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C. M. Murray (Univ. of New Mexico), T. A. Oosterloo, R. Morganti (NFRA)
Early-type galaxies constitute quite a heterogeneous group of galaxies. A key element for explaining the range in their observed properties is the difference in their gas content. The renewed interest in the study of the cold ISM, and in particular of the neutral hydrogen in early-type galaxies has brought the realization that some of these galaxies have a surprisingly large amount of \ion{H}{1}. Even more interesting, this gas can be distributed in very regular structures (i.e. disks or rings).
One spectacular example is the elliptical galaxy NGC~807, classified as an E3. Deep \ion{H}{1} observations were obtained with the D-array configuration of the VLA (archive data) and new higher spatial and velocity resolution have been obtained with the Westerbork Synthesis Radio Telescope. These observations reveal that a low-surface brightness \ion{H}{1} disk with more than \( 1010M\odot \) of neutral gas extending to 60 kpc radius is present in this galaxy. The disk shows a very regular rotation. Though the surface density of NGC~807 is much lower than late-type spirals (peak surface density below 2 M\odot pc-2), there is a great similarity between the rotation curves of this galaxy and luminous late-type spirals.
The large amount of neutral gas (M\rm HI/L ~0.2) in NGC~807 suggests that it is the result of a major merger, i.e. a merger of two spiral galaxies. However the regularity of the \ion{H}{1} suggests that the disk, if originally accreted, is the result of a very old accretion (>109 years ago) and it therefore is a long-lived structure. This is similar to what has been observed in a handful of other elliptical galaxies (e.g. NGC~5266, NGC~3108). This long-lived, cool interstellar medium must play an important role in the evolution of these galaxies.
This work was done as part of the Summer Student Program of the Netherlands Foundation for Research in Astronomy.