Recent research findings on M87 (NGC 4486)

M87 (Messier 87), also known as NGC 4486, is a giant elliptical galaxy, located about 53.5 million light-years away. It is noteworthy for several reasons, including the presence of an unusually large supermassive black hole (SMBH) in its active galactic nucleus, with an estimated mass of about 6.4×10⁹ times the mass of the Sun (M_⊙), two plasma jets that emit strongly at radio frequencies and extend at least 5000 light-years from the SMBH (although only the jet pointed more towards us is readily detectable), and a population of about 15,000 globular clusters.

The total mass of M87 is difficult to estimate, because elliptical galaxies like M87, and unlike spiral galaxies, do not tend to follow the Tully-Fisher relation between intrinsic luminosity and total mass calculated from rotation curves – which therefore includes dark matter. Estimates of the total mass of M87, including dark matter, come in around 6×10¹² M_⊙ within a radius of 150,000 light-years from the center. This compares with about 7×10¹¹ M_⊙ for the Milky Way, but M87 could be more than 10 times as massive.

In other comparisons, the Milky Way has only about 160 globular clusters, and a central black hole (Sagittarius A*) with a mass of about 4.2×10⁶ M_⊙. So M87's central black hole is about 1500 times as massive as the Milky Way's. Pretty impressive difference.



M87 – click for 640×480 image

Besides the recent research listed below, I've written about earlier research on M87 in these articles: Galactic black holes may be more massive than thought, Stellar birth control by supermassive black holes, Black holes in the news.

You might also be interested in some articles from the past year on the general subject of active galaxies: Active galaxies and supermassive black hole jets, Where the action is in black hole jets, Quasars in the very early universe.

Feedback under the microscope: thermodynamic structure and AGN driven shocks in M87 (6/29/10) – arXiv paper

Feedback under the microscope II: heating, gas uplift, and mixing in the nearest cluster core (3/28/10) – arXiv paper

Activity of the SMBH in M87 has a significant effect not only on the host galaxy, but also on the Virgo cluster of galaxies in which M87 is near the center. Energetic outflows of matter from near the black hole force plumes of gas out of the galaxy into the hotter intergalactic medium. The mass transported in this way represents about as much gas as is contained within 12,000 light-years of M87's center. (However, that's only about 2.5% of M87's 500,000 light-year radius.) If it had not been expelled, the gas could have formed hundreds of millions of stars.

The first paper reports on studies using the Chandra X-ray Observatory to measure gas temperatures around M87's center. The findings include detection of 2 distinct shock wave fronts about 46 thousand light-years and 10 thousand light years from the center. This indicates that explosive events occurred about 150 million and 11 million years ago, respectively.

The second paper uses observations from Chandra, XMM-Newton, and optical spectra to distinguish different phases of the hot gas surrounding M87's SMBH.

Refs:
• Galactic 'Super-Volcano' in Action (8/20/10) – Science Daily (press release)
• Galactic Supervolcano Erupts From Black Hole (8/20/10) – Wired.com
• Galactic 'Supervolcano' Seen Erupting With X-Rays (9/6/10) – Space.com

A correlation between central supermassive black holes and the globular cluster systems of early-type galaxies (8/13/10) – arXiv paper

A study of 13 galaxies, including M87, has found a correlation between the size of a galaxy's SMBH and the number of the galaxy's globular clusters. The types of galaxies studied included nine giant ellipticals (like M87), a tight spiral, and 3 galaxies intermediate in type between spiral and elliptical. The smallness of the sample is due to the exclusion of open spiral galaxies and the further limitation to cases where good estimates of the number of globular clusters and mass of the central black hole existed.

The correlation, in which the number of globular clusters is proportional to the black hole mass, is actually stronger than correlations between black hole mass and other galaxy properties previously studied for correlation, such as stellar velocity dispersion (an indicator of total mass), and luminosity of the galaxy's central bulge or whole galaxy (for ellipticals).

