GeminiFocus October 2014 | Page 11

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 9