2 Observations and data reduction

Most of the observations presented here were taken at the German-Spanish Astronomical Centre on Calar Alto, Spain. A summary of relevant data for all observations is given in Table 1.

 

 

Table 1: Log of observations.

filter/	$t_{\rm exp}$	instrument	epoch
grism	[s]
J	360	ALFA/Omega-Cass	Sep.98
H	180	ALFA/Omega-Cass	Sep.98
K'	90	ALFA/Omega-Cass	Sep.98
-	700	CAFOS	Jul.98
B	4560	CAFOS	Jul.98, 00
R	3700	CAFOS	Jul.99, 00
I	820	CAFOS	Jul.00
B400	1200 (G1)	CAFOS	Jul.99
B400	3000 (G2)	CAFOS	Jul.99
R	1200	TLS Schmidt	Jan.99
I	960	TLS Schmidt	Dec.99

Deep direct optical imaging was performed with the focal reducer camera CAFOS at the 2.2 m telescope in the B, R, and I band and without filters. CAFOS was equipped with a SITe CCD with a scale of $0\hbox{$.\!\!^{\prime\prime}$ }5$/pixel. The conditions were always photometric with a seeing typically of about 1''. Unfortunately, IRAS03158+4227 is located only about 2' away from the bright foreground star HD20489 (V=8.6) and 7 $.\!\!^{\prime\prime}$4 from the USNO-2 star No.275-02191303 (B=16.2, R=14.9). Therefore, we took several sets of relatively short exposures (typically between 100 and 300 s) to avoid saturation effects of the CCD. The total integration time amounts to 2.8 hours. MIDAS standard algorithms were applied for the data reduction. The combined image was PSF-deconvolved using the Lucy-Richardson method (MIDAS procedure deconvolve/flucy).

High-resolution imaging in the J, H, and K' bands was performed using the adaptive optics system ALFA in combination with the Omega-Cass camera (Hippler et al. 1998) at the 3.5 m Calar Alto telescope. Omega-Cass utilizes a 1024$\times$1024 HAWAII detector and was operated at the pixel scale of 0 $.\!\!^{\prime\prime}$08. Since there is no sufficiently bright star close to IRAS 03158+4227 which could be used for wavefront sensing and with the laser guide star being not operational at the time of the observations, we corrected the static aberrations of the telescope on a nearby star and imaged the target with the deformable mirror "frozen''. Although this procedure does not yield diffraction-limited resolution, it improved the image quality considerably, leading to sub-arcsecond resolution. Two adjacent fields were observed for deriving the sky frames. During the data processing, the images were rebinned in order to enhance the signal-to-noise ratio which led to a final pixel scale (from the astrometric solution) of 0 $.\!\!^{\prime\prime}$155. After correction for flat field and bad pixels, the images were filtered using the wavelet algorithm of Pantin & Starck (1996) to minimize noise amplification in the subsequent Richardson-Lucy deconvolution. The USNO-2 star No.275-02191303 served as PSF reference for the deconvolution. JHK' photometry was derived from the non-deconvolved images and tied to the JHK' magnitudes of the PSF star according to its entry in the 2MASS Second Incremental Release Point Source Catalog (Cutrie et al. 2000). The derived fluxes (Fig. 2) refer to a synthetic aperture of 7 $.\!\!^{\prime\prime}$75 diameter. The photometric error amounts to 0.06 mag. In order to assess the separation of a possible double nucleus, the FWHM of the images of the ULIRG in the three filters were compared to those of stars in the field. The average stellar FWHMs derived from Gaussian fits amount to 0 $.\!\!^{\prime\prime}$60, 0 $.\!\!^{\prime\prime}$57, and 0 $.\!\!^{\prime\prime}$58 for J, H, and K', respectively. The FWHMs of the ULIRG are 0 $.\!\!^{\prime\prime}$74, 0 $.\!\!^{\prime\prime}$70, and 0 $.\!\!^{\prime\prime}$71. This leads to beam-deconvolved sizes of 0 $.\!\!^{\prime\prime}$44, 0 $.\!\!^{\prime\prime}$40, and 0 $.\!\!^{\prime\prime}$40 for the angular extent of the emitting core region of the ULIRG.

Low-resolution spectra of both IRAS03158+4227 and its nearest neighbour galaxy were taken with CAFOS equipped with the grism B400 which is suitable for the wavelength range $\lambda = 3200\,...\,8000$ Å. With a slit width of $1\hbox{$.\!\!^{\prime\prime}$ }2$ the spectral resolution is about 20 Å.

Finally, we observed the field of IRAS03158+4227 with the Tautenburg Schmidt telescope at moderate seeing of about 2''. The Schmidt camera was equipped with a 2 k $\times$ 2 k SITe CCD with pixel size of 24 $\mu$m $\times$ 24 $\mu$m which yields a field size of $40' \times 40'$. These images were used only to evaluate the large-scale environment of IRAS03158+4227.