Astronomers have used NASA's Chandra X-ray Observatory
and ESA's XMM-Newton X-ray observatory to discover an extremely
luminous, variable X-ray source located outside the center of its parent
galaxy. This peculiar object could be a wandering black hole that came from a small galaxy falling into a larger one.
Astronomers think that supermassive black holes,
with some 100,000 to 10 billion times the Sun's mass, are in the
centers of most galaxies. There is also evidence for the existence of
so-called intermediate mass black holes, which have lower masses ranging
between about 100 and 100,000 times that of the Sun.
Both of these types of objects may be found away from the center of a
galaxy following a collision and merger with another galaxy containing a
massive black hole. As the stars, gas and dust from the second galaxy
move through the first one, its black hole would move with it.
A new study reports the discovery of one of these "wandering" black
holes toward the edge of the lenticular galaxy SDSS J141711.07+522540.8
(or, GJ1417+52 for short), which is located about 4.5 billion light
years from Earth. This object, referred to as XJ1417+52, was discovered
during long observations of a special region, the so-called Extended
Groth Strip, with XMM-Newton
and Chandra data obtained between 2000 and 2002. Its extreme brightness
makes it likely that it is a black hole with a mass estimated to be
about 100,000 times that of the Sun, assuming that the radiation force
on surrounding matter equals the gravitational force.
The main panel of this graphic has a wide-field, optical light image
from the Hubble Space Telescope. The black hole and its host galaxy are
located within the box in the upper left. The inset on the left contains
Hubble's close-up view of GJ1417+52. Within this inset the circle shows
a point-like source on the northern outskirts of the galaxy that may be
associated with XJ1417+52.
The inset on the right is Chandra's X-ray image of XJ1417+52 in
purple, covering the same region as the Hubble close-up. This is a point
source, with no evidence seen for extended X-ray emission.
The Chandra and XMM-Newton observations show the X-ray output of
XJ1417+52 is so high that astronomers classify this object as a
"hyper-luminous X-ray source" (HLX). These are objects that are 10,000
to 100,000 times more luminous in X-rays than stellar black holes, and
10 to 100 times more powerful than ultraluminous X-ray sources, or ULXs.
At its peak XJ1417+52 is about ten times more luminous than the
brightest X-ray source ever seen for a wandering black hole. It is also
about 10 times more distant than the previous record holder for a
wandering black hole.
The bright X-ray emission
from this type of black hole comes from material falling toward it. The
X-rays from XJ1417+52 reached peak brightness in X-rays between 2000
and 2002. The source was not detected in later Chandra and XMM
observations obtained in 2005, 2014 and 2015. Overall, the X-ray
brightness of the source has declined by at least a factor of 14 between
2000 and 2015.
The authors theorize that the X-ray outburst seen in 2000 and 2002
occurred when a star passed too close to the black hole and was torn
apart by tidal forces. Some of the gaseous debris would have been heated
and become bright in X-rays as it fell towards the black hole, causing
the spike in emission.
The location and brightness of the optical source in the Hubble image
that may be associated with XJ1417+52 suggest that the black hole could
have originally belonged to a small galaxy that plowed into the larger
GJ1417+52 galaxy, stripping away most of the galaxy's stars but leaving
behind the black hole and its surrounding stars at the center of the
small galaxy. If this idea is correct the surrounding stars are what is
seen in the Hubble image.
A paper by Dacheng Lin (University of New Hampshire) and colleagues
describing this result appears in The Astrophysical Journal and is
available online. NASA's Marshall Space Flight Center in Huntsville,
Alabama, manages the Chandra program for NASA's Science Mission
Directorate in Washington. The Smithsonian Astrophysical Observatory in
Cambridge, Massachusetts, controls Chandra's science and flight
operations.
Fast Facts for XJ1417+52:
Scale: Image is about 2.9 arcmin across (about 3.13 million light years)
Category: Black Holes
Coordinates (J2000): RA 14h 17m 11.10s | Dec +52° 25' 42.30"
Constellation: Boötes
Observation Date: 7 pointings between Aug 2002 and Oct 2014
Observation Time: 85 hours 22 min (3 days 13 hours 22 min).
Obs. ID: 3305, 4357, 4365, 5851, 7181, 16027, 17487
Instrument: ACIS
References: Lin, D. et al, 2016, ApJ, 821, 25; arXiv:1603.00455
Color Code: X-ray (Purple); Optical (Red, Green, Blue)
Distance Estimate: About 4.47 billion light years (z=0.41827)
Source: NASA’s Chandra X-ray Observatory
