An optical and H I study of NGC 5850: Victim of a high-speed encounter?
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Abstract
We present optical CCD surface photometry and VLA H I observations of NGC 5850, one of the largest and brightest barred spirals of the inner ring variety in the sky. The broadband images reveal numerous morphological peculiarities, particularly in the spiral arms. Structural asymmetries are more obvious in H I, the most pronounced being a large-scale displacement of gas to the west and northwest of the nucleus. Most of the (3.3 +/- 0.1) x 10(9) M-circle dot of atomic hydrogen is concentrated in the prominent optical ring and faint spiral arms, with very low H I surface densities (Sigma(HI)) in the bulge and interarm regions. We detect approximately 2 x 10(7) M-circle dot of H I in the southwestern half of the nuclear ring. The H I surface density drops rapidly outside the arms, and we find no evidence for either large-scale tidal features or an extended gas disk above 0.05 M-circle dot pc(-2) (3 sigma). Overall, the intensity-weighted H I velocity field appears fairly regular, yet still shows clear deviations from circular rotation that we attribute to a warped oval disk and streaming motions across the arms. Radio continuum emission at 20 cm is dominated by a faint bulge component that peaks at the optical nucleus. The spiral arms are not detected. The absence of extended X-ray emission throughout the NGC 5846 group and the close similarity between NGC 5850's optical and H I morphologies argue against ram pressure stripping through a dense intergalactic medium as the cause of the galaxy's peculiar morphology. We attribute it instead to a high-speed encounter with the massive elliptical galaxy NGC 5846. We identify the peculiar spiral arms as a disrupted outer pseudoring. Star formation in NGC 5850 has not been enhanced relative to other intermediate field spirals, nor have significant gas masses been transported to the nucleus. This may be attributed to the recent (less than or similar to 200 Myr) nature of the interaction.