Young galaxies typically go through a period of rapid star production. For example, dusty starburst galaxies produce stars so rapidly that it would consume all of its gas and dust in about 10 million years at that rate. But before that early star production period, a galaxy also has a period of rapid formation into a galaxy. In this period the central black hole of the galaxy is particularly active. As a result these galaxies are bright in the infrared, and are known as luminous infrared galaxies (LIRGs).

Recently, analysis of data from the WISE infrared satellite has found dozens of LIRGs, including the brightest galaxy ever discovered, known as WISE J224607.57-052635.0. The light from this particular galaxy has traveled for about 12.5 billion years, which means it comes from a time when the universe was only 1.3 billion years old.  It has a luminosity equivalent to 350 trillion Suns, which is surprising since it is smaller than our own Milky Way.

The reason it is so bright in the infrared is that it is surrounded by a halo of gas and dust. As the central black hole gorges itself it emits light at a range of wavelengths from x-rays to ultraviolet. Most of that light is absorbed by the surrounding halo, which is heated by the light and emits infrared. But given just how bright this galaxy is, the central black hole must be consuming matter at a prodigious rate. So much that it would exceed a theoretical limit known as the Eddington limit. Basically as a black hole consumes matter the light it emits should push back against infalling matter, thus limiting how much can be captured.

There are ways that the Eddington limit can be bypassed, but the fact that this would occur in a young galaxy indicates that supermassive black holes in the centers of galaxies may have formed earlier and faster than we once supposed.

Paper: Chao-Wei Tsai et al. The Most Luminous Galaxies Discovered by WISE. ApJ 805 90. doi:10.1088/0004-637X/805/2/90 (2015)

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In an earlier post I talked about ultraluminous x-ray sources, and how they are typically powered by stellar-mass black holes. The source of these intense x-rays is the superheated material surrounding the black hole. By observing the intensity of the x-rays, we can get a handle on just how much mass a black hole is actively accumulating. The x-ray intensity has also been used as a measure of the black hole’s mass, since there is a rough limit to just how much matter a black hole of a given size can accrete. It’s known as the Eddington limit, but as we’ve seen, that limit isn’t absolute.

The basic idea of the Eddington limit is that the very x-rays produced as a black hole accretes matter also work to prevent matter from being accreted. That’s because the emitted light can exert an outward pressure on the infalling matter, thus reducing the amount that reaches the black hole. At some point the intensity of light would be so great that the surrounding matter would be pushed away from the black hole, preventing matter from accreting altogether. This is what’s known as the Eddington limit.

Which brings us to the case of a black hole known as P13. Like most ultraluminous black holes, P13 has a binary companion, in this case a blue supergiant of about 20 solar masses. The black hole captures material from its companion, which is how it gets the matter it accretes. As the black hole and star orbit each other, their light is Doppler shifted due to their orbital motion. From this we know that the two orbit each other with a period of about 64 days.

That information by itself isn’t enough to determine the mass of the black hole, but in a recent paper in Nature, new data is used to determine the mass of P13. As the authors point out, the intense x-rays of the black hole heat the facing side of the companion star. This affects the spectrum emitted by the star, which can be observed. My modeling the x-ray heating and matching it to the observed spectrum, the team could get a measure of their distance of separation. From this they determined the mass of P13 to be less than 15 solar masses.

That’s a great result by itself, but what’s surprising is that this is much smaller than the value expected given its luminosity. According to the Eddington limit, a black hole of this size shouldn’t be nearly so bright. Given past violations of the Eddington limit, that might not be too surprising, but the rate of accretion is huge. This black hole is eating an Earth’s worth of mass every three years.

Just how that occurs is not clear. What is clear is that P13 is a hungry hippo indeed.

Paper: C. Motch, et al. A mass of less than 15 solar masses for the black hole in an ultraluminous X-ray source. Nature 514, 198–201 (2014)

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Yesterday I mentioned that hypernovae (super-supernovae) are the result of the explosion of a star that’s about as massive as a star can be (about 150-200 solar masses). But how exactly do we know that this is an upper limit?

The first clue comes from a derivation by Arthur Eddington. In 1916, Eddington demonstrated that there was a limit to how bright a stable star could be. The basic idea is that the atmosphere of a star is being gravitationally attracted by the mass of the star (giving it weight), and this weight is balanced by the pressure of the deeper layer of the star. For a star to be stable, the weight and pressure must be equal, so the star doesn’t collapse inward or push the atmosphere outward.

