Quote:
Originally Posted by ngc3314
OK, here's one that went the other way. Show of hands, everyone who's heard of NGC 4569. Messier heard of it and labelled it 90. It's what van den Bergh has called an anemic spiral, right in the core of the Virgo cluster, with truncated H I distribution and smooth outer arms.
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I've heard of it. It has a very high HI deficiency which makes it unreliable to use for the Tully-Fisher relation when hydrogen linewidths are used to measure the rotational velocity. The HI linewidth for NGC 4569 underestimates the rotational velocity and so using the TFR you get a distance of ~10 Mpc. However, the optical rotation curve data gives a larger rotational velocity and therefore a larger TFR distance - more in line with the Cepheid distances to Virgo spirals.
A few years ago I was comparing the HI deficiency measures for Virgo cluster galaxies from a paper by Solanes (I think it was) against the difference between the HI rotational velocity and the optical rotation curve velocity. It was pretty clear that in general the more HI deficient the Virgo spiral was, the more difference there was between the HI and optical rotation velocities such that the HI rotation velocities are smaller.
It is certainly something to bear in mind as a possible source of error in TFR distances based upon HI linewidths.