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[astro-ph/0601580] A Successful Targeted Search for Hypervel

Posted: January 28 2006
by Neal Dalal
This paper discusses new hyper-velocity stars recently detected with radial velocities of order 600 km/s. These are quite interesting, since it's believed that they were ejected by the supermassive black hole at the Galactic center. One use of such stars, discussed in astro-ph/0506739 by Gnedin et al., is to measure the triaxiality of our halo : if we know they came from the Galactic center, any deviation from purely radial motion comes from non-sphericity of the Galactic potential.

The thing which confuses me is the claim that these must be ejected by a supermassive BH (e.g. with M > 10^6 M_\odot). The idea is that the ejected stars originated in tight binaries which were tidally disrupted by the SMBH. If I'm reproducing the argument correctly, if the original binary had mass m and binary velocity v, then the ejected star can have a velocity at infinity of v_\infty = v sqrt(2) * (M_BH/m)^(1/6). At most, v can be the escape velocity at the star's surface, e.g. 300 km/s. But if it is this high, then I don't need BH masses of 10^6 to reach the observed velocities of 550-700 km/s -- even intermediate mass BH's with 100-1000 M_\odot are sufficient to do the job. So why is there such certainty that they came from the Galactic center? Am I missing something?

We talked about this at our coffee at CITA, without a successful resolution, so I was wondering if other people knew the answer. Another reason this could be important is that a similar hypervelocity star was seen ejected from the LMC. If a SMBH is required there too, this would be the first evidence for a SMBH in the LMC, and I think it would violate the M-\sigma relationship.