Star Birth Dominates Energy Production in Ultra-Luminous Galaxies

Artists impression of an ultra-luminous galaxy heating the surrounding dust (JAXA/ISAS/LIRA)
Artists impression of an ultra-luminous galaxy heating the surrounding dust (JAXA/ISAS/LIRA)

In the early 1980’s, NASA’s Infrared Astronomical Satellite (IRAS) detected a number of unknown objects lurking in the depths of the cosmos.

At the time, these IRAS objects stirred speculation in the press. Were the infrared signals being emitted by comets inside the Solar System? Or were they failed stars (brown dwarfs) lurking beyond the orbit of Pluto? The latter theory spawned the idea that the hunt for Planet X was back on (stoking the smoldering conspiracy embers of the flawed doomsday theory that Nibiru is coming to get us). Alas, it was neither, these intense infrared signals were coming from much, much further away.

It turned out that the infrared emissions were being generated by galaxies that, bizarrely, had little optical signal. Although a high proportion of them were known to be interacting galaxies (i.e. they were colliding with other galaxies), the exact energy mechanism driving their emissions was not known.

Ultra-luminous galaxies have the luminocity of a trillion Suns, whereas our galaxy has the luminosity of a pedestrian ten billion Suns. Obviously, ultra-luminous galaxies are different animals to the Milky Way, but a galaxy is a galaxy and the energy sources are similar whether they are ultra-luminous or not. It would appear that the only difference is how active the galaxy is.

The first obvious energy source in a galaxy is star formation; the more stars that are forming, the brighter the galaxy. Secondly — as with our galaxy — the central supermassive black hole’s accretion rate contributes to the galaxy’s energy budget; the more matter being accreted by the black hole, the more energy is being generated (and therefore the brighter the galaxy).

So, when observing these ultra-luminous galaxies, surely it should be an easy task to work out where all this energy is coming from? Actually, this isn’t the case, astronomers are having a difficult job in understanding the nature of IRAS galaxies and the reason for this comes from the source of the infrared emissions. Galactic dust is being heated by the energy source, but this dust obscures the source of this heating (it is opaque to optical wavelengths).

Smithsonian Astrophysical Observatory (SAO) researcher Guido Risaliti and his team have been analyzing Spitzer data to try to characterize the infrared emissions from 71 ultra-luminous galaxies. Using a “dust emission diagnostic technique,” the team have deduced that approximately 70% of the galaxies have active nuclei (i.e. their supermassive black holes have high accretion rates). Although most of the galactic nuclei are active, it is star formation that dominates the energy production in two-thirds of the galaxies. Also, these account for the highest fraction of the brightest galaxies.

This is a significant finding as it demonstrates how a galaxy reacts when it interacts with another galaxy. It would appear that the black hole in the core of the galactic bulge is kick-started during the massive gravitational interaction, boosting energy output as it eats more matter. The interaction also boosts star birth and this energy source becomes a dominant factor. Both energy sources heat up interstellar dust, making the galaxy glow in infrared wavelengths while optical light is masked.

Source: SAO (Harvard)

Unexpectedly Large Black Holes and Dark Matter

The M87 black hole blasts relativistic plumes of gas 5000 ly from the centre of the galaxy (NASA)
The M87 black hole blasts relativistic plumes of gas 5000 ly from the centre of the galaxy (NASA)

I just spent 5 minutes trying to think up a title to this post. I knew what I wanted to say, but the subject is so “out there” I’m not sure if any title would be adequate. As it turns out, the title doesn’t really matter, so I opted for something more descriptive…

So what’s this about? Astronomers think they will be able to “see” a supermassive black hole in a galaxy 55 million light years away? Surely that isn’t possible. Actually, it might be.

When Very Long Baseline Interferometry is King

Back in June, I reported that radio astronomers may be able to use a future network of radio antennae as part of a very long baseline interferometry (VLBI) campaign. With enough observatories, we may be able to resolve the event horizon of the supermassive black hole lurking at the centre of the Milky Way, some 26,000 light years away from the Solar System.

The most exciting thing is that existing sub-millimeter observations of Sgr. A* (the radio source at the centre of our galaxy where the 4 million solar mass black hole lives) suggest there is some kind of active structure surrounding the black hole’s event horizon. If this is the case, a modest 7-antennae VLBI could observe dynamic flares as matter falls into the event horizon.

It would be a phenomenal scientific achievement to see a flare-up after a star is eaten by Sgr. A*, or to see the rotation of a possibly spinning black hole event horizon.

All of this may be a possibility, and through a combination of Sgr. A*’s mass and relatively close proximity to Earth, our galaxy’s supermassive black hole is predicted to have the largest apparent event horizon in the sky.

Or does it?

M87 Might be a Long Way Away, But…

As it turns out, there could be another challenger to Sgr. A*’s “largest apparent event horizon” crown. Sitting in the centre of the active galaxy called M87, 55 million light years away (that’s over 2,000 times further away than Sgr. A*), is a black hole behemoth.

M87’s supermassive black hole consumes vast amounts of matter, spewing jets of gas 5,000 light years from the core of the giant elliptical galaxy. And until now, astronomers have underestimated the size of this monster.

Karl Gebhardt (Univ. of Texas at Austin) and Thomas Jens (Max Planck Institute for Extraterrestrial Physics in Garching, Germany) took another look at M87 and weighed the galaxy by sifting through observational data with a supercomputer model. This new model accounted for the theorized halo of invisible dark matter surrounding M87. This analysis yielded a shocking result; the central supermassive black hole should have a mass of 6.4 billion Suns, double the mass of previous estimates.

Therefore, the M87 black hole is around 1,600 times more massive than our galaxy’s supermassive black hole.

A Measure for Dark Matter?

Now that the M87 black hole is much bigger than previously thought, there’s the tantalizing possibility of using the proposed VLBI to image M87’s black hole as well as Sgr. A*, as they should both have comparable event horizon dimensions when viewed from Earth.

Another possibility also comes to mind. Once an international VLBI is tested and proven to be an “event horizon telescope,” if we are able to measure the size of the M87 black hole, and its mass is confirmed to be in agreement with the Gebhardt-Jens model, perhaps we’ll have one of the first indirect methods to measure the mass of dark matter surrounding a galaxy…

Oh yes, this should be good.

UPDATE! How amiss of me, I forgot to include the best black hole tune ever:

Publication: The Black Hole Mass, Stellar Mass-to-Light Ratio, and Dark Matter Halo in M87, Karl Gebhardt et al 2009 ApJ 700 1690-1701, doi: 10.1088/0004-637X/700/2/1690.
Via: New Scientist

Mystery Blob Detected 12.9 Billion Light Years Away

The Himiko object, the most massive object ever discovered in the early universe (M. Ouchi et al.)

Take a good look, this is one of the most mysterious, massive objects ever discovered in the cosmos. We don’t really know what it is, but this thing is huge, spanning 55,000 light years across (the approximate radius of our Milky Way). What makes this all the more intriguing is the fact that this object formed only 800 million years after the Big Bang and it is 10 times more massive than the next biggest object observed in the early Universe. But what is it?
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