500 Billion –A Universe of Galaxies: Some Older than Milky Way

By Yidir K. Published on June 10, 2013 14:08

he Hubble Space Telescope site estimates there are hundreds of billions of galaxies in the universe. A recent German super-computer simulation estimates that the number may be as high as 500 billion, with many older than the Milky Way. Common obervational wisdom among astronomers is that there are 17 billion Earth-sized planets in our galaxy. They don't yet know how many of these worlds are in habitable zones, but the implications of this discovery are astounding. Simply put: If there are 17 billion Earth-sized worlds in our galaxy alone, it's clear that the Universe has the potential to be teeming with life.

A team of scientists lead by Francois Fressin of the Harvard-Smithsonian Center for Astrophysics, used the latest data from NASA's Kepler mission to find that one in six stars have "a planet 0.8 - 1.25 times the size of Earth in an orbit of 85 days or less." Extrapolated out to the Universe as a whole, the potential number becomes mind-boggling.
With the advent of powerful space infrared telescopes like the Spitzer Space Telescope and the (recently deceased) Herschel Space Telescope, astronomers have been able to study the properties of dust in galaxies so remote that their light has been traveling towards us for over ninety percent of the age of the universe. That these distant objects are detected at all is because they are very bright in the infrared, and they are bright because they are making huge numbers of stars whose light warms the dust that in turn radiates at infrared wavelengths.

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In the image below A field of distant galaxies as seen at long infrared wavelengths over a region about one-third the size of the moon. The poor spatial resolution of infrared telescopes coupled with the great distances of the galaxies preclude seeing their spiral (or other) structures, but their different colors are apparent. A new study of 2500 distant infrared galaxies concludes they are more varied than local galaxies, probably due to different kinds of dust and dusty conditions in these early objects.

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Local galaxies - those only hundreds of millions of light-years away in our cosmic neighborhood - provide a template for understanding how galaxies behave, and are the basis for models of their distant cousins. It has been known for decades that the early universe was actively making stars in galaxies. A key question for astronomers is whether distant galaxies are different enough from local ones that different physical processes need to be included in the models, or whether comparisons with local objects are valid.

CfA astronomer Ho Seong Hwang and a large team of his collaborators have analyzed a large sample of distant galaxies to address this question. The Herschel Space Telescope during its lifetime observed many distant infrared galaxies. The astronomers selected 2500 of them from a set of over fifty thousand, based on their having clear detections at several infrared wavelengths with ancillary data from other missions. The sample was selected in a way that was independent of observer preferences, like extreme brightness, that might compromise the conclusions, the first time this has been done for such a large sample.

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The results were surprising. The scientists found that the dust in remote luminous galaxies tended to be warmer than it is in local galaxies of the same luminosity. Together with other indicators, the data suggest that the character of the dust and its environments have evolved with time in ways that are still not well known. Probably as a result of the dust variations there also appears to be a greater diversity of types of galaxies in the early universe.

Finally, the new paper notes, in accord with other recent papers, that there are indications that these galaxies may have started forming sooner after the big bang than had been anticipated in some old models.

In 2012, the Brightest of Reionizing Galaxies (BoRG) survey, which uses Hubble's WFC3 to search for the brightest galaxies around 13 billion years ago, when light from the first stars burned off a fog of cold hydrogen in a process called reionisation located five clustered galaxies so distant that their light has taken 13.1 billion years to reach us. We are seeing them just 600 million years after the Universe's birth in the Big Bang.

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Galaxy clusters are the largest structures in the Universe, comprising hundreds to thousands of galaxies bound together by gravity. This developing cluster, or protocluster, seen as it looked 13 billion years ago, presumably has grown into one of today's massive cities of galaxies, comparable to the nearby Virgo cluster of more than 2000 galaxies.

"These galaxies formed during the earliest stages of galaxy assembly, when galaxies had just started to cluster together," says the study's leader, Michele Trenti (University of Cambridge, UK and University of Colorado). "The result confirms our theoretical understanding of the buildup of galaxy clusters. And, Hubble is just powerful enough to find the first examples of them at this distance."

Most galaxies in the Universe reside in groups and clusters, and astronomers have probed many of these in detail at a range of distances. But finding clusters in the early phases of construction has been challenging because they are rare and dim.

"We need to look in many different areas because the odds of finding something this rare are very small," says Trenti who used Hubble's sharp-eyed Wide Field Camera 3 (WFC3) to pinpoint the galaxy clusters. "It's like playing a game of Battleship: the search is hit and miss. Typically, a region has nothing, but if we hit the right spot, we can find multiple galaxies."

Because these distant, fledgling clusters are so dim, the team hunted for the systems' brightest galaxies. These brilliant galaxies act as billboards, advertising cluster construction zones. From simulations, the astronomers expect galaxies at early epochs to be clustered together. Because brightness correlates with mass, the most luminous galaxies pinpoint the location of developing clusters.

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These powerful light beacons are found in deep wells of dark matter, an invisible form of matter that makes up the underlying gravitational scaffolding for galaxy formation. The team expects many fainter galaxies that were not seen in these observations to inhabit the same neighborhood.

The five bright galaxies spotted by Hubble are about one-half to one-tenth the size of our Milky Way, yet are comparable in brightness. The galaxies are bright and massive because they are being fed large amounts of gas through mergers with other galaxies. The team's simulations show that the galaxies will eventually merge and form the brightest central galaxy in the cluster, a giant elliptical radio similar to the Virgo Cluster's Messier 87 (image at top of page).

These observations demonstrate the progressive buildup of galaxies and provide further support for the hierarchical model of galaxy assembly, in which small objects accrete mass, or merge, to form bigger objects over a smooth and steady, but dramatic, process of collision and collection.

The team estimated the distance to the newly found galaxies based on their colors. Astronomers now plan to follow up with spectroscopic observations, which will help them precisely calculate the cluster's distance. These observations will also yield the velocities of the galaxies and show whether they are gravitationally bound to each other.

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The image below at left, taken in visible and near-infrared light, reveals the location of five galaxies clustered together just 600 million years after the Universe’s birth in the Big Bang. The circles pinpoint the galaxies. The sharp-eyed Wide Field Camera 3 aboard the NASA/ESA Hubble Space Telescope spied the galaxies in a random sky survey.

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The developing cluster is the most distant ever observed. The average distance between them is comparable to that of the galaxies in the Local Group, consisting of two large spiral galaxies, the Milky Way and the Andromeda Galaxy, and a few dozen small dwarf galaxies. The close-up images at right, taken in near-infrared light, show the galaxies.

Simulations show that the galaxies will eventually merge and form the brightest central galaxy in the cluster, a giant elliptical similar to the Virgo cluster’s Messier 87. Galaxy clusters are the largest structures in the Universe, comprising hundreds to thousands of galaxies bound together by gravity. The developing cluster presumably will grow into a massive galactic city, similar in size to the nearby Virgo Cluster, a collection of more than 2000 galaxies.

The Daily Galaxy via Cfa and ESA/Hubble Information Center

Image Credit: NASA, ESA, M. Trenti (University of Cambridge, UK and University of Colorado, Boulder, USA), L. Bradley (STScI), and the BoRG team; ESA GOOD-S

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