Astrophysicists have found evidence of black holes destroying stars, a long-sought phenomenon that provides a new window into general relativity. The research also opens up a method to search for the possible existence of a large population of presently undetectable “intermediate mass” black holes which are hypothesized to be precursors to the super-massive black holes at the centers of most large galaxies. Image: iStockPhoto.com/Christian Miller |
Astrophysicists have found
evidence of black holes destroying stars, a long-sought phenomenon that
provides a new window into general relativity. The research, reported in Astrophysical Journal, also opens up a
method to search for the possible existence of a large population of presently
undetectable “intermediate mass” black holes which are hypothesized to be
precursors to the super-massive black holes at the centers of most large
galaxies.
The study was carried out
primarily by Glennys Farrar and Sjoert van Velzen at New York University’s
Center for Cosmology and Particle Physics, and also included the following
researchers: Suvi Gezari of Johns Hopkins University’s Department of Physics
and Astronomy; Linda Ostman of Spain’s Universitat Autònoma de Barcelona; Nidia
Morrell of the Las Campanas Observatory in Chile; Dennis Zaritsky of the
University of Arizona; Matthew Smith of South Africa’s University of Cape Town;
Joseph Gelfand of NYU-Abu Dhabi; and Andrew Drake of Caltech. Van Velzen is
currently a doctoral candidate at Radboud
University in the Netherlands.
Cosmologists have calculated
that, on occasion, a star’s orbit will be disturbed in such a way that it
passes very near the super-massive black hole at the center of its galaxy—but
not so close that it is captured whole. Such a star will be torn apart by the
extreme tidal forces it experiences: the force of gravity on the near side of
the star is so much stronger than that on the far side that the gravitational
force holding the star together is overwhelmed, causing the star to simply come
apart. While some of the star’s matter falls into the black hole, much of it
continues in chaotic orbits, crashing into itself and producing intense
radiation lasting days to months. These phenomena are called stellar tidal
disruption flares, or TDFs.
Although discovering evidence of
TDFs has been a high priority of astrophysicists for many years, and several
possible examples have been found using X-ray and UV satellites, discovering
TDFs in a large-scale, systematic survey using ground-based optical telescopes
as has now been achieved, is critical to controlling bias and avoiding
misidentifications.
The difficulty in detecting TDFs
is largely due to the challenge of distinguishing them from more common types
of flares such as supernovae. (For every TDF there are about 1,000 supernovae.)
In addition, some super-massive black holes have an “accretion disk”
surrounding them—gas and dust, often left from an earlier merger with another
galaxy—which is continuously feeding the hole. Such accreting black holes are
usually evident from the bright emission they produce and are known as quasars
or Active Galactic Nuclei (AGN). However, a hiccup in the accretion of an
undetected active black hole could produce a flare that might be mistakenly
identified as a TDF.
The researchers on the Astrophysical Journal study uncovered
sound evidence for the presence of two TDFs through a rigorous analysis of
archival data from the Sloan Digital Sky Survey (SDSS).
To do so, they sifted through
voluminous SDSS data, in which more than 2 million galaxies were repeatedly
observed over 10 years. By very carefully registering the images and looking at
differences between consecutive images, they obtained a sample of 342 intense
and well-measured flares.
Of these, almost all could be
classified into supernovae and AGN flares. However, two cases were left that
did not fit either classification. By relying on multi-year observations, the
researchers could see that the two flares’ host galaxies showed no other
flaring activity, as would be the case if the flares came from a hidden
variable AGN. This means the possibility these two flares were produced by undetected
AGNs is extremely small.
In addition, the researchers located
these flares at the nucleus of their galaxy with high precision, which reduces
the likelihood that they are supernovae to less than 1% since supernovae are
randomly distributed through galaxies.
Finally, the properties of these
flares are very different from flares of AGNs and supernovae—and their spectra
are unlike any supernovae observed to date. Supernovae flares are
characteristically very blue at first but become red as they cool and rapidly
decay, whereas the TDF flares are very blue throughout—slowly decaying without
changing color. This behavior is consistent with expectations for a TDF—the
debris from the star should rapidly form an accretion disk and look like a
short-lived AGN.
Sjoert van Velzen, the study’s
lead author, was a Dutch first-year graduate student who came to NYU to work
under the direction of Glennys Farrar, a Professor of Physics at NYU and senior
scientist of the project. Van Velzen is now completing his PhD in Holland.
About his first encounter with
real scientific work, van Velzen says, “Searching through 2.6 million galaxies
was actually a lot of fun—there is so much to discover! Based on our search
criteria and observing two TDFs that met those criteria, the rate of TDFs is
about once per 100,000 years, per galaxy. It’s quite thrilling to have been able
to make such a measurement.”
“The next step is to develop
models to explain in detail the flares’ properties and duration, and address
the question of whether TDFs could be responsible for producing Ultrahigh
Energy Cosmic Rays, whose sources have been elusive up to now,” says Farrar. “It is very exciting that we are on the verge of obtaining a large and
better-observed sample of TDFs to study—though a more sensitive search of SDSS
archival data and the new generation of transient surveys which will observe
more flares in real-time and with multi-wavelength follow-up. A large sample
will be invaluable to understanding many outstanding questions in astrophysics.”