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Astron. Astrophys. 345, 36-42 (1999)

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5. The tidal tail in the interacting galaxies NGC 4485/4490

The most severe discrepancy between [FORMULA] and [FORMULA] occurs for systems with small accelerations. So that an extremely low [FORMULA] is expected for gas-rich tidal tails around small galaxies.

A tidal tail of this type has been reported in the closely interacting galaxies NGC 4485 and NGC 4490 which are separated a distance of 7.7 kpc at an assumed distance of 7.8 Mpc. It turns out an interesting case to compare the predictions of MOND with observations.

Following the observations reported by Elmegreen et al. (1998), the tail has an area of approximately [FORMULA] ([FORMULA] pc2), and a total mass of [FORMULA] M[FORMULA]. Correspondingly, the mass column density of the tail is [FORMULA] g cm-2.

The epicyclic frequency, [FORMULA], suffers from large uncertainties in the vicinity of the tail due to a possible warp in the disc, which lead to an uncertain rotation curve. From the observed rotation curve of NGC 4490 (Fig. 6 of Elmegreen et al.), [FORMULA] km s-1kpc-1 and [FORMULA] at the galactocentric radius of 7.5 kpc. But these values could be somewhat different ([FORMULA] km s-1kpc-1, [FORMULA]) if we extrapolate the rotation curve as a flat curve. Assuming that the velocity dispersion of the gas in the tail is 10 km/s as observed in the disc of NGC 4490, the range of values for [FORMULA] and [FORMULA] yield:

[EQUATION]

This high level of instability predicted by MOND seems to be in disagreement with the observations which show that the ages of the youngest regions in the tail are similar to the time since perigalacticon compression by the companion approach, approximately [FORMULA] yr, suggesting a remarkable stability of the tail.

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© European Southern Observatory (ESO) 1999

Online publication: April 12, 1999
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