In some cases the correlation of black hole mass with total luminosity was especially weak, but better with number of globular clusters. For instance, Fornax A (NGC 1316) is a giant lenticular galaxy with luminosity comparable to that of M87. Yet its central black hole has a mass of 1.5×10⁸ M_⊙, 2.3% that of M87's black hole. It has 1200 globular clusters, 8% of M87's count. Clearly this is not a linear relation. Rather, the study found that the best fit was a power law with M_• ≈ (1.7×10⁵)×N^1.08±0.04, where M_• is black hole mass in units of M_⊙ and N is number of globular clusters. This relation predicts a SMBH mass of 5.5×10⁹ M_⊙ for M87, which is very close, and 3.6×10⁸ M_⊙ for the SMBH mass of NGC 1316, which is high – but the SMBH mass of NGC 1316 is also unusually low in comparison with its luminosity and velocity dispersion.

By contrast, the relation predicts that the Milky Way with a SMBH mass of 4.2×10⁶ M_⊙ should have only about 20 globular clusters, while the actual number is about 160. However, the Milky Way is a loose spiral, not one of the types that was studied, which may account for the much worse correlation. The fit is much better if only globular clusters associated with the central bulge (about 30) are considered.

The obvious question is about why this relation between SMBH mass and number of globular clusters exists. Presumably it has much to do with the typical history of a large galaxy, which is expected to include frequent mergers with other galaxies. The existence of the relationship should provide clues to galactic history, and especially how this may be different for loose spirals like the Milky Way, in comparison with more compact galaxies.

Refs:
• A correlation between central supermassive black holes and the globular cluster systems of early type galaxies (8/11/10) – The Astrophysical Journal
• Supermassive black holes reveal a surprising clue (5/25/10) – Physicsworld.com

A Displaced Supermassive Black Hole in M87 (6/16/10) – arXiv paper

It has generally been assumed that a galaxy's central SMBH is very close to the actual center of mass of the galaxy, because that is (by definition) the gravitational equilibrium point. This central point should be essentially the same as the photometric center of the galaxy, since the galaxy's stars should be distributed symmetrically around the center. Consequently, astronomers have not carefully searched for cases where a SMBH is not very near the galactic center. This lack of extensive investigation is also a result of the fact that the SMBH is often hidden inside a dense cloud of dust, so its exact position is difficult to determine. M87's SMBH (more precisely, the accretion disk around the SMBH), however, is clearly visible, and the research reported in this paper finds it is actually located about 22 light-years from the apparent galactic center.

There are various possible reasons for this much displacement from the center, and not a lot of evidence to identify the most likely reason. Possible reasons include: (1) The SMBH is part of a binary system in which the other member is not detected. (2) The SMBH could have been gravitationally perturbed by a massive object such as a globular cluster. (3) There is a significant asymmetry of the jets. (4) The SMBH has relatively recently merged with another SMBH, subsequent to an earlier merger of another galaxy with M87.

The displacement of the SMBH is in the direction opposite the visible jet, so the last two possibilities are more likely than the others. However, possibility (3) depends on the jet structure having existed at least 100 million years and the density of matter at the center of M87 being low enough to provide insufficient restoring force. Possibility (4) is viable if the SMBH is still oscillating around the center following a galactic merger within the past billion years.

Refs:
• A Displaced Supermassive Black Hole in M87 (6/9/10) – The Astrophysical Journal Letters
• Black Hole Shoved Aside, Along With 'central' Dogma (5/25/10) – Science News
• Black Hole Found in Unexpected Place (5/25/10) – Wired.com
• Supermassive black holes may frequently roam galaxy centers (5/25/10) – Physog.com (press release)
• Bizarre Behavior of Two Giant Black Holes Surprises Scientists (5/25/10) – Space.com
• Galactic Black Holes Can Migrate or Quickly Awaken from Quiescence (5/26/10) – Scientific American




M87 jet

Radio Imaging of the Very-High-Energy γ-Ray Emission Region in the Central Engine of a Radio Galaxy (7/24/09) – Science

Energetic plasma jets, in which matter is accelerated close to the speed of light, combined with intense electromagnetic emissions, especially at radio frequencies, are prominent in about 10% of active galaxies, including M87. However, little has been well established about what processes are responsible for the emissions, or more generally how the jets are powered, accelerated, and focused into narrow beams. Because of the relative proximity of M87 and the fact that the jet we observe is angled from 15° to 25° to our line of sight, M87 is one of the best objects to study in order to learn more about how jets work.

