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RADIO galaxies are divided into two broad classes1 according to whether they show extended radio lobes ending in hotspots (compact bright sources of radio emission) at some distance from the nucleus (Fanaroff–Riley class II) or instead have bright regions or knots close to the nucleus (FR I). Hotspots and knots emit synchrotron radiation, sometimes even in the optical range2,3. The galaxy M87 (NGC4486) in Virgo is the closest giant elliptical galaxy showing a jet and double-lobe radio structure, and is classified as FR I mainly because of the morphology of the jet-dominated west radio lobe, even though the east radio lobe resembles typical FR II objects and shows a hotspot4. We report here the detection of the optical counterpart of this hotspot. Because the lifetime of optically emitting synchrotron electrons is short, we suggest that this hotspot is fed by an unseen counterjet. This conclusion adds weight to the idea that apparently one-sided jet sources are intrinsically two-sided, with the visibility of the receding jet reduced by beaming effects.

THE jet in the nearby elliptical galaxy NGC4486 (M87) is a prototype of this kind of nuclear activity1. High-resolution observations over a wide wavelength range have been used to investigate the propagation of energy along the jet and the radiation mechanism, but these studies have been hampered by the much lower resolution available at optical compared with radio wavelengths. Here we present high-resolution multi-colour images of the jet, obtained using image reconstruction techniques. Morphological features in the jet known from radio observations are seen in our optical images. In particular, we find that the radio-bright 'knots' are spatially extended, and that there is significant emission in the regions between the knots. There is also evidence for limb-brightening at a number of positions along the jet. Taken together, our observations suggest that the jet propagates essentially free of losses, generating radio and optical emission at its edges, where interstellar material may be encountered.

Decoupling of angular momenta of gas and stars in elliptical galaxies constitutes evidence for the acquisition of external material by these galaxies later in their history. In minor-axis dust-lane ellipticals the angular momenta of stellar and gaseous components are orthogonal. Here we present new observations on four major-axis dust-lane ellipticals, Anon 1029-459, NGC3528, NGC4370 and NGC5745. In the first two of these we observe counter-rotation of gas and stars, whereas the two components are co-rotating in NGC4370 and NGC5745; these results are consistent with the acquisition hypothesis. The same accretion events occur also in S0 galaxies, where they give rise to polar rings or to counter- and co-rotating gas in the equatorial plane. As dynamic decoupling in these latter galaxies appears common, we suggest that in all S0s with extended emission lines the gas is of external origin.