Astron. Astrophys. 359, 1139-1146 (2000)

3. Near-infrared observations and data reduction

Near-infrared observations of the region of N 66 were secured with NICMOS cameras at Cerro Tololo and Las Campanas observatories. The Cerro Tololo observations were obtained at the 1.5m telescope with the 256256 NICMOSIII infrared camera CIRIM during December 1997. These images were made through narrow filters centered on the v=(1-0) S(1) line of H₂ at 2.12 µm and through a similar continuum filter centered at 2.14 µm. This combination provides a continuum subtracted H₂ image with limited stellar contamination. Individual images have a field size of 5´5´ and a scale of 1.16 arcsec/pixel. The total integration time was 2000 seconds on-source for each of the 2.12 µm and the 2.14 µm data sets.

In each filter, individual images were sky subtracted and flatfielded, then combined by median averaging, using IRAF routines. To produce an image with only the molecular gas emission, the 2.14 µm continuum image was scaled to the 2.12 µm image by equalizing the average flux of bright stars away from the field center. Differences in the stellar psf 's between the two images, due to seeing, tracking, and slight focus errors were partially adjusted by an 0.5 pixel gaussian smoothing of the 2.12 µm image. Small focus changes with CIRIM produce aberrated images which have no readily modelled dependence on field position. The resulting 2.12 µm continuum-subtracted image is shown in Fig. 6. Bright stars exhibit residual improperly cancelled flux at the 2 percent level due to slight psf mismatches.

Fig. 6. Continuum-subtracted image of N 66 in the v=(1-0) S(1) line of H2 at 2.12 µm, obtained at Cerro Tololo. The pixel size is 1.16 arc second.

Broad band J, H and Ks images of the molecular cloud to the NE of the N 66 HII region were made at Cerro Tololo using the 1.5m telescope with CIRIM in two observing runs (1995 and 1996). The observations covered a 2.5´2.5´ area with a resolution of 0.64 arcsec/pixel. We have also obtained in November 1997 at the Las Campanas Observatory J, H and Ks images of the mid-IR peaks of emission described in Paper I. These observations were performed with the 256256 NICMOSIII IR camera IRCAM attached to the 2.5m Dupont telescope. They cover a 1.5´1.5´ field with a resolution of 0.35 arcsec/pixel. These images will be discussed in a future paper. Here, we have only looked at them qualitatively in order to find embedded stars.

Fig. 6 and Fig. 7 show the H₂ and 2.14 µm continuum images respectively. The continuum image is dominated by the emission of the exciting stars, with some contribution of field stars. A number of embedded stars which are not or only barely visible optically can also be seen on this image. They have been confirmed by examination of the J, H and Ks images. They are located in Peaks C, D, E, I, and in the compact mid-IR source at the northern tip of the spur as shown in Fig. 7. Also, fainter IR sources are found towards peaks B, H and F.

Fig. 7. Continuum image of N 66 in the continuum at 2.14 µm obtained at Cerro Tololo. The pixel size is 1.16 arc second. The embedded stars or star clusters are designated by numbers, together with the designation of the peaks to which they belong (see Fig. 8 for identification of the peaks).

The H₂ image shows a filamentary structure. The filaments to the S-W form an arc which coincides perfectly with the absorption lanes seen on Fig. 1. They also coincide with the emission of the AIBs between 5.0 and 8.0 µm mapped with ISOCAM in the LW2 filter, as shown by Fig. 8. This is characteristic of a photodissociation region (PDR) that is obviously seen edge-on in the present case. Fig. 8 also demonstrates the excellent general correlation between the v=(1-0) S(1) line of H₂ and the AIBs. Fig. 10 displays the relation between the H₂ line and the CO line, which is also very well marked.

Fig. 8. Contours of the LW2 (5.0-8.0 µm) image of the region of N 66 (resolution 6") superimposed over the continuum-subtracted image in the v=(1-0) S(1) line of H2 at 2.12 µm smoothed at the same resolution. Contour levels are 0.024, 0.042, 0.059, 0.087, 0.14, 0.17, 0.21, 0.28, 0.35, 0.42 and 0.52 in units of mJy per square arc second. The emission peaks discussed in Paper I are identified by letters.

© European Southern Observatory (ESO) 2000

Online publication: July 13, 2000
helpdesk.link@springer.de