We typically think of pressure as being due to gas and such, but light can also exert pressure on a material. We don’t notice light pressure in our daily lives because it is so small. Even in our Sun, the pressure on the atmosphere is relatively small, so the weight of our Sun’s atmosphere is mostly balanced by the pressure of the plasma in the layer underneath it. But if the Sun were brighter, the light it emits would push harder against the particles of the atmosphere. What Eddington showed is that there is a limit where the pressure of a star’s light on the atmosphere is large enough to balance the gravitational weight of the stellar atmosphere entirely, known as the Eddington luminosity limit. If the star were any brighter, the light of the star would push away the outer layers of the atmosphere, thus causing the star to lose mass.

When Eddington first derived this limit, he found that the maximum luminosity (brightness) of a star was proportional to the mass of a star. This meant that more massive stars could be brighter than less massive stars, but it didn’t say anything about an upper limit on mass. Then in 1924, Eddington discovered a relationship between the mass of a star and its luminosity, specifically that the brightness of a star is roughly proportional to the mass cubed.

This meant the brightness of a star increased with mass faster than the luminosity limit, so there must be an upper limit on a star’s mass. Stars with larger masses would be so bright that they would burn away their outer layers. With Eddington’s calculation, this limit is around 65 solar masses. Later, more detailed calculations put this limit at around 150 solar masses, which is generally considered an upper limit for stable stars.

In 2007, a research team made a study of the Aches cluster, which is the densest known star cluster in our galaxy. Looking at the brightest stars in this cluster, they found no stars greater than about 120 solar masses. Using their observations to make a statistical extrapolation, they found that the upper limit for stars is likely 150 solar masses.

But recently new evidence has questioned that limit. Theoretical work has shown that it is possible to have stable stars with a brightness greater than the Eddington luminosity limit. Effects such as turbulence within the atmosphere and photon bubbles, where light could pass through the stellar atmosphere more easily would allow super-luminous stars to remain stable. Then there are calculations from hypernova explosions that estimate the progenitor (the star that exploded) had a mass of about 200 solar masses. Finally, there is a star known as R136a1. Discovered in 2010 which is currently the most luminous known star, and has an estimated mass of about 265 solar masses.

So while 150 solar masses is generally considered an upper limit, that limit seems to be more of a guideline.

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In 1916, Eddington demonstrated that there was a limit to how bright a stable star could be.  The basic idea is that the atmosphere of a star is being gravitationally attracted by the mass of the star (giving it weight), and this weight is balanced by the pressure of the deeper layer of the star.  For a star to be stable, the weight and pressure must be equal, so the star doesn’t collapse inward or push the atmosphere outward.
We typically think of pressure as being due to gas and such, but light can also exert pressure on a material.  We don’t notice light pressure in our daily lives because it is so small.  Even in our Sun, the pressure on the atmosphere is relatively small, so the weight of our Sun’s atmosphere is mostly balanced by the pressure of the plasma in the layer underneath it.  But if the Sun were brighter, the light it emits would push harder against the particles of the atmosphere.  What Eddington showed is that there is a limit where the pressure of a star’s light on the atmosphere is large enough to balance the gravitational weight of the stellar atmosphere entirely, known as the Eddington luminosity limit.  If the star were any brighter, the light of the star would push away the outer layers of the atmosphere, thus causing the star to lose mass.

The galaxy M83. Credit: ESO/IDA/Danish 1.5 m/R. Gendler, S. Guisard and C. Thöne

This same limit is often thought to hold for other objects, such as active galactic nuclei (AGNs) powered by black holes, but it also isn’t an absolute limit.  When Eddington derived the limit he assumed a star that is spherical and non-rotating.  Black holes are known to rotate, and their accretion disks are not spherical, so there have been proposed models that allow AGNs and other black holes to emit more power than the Eddington limit.  There have been searches for such super-Eddington luminosity, but so far results have been inconclusive.

Now a new paper in Science has analyzed the energy output of a black hole in the galaxy M83, and found that it has emitted sustained levels of energy beyond the Eddington limit.  Since the black hole was in a quiet phase when it was observed, the team could make an accurate determination of its mass by analyzing its accretion disk.  They then looked at the effect of its jets, which were produced in an active period.  They found that the energy of the jets clearly exceeded the Eddington limit.

By demonstrating that energy generated near black holes can exceed the Eddington limit, the authors have demonstrated that black holes can affect their environment in a more powerful way that originally thought.

Paper:  Soria R, Long KS, Blair WP, et al. Super-Eddington Mechanical Power of an Accreting Black Hole in M83. Science. (2014)

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