Gamma rays, because of their very high energies (greater than 100 keV per photon), are not continuously produced in active galaxy jets, but are occasionally observed in short bursts lasting only a few days. One such event occurred in M87 in February 2008. At the same time, the intensity of radiation at all other wavelengths increased substantially. Such flares, at lower energies, are not unusual, since the energy output of most jets is somewhat variable in time. The flare persisted for much longer at energies below the gamma-ray band, indicating that the disturbance continued to propagate along the jet even after the gamma-ray flare subsided. However, although we don't know what the cause was, the coincidence in time of the gamma-ray emissions and the beginning of the extended flare makes it very likely that the events had the same source.

This is significant information, because our technology for detecting gamma-ray events has very poor angular resolution (~0.1°), since gamma rays can be detected on the ground only by secondary effects that a gamma ray produces in our upper atmosphere. More than 6 orders of magnitude finer resolution can be achieved at radio frequencies, using very long baseline interferometry. With that technology, it was possible to locate the origin of the disturbance that caused both gamma ray and lower energy flaring to a region within about 100 Schwarzschild radii (R_s) of the SMBH. Since R_s = 2G×M_•/c², R_s for the M87 SMBH is about 1.9×10¹⁰ km, or more than twice the radius of the solar system. So 100R_s is about 70 light-days – which is pretty small compared to the 53.5 million light-year distance to M87.

It's also significant that the gamma-ray event occurred so close to the SMBH, because the cause must be unlike whatever is responsible for the flaring described in the following research.

Refs:
• VLBA locates superenergetic bursts near giant black hole (7/2/09) – Physorg.com (press release)
• Mysterious Light Originates Near A Galaxy's Black Hole (7/2/09) – Space.com
• A Flare for Acceleration (7/24/09) – Science
• High Energy Galactic Particle Accelerator Located (9/14/09) – Science Daily (press release)

Hubble Space Telescope observations of an extraordinary flare in the M87 jet (4/22/09) – arXiv paper

Electromagnetic radiation from SMBH jets is fairly variable in both time and location along the jet. In the case of M87, high-resolution images at various wavelengths have shown the existence of many regions of enhanced emissions within the jet. One of the most prominent of these even has a name: HST-1, so-named because it was discovered by the Hubble Space Telescope. It occupies a stationary position on the jet, about a million Schwarzschild radii from the center, i. e. about 2000 light-years from the SMBH.

HST-1 has been observable for some time, but until February 2000 it was relatively dormant. After that it began to flare more brightly across the electromagnetic spectrum up to X-rays. In 2003 it became more variable, and it reached its greatest brightness in May 2005, when the flux in near ultraviolet was 4 times as great as that of M87's central energy source, the SMBH accretion disk. This represents a brightness increase at that wavelength of a factor of 90. The X-ray flux increased by a factor of 50, and similar, synchronized changes occurred at other wavelengths. The synchronization indicates that one mechanism is responsible for the variability at all wavelengths.

What the actual cause of the disturbance may be is not clear. Because of the great distance of HST-1 from the SMBH, its basic energy source must not be the central accretion disk itself. More likely HST-1 is a result of constriction of magnetic field lines, resulting in further acceleration of the particles making up the jet. Acceleration of charged particles causes radiation by the synchrotron process, and is evidenced by polarization of the emitted photons. Constriction of the jet may be a result of passage through a region of higher density of stars. The increased variability could mean that the jet has encountered a region of higher but varying stellar density. Alternatively, the jet may be passing through a patch of thick gas or dust, with excess radiation produced by the resulting particle collisions.

These results could explain the variability of light from other, more distant active galaxies, at least those which have strong jets, given that it's possible for a small region of the jet far from the SMBH to outshine the central source. However, another source of variability occurs when a jet is viewed at a very low angle to our line of sight, in which case any slight change of direction could cause an apparent change of brightness.

Refs:
• Hubble Space Telescope observations of an extraordinary flare in the M87 jet (3/6/09) – The Astronomical Journal
• Hubble Witnesses Spectacular Flaring in Gas Jet from M87's Black Hole (4/14/09) – Physorg.com (press release)
• Black Hole Creates Spectacular Light Show (4/14/09) – Space.com
• Black hole jet brightens mysteriously (4/15/09) – New Scientist
• Black hole spews out impressive light show (4/20/09) – Cosmos Magazine