CO Observations and a New Interpretation of the Anomalous Arms of NGC 4258

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Abstract
We present CO (2-1) maps of the molecular gas associated with the inner parts of the anomalous arms of NGC 4258. The CO emission is found to be well correlated with the arms, and it extends out to ~2 kpc from the center. Together with CO (1-0) and (3-2) measurements and the detection of 13CO (1-0) and CS (3-2) toward the center of NGC 4528, we derive the physical conditions of the molecular gas along these arms and analyze in detail the dynamics of the gas. The molecular gas is relatively dense (103 cm–3 on average) and warm (50-100 K), with a total H2 mass of 109 M☉. The CO distribution shows a characteristic S-shaped morphology, along which the molecular gas is found to move toward the nucleus, not away from it. Contrary to previous interpretations, we suggest that the anomalous arms in NGC 4258 trace the gas flow due to a bar rather than being the manifestation of a jet. Both the S-shaped morphology and the velocity dispersion revealed by the present CO measurements are characteristic of barred galaxies. In the bar of NGC 4258, most of the gas is molecular, not atomic. The molecular gas is bounded by the sharp leading edge traced in the radio continuum that is probably the bar shock with its compressed magnetic field. The shocked Hα emission along the anomalous arms is probably a secondary manifestation of the bar shock. In the X-ray-emitting gas associated with the anomalous arms, the temperature is consistent with velocities and densities in bar-shock fronts rather than in jets of radio galaxies. We suggest that the X-rays, like the radio continuum and optical line emission, are produced via the bar shock and are unrelated to a jet from the black hole. The large extension of both the Hα and radio emission out to a projected distance of ~7 kpc probably trace the hot (106 K) thermal X-ray gas produced by the bar shocks, which leaks out of the bar structure and eventually escapes from the disk in the z-direction. We predict that, with sufficient sensitivity and angular resolution, similar radio and X-ray emission will also be found in the shock fronts of other, more distant, barred galaxies.