AGES was such a fun survey we decided we didn't want to stop, so in effect we've kept going. The new survey, WAVES (Wide-field Arecibo Virgo Extragalactic Survey) will only cover the Virgo cluster, but with the ultimate goal of giving full cluster coverage to the AGES sensitivity level or nearly so. That will, amongst many other things, let us work out how common all those optically dark clouds really are, where they're found etc., and so give much better constraints on what the the little blighters actually are.
But there's much more to Virgo than poxy little clouds. For one thing, there's the massive Kent complex, the main target of the first WAVES field. This was discovered using the shallower ALFALFA survey and had some follow-up with the VLA. As we all know, interferometers just aren't as sensitive as single-dish telescopes. So since this is a big, extended feature, surveying it with Arecibo but to a higher sensitivity level (a factor of four) seemed like a good idea. And it was.
The Kent complex has a unique set of properties. It's 150 kpc in projected length, though there are other clouds known which are longer. It's got a gas mass comparable to a large galaxy, but other clouds of similar masses are also known. It's got complex kinematics, but so do others. It doesn't have an obvious parent galaxy, but neither do other clouds. But none of the other features have all of these properties - in Virgo at least, this is unique.
Our observations show that it's probably weirder than Kent gave it credit for. They show that the mass is more than a billion times the Sun, more than doubling the value from Kent. In our data it looks for all the world like a collection of galaxies, except that there's nothing visible at optical wavelengths at all. Importantly, Kent's VLA data rules out the dark galaxy hypothesis because there's no sign of ordered motions either of the complex as a whole or within individual clumps. We also found that much of the gas is in diffuse, extended emission, as well as in the discrete clouds that Kent found. We show that these clouds are indeed directly connected to each other, embedded in a sort of common envelope. And the whole thing looks a lot like a rhino.
How does this change how we interpret the cloud ? Kent leaned towards either NGC 4445 or NGC 4424, both reasonably nearby spiral galaxies, as the possible parents. Either ram pressure or tidal harassment (or both) could potentially have removed the gas. But honestly, neither of these candidates are great. They don't line up with the major axis of the cloud at all - in fact, they're basically orthogonal to it. That doesn't match any other similar features, and the fact that they're completely detached is also weird : ram pressure doesn't seem to do that, and removing this much has through tides is extremly difficult. We considered a scenario where one of the galaxy lost all its gas very rapidly, which then formed an expanding cloud, but while that might work (actually we tried quite a few models which didn't make it into the paper - we might do a separate paper in future), it's hard to physically justify. And NGC 4424 has a tail which points in exactly the wrong direction, so it's almost certainly not that one.
There's another galaxy in this region which is one of the iconic images for ram pressure stripping : NGC 4522. This is already known from VLA data to have a short tail pointing towards the Kent complex. We found it has a second tail, of similar alignment, several times longer, but at a different velocity. The direction alignment towards the complex is awfully tempting... but the velocity of the galaxy is 1,800 km/s different to the cloud. Even in a cluster, where galaxies swarm like angry bees, that velocity difference is just too high for the two features to be associated.
The final weird thing is just how massive the Kent complex is. In almost all galaxies with hydrogen streams, the mass in the tail is only a few percent or so of the mass in the disc. For reasons we don't fully understand, the gas appears to rapidly evaporate and/or disperse once removed from its host, likely because of the very hot intracluster medium. The problem is that for any of the plausible donor galaxies, hardly any gas at all would have evaporated. And there's no obvious reason at all why the Kent complex should retain almost all of its gas as detectable atomic hydrogen whereas in all other cases the majority of it becomes undetectable.
So what the hell is it ? We don't know. NGC 4445 remains the least worst candidate parent galaxy, but we mean that literally : it's only the best of a bad bunch, and not at all convincing. As for how the cloud formed, we just don't know.
The Widefield Arecibo Virgo Extragalactic Survey I: New structures in the Kent cloud complex and an extended tail on NGC 4522
We are carrying out a sensitive blind survey for neutral hydrogen (HI) in the Virgo cluster and report here on the first 5° x 1° area covered, which includes two optically-dark gas features: a five-cloud HI complex (Kent et al. 2007, 2009) and the stripped tail of NGC 4522 (Kenney et al.
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