Astron. Astrophys. 327, 299-308 (1997)

2. Observations and results

2.1. Near-infrared images

The broad and narrow-band near-IR images were made on the night of 16 June 1994 with the Las Campanas Observatory Near-Infrared Camera (Persson et al. 1992) on the 2.5m DuPont telescope under sub-arcsec seeing conditions. The camera has a NICMOS3 256 256 HgCdTe detector and the scale was 0.35 /pixel.

The narrow-band filters are centered at (), which includes the H₂ v=1-0 S(1) line, and at () for measuring the neighboring continuum.

For each filter, five images were taken in the immediate vicinity of the region, some of them also including the program object in different positions to increase the on-source integration time and at the same time, increasing the covered area. The individual frames were reduced by sky subtracting and flat fielding using the median of all images. Flux calibration was done by observing standards from the lists of Elias et al. (1982), of UKIRT (Users Manual), and of Las Campanas Observatory. Stellar photometry was performed with DAOPHOT (Stetson 1987) within IRAF, with an aperture of 3 . Astrometry of the sources was obtained using the digitized Palomar Sky Survey. The limiting magnitudes are 19.6, 18.7 and 17.0 at J, H and K, respectively

J(1.25 ), H(1.65 ), K (2.2 ) and H₂ (2.125 ) mosaics of dimensions 104 104 square arcsec are shown in Fig.1

Fig. 1. Mosaics (each of five frames) covering 104 × 104 square arcsec in the J, H, K and H2 of G 35.20-1.74. North is at top, east to the left.

The images, especially in K, are characterized by the presence of an extended IR nebula (size 40 ) at the west border of which is located a bright source identified with the UC HII region. A smaller IR diffuse nebulosity is observed in the SW part of the image. An infrared source with a localized nebulosity is seen at the position of HC1 and is detected only at K. A weaker near IR source maybe associated with HC2. Finally, the comparison between the H₂ image and the adjacent continuum does not show any molecular hydrogen emission above the noise level of 10^-15 ergs cm^-2 s^-1. Consequently, no image of the H₂ minus continuum is shown.

2.2. Mid-infrared image

The narrow-band image at 11.2 () was obtained during an observing run on July 1996 at the Canadian-France-Hawaii-Telescope (CHFT) with the mid-IR camera CAMIRAS, and is shown in Fig.2. CAMIRAS was developed at the Service d'Astrophysique at Saclay (Lagage et al. 1992) and operates with a 192 128 Si:Ga/DVR detector array, manufactured by the Laboratoire Infrarouge (LIR) at Grenoble. We used a scale of 0.31 /pix, and the measured point-spread function was of 0.9-1.0 . The sky and telescope emissions were subtracted using chopping and nodding technique. The absolute calibration of the image was obtained observing the standard stars Aql and Sgr at approximately the same air mass. The astrometry of the MIR image was made comparing it with the K image.

Fig. 2. 11.2 image of G 35.20-1.74. The image has dimension 59.5 39.7 square arcsec. The positions of the 6 MIR sources are indicated.

We detected six MIR sources and a diffuse nebulosity east of the UC HII region, with embedded at least three of the MIR sources. The bright source MIR3 has an extension of 8 and coincides with the brightest source seen in K and with the UC HII region. The positions and the flux densities of the sources are given in Table I. The statistical photometric errors are also reported in Table I for the 11.2 flux densities.

We note that IRAS18592+0108, which is usually associated with the UC HII region, is located more than 10 to the south of it. The IRAS 12 flux (114 Jy) is about a factor 4 greater than that of the 11.2 UC HII region (and also of the entire flux density present in our 11.2 map). Consequently, the IRAS fluxes (together with those at longer wavelengths) are representative of the entire SFR and should not be directly associated with the UC HII region. The difference in fluxes suggests that the contribution to the FIR emission from an extended dust cloud is much larger than that of the UC HII region.

© European Southern Observatory (ESO) 1997

Online publication: April 8, 1998
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