Astron. Astrophys. 358, 88-94 (2000)

1. Introduction

The study of the galaxy population at high redshifts has progressed rapidly during the last decade. Through the Lyman-break technique hundreds of normal (i.e. not dominated by active galactic nuclei), star forming galaxies at z=2-4 have been detected and studied with imaging as well as spectroscopy (Steidel et al. 1996). These so called Lyman-Break Galaxies (LBGs) have star formation rates (SFRs) in the range 4-55h^-2 yr^-1 for =1.0 or 20-270 yr^-1 for =0.2 (Pettini et al. 1998). Also, via the study of the class of high column density QSO absorption lines systems known as Damped Ly Absorbers (DLAs) a wealth of information on the early chemical evolution of galaxies at z=2-4 has been obtained (e.g. Lu et al. 1996). The DLAs are in general forming stars at a significantly lower rate than the LBGs (Moller & Warren 1998, Fynbo et al. 1999).

Independent information about the formation of the brightest galaxies comes from detailed studies of the stellar populations of present day bright cluster ellipticals. These populations seems to have formed early (z2) in strong burst of star formation (Bower et al. 1992). Studies of the fundamental plane for elliptical and lenticular galaxies in rich clusters at intermediate redshifts also indicate early formation times (z5 for =1, Jorgensen et al. 1999), and the fundamental plane for field ellipticals at similar redshifts is consistent with being the same as in clusters (Treu et al. 1999a). Studies of the globular cluster populations of faint elliptical galaxies also indicate rather early formation times (z1), whereas for bright cluster ellipticals the globular cluster populations do not strongly constrain the possible formation scenarios (Kissler-Patig et al. 1998). Furthermore, the presence of seemingly old stellar populations in elliptical galaxies at z1 proves that at least some elliptical galaxies formed very early in strong bursts of star formation (Spinrad et al. 1997, Treu et al. 1999b, see also Jimenez et al. 1999). For first-rank ellipticals star formation rates as high as SFR10³ yr^-1 would then be possible. A reason why such high star formation rates have not been detected in galaxies at high redshift may be that these galaxies are the hosts of powerful QSOs and hence are hidden by the light from the QSOs (e.g. Terlevich & Boyle 1993). Support for a connection between QSOs and bright elliptical galaxies comes from the fact that radio quiet QSOs as well as radio loud QSOs and radio galaxies at z=0.1-0.3 are hosted by galaxies for which the light profiles are best fit by de Vaucouleurs profiles indicating that they are early stages of massive ellipticals (McLure et al. 1999). There is increasing evidence that QSOs at redshifts z2 are embedded in extended emission that is consistent with the presence of a stellar population in the QSO host galaxies. In the case of radio loud QSOs host galaxies have been detected in the optical and infrared by Lehnert et al. (1992) and Carballo et al. (1998), and in the case of radio quiet QSOs host galaxies have been detected in the optical and near infrared by Aretxaga et al. (1998a,b). There does not seem to be any systematic differences between the host galaxies of radio loud and radio quiet QSOs. Both populations of host galaxies are extremely bright, R21-22, and have optical-to-infrared colours in the range R-K3-5. However, measured polarisation of the light from some radio galaxies show that scattered QSO light can also contribute significantly to the observed extended emission (e.g. Cimatti et al. 1998).

In 1996 we performed a narrow band study of the Damped Ly Absorber (DLA, Wolfe et al. 1986) towards Q0151+048A using the 2.56-m Nordic Optical Telescope (NOT) (Fynbo et al. 1999). The main result of this study was the detection of extended Ly emission from the DLA. The Ly emission line had prior to this been detected in a spectroscopic study of Q0151+048A (Moller et al. 1998), but the large extended nature of the DLA absorber was quite unexpected. U band data, also from the 1996 run, hinted at the existence of an extended broad band object, but the signal-to-noise ratio of the object was low. We have therefore obtained deeper imaging of Q0151+048 in broad band U, B and I filters in order to confirm or reject our tentative detection, and to measure the extend and luminosity of the broad band source if real.

In Sect. 2 below we describe our new observations. In Sect. 3 we describe in detail the two independent methods we have used to search for extended objects close to the quasar. First we describe the image-deconvolution, where we used the Magain et al. (1998, hereafter MCS) algorithm, secondly we describe the direct PSF subtraction, and Sect. 4 we discuss our results.

In this paper we adopt H=100 h km s^-1 Mpc^-1, =1.0 and =0 unless otherwise stated.

© European Southern Observatory (ESO) 2000

Online publication: June 26, 2000
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