warm molecular gas, as is the case for starburst and active galaxies, the rollover in intensity is likely to occur somewhere in the mid-J transition range.

Figure 2. Map of the H13CN J=8-7 line in the Orion core. The temperature scale is Tr*, with eta(fss)=0.2, appropriate for coupling to a Jupiter-sized source. The map centre is Irc2: RA(1950) = 5h 32m 47s, Dec (1950) = -5d 24' 23".

With the increased sensitivity of FANATIC over the RxG Schottky system, it has become possible to more easily map out the distribution of the warm, dense gas as traced by the CO 6-5 line in a number of nearby bright galaxies. Previously, emission from the 6-5 line had been detected in the nuclei of 3 nearby starburst galaxies: M82, NGC 253 and IC 342 (Harris etal 1991). The intensities of the 6-5 emission in these galaxies indicate a large amount of warm gas in their nuclei. We have now studied the nucleus of M82 in more detail by mapping the distribution of the 6-5 line over the central 25(??) arcsec in its nuclear region.

We were able to map around both the NE and SW low-J CO lobes, in the nucleus itself, and along the major axis. Through line pointing on M82 itself with RxB3i in the 3-2 line, we were able to establish the relative pointing from night to night. The extent of the 6-5 emission is large. We detect lines throughout the central 20" of this galaxy, in the nucleus as well as in the CO lobes (Figure 3).

Figure 3. Sample spectra from our 12CO J=6-5 map in M82: (a) the peak of the NE CO lobe; (b) the nucleus; and (c) the peak of the SW CO lobe. The temperature scale is Tr*, with eta(fss)=0.2, appropriate for coupling to a Jupiter-sized source.

The 6-5 lines are peaked at the positions of the NE and SW lobes, similar to the distribution of low-J 12CO (e.g. Lo etal 1987; Tilanus etal 1991; Wild etal 1992). The emission is distributed throughout the nucleus, not conentrated in a nuclear core, or associated with the clusters of supernova remnants, as traced out by the 6-cm radio continuum emission (Kronberg, Biermann and Schwab 1985).

Such a distribution and the large extent of the 6-5 emission indicates that the gas is primarily being heated by large scale processes, and not by the input of energetic photons into the ISM by supernovae. The fact that the emission is so widespread means that clouds with bulk temperatures of about 50 K are prevalent in the central regions of M82. M82 may not be unique among starburst galaxies. Previous 6-5 observiations of NGC 253 with the Schottky RxG show that the 6-5 emission is also extended in that galaxy. Indeed, the gas which gives rise to the mid-J CO emission may well comprise a significant percentage of the molecular gas from galaxy nuclei.

Special thanks go to Lorne Avery for his help at the telescope, and to K.-H. Gundlach and B. Plathner for assistance with the junction fabrication at IRAM. As always the mechanical and software support during setup at the JCMT was outstanding. We also thank Per Friberg for his extensive help as the FANATIC JCMT support scientist.

References:

Blake, G.A., Sutton, E.C., Masson, C.R., and Phillips, T.G. 1987, Ap.J., 315, 621.

Genzel, R., Downes, D., Ho, P.T.P., and Bieging, J. 1982, Ap.J., 259, L103.

Harris, A.I., Hills, R.E., Stutzki, J., Graf, U.U., Russell, A.P.G., and Genzel, R. 1991, Ap.J., 382, L75.

Harris, A.I., Schuster, K.-F., Gundlach, K.-H., and Plathner, B. 1994, Internat. J. IR and mm Waves, in press.

Kronberg, P.P, Biermann, P., and Schwab, F.R. 1985, Ap.J., 291, 693.

Lo, K.Y., Cheung, K.W., Masson, C.R., Phillips, T.G., Scott, S.L., and Woody, D.P. 1987, Ap.J., 312, 574.

Schilke, P., Walmsley, C.M., Pineau des Forets, G., Roueff, E., Flower, D.R., and Guilloteau, S. 1992, A&A, 256, 595.

Schuster, K.-F., Harris, A.I., and Gundlach, K.-H. 1993, Internat. J. IR and mm Waves, 15, 1867.

Tilanus, R.P.J., Tacconi, L.J., Sutton, E.C., Zhou, S., Sanders, D.B., Wynn-Williams, C.G., Lo, K.Y., and Stephens, S.A. 1991, Ap.J., 376, 500.

Wild, W., Harris, A.I., Eckart, A., Genzel, R., Graf, U.U., Jackson, J.M., Russell, A.P.G., and Stutzki, J. 1992, A&A, 265, 447.

Wright, M.C.H. and Vogel, S.N. 1985, Ap.J., 297, L11.

Andrew Harris, Linda Tacconi, Karl Schuster & Reinhard Genzel,

Max-Planck-Institut fur extraterrestrische Physik, Garching, Germany