Dwarf galaxies provide clues to origin of supermassive black holes

Dwarf galaxy NGC 4395, about 13 million light-years from Earth, known to harbour a black hole some 300,000 times more massive than the Sun. 

It is a prototypical example of a small galaxy once thought to be too small to contain such a black hole. 

Credit: David W. Hogg, Michael R. Blanton, and the Sloan Digital Sky Survey Collaboration; NRAO/AUI/NSF.

Pouring through data from a large sky survey, astronomers have found more than 100 small, dwarf galaxies with characteristics indicating that they harbor massive black holes feeding on surrounding gas.

The discovery confounds a common assumption that only much larger galaxies hold such monsters and may help resolve the question of how such black holes originated and grew in the early universe.

Amy Reines

"We've shown that even small galaxies can have massive black holes and that they may be more common than previously thought," said Amy Reines, of the National Radio Astronomy Observatory (NRAO).

"This is really exciting because these little galaxies hold the clues to the origin of the first 'seeds' of supermassive black holes in the early universe," she said. Reines and her colleagues presented their findings to the American Astronomical Society's meeting in Washington, DC.

Black holes are concentrations of mass so dense that not even light can escape their gravitational pull.

Nearly all "full-sized" galaxies are known to have supermassive black holes, millions or billions of times more massive than the Sun, at their cores.

Until recently, however, smaller galaxies were thought not to harbor massive black holes.

Marla Geha

Reines, along with Jenny Greene of Princeton University and Marla Geha of Yale University, analyzed data from the Sloan Digital Sky Survey and found more than 100 dwarf galaxies whose patterns of light emission indicated the presence of massive black holes and their feeding process.

"The galaxies are comparable in size to the Magellanic Clouds, dwarf satellite galaxies of the Milky Way," Geha said.

"Previously, such galaxies were thought to be too small to have such massive black holes," she added.

In the nearby universe, astronomers have found a direct relationship between the mass of a galaxy's central black hole and a "bulge" in its center.

This indicates that the black holes and the bulges may have affected each others' growth.

NASA /ESA Hubble Image: The DDO 190 a Galactic Island

Image Credit: ESA/Hubble & NASA

In terms of intergalactic real estate, our solar system has a prime location as part of a big, spiral galaxy, the Milky Way.

Numerous, less glamorous dwarf galaxies keep the Milky Way company.

Many galaxies, however, are comparatively isolated, without close neighbours.

One such example is the small galaxy known as DDO 190, snapped here in a new image from the NASA/ESA Hubble Space Telescope.

(“DDO" stands for the David Dunlap Observatory, now managed by the Royal Astronomical Society of Canada, where the catalogue was created).

DDO 190 is classified as a dwarf irregular galaxy as it is relatively small and lacks clear structure.

Older, reddish stars mostly populate DDO 190's outskirts, while some younger, bluish stars gleam in DDO 190's more crowded interior.

Some pockets of ionized gas heated up by stars appear here and there, with the most noticeable one shining towards the bottom of DDO 190 in this picture.

Meanwhile, a great number of distant galaxies with evident spiral, elliptical and less-defined shapes glow in the background.

DDO 190 lies around 9 million light years away from our solar system. It is considered part of the loosely associated Messier 94 group of galaxies, not far from the Local Group of galaxies that includes the Milky Way.

Canadian astronomer Sidney van der Bergh was the first to record DDO 190 in 1959 as part of the DDO catalogue of dwarf galaxies.

Although within the Messier 94 group, DDO 190 is on its own. The galaxy's nearest dwarf galaxy neighbour, DDO 187, is thought to be no closer than 3 million light years away.

In contrast, many of the Milky Way’s companion galaxies, such as the Large and Small Magellanic Clouds, reside within a fifth or so of that distance, and even the giant spiral of the Andromeda Galaxy is closer to the Milky Way than DDO 190 is to its nearest neighbour.

ESA/Hubble Image: A Lonely Galactic Island

In terms of intergalactic real estate, our Solar System has a plumb location as part of a big, spiral galaxy, the Milky Way.

Numerous, less glamorous dwarf galaxies, keep the Milky Way company. 

Many galaxies, however, are comparatively isolated, without close neighbours. 

One such example is the small galaxy known as DDO 190, snapped here in a new image from the NASA/ESA Hubble Space Telescope.

Astronomers Find Large Amounts of Dark Matter near the Sun

The high resolution simulation of the Milky Way used to test the mass-measuring technique. Credit: Dr A. Hobbs

Astronomers at the University of Zürich, the ETH Zurich, the University of Leicester and NAOC Beijing have found large amounts of invisible "dark matter" near the Sun.

Their results are consistent with the theory that the Milky Way Galaxy is surrounded by a massive "halo" of dark matter, but this is the first study of its kind to use a method rigorously tested against mock data from high quality simulations.

The authors also find tantalising hints of a new dark matter component in our Galaxy.

