Jets and explosions in NGC 7793

NGC 7793

Credit: ESA/Hubble & NASA

Acknowledgement: D. Calzetti (University of Massachusetts) and the LEGUS Team

This new image from the NASA/ESA Hubble Space Telescope shows NGC 7793, a spiral galaxy in the constellation of Sculptor some 13 million light-years away from Earth. NGC 7793 is one of the brightest galaxies in the Sculptor Group, and one of the closest groups of galaxies to the Local Group — the group of galaxies containing our galaxy, the Milky Way and the Magellanic Clouds.

The image shows NGC 7793’s spiral arms and small central bulge. Unlike some other spirals, NGC 7793 doesn’t have a very pronounced spiral structure, and its shape is further muddled by the mottled pattern of dark dust that stretches across the frame. The occasional burst of bright pink can be seen in the galaxy, highlighting stellar nurseries containing newly-forming baby stars.

Although it may look serene and beautiful from our perspective, this galaxy is actually a very dramatic and violent place. Astronomers have discovered a powerful microquasar within NGC 7793 — a system containing a black hole actively feeding on material from a companion star. While many full-sized quasars are known at the cores of other galaxies, it is unusual to find a quasar in a galaxy’s disc rather than at its centre.

Micro-quasars are almost like scale models — they allow astronomers to study quasars in detail. As material falls inwards towards this black hole, it creates a swirling disc around it. Some of the infalling gas is propelled violently outwards at extremely high speeds, creating jets streaking out into space in opposite directions. In the case of NGC 7793, these jets are incredibly powerful, and are in the process of creating an expanding bubble of hot gas some 1000 light-years across.

Source: ESA/Hubble - Space Telescope

Black Hole True Power Revealed

A composite image showing the position of the 'miniature galaxy' S26 in the galaxy NGC 7793. The image of S26 is a radio image, made with a CSIRO telescope while the image of the galaxy is made from combined X-ray and optical data. Image credit - Soria et al / CSIRO / ATCA; NGC 7793 - NASA, ESO and NOAO.

Following a study of what is in effect a miniature galaxy buried inside a normal-sized one - like a Russian doll - astronomers using a CSIRO telescope have concluded that massive black holes are more powerful than we thought.

An international team of astronomers led by Dr Manfred Pakull at the University of Strasbourg in France has discovered a 'microquasar' - a small black hole, weighing only as much as a star, that shoots jets of radio-emitting particles into space.

Called S26, the black hole sits inside a regular galaxy called NGC 7793, which is 13M light-years away in the Southern constellation of Sculptor.

Earlier this year Pakull and colleagues observed S26 with optical and X-ray telescopes (the European Southern Observatory's Very Large Telescope and NASA's Chandra space telescope).

Now they have made new observations with CSIRO's Compact Array radio telescope near Narrabri, NSW. These show that S26 is a near-perfect analogue of the much larger 'radio galaxies' and 'radio quasars'. Called S26, the black hole sits inside a regular galaxy called NGC 7793, which is 13M light-years away in the Southern constellation of Sculptor.

Powerful radio galaxies and quasars are almost extinct today, but they dominated the early Universe, billions of years ago, like cosmic dinosaurs. They contain big black holes, billions of times more massive than the Sun, and shoot out huge radio jets that can stretch millions of light-years into space.

Astronomers have been working for decades to understand how these black holes form their giant jets, and how much of the black hole's energy those jets transmit to the gas they travel through. That gas is the raw material for forming stars, and the effects of jets on star-formation have been hotly debated.

"Measuring the power of black hole jets, and therefore their heating effect, is usually very difficult," said co-author Roberto Soria (University College London), who carried out the radio observations.

"With this unusual object, a bonsai radio quasar in our own backyard, we have a unique opportunity to study the energetics of the jets."

Using their combined optical, X-ray and radio data, the scientists were able to determine how much of the jet's energy went into heating the gas around it, and how much went into making the jet glow at radio wavelengths.

They concluded that only about a thousandth of the energy went into creating the radio glow.

"This suggests that in bigger galaxies too the jets are about a thousand times more powerful than we'd estimate from their radio glow alone," said Dr Tasso Tzioumis of CSIRO Astronomy and Space Science.

"That means that black hole jets can be both more powerful and more efficient than we thought, and that their heating effect on the galaxies they live in can be stronger."

The study was made possible by a recent upgrade to the Compact Array, which can now do work of this kind five times faster than before.

Publication: Roberto Soria, Manfred W. Pakull, Jess W. Broderick, Stephane Corbel, and Christian Motch. "Radio lobes and X-ray hotspots in the microquasar S26." In press in Monthly Notices of the Royal Astronomical Society. Available online on the MNRAS website and at http://arxiv.org/abs/1008.0394.

NGC 7793: Black Hole Blows Big Bubble

Credit X-ray (NASA/CXC/Univ of Strasbourg/M. Pakull et al); Optical (ESO/VLT/Univ of Strasbourg/M. Pakull et al); H-alpha (NOAO/AURA/NSF/CTIO 1.5m)

JPEG (356.2 kb) - Tiff (37.9 MB) - PS (26.4 MB)

More Images - Zoom-In (flash)

This composite image shows a powerful microquasar containing a black hole in the outskirts of the nearby (12.7 million light years) galaxy NGC 7793. The large image contains data from the Chandra X-ray Observatory in red, green and blue, optical data from the Very Large Telescope in light blue, and optical emission by hydrogen ("H-alpha") from the CTIO 1.5-m telescope in gold.

The upper inset shows a close-up of the X-ray image of the microquasar, which is a system containing a stellar-mass black hole being fed by a companion star. Gas swirling toward the black hole forms a disk around the black hole. Twisted magnetic fields in the disk generate strong electromagnetic forces that propel some of the gas away from the disk at high speeds in two jets, creating a huge bubble of hot gas about 1,000 light years across. The faint green/blue source near the middle of the upper inset image corresponds to the position of the black hole, while the red/yellow (upper right) and yellow (lower left) sources correspond to spots where the jets are plowing into surrounding gas and heating it. The nebula produced by energy from the jets is clearly seen in the H-alpha image shown in the lower inset.

Credit: NASA/CXC/Univ of Strasbourg/M. Pakull et al

Labeled X-ray Close-up

The jets in the NGC 7793 microquasar are the most powerful ever seen from a stellar-mass black hole and the data show that a surprising amount of energy from the black hole is being carried away by the jets, rather than by radiation from material being pulled inward. The power of the jets is estimated to be about ten times larger than that of the very powerful ones seen from the famous microquasar in our own galaxy, SS433. This system in NGC 7793 is a miniature version of the powerful quasars and radio galaxies, which contain black holes that range from millions to billions of times the mass of the Sun.

A paper describing this work is being published in the July 8th, 2010, issue of Nature. The authors are Manfred Pakull from the University of Strasbourg in France, Roberto Soria from University College London, and Christian Motch, also from the University of Strasbourg.

Fast Facts for NGC 7793:

Scale: Wide field image is 9 arcmin across (about 34,000 light years); Inset image is 45 arcsec wide (about 2,800 light years)

Category: Black Holes
Coordinates: (J2000) RA 23h 57m 49.8s | Dec -32° 35' 29.5"
Constellation: Sculptor
Observation Date: Sep 6, 2003
Observation Time: 14 hours
Obs. ID: 3954

Color Code: X-ray (red: 0.2-1.0 keV, green: 1.0-2.0 keV, blue: 2.0-8.0 keV), Optical (Cyan), H-alpha (Gold)

Instrument: ACIS
References: M.Pakull et al., 2010, Nature, in press.
Distance Estimate: About 12.7 million light years