by Matt Williams
Imagine, if you will, that the Universe was once a much
dirtier place than it is today. Imagine also that what we see around us, a
relatively clean and unobscured Universe, is the result of billions of years of
stars behaving like giant celestial Roombas, cleaning up the space around them
in preparation for our arrival. According to a set of recently published
catalogues, which detail the latest findings from the ESA’s Herschel Space
Observatory, this description is actually quite fitting.
These catalogues represents the work of an international team of
over 100 astronomers who have spent the past seven years analyzing the infrared
images taken by the Herschel Astrophysical Terahertz Large Area Survey (Herschel-ATLAS).
Presented earlier this week at theNational
Astronomy Meeting in Nottingham , this catalogue revealed that as early as 1
billion years ago, the Universe looked much different than it does today.
In order to put this research into context, it is important to
understand the important of infrared astronomy. Prior to the deployment of
missions like Herschel (which was launched in 2009), astronomers were unable to
see a good portion of the light emitted by stars and galaxies. With roughly
half of this light being absorbed by interstellar dust grains, research into
the birth and lives of galaxies was difficult.
Artist’s impression of the Herschel
Space Telescope. Credit: ESA/AOES Medialab/NASA/ESA/STScI
But thanks to surveys like Herschel ATLAS – as well NASA’s Spitzer Space
Telescope and the Wide-field Infrared Survey Explorer (WISE) – astronomers have been able to
account for this missing energy. And what they have seen (especially from this
latest survey) has been quite remarkable, presenting a Universe that is far
denser than previously expected.
Last week (Friday, June 29th), during the final day of the
National Astronomy Meeting, the first of the catalogues was presented. The
images they showed gave all those present a glimpse of the unseen stars and
galaxies that have existed over the last 12 billion years of cosmic history. In
sum, over half-a-million far-infrared sources have been spotted by the
Herschel-ATLAS survey, and what they revealed was fascinating.
Many of these sources were galaxies that are nearby and similar
to our own, and which are detectable using using conventional telescopes. The
others were much more distant, their light taking billions of years to reach
us, and were obscured by concentrations of cosmic dust. The most distant of
these galaxies were roughly 12 billion light-years away, which means that they
appeared as they would have 12 billion years ago.
Ergo, astronomers now know that 12 billion years ago (i.e.
shortly after the Big Bang)., stars and galaxies were much dustier than they
are now. They further concluded that the evolution of our galaxies since
shortly after the Big Bang has essentially been a major clean-up effort, as
stars gradually absorbed the dust that obscured their light, thus making it the
more “visible” place it is today.
Herschel fig2smallAn illustration of
the time reach of the Herschel ATLAS and the kinds of objects it has
discovered. Credit: Herschel-ATLAS/ESA/ALMA/ NRAO
The data released by the survey includes several maps and
additional files which were described in an article produced by Dr. Elisabetta
Valiante and a research team from Cardiff University – titled “The Herschel-ATLAS Data Release 1 Paper I: