WHAT IS BLACK HOLE SCIENCE?

The gravity of a celestial body is essentially a measure of how fast a rocket must be fired in order to escape the body's attraction. For example, the escape velocity from Earth is of about 25000 mph (40000 kmh).

Nowhere gravity is stronger than near the objects we call `black holes'. Matter falling toward a black hole can become extremely hot and luminous though, making it possible for us to infer the presence of such an object. Even more surprisingly, part of the in-falling matter can actually be turned around and propelled outward in form of narrow bipolar streams of outflowing matter and energy. As the escape velocity from very near a black hole is close to that of light, these outflows have to be incredibly fast -- millions of mph! -- or even more. These phenomena, called `relativistic jets', are amongst the most spectacular objects in the Universe.

Black holes come in different sizes. Stellar mass black holes are formed when a massive star -- some ten times more massive than our Sun - dies. Once it has exhausted its fuel, the core of a massive star can no longer sustain its own gravity, and collapses into a black hole. As most of the stars in our Galaxy live in binary systems, this black hole is left with a close companion star, and starts to swallow material from it (this phenomenon is called `accretion'). At the same time, part of the in-falling material manages to escape in the form of a relativistic jet.

Similarly, black holes with masses as big as millions of Suns lurk at the center of virtually every galaxy, including our own Milky Way (which is home to a 3.6 million solar mass black hole). The process by which these beasts form and evolve remains unclear. Certainly, super-massive black holes were formed early in the Universe, possibly through repeated merging of bigger and bigger matter over-densities. Remarkably though, while the 'sphere of influence' of a black hole is incredibly small compared with the size of its host galaxy, its presence is known to be intimately related to the evolution of the entire galaxy. Somewhat surprisingly, stars orbiting around a galaxy at tens of thousands of light years from the black hole still feel its influence. Namely, their motion is somewhat regulated by the black hole mass: the more massive the hole, the higher their dispersion in velocity (about a mean value).

At the same time, the power deposited by black hole jets into the surroundings may be a fundamental source of heat for the whole galaxy. By powering relativistic jets, and injecting heat into the surroundings black holes may be able to prevent of collapse of cold gas and to quench the formation of young - and thus blue - stars. As a result, they could help explain why massive nearby galaxies look red. Current models for the formation and evolution of cosmic structures have suggested that the so called black hole energy 'feedback' is an essential ingredient to reproduce the observed red colors of galaxies. Roughly speaking, we literally need black holes in order explain the Universe as we know it.

How can we study such objects, living hundreds of thousands of light years from us? By means of space- and ground-based telescopes: we observe the in-falling (or accreting) matter in form of infrared/ultraviolet and X-ray radiation, and the relativistic jet in form of radio waves. The former are either attenuated or blocked by the Earth atmosphere, and we need telescopes flying around our planet to detect them: NASA's great observatories: the Spitzer Space Telescope (infrared), the Hubble Space Telescope (optical) and the Chandra X-ray Observatory (X-ray). The latter pass through the atmosphere undisturbed, and we employ clever arrays of radio telescopes (such has the Very Large Array) allowing us to distinguish them from radio communication signals.

AMUSE-Virgo is an extensive observational campaign that makes use of NASA's great observatories and the Very Large Array to study the properties of super-massive black holes at the center of 100 galaxies in the Virgo cluster. At a distance of more than 50 millions of light years from us, the Virgo cluster comprises hundreds of galaxies that are bound together to form a single system permeated with hot (millions of degrees) gas.

This project aims to quantify the role of faint accreting black holes in influencing the evolution of the whole host galaxy. Specifically, we we wish to address questions such as: What is the fraction of super-massive black holes that shines in X-rays? How much energy are they depositing into their surroundings? For instance, by observing an homogeneous sample of 100 galaxies in the Virgo cluster, we may find that the fraction of X-ray active galaxies depends on the mass of the central BH. By combining information from different bands of the electromagnetic spectrum (from radio up to X-rays), we may be able to quantify the fraction of power that comes out from the black hole in the form relativistic jets. For instance, if the the relative amount of power dissipated in the form of jets increases as the X-ray luminosity decreases, this would mean that, while super-massive black holes appear as `dormant' at high energies (optical, X-rays), the are actually depositing an increasingly higher fraction of heat into their surroundings in a `dark' fashion. That is, before swallowing matter trough their event horizon, black holes give back vast amount of energy by turning part of that matter away, back to the Universe at large. After all, black holes may well turn out to be much less hostile physical entities than we thought.