they come from galaxies that have lost their
extended components? The extremely large
black hole mass fraction and relatively normal stellar mass-to-light ratio of M60-UCD1
suggest the latter — that it is the remains of
a tidally-stripped galaxy. This galaxy would
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 X-ray
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 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
Figure 2.
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).
October 2014
GeminiFocus
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