Astronomers have completed conducting the broadest survey to date of galaxies from about 800 million years after the Big Bang. They found 22 early galaxies and confirmed the age of one by its characteristic hydrogen signature at 787 million years post Big Bang. The finding is the first age-confirmation of a so-called dropout galaxy at that distant time and pinpoints when an era called the reionization epoch likely began.
Astronomers have long wondered whether the universe underwent reionization instantaneously or gradually over time, but more importantly, they have tried to isolate when the universe began reionization. Galaxy density and brightness measurements are key to calculating star-formation rates, which tell a lot about what happened when. The astronomers looked at star-formation rates and the rate at which hydrogen was ionized.
Using data from their study and others, they determined that the star-formation rates were dramatically lower from 800 million years to about one billion years after the Big Bang, than thereafter. Accordingly, they calculated that the rate of ionization would be very slow during this early time, because of this low star-formation rate.
"We were really surprised that the rate of ionization seems so low, which would constitute a contradiction with the claim of NASA's WMAP satellite. It concluded that reionization started no later than 600 million years after the Big Bang," remarked Ouchi. "We think this riddle might be explained by more efficient ionizing photon production rates in early galaxies. The formation of massive stars may have been much more vigorous then than in today's galaxies. Fewer, massive stars produce more ionizing photons than many smaller stars," he explained.
With recent technological advancements, such as the Wide-Field Camera 3 on the Hubble Space Telescope, there has been an explosion of research of the reionization period, the farthest back in time that astronomers can observe.
The Big Bang, 13.7 billion years ago, created a hot, murky universe. Some 400,000 years later, temperatures cooled, electrons and protons joined to form neutral hydrogen, and the murk cleared. Some time before 1 billion years after the Big Bang, neutral hydrogen began to form stars in the first galaxies, which radiated energy and changed the hydrogen back to being ionized. Although not the thick plasma soup of the earlier period just after the Big Bang, this star formation started the reionization epoch. Astronomers know that this era ended about 1 billion years after the Big Bang, but when it began has eluded them and intrigued researchers like lead author Masami Ouchi of the Carnegie Observatories.
The U.S. and Japanese team led by Ouchi used a technique for finding these extremely distant galaxies. "We look for 'dropout' galaxies," explained Ouchi. "We use progressively redder filters that reveal increasing wavelengths of light and watch which galaxies disappear from or 'dropout' of images made using those filters. Older, more distant galaxies 'dropout' of progressively redder filters and the specific wavelengths can tell us the galaxies' distance and age. What makes this study different is that we surveyed an area that is over 100 times larger than previous ones and, as a result, had a larger sample of early galaxies (22) than past surveys. Plus, we were able to confirm one galaxy's age," he continued. "Since all the galaxies were found using the same dropout technique, they are likely to be the same age."
It helps to put things in perspective here on our frenetic little
planet with a look at this extraordinarily powerful and moving video of
the Hubble Space Telescope mapping of the Universe, whose known size is
78 billion light years across.
The video of the images is the equivalent of using a “time machine” to
look into the past to witness the early formation of galaxies, perhaps
less than one billion years after the universe’s birth in the Big Bang.
The Hubble Deep Field video below video includes mankind’s deepest, most detailed optical view of
the universe. One of the stunning
images was assembled from 342 separate exposures taken with the Wide
Field and Planetary Camera 2 (WFPC2) for ten consecutive days.
Representing a narrow “keyhole” view stretching to the visible
horizon of the universe, the HDF image covers a speck of the sky only
about the width of a dime located 75 feet away. Though the field is a
very small sample of the heavens, it is considered representative of
the typical distribution of galaxies in space because the universe,
statistically, looks largely the same in all directions. Gazing into
this small field, Hubble uncovered a bewildering assortment of at least
1,500 galaxies at various stages of evolution.
Most of the galaxies are so faint (nearly 30th magnitude or about
four-billion times fainter than can be seen by the human eye) they have
never before been seen by even the largest telescopes. Some fraction of
the galaxies in this menagerie probably date back to nearly the
beginning of the universe.
“The variety of galaxies we see is amazing. In time these Hubble data
could turn out to be the double helix of galaxy formation. We are
clearly seeing some of the galaxies as they were more than ten billion
years ago, in the process of formation,” said Robert Williams, Director
of the Space Telescope Science Institute Baltimore, Maryland. “As the
images have come up on our screens, we have not been able to keep from
wondering if we might somehow be seeing our own origins in all of this.”
Essentially a narrow, deep “core sample” of sky, the HDF is analogous
to a geologic core sample of the Earth’s crust. Just as a terrestrial
core sample is a history of events which took place as Earth’s surface
evolved, the HDF image contains information about the universe at many
different stages in time. Unlike a geologic sample though, it is not
clear what galaxies are nearby and therefore old, and what fraction are
very distant and therefore existed when the universe was newborn. “It’s
like looking down a long tube and seeing all the galaxies along that
line of sight. They’re all stacked up against one another in this
picture and the challenge now is to disentangle them,” said Mark
Dickinson of the HDF team.
Nearly a year of preparation preceded the observation. The HDF team
selected a piece of sky near the handle of the Big Dipper (part of the
northern circumpolar constellation Ursa Major, the Great Bear). The
field is far from the plane of our Galaxy and so is “uncluttered” of
nearby objects, such as foreground stars. The field provides a
“peephole” out of the galaxy that allows for a clear view all the way
to the horizon of the universe.
You may have asked, “How can the universe be 78 billion light years
across when the age of the universe is only about 13 billion years?”
How
can something be larger than then distance travelled at the speed of
light? Since light from the beginning of the universe has only had 13
billion years to travel (not 78 billion), then shouldn’t the universe
be only 13 billion light years across? That’s a pretty intuitive
thought.
But it doesn’t take into account that the entire
universe itself is also expanding. When a photon of light leaves it’s
point of origin, it does so at the speed of light, so in a universe
that doesn’t expand, a photon traveling for 13 billion years traverses
13 billion light years.
In a universe that does expand, all of
the distance covered by the photon gets increased by a scale factor
equal to the rate of expansion of the universe.
Casey Kazan via The Carnegie Institution
http://www.ciw.edu/news/dropouts_pinpoint_earliest_galaxies
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