X-ray emission from dwarf galaxy J1329+3234
Copyright: ESA/XMM-Newton/N. Secrest, et al. (2015)
The galaxy, an irregular dwarf named J1329+3234, is one of the smallest
galaxies yet to contain evidence of a massive black hole. Located over
200 million light-years away, the galaxy is similar in size to the Small
Magellanic Cloud, one of our nearest neighbouring galaxies, and
contains a few hundred million stars.
In 2013, an international team of astronomers was intrigued to discover
infrared signatures of an accreting black hole within J1329+3234 when
they studied it with the Wide-Field Infrared Survey Explorer (WISE).
The same team has now investigated the galaxy further, using ESA's XMM-Newton to hunt for this black hole in X-rays – and found something very surprising.
The same team has now investigated the galaxy further, using ESA's XMM-Newton to hunt for this black hole in X-rays – and found something very surprising.
"The X-ray emission from J1329+3234 is over 100 times stronger than expected for this galaxy,"
says Nathan Secrest of George Mason University in Virginia, USA, lead
author of the new study published in The Astrophysical Journal. "We
would typically expect to find low-level X-ray emission from
stellar-mass black holes within the galaxy, but what we found instead
was emission consistent with a very massive black hole."
The combined X-ray and infrared properties of this galaxy can only be
explained by the presence of a massive black hole residing in
J1329+3234, similar to the supermassive black holes found at the centres
of much more massive galaxies.
While the exact mass of the black hole is
not known, it must be at least 3000 times as massive as the Sun,
although it is likely to be much more massive than that. If the black
hole in J1329+3234 is similar to known low-mass supermassive black
holes, then it has a mass of around 150 000 times that of the Sun.
A feeding black hole at the centre of a galaxy is known as an active galactic nucleus, or AGN. In the region surrounding the black hole, material from the galaxy emits intensely bright radiation as it swirls inwards towards the centre of the galaxy and is devoured by the black hole. AGNs powered by massive black holes are commonplace in large galaxies, but they appear to be rarer in galaxies without a central "bulge" of stars – dwarf galaxies being a key example.
"This is a really important discovery," says co-author Shobita Satyapal, also from George Mason University. "It's interesting enough that such a tiny galaxy has such a large black hole, but this also raises questions about how these black holes form in the first place."
Astronomers believe that the "seeds" of massive black holes formed very
early on in the Universe, along with the first generation of stars.
These seed black holes then grew into massive black holes via a string
of galaxy mergers. As the galaxies merged, so did their central black
holes.
The turbulent merging process would feed the accreting black holes with
copious amounts of material while simultaneously building up large,
bulge-dominated galaxies. However, with each successive merger
information about the properties of the original black hole is lost,
meaning that astronomers cannot determine the mass of the original seeds
by looking at massive bulge-dominated galaxies – instead they probe
their dwarf and bulgeless relatives, such as J1329+3234, for clues.
Finding a massive black hole within such a tiny bulgeless galaxy
provides support for the theory that black holes may have grown very
efficiently within the gaseous haloes of forming galaxies, originating
in massive, collapsing clouds of primordial gas.
Along with J1329+3234, Secrest and his colleagues found several hundred
other bulgeless galaxies from the WISE survey that also show intriguing
infrared properties – many of which, like J1329+3234, display no
evidence for AGNs in optical light.
In recent years, growing numbers of massive black holes have been
identified within dwarf and bulgeless galaxies. However, it is much
harder to find them than it is to find their supermassive counterparts –
they are less likely to show up in optical studies since they are often
obscured by dust and are usually much dimmer, making them difficult to
detect above surrounding light.
This emphasises the importance of multi-wavelength sky surveys, says ESA's XMM-Newton project scientist Norbert Schartel. "Using a mix of optical, infrared, and X-ray observations was vital here," he adds. "The sensitivity of XMM-Newton made it possible not only to discover this black hole but to also fully characterise its spectrum, meaning we can say with much more certainty that it's a black-hole-fuelled AGN."
This emphasises the importance of multi-wavelength sky surveys, says ESA's XMM-Newton project scientist Norbert Schartel. "Using a mix of optical, infrared, and X-ray observations was vital here," he adds. "The sensitivity of XMM-Newton made it possible not only to discover this black hole but to also fully characterise its spectrum, meaning we can say with much more certainty that it's a black-hole-fuelled AGN."
More information
"An optically obscured AGN in a low mass, irregular dwarf galaxy: A multi-wavelength analysis of J1329+3234" by N. Secrest et al. is published in The Astrophysical Journal.
The European Space Agency's X-ray Multi-Mirror Mission, XMM-Newton, was launched in December 1999. The largest scientific satellite to have been built in Europe, it is also one of the most sensitive X-ray observatories ever flown. More than 170 wafer-thin, cylindrical mirrors direct incoming radiation into three high-throughput X-ray telescopes. XMM-Newton's orbit takes it almost a third of the way to the Moon, allowing for long, uninterrupted views of celestial objects.
Contacts
Nathan SecrestGeorge Mason University
Fairfax, Virginia, USA
Email: nsecrest@masonlive.gmu.edu
Phone: +1-301-760-0210
Shobita Satyapal
George Mason University
Fairfax, Virginia, USA
Email: ssatyapa@gmu.edu
Phone: +1-703-993-1283
Norbert Schartel
ESA XMM-Newton Project Scientist
Directorate of Science and Robotic Exploration
European Space Agency
Email: Norbert.Schartel@esa.int
Phone: +34-91-8131-184
