Giant black hole could upset galaxy evolution models
A group of astronomers led by Remco van den Bosch from the Max Planck Institute for Astronomy (MPIA) have discovered a black hole that could shake the foundations of current models of galaxy evolution. At 17 billion times the mass of the Sun, its mass is much greater than current models predict – in particular in relation to the mass of its host galaxy. This could be the most massive black hole found to date.
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To the best of our astronomical knowledge, almost every galaxy should contain in its central region what is called a supermassive black hole: a black hole with a mass between that of hundreds of thousands and billions of Suns. The best-studied super-massive black hole sits in the center of our home galaxy, the Milky Way, with a mass of about four million Suns.
For the masses of galaxies and their central black holes, an intriguing trend has emerged: a direct relationship between the mass of a galaxy's black hole and that of the galaxy's stars.
Typically, the black hole mass is a tiny fraction of the galaxy's total mass. But now a search led by the Dutch astronomer Remco van den Bosch (MPIA) has discovered a massive black hole that could upset the accepted relationship between black hole mass and galaxy mass, which plays a key role in all current theories of galaxy evolution. The observations used the Hobby-Eberly Telescope and existing images from the Hubble Space Telescope.
With a mass 17 billion times that of the Sun, the newly discovered black hole in the center of the disk galaxy NGC 1277 might even be the biggest known black hole of all: the mass of the current record holder is estimated to lie between 6 and 37 billion solar masses (McConnell et al. 2011); if the true value lies towards the lower end of that range, NGC 1277 breaks the record. At the least, NGC 1277 harbors the second-biggest known black hole.
The big surprise is that the black hole mass for NGC 1277 amounts to 14% of the total galaxy mass, instead of usual values around 0,1%. This beats the old record by more than a factor 10. Astronomers would have expected a black hole of this size inside blob-like ("elliptical") galaxies ten times larger. Instead, this black hole sits inside a fairly small disk galaxy.
Is this surprisingly massive black hole a freak accident? Preliminary analysis of additional data suggests otherwise – so far, the search has uncovered five additional galaxies that are comparatively small, yet, going by first estimates, seemed to harbor unusually large black holes too. More definite conclusions have to await detailed images of these galaxies.
If the additional candidates are confirmed, and there are indeed more black holes like this, astronomers will need to rethink fundamentally their models of galaxy evolution. In particular, they will need to look at the early universe: The galaxy hosting the new black hole appears to have formed more than 8 billion years ago, and does not appear to have changed much since then. Whatever created this giant black hole must have happened a long time ago.
The work described here will be published as van den Bosch et al., "An over-massive black hole in the compact lenticular galaxy NGC 1277", in the November 29 edition of the journal Nature.
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The co-authors are Remco C. E. van den Bosch (Max Planck Institute for Astronomy; MPIA), Karl Gebhardt (University of Texas at Austin), Kayhan Gültekin (University of Michigan, Ann Arbor), Glenn van de Ven, Arjen van der Wel (both MPIA) and Jonelle L. Walsh (University of Texas at Austin).
What was the motivation for the present study?
The accepted relationship between the mass of a galaxy and the mass of its central black hole is not completely understood - at least three completely different models have been put forth to explain the connection. One of the reasons we lack a complete picture of the black hole mass-galaxy mass relation is the paucity of data points: there are less than a hundred galaxies for which the central black hole mass can be measured.
A good way of testing a relationship is to look at the extremes. For the correlation of black hole and galaxy mass, little was known about the very biggest masses. That is why, in 2010, Remco van den Bosch began a systematic search for the most massive black holes in the cosmos. For black holes of this mass, it should be possible to trace stellar motion (and hence measure black hole masses) out to distances of hundreds of millions of light-years.
The initial step of the systematic search uses the Hobby-Eberly Telescope at McDonald Observatory in Texas. This telescope has a mirror of unrivaled size, with a total area of 11 by 9.8 meters, composed of 91 hexagonal mirrors. The total size makes the telescope particularly well suited for survey work of this kind, as observations for each galaxy can be completed fairly quickly. Using this telescope, van den Bosch tackled the task of taking spectra of nearly 700 nearby galaxies.
The result reported here is one of the first from this systematic search; additional results will be published as follow-up observations and black hole mass-modeling are completed for additional galaxies.
From the spectra taken with the Hobby-Eberly Telescope alone, van den Bosch and his colleagues derived a first estimate, using a well-known relation between the broadness of certain spectral lines (indicating the "velocity dispersion", roughly the amount by which stellar velocities deviate from the average) and central black hole mass. This uncovered a total of six candidates of relatively small galaxies with very large black holes. For only one of these six galaxies high spatial resolution imaging is available and it was thus the focus of the black hole mass measurement. For the five other galaxies more observations are required to measure the distribution of stars in their centers.
How was the black hole mass determined?
In order to measure the mass of the central black hole, astronomers need to track the motion of the galaxy's innermost stars – those whose orbits are strongly influenced by the black hole's gravity. The greater the black hole mass, the greater its influence and the speed of the stars in orbit around it.
Aspects of stellar motion can be measured by looking at the spectrum of light emitted in the galaxy's central region. Movement influences specific features ("Doppler shifts of spectral lines") in the galaxy's light in a systematic way, and these changes can be detected in the spectrum, allowing astronomers to reconstruct stellar motion.
The speeds and direction in which the stars move is influenced by the distribution of mass in the galaxy. The heavier the black hole, the faster the stars move in the center. The centers of galaxies are too dense and too distant to resolve the individual stars, and so we can only measure the distribution of velocities of the spectral lines.
To measure the black hole mass, van den Bosch et al. create a dynamical model of the galaxies that consists of all possible orbits along which stars can travel. Through a systematic search, they then find out which combination of orbits and black hole mass fit the observed distribution of stellar velocities best. In the case of NGC 1277, van den Bosch found the black hole mass to be 17±3 billion times that of the Sun, while the galaxy as a whole weighs in at 120 billion solar masses.
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