Scientific highlights with SPIFI on the JCMT

Figure 1. (left) SPIFI CO (7-6) contours (April 1999 run) overlain on a 37.7um continuum image. The inset shows our 5x5 array footprint and individual pixel sizes. Blue dashed box roughly indicates the region mapped in 2001 (right). Sample footprints from our April 2001 run with arrows indicating positions for cross-marked pixel.

The data from our April 2001 run, is much more extensive. We made 43 distinct pointings of the 25 element SPIFI array (21 detectors operational) for about 900 distinct spectra in each line. These spectra have a velocity resolution of about 150 km s^-1, and cover 1680 km s^-1, so that both the [CI] and CO(7®6) lines are detected in one scan. In Figure 1 (right), we present two sample footprints from this data set. Quite apparent are changes in the CO(7®6) to [CI] line intensity ratio. Since the critical density for the CO line is about 300 times larger than that of the [CI] line, changes in this ratio largely reflect density enhancements (clumps or shocks) within the ring. Notice that the CO line is relatively much stronger than the [CI] line at the inner edge of the CND reflecting the intense UV fields and shocks caused by cloud-cloud collisions there. However, the CO line intensity falls off quite rapidly with radial distance from the far-IR ring. In contrast, the [CI] line is relatively weak at the far-IR ring, gets brighter just a few tenths of pc into the ring, and stays bright out to the furthest regions that we have mapped so far (r ~ 3 pc). It is clear that the [CI] emission is not dominated by the photodissciated inner edge of the CND, but is dominated by emission from the outer parsecs of the CND. This was known from previous mapping of the [CI] 610 mm line. However, our new 370 mm observations indicate that this extended [CI] emitting gas is also quite warm: T_ex ~ 100 K. The data analysis is still progressing, but we expect to use the line ratio map to pick out localized clumps and shocks, especially for regions where streamers entering the central cavity collide with the CND. The imaging array yields near perfect registration and relative calibration between pixels and spectral lines in a map, greatly facilitating the analysis.

From our first run, we observed the CO(7®6) line from M82 and NGC 253 – two nearby starburst nuclei. The NGC 253 data has appeared in C.M. Bradford’s thesis, and will soon be submitted to ApJ. Our NGC 253 data indicates the high J emission is widespread - the line is clearly detected over most of the array footprint. Comparing our CO(7®6) map with low J maps from the literature characterizes the physical conditions of the molecular ISM. The bright CO(7®6) line indicates the emitting gas is remarkably warm (T_gas ~ 180 K), and dense, n ~ 4 ´ 10⁴ cm^-3 - highly excited by the strong UV fields of the starburst. This suggest the starbursts is self-limiting, in that the effect of the large UV fields is to heat and fragment the ambient molecular ISM thereby inhibiting further star formation.

In April 2001, we obtained spectra including both the CO(7®6) and [CI] lines on NGC 253, M82, NGC 6946, M51 and NGC 4038/4039. Both lines were detected from the nuclei of NGC 253, M82 (Figure2) and M51, while only the [CI] line was detected from NGC 6946 (Figure 3). The ratio of the lines is indicative of the excitation of the ambient interstellar medium. The [CI] line is relatively easy to excite, so that it is strong in quiescent spiral galaxies such as NGC 6946 (and the Milky Way). However, the CO(7®6) line has much higher excitation requirements and is only strong on large scales in the nuclei of starburst galaxies. The line ratio therefore traces the excitation of the interstellar medium. For starburst galaxies, the C^o/ CO abundance ratio is often greatly enhanced. The [CI] 370 mm observations can ascertain the origins of the [CI] emission: strong 370 mm line emission signals PDR origins, since the gas in PDRs is warm. Cosmic ray enhancement of abundances would not lead to enhanced 370 mm line emission since cosmic rays are much less effective at heating the gas. The brightness of the [CI] lines that we observe in M82, NGC 253, and M51 favor a PDR origin.

Further information on SPIFI can be found in Bradford et al. 2002 (to appear in Applied Optics) and at our web site: http://www.astro.cornell.edu/SPIFI/new/spifi.html.

Many people have contributed to the success of SPIFI. We are extremely grateful to the management and staff of the JCMT for giving us the opportunity to bring a complicated new instrument onto the superb JCMT telescope, and for their help with logistics, emergency repairs to components of the instrument and mount, software and interfacing, and shipping and packing. We thank the Director, Ian Robson for his continued support and enthusiasm for the SPIFI project. Perhaps most important is the wonderful support we received from Wayne Holland, and Richard Prestage at the telescope during the first nights of observing in April 1999.

People involved with the instrument or observing runs include Gordon Stacey, Matt Bradford, Thomas Nikola, Laurie Hall , Jim Jackson, Alberto Bolatto, Frank Israel, Kate Isaak, Peter Ade, Jackie Davidson, Maureen Savage, Sarah Unger, and Christine Wilson. We are grateful for the GSFC arrays out of S. Harvey Moseley's group.

This work was supported by NASA grants NAGW-4503 and NAGW-3925, and NSF Grants OPP-8920223, and OPP-0085812.