Astron. Astrophys. 336, 433-444 (1998)

2. Observations

2.1. Broad-band images

The J, H and K-band images were obtained on 1994 May 18at UKIRT under non-photometric conditions using IRCAM3, the UKIRT near-infrared imaging camera, through standard J, H and K-band filters (1.25, 1.65 and 2.2 µm respectively), on a Santa Barbara Research Corporation InSb array of 256256 pixels, with a pixel scale of on the sky, resulting in a field. The image of a foreground star at , was used to align the different frames. The flux calibration of the individual images was derived from the synthetic aperture photometry by Forbes et al. (1992), using their largest () aperture values in order to minimize the effects of positional differences. Consistency checks using their smaller aperture values revealed deviations of up to 3%, which can be regarded as the uncertainty of our flux calibration procedure. The effective resolution of the final images, measured from the foreground star referred to above, is

2.2. Molecular hydrogen vibrational emission line images

The H₂ S(1) images were obtained in May 1992 using the near-infrared Fabry-Perot imaging spectrometer FAST (Krabbe et al. 1993) at the Cassegrain focus of the 4.2 m William Herschel Telescope (WHT) at Roque de los Muchachos at La Palma, Spain. The FAST camera used a Santa Barbara Reserarch Corporation 5862 InSb array with a pixel scale of and a field of about . Dispersion was provided by a Queensgate scanning Fabry-Perot interferometer with a spectral resolution () of 950 at m (corresponding to a velocity resolution of 315 km s^-1), used in tandem with a = 45 cold circular variable filter (CVF) as order sorter. The seeing was about . Because the S(1) line has a full width at 20 intensity of , we took line images not only at the systemic velocity but also at velocity offsets of , as well as line-free continuum images at velocity offsets , i.e. at five velocity settings in total. Several sets of images were obtained with exposure times of sec each. Sky frames were obtained at a position east of the nucleus. After subtraction of the dark current, the individual frames were flatfielded and sky-subtracted. The resulting line-plus-continuum images were corrected for atmospheric transmission and instrumental response with the use of the standard stars HR 3888 and HR 4550. Finally, the mean of the continuum on either side of the line (velocity offsets ) was subtracted. The resulting line images were co-added, yielding line flux maps with a total integration time of 1650 sec at the systemic velocity and 750 sec each for the images offset in velocity by .

We also obtained images at the wavelengths of Br (m) and [] (m), centered on the systemic velocity under good conditions (seeing ). Total integration times were 2000 sec and 1500 sec respectively. Resulting r.m.s. noise figures are W m^-2 sr^-1 for Br and W m^-2 sr^-1 for [].

No [] emission was detected, but the Br image shows weak emission centred on the nucleus, just above the noise level and extended over about 3". The total measured flux of about W m^-2, is reasonably consistent with the marginal detections by HIGW: W m^-2 in a large aperture CVF spectrum and W m^-2 in a 3" UKIRT CGS4 aperture. Taken at face value, these rather uncertain data suggest the presence of weak and diffuse ionized hydrogen extended over the inner kpc, and somewhat concentrated near the nucleus.

© European Southern Observatory (ESO) 1998

Online publication: July 20, 1998
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