Galaxies

Galaxies

How Many Galaxies Have We Discovered?

Astronomers think that there are hundreds of billions galaxies in the universe, however the exact number is not known. But astronomers should know how many galaxies we've actually seen and discovered, right?

Well, not necessarily.

“We don’t know,” says Ed Churchwell, professor of astronomy at the University of Wisconsin-Madison. “We know it’s a very large number.”

In just one image for example, the Hubble Ultra Deep Field, above, there are about 10,000 galaxies visible.

In our own galaxy, There are between 4 billion 100-300 billion stars in the Milky Way. At most, 8,479 of them are visible from Earth. Roughly 2,500 stars are available to the unaided eye in ideal conditions from a single spot at a given time.

But the number of galaxies will keep growing as our telescopes get better and can look out and back farther in time.


“To count them all, you have to be able to look far enough back in time or deep enough in space to see when galaxies were formed,” Churchwell says. “We haven’t reached that point yet. It’s not a well-determined number, but at some point we’re going to reach it.”

The estimate of how many galaxies there are in the universe is done by counting how many galaxies we can see in a small area of the sky. This number is then used to guess how many galaxies there are in the entire sky.


For the time being, the hundreds of billions in the tally are extrapolated from the Hubble Ultra Deep Field, taken over a time period in 2003 and 2004. Pointed at a single piece of space for several months — a spot covering less than one-tenth of one-millionth of the sky — Hubble returned an image of galaxies 13 billion light years away.

"Você olha para isso e dizer, 'Como muitas galáxias posso ver?" Churchwell explica. “And that turns out to be a very large number.” "E isso acaba por ser um número muito grande."



“Then you take that number of galaxies from that postage-stamp-sized piece of the sky and multiply it by the number of postage-stamp-sized pieces of sky,” Churchwell says. "Então você pega o número de galáxias do que um selo de tamanho pedaço do céu e multiplicá-lo pelo número de um selo pedaços do céu", Churchwell diz. “And that turns out to be a much larger number.” "E isso acaba por ser um número muito maior."

In the first Hubble Deep Field image , taken in 1995, there are about 3,000 galaxies visible in the image. Na primeira imagem do Hubble Deep Field, tomada em 1995, existem cerca de 3.000 galáxias visíveis na imagem.

Source: UW-M






Cosmic mystery



Researchers propose a new explanation for why some tiny galaxies have more than their fair share of dark matter.

Literally cloaked in darkness, the faintest galaxies in the universe have remained a mystery since their discovery more than two decades ago. Now a team of theorists has come up with a new explanation for the origin of these dim bodies. Known as dwarf spheroidal galaxies, these ancient stellar groupings not only serve as fossil remains of the early universe but have the highest known ratio of dark matter to ordinary, visible matter.

In the most widely accepted model of galaxy formation, an exotic type of invisible material, known as cold dark matter, provides the gravitational glue that draws together stars and gas and keeps galaxies, along with galaxy clusters, from flying apart. It would seem that all galaxies ought to have about the same ratio of dark matter to visible matter, because gravity builds all galaxies in the same way. Yet dwarf spheroidals are the most dark matter–dominated galaxies known, with 10 to 30 times the ratio of dark to visible matter as large galaxies including the Milky Way.

That’s the puzzle that Elena D’Onghia of the University of Zurich and the Harvard-Smithsonian Center for Astrophysics in Cambridge, Mass., and her colleagues set out to solve in a study posted online July 16 (http://arxiv.org/abs/0907.2442) and in an upcoming Nature.

Like other researchers, she and her collaborators assume that dwarf spheroidals were born with a lower, more typical ratio of dark to visible matter, but that much of the visible stuff somehow got pulled out.

Previous models suggest a complex, two-step process to explain the high ratio. But these models require a dwarf spheroidal to lie close to a galaxy as large as the Milky Way. In reality, some spheroidals lie far from such galaxies. Also, these models don’t easily explain the spherical shape of these galaxies or the diversity of their dark matter ratios.

In contrast, the new model proposed by D’Onghia’s team relies on the assumption that stars and gas rotate in fledgling galaxies, a property which the underlying dark matter model of formation requires.

If the rotation and orbit of stars in a dwarf spheroidal are in sync with the rotation of a slightly larger, nearby galaxy — possibly even just another dwarf spheroidal — the gravitational influences of the two galaxies on each other are enhanced, D’Onghia says.

Within 2 to 3 billion years, the gravitational pull would remove many stars from the lower-mass dwarf, D’Onghia says. Because dark matter does not rotate, it would be left behind in the dwarf galaxy. Depending on how closely the rotation of stars and gas aligns in neighboring galaxies, the dwarf spheroidals would end up with varying, but always high, ratios of dark to visible matter.

The proposed interaction could account for dwarf spheroidals, such as the recently discovered galactic duo Leo IV and Leo V, that don’t reside close to a large galaxy like the Milky Way, D’Onghia asserts.

“Certainly this is an idea that needs to be taken very seriously,” comments theorist James Bullock of the University of California, Irvine. “I bet some of the [dwarf spheroidals] formed this way, but I’m not sure if the numbers work out to explain all of them,” he adds.

D’Onghia and her collaborators simulate only the interaction of stars, not gas, cautions Rosemary Wyse of Johns Hopkins University in Baltimore, Md. But D’Onghia says that the rotating gas in a dwarf spheroidal, although more difficult to model than the stars, ought to be removed in a similar manner.

Jorge Peñarrubia of the University of Cambridge in England takes a contrarian view. “In my opinion, the whole problem may be a theoretical misconception,” due to uncertainties about star formation in galaxies, he says. Although dark matter models require that stars form in rotating disks, star-forming regions in the Milky Way indicate that most stars form in clusters instead. If stars in dwarf spheroidals don’t form in rotating disks, the scenario proposed by D’Onghia and her collaborators wouldn’t provide an explanation, he says.