have lost many of its stars in encounters in
the dense environment around the massive
elliptical galaxy M60.
The research team suggests the UCD black
holes are indeed common, doubling the
number of known supermassive black holes
in galaxy clusters and therefore greatly increasing the number density of black holes
overall in the local universe.
Complete results are published in the journal Nature (view here), and more highlights
and images are available on the Gemini web
page (view here).
A Tidal Disruption Event Due to a
Low-mass Black Hole
Archival data from the Chandra X-ray Observatory show an X-ray flare near the galaxy
cluster Abell 1795, first detected in observations from 1999. A number of different
processes could plausibly explain such Xray variability, including a flare of an active
galactic nucleus in the field or the tidal disruption of a star in a nearby galaxy. Peter
Maksym (University of Alabama) and collaborators used the Gemini Multi-Object
Spectrograph on Gemini North (Figure 6) to
obtain a deep observation of the field and
identify the flare’s host as an inactive dwarf
galaxy that is a member of the Abell cluster.
They conclude that a tidal disruption event
triggered the flare, occurring as a star approached too close to the black hole at the
center of the dwarf galaxy to survive.
The Gemini observations show that the host
is located at a redshift of z = 0.065, confirming it as a member of Abell 1795. Determining the distance also confirms the stellar
luminosity and therefore low stellar mass,
around 3 x 108 solar masses. Applying standard relationships between bulge luminosity and central black hole mass sets an upper limit for the black hole, MBH < 7 x 105 MSun.
Associating an earlier bright flare with the
same host galaxy sets a lower limit, MBH > 2
x 105 MSun, assuming that this event did not
exceed the Eddington luminosity. Thus, the
central black hole is relatively low mass, and
analysis of this source type can help bridge
the gap between the more well-studied stellar-mass and supermassive varieties.
Full results appear in Monthly Notices of the
Royal Astronomical Society, (viewable here).
Figure 6.
The Gemini spectrum
of the tidal flare source
(black), with bestfitting model (red)
and residuals (blue),
demonstrates that
the host is a low-mass
quiescent member of
Abell 1795. Spectra
of nearby early-type
galaxies are also
plotted (orange and
purple at the top).
January 2015
2014 Year in Review
GeminiFocus
27