Morphology

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Morphology

Published optical imagery and spectroscopy have detected two ionized gas components around the quasar MR 2251-178: an elongated, highly-ionized circumnuclear component of diameter $\sim27$ kpc, and an extended envelope of faint H $\alpha$ - and [OIII]-emitting filaments out to a radius of $\sim 110$ kpc ([Bergeron et al. 1983]; [Hansen et al. 1984]; [Alighieri et al. 1984]; [Macchetto et al. 1990]). Our observations confirm these findings and reveal a number of new features.

The circumnuclear component of ionized gas is evidenced by strong H $\alpha$ emission directly surrounding the quasar, extending out to a radius of $\sim 20$ kpc. This emission is slightly elongated in the east-west direction, i.e., along the axis of the quasar's radio jet ( $PA \sim 102\arcdeg$ ; [Macchetto et al. 1990]), and has been identified as an ``extended emission-line region'' (EELR) of the quasar host galaxy ([Mulder & Valentijn 1992]). As others have reported on characteristics of the EELR in some detail, we will say nothing more of it herein.

The extended ionized gas component is observed as a much larger network of diffuse and filamentary emission exterior to the EELR. Above a flux level of $1.8 \times 10^{-17}$ erg s^-1 cm^-2 arcsec^-2, diffuse H $\alpha$ emission is visible surrounding the galaxy to a radius of at least 50 kpc. Knots and filaments of ionized gas are visible to radii up to 120 kpc towards the northeast, southeast, and southwest. The filaments display a spiral morphology and exhibit a remarkable azimuthal symmetry that has not been previously seen. The total extent of this complex ( $\sim200$ kpc) makes it the largest known around a quasar. We derive a total flux from the extended ionized gas component of $1.4 \times 10^{-13}$ erg s^-1 cm^-2, or a luminosity of $2.5 \times 10^{42}$ erg s^-1 at the redshift distance of the quasar (384 Mpc). This value, as well as the corresponding upper limit on the gas mass of $6\times10^{10}$ M $_{\sun}$ , is comparable or slightly greater than previously published results (e.g., [Nørgaard-Nielsen et al. 1986]; [Macchetto et al. 1990]).

A few of the individual knots and filaments have been previously noted; these are indicated in Figure 1d, using the denotations from Macchetto et al. (1990). Several additional knots also noted by these authors and others (e.g., [Alighieri et al. 1984]) are conspicuous in their absence from our imagery. Most notable among these are the ``E'' knots interior to the eastern filament in Figure 1. As will be noted in the following section, this absence is probably due to the large range of velocity spanned by the filament emission, relative to our narrow bandpasses.

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Patrick Shopbell
1999-08-26