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Super Massive Black Holes “Eat” Less Material than Expected


An image shows the emission around the Milky Way’s supermassive black hole (SMBH) in X-rays (blue) and infrared (purple and yellow). The inset is an X-ray close-up of the black hole’s immediate neighborhood (half a light-year wide). *Image source: Sky & Telescope.

Taking advantage of the NASA’ super-sensitive Chandra X-ray telescope, astronomers successfully solved a puzzle about the nearest SMBH located at the center of the galaxy. The new findings could be used to explain why the black holes – supposed to swallow anything approaching – have such a low accretion rate and swallow very little of the cosmic gases available, as if they are on a severe diet. The advance has been described in the Journal Science in August.

SMBH are the largest black holes in galaxies. Scientist believe that almost every galaxy has an SMBH, so does our galaxy. The nearest SMBH, Sagittarius A* (Sgt A*), which is 4 million times heavier than the sun, lies around 26,000 light years away at the center of our own Milky Way galaxy. It has long been a great challenges to observe SMBH, since nothing can escape from the black holes – for us, their internal structures are totally unknown.

Astronomers can only observe and study things around black holes, for example, speculate their characteristics by studying the cosmic gases that are swirling at high speed. The process of swallowing hot gases blown from near stars into SMBH is called accretion. Most SMBHs have low accretion rate, thus they appear to be dark, so does Sgt A*.

Now, with the help of the Chandra instrument, Daniel Q. Wang, astronomer from University of Massachusetts, recorded the intensities of gases released from thousands of stars “close” to Sgt A*. These stars erupt extremely high winds and the intensities of these winds are much higher than the solar winds produced by the sun. If we add the gases up, the SMBH should devour a mass of hundred thousandth that of the sun each year.

The question is, such accretion rate will make Sgt A* much brighter, 100 million times brighter than it actually is. Hence, the possible explanation is that either the massive gases never arrive at the black hole, or these gases don’t release any energy when falling into the black hole. “It’s as though you filled up your car’s gas tank, drove two feet, and ran out of gas. Is there a massive leak in the fuel line, or is the mileage really horrible?” Said Jeremy Schnittman at NASA Goddard.

After observation using the extraordinary Chandra instrument, Daniel Wang and colleagues were able to predict the temperature and density of gases around Sgt A*. They proved that more than 99% of the initially captured materials never makes it into the innermost region, instead, they escape as an outflow – this is in good accordance with another prediction of the RIAF model. Astronomers believe that most accretion occur near SMBHs resemble this kind of inefficient accretion.

In the near future, a large gas cloud will collide with Sgt A* and at that time, the SMBH might become millions of times brighter. Astronomers will seize the chance and take a close glimpse on this big guy hidden in the center of the galaxy.

SourceEurekAlert!; Sky & Telescope