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.
[astro-ph/0601580] A Successful Targeted Search for Hypervelocity Stars
|Authors:||Warren R. Brown, Margaret J. Geller, Scott J.Kenyon, Michael J. Kurtz (Smithsonian Astrophysical Observatory)|
|Abstract:||Hypervelocity stars (HVSs) travel with velocities so extreme that dynamical ejection from a massive black hole is their only suggested origin. Following our discovery of the first HVS, we have undertaken a dedicated survey for more HVSs in the Galactic halo and present here the resulting discovery of two new HVSs: SDSS J091301.0+305120 and SDSS J091759.5+672238, traveling with Galactic rest-frame velocities at least +558+-12 and +638+-12 km/s, respectively. Assuming the HVSs are B8 main sequence stars, they are at distances ~75 and ~55 kpc, respectively, and have travel times from the Galactic Center consistent with their lifetimes. The existence of two B8 HVSs in our 1900 deg^2 survey, combined with the Yu & Tremaine HVS rate estimates, is consistent with HVSs drawn from a standard initial mass function but inconsistent with HVS drawn from a truncated mass function like the one in the top-heavy Arches cluster. The travel times of the five currently known HVSs provide no evidence for a burst of HVSs from a major in-fall event at the Galactic Center in the last \~160 Myr.|
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