The team's results will be published in the journal Monthly Notices of the Royal Astronomical Society. Dark matter was first proposed by the Swiss astronomer Fritz Zwicky in the 1930s.

He found that clusters of galaxies were filled with a mysterious dark matter that kept them from flying apart.

Jan Oort

At nearly the same time, Jan Oort in the Netherlands discovered that the density of matter near the Sun was nearly twice what could be explained by the presence of stars and gas alone.

In the intervening decades, astronomers developed a theory of dark matter and structure formation that explains the properties of clusters and galaxies in the Universe, but the amount of dark matter in the solar neighbourhood has remained more mysterious.

For decades after Oort's measurement, studies found 3-6 times more dark matter than expected. Then last year new data and a new method claimed far less than expected.

The community was left puzzled, generally believing that the observations and analyses simply weren't sensitive enough to perform a reliable measurement.

In this latest study, the authors are much more confident in their measurement and its uncertainties. This is because they used a state-of-the-art simulation of our Galaxy to test their mass-measuring technique before applying it to real data.

Quelle Supris
This threw up a number of surprises. They found that standard techniques used over the past 20 years were biased, always tending to underestimate the amount of dark matter.

They then devised a new unbiased technique that recovered the correct answer from the simulated data.

Applying their technique to the positions and velocities of thousands of orange K dwarf stars near the Sun, they obtained a new measure of the local dark matter density.

Lead author Silvia Garbari says: "We are 99% confident that there is dark matter near the Sun. In fact, our favoured dark matter density is a little high.

There is a 10% chance that this is merely a statistical fluke. But with 90% confidence, we find more dark matter than expected.

If future data confirms this high value, the implications are exciting. It could be the first evidence for a "disc" of dark matter in our Galaxy, as recently predicted by theory and numerical simulations of galaxy formation. Or it could be that the dark matter halo of our Galaxy is squashed, boosting the local dark matter density."

Nasa Scientists Solve Mystery of Ghost Galaxies / Images

These computer simulations show a swarm of dark matter clumps around our Milky Way galaxy. Some of the dark-matter concentrations are massive enough to spark star formation.

Credit: Nasa

The green blobs are those dark-matter chunks massive enough to obtain gas from the intergalactic medium and trigger ongoing star formation, eventually creating dwarf galaxies.

The red blobs are ultra-faint dwarf galaxies that stopped forming stars long ago.

These Hubble images show the dim, star-starved dwarf galaxy Leo IV. The image shows only few stars in Leo IV galaxy.

The small and faint star-starved dwarf galaxy, Leo IV, is one of more than a dozen ultra-faint dwarf galaxies found lurking around the Milky Way. These galaxies are dominated by dark matter.

 

For several years, astronomers were puzzled as to why ghost galaxies contained only a few stars. But now scientists have solved the mystery, according to a Nasa report.

Nasa scientists have found that more than 13 billion years ago these galaxies started forming stars but the star formation abruptly stopped because of the big bang.

The discovery was made while analysing data retrieved from the Hubble space telescope. The Hubble telescope had studied three galaxies, Hercules, Leo IV, and Ursa Major. The galaxies' distance from earth ranges from 330,000 light-years to 490,000 light-years.

Ghost galaxies are the tiniest, the oldest and the most pristine galaxies in the universe. They have been discovered over the past decade by scientists using automated computer techniques to search through images of the Sloan Digital Sky Survey but scientists did not know as to why these galaxies had very few stars. Now they know that it was because of the big bang.

Reionisation of the universe began in the first billion years after the big bang. During this period, radiation from the first stars knocked electrons off primeval hydrogen atoms, ionising the cool hydrogen gas. This process allowed hydrogen gas to become transparent to ultraviolet light.

Ironically, the same radiation that sparked universal reionisation appears to have squelched star-making activities in dwarf galaxies.

The small irregular galaxies were born about 100 million years before reionisation began and they had just started to churn out stars.

Roughly 2,000 light-years wide, the galaxies are the smaller cousins of the more luminous star-making dwarf galaxies near our Milky Way.

Unlike their larger relatives, the tiny galaxies were not massive enough to shield themselves from the harsh ultraviolet light.

What little gas they had was stripped away as the flood of ultraviolet light rushed through them. Their gas supply depleted and the galaxies could not make new stars.

The ghost galaxies have very little stellar pollution and have only a few thousand stars. Even though they have very few stars, scientists claim that they have an abundance of dark matter around them.

Normal dwarf galaxies near the Milky Way contain 10 times more dark matter than the ordinary matter that makes up gas and stars. In ultra-faint dwarf galaxies, dark matter outweighs ordinary matter by at least a factor of 100.

"The small galaxies in our study are made up mostly of dark matter because their hydrogen gas was ionised and the stars got turned off," Brown explained.

Scientists claim that this discovery will help them solve another mystery about the missing satellite galaxies.

Computer simulation has shown thousands of satellite galaxies existing in the Milky Way but till now scientists have found only a few dozen galaxies.

Scientists are baffled as to how several satellite galaxies have gone missing.