Which Came First: The Black Hole or the Galaxy? Challenges to Black Hole and Galactic Growth Models
Every galaxy seems to harbor a supermassive black hole (SMBH) at the center. This engine of destruction is thought to grow with galaxies containing a central bulge, for the majority of them seem to be 3-5% of the mass of their residency. It is through mergers of galaxies that SMBH grow along with material from the host galaxy. Population III stars, whose from the first formation about 200 million years post Big Bang, collapsed into roughly 100 solar mass black holes. Because those stars formed in clusters, plenty of material was around for the black holes to grow and merge. However, some recent finding have cast this long-held view into question, and the answers only seem to lead to even more questions… (Natarajan 26-7)
A Mini-SMBH from Beyond
Spiral galaxy NGC 4178, located 55 million light years away, does not contain a central bulge, which means it shouldn’t have a central SMBH, and yet one was found. Data from the Chandra X-Ray Telescope, Spitzer Space Telescope, and the Very Large Array place the SMBH at the lowest end of the possible mass spectrum for SMBHs, with a total a little less than 200,000 suns. Along with 4178, four other galaxies with similar conditions have been found including NGC 4561 and NGC 4395. This could imply that SMBH form under other or perhaps even different circumstances than previously thought (Chandra “Revealing”).
A Giant SMBH from the Past
Now here we have a nearly polar opposite case: one of the largest SMBHs ever seen (17 billion suns) that happens to reside in a galaxy that is too small for it. A team from the Max Planck Institute for Astronomy in Heidelberg, Germany used data from the Hobby-Eberly Telescope and archived data from Hubble to determine that the SMBH in NGC 1277 is 17% of the mass of its host galaxy, even though the elliptical galaxy of such size should only have one which is 0.1%. And guess what: four other galaxies have been found to exhibit similar conditions to 1277. Because ellipticals are older galaxies that have merged with other galaxies, perhaps the SMBHs did as well and thus grew as they became and ate gas and dust from around them (Max Planck Institute, Scoles).
And then there are Ultra Compact Dwarfs (UCD), which are 500 times smaller than our Milky Way. And in M60-UCD-1, found by Anil C. Seth of the University of Utah and detailed in a September 17, 2014 issue of Nature, is the lightest object known to have a SMBH. Scientists also suspect that these could have arisen from galactic collisions, but these are even denser with stars that elliptical galaxies. The determining factor of is a SMBH was present was star motion around the core of the galaxy, which according to data from Hubble and the Gemini North put the stars at a velocity of 100 kilometers per second (as compared to the outer stars which moved at 50 kilometers per second. The mass of the SMBH is clocked in at 15% that of M60 (Freeman, Rzetelny).
Galaxy CID-947 is similar in premise. Located around 11 billion light-years away, its SMBH clocks in at 7 billion solar masses and is from a time when the Universe was less than 2 billion years old. This should be way too early for such an object to exist and the fact that its about 10% the mass of its host galaxy upsets the usual observation of 1% for black holes of that era. For something with that large a mass, it should be done forming stars and yet evidence shows the contrary. This is a sign that something is wrong with our models (Keck).
No So Fast
NGC 4342 and NGC 4291 seem to be two galaxies with SMBHs too big to have formed there. So they looked toward tidal striping from a past encounter with another galaxy as a possible formation or introduction. When dark matter readings based off Chandra's data showed no such interaction, scientists then began to wonder if an active phase in the past led to blasts of radiation that has obscured some of the mass from our telescopes. This could perhaps be a reason for the seemingly miscorrelation of some SMBH to their galaxy. If some of the mass is hidden, then the host galaxy could be larger than suspected and thus the ratio could be correct (Chandra “Black Hole Growth”).
And then there are ancient blazars, or highly active SMBHs. Many have been seen 1.4 - 2.1 billion years post Big Bang, a time frame that many consider to be too early for them to have formed, especially with the low number of galaxies around them. Data from the Fermi Gamma Ray Observatory found some so large that they were a billion times more massive than our own sun! 2 other candidates from the early Universe found by Chandra point to a direct collapse of gas millions of times the mass of the sun rather than any known supernova explosion (Klotz, Haynes).
But it gets worse. Quasar J1342+0928, found by Eduardo Banados at The Carnegie Institution for Science in Pasadena, was spotted at a time when the Universe was only 690 million years old, yet it has a mass of 780 million solar masses. This is just too big to explain away easily, for it violates the Eddington rate of black hole growth which limits their development as the radiation leaving a black hole pushes material entering it away. But a solution may be at play. Some theories of the early Universe hold that at this time, known as the Epoch of Reionization, black holes of 100,000 solar masses formed with ease. How this occurred is still not well understood (it may have to do with all the gas hanging around, but many special conditions would be required to prevent star formation preceding black hole formation) but the Universe at that time was just becoming ionized again. The area around J1342 is about half neutral and half ionized, meaning it was around during the Epoch before charges could be totally stripped or that the Epoch was a later event than previously thought. Updating this data to the model may give insight into how such large black holes can appear at so early a stage in the Universe (Klesman "Lighting", Sokol, Klesman "Farthest").
Some researchers tried a new way to account for black hole growth in the early universe and they soon realized that dark matter may play a role since its important to general galactic integrity. A study by the Max Planck Institute, the University of Observatory Germany, the University of Observatory Munich, and the University of Texas at Austin looked at galactic properties like mass, bulge, SMBH, and dark matter content to see if any correlations were there. They found that dark matter doesn't play a role but the bulge does seem directly tied to the growth of the SMBH, which makes sense. That is where all the material it needs to feed on is present, so the more that is there to eat then the more it can grow. But how can they grow so quickly? (Max Planck)
Maybe via direct collapse. Most models require a star to start a black hole via a supernova, but certain models indicate that if enough material is floating around then the gravitational pull can skip the star, avoid the spiraling in and therefore the Eddington limit of growth (the fight between gravity and outward radiation) and collapse directly into a black hole. Models indicate that it might just take 10,000 to 100,000 solar masses of gas to create SMBHs in as little as 100 million years. The key is to create an instability in the dense cloud of gas, and that would seem to be natural hydrogen versus periodic hydrogen. The difference? Natural hydrogen has two bonded together while periodic is singular and without an electron. Radiation can excite natural hydrogen to split, meaning that conditions heat up as energy is released and so prevents stars from forming and instead let enough material gather to cause a direct collapse. Scientists are looking for high infrared readings from 1 to 30 microns due to the high energy photons from the collapsing event losing energy to the surrounding material then becoming redshifted. Another place to look at are Population II clusters and satellite galaxies which are high in that star count. Hubble, Chandra, and Spitzer data shows several candidates from when the Universe was less than a billion years old, but finding more has been elusive (Timmer, Natarajan 26-8, BEC, STScl).
No easy answers, folks.
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© 2017 Leonard Kelley