Active galaxies contain a supermassive black hole (SMBH) that causes vigorous radiation of electromagnetic energy as a result of rapid accretion of gas and dust. While almost all galaxies except dwarfs contain an SMBH in the center, active galaxies are rare – fewer that 1% of galaxies in the present universe. A very few active galaxies contain two active SMBHs. Even fewer have three. Before the latest discovery, only two examples had ever been documented in the literature.
The most recent example was so obscure it doesn’t even have a name. This example is unusual in other respects as well. It is quite distant, having a redshift of z=1.35. That means its light has taken 8.92 billion years to reach us. We see it as it looked about 4.75 billion years after the big bang. Most galaxies at that time were fairly mature. Not this one. It seems to be quite young and irregular in form, with 4 separate components. Three of those appear to contain active SMBHs.
Some of the details about this galaxy are not known very precisely. It’s (barely) possible that one or more of the apparent active SMBHs are actually bulges hosting very active star formation. But the spectroscopic evidence is heavily against that.
The three SMBHs are not especially large as such things go. The masses could be as much as 3.1×106 M⊙, 1.0×107 M⊙, and 1.2×107 M⊙. However, these are upper bounds, and the actual masses could be only 20% as large. For comparison, the Milky Way’s SMBH is 4.2×106 M⊙, but many SMBHs are in the 109 to 1010 M⊙ range.
There are several intriguing questions about this object. The first: how did it form? Is it a 3-way merger of smaller galaxies of similar size, each of which had its own SMBH already? In the modern universe, such mergers are very rare. The evidence is that they were also rare in the era of 5 billion years after the big bang. It’s a lot more likely that this galaxy is still rather young and in an early stage of formation.
If that is the case, then there are very interesting questions about how the object can have three active nuclei.
There has to be an abundance of available gas and dust to accrete around each SMBH and fuel its activity. Given that, it’s not too hard for the SMBHs to have grown to their present size in a billion years, or possibly much less, especially if they are a lot less massive than the high estimate.
There’s a limit – the “Eddington limit” – to how rapidly a massive object (such as a protostar or an active SMBH) can accrete gas. The limit exists because the more rapidly accretion occurs, the hotter and more luminous the gas becomes. At a sufficiently high accretion rate, the outward radiation pressure will balance the inward gravitational force and limit accretion to the rate at which the gas cools from radiative loss of energy.
If the mass of an accreting SMBH is M(t) at a time t (after t=0), then if the SMBH is growing at the Eddington limit
Here ε is the efficiency of conversion of mass to energy (usually ε≅0.1), and te = 0.45 billion years is the characteristic time scale. M(0) is the mass of the black hole “seed” at t=0. It can be anything from the size of the smallest stellar remnant black hole (about 3 M⊙) on up. In fact, the nature of black hole seeds and their masses is an intriguing question itself. In any case, starting from seeds as small as 10 to 100 M⊙ it’s easy for a black hole to grow to 107 M⊙ in 500-800 million years if there’s enough available gas.
Where the black hole seeds come from may be the most interesting question of all. It turns out that the formation of black hole seeds is a lot harder to explain 4 or 5 billion years after the big bang than it is within the first 1 or 2 billion years. This is because at the later time almost all the available gas for star formation contains significant amounts of elements heavier than hydrogen and helium. These heavier elements – referred to as “metals” – make it possible for condensing clouds of gas to cool down more easily than clouds consisting almost entirely of hydrogen and helium. In gas clouds mostly devoid of heavy elements, cooling is less efficient because there are fewer possible electron energy level transitions to produce radiation photons.
The Eddington limit applies not only to accretion of gas around black holes, but also in the formation of stars from interstellar gas. Since cooling is less efficient in primordial gas that originated in the big bang itself and has almost no heavy elements, greater quantities of gas are required in order to have sufficient mass that gravity can overcome radiation pressure and allow stars to form at all. Consequently, the first stars that formed in the universe, several hundred million years after the big bang – “Population III” stars – tended to be very large, with masses of from 40 or 50 M⊙ up to perhaps 100 to 200 M⊙ or more. Such stars have short lives and quickly expire as supernovae. They leave behind substantial black hole seeds in the 10 to 100 M⊙ range and above. It’s also possible for large primordial gas clouds to collapse directly to black holes with mass in the same range, without forming stars or supernovae at all.
Supernovae also produce and scatter into the interstellar medium significant amounts of heavier elements. Over hundreds of millions of years, this process enriches interstellar gas with heavier elements that eventually curtail the formation of very massive stars in later generations, and, therefore, also black holes that can act as seeds that later grow into SMBHs. Only stars with mass more than about 8 M⊙ will explode as core collapse supernovae at all and thus leave black hole remnants.
It is expected that 4 to 5 billion years after the big bang almost no primordial gas essentially free of heavy elements will remain. Indeed even ~2 billion years after the big bang there should be very little primordial gas. Recent research did identify one pocket of primordial gas near the 2 billion year point. But such pockets seem to be very rare at that time or later.
Obviously, a few extremely large stars, which will eventually leave behind substantial black holes, continue to form even in the present universe. For instance one of the largest stars in the Milky Way, Eta Carinae, may have a mass around 100 M⊙. But such large stars are extremely rare now. The Milky Way may contain only a few dozen of that size. So it’s rather odd that a much smaller galaxy just in the process of forming 4.75 billion years after the big bang could have hosted three sufficiently large stars whose black hole remnants were able to grow into the SMBHs having masses in the range that seems to be there.
This new discovery could be just a statistical fluke, however unlikely. It might also be a 3-way merger, as seems to be the case with the only 2 other known instances of galaxies with 3 active SMBHs, but that’s quite unlikely as well. A third possibility is that the SMBHs formed at a much earlier time, and have simply become active again as they are now seen, as a result of some sudden new supply of gas to fuel their activity. Further study with more powerful equipment is going to be required to determine which of the unlikely possibilities is in evidence here.
|Schawinski, K., Urry, M., Treister, E., Simmons, B., Natarajan, P., & Glikman, E. (2011). EVIDENCE FOR THREE ACCRETING BLACK HOLES IN A GALAXY AT z~1.35: A SNAPSHOT OF RECENTLY FORMED BLACK HOLE SEEDS? The Astrophysical Journal, 743 (2) DOI: 10.1088/2041-8205/743/2/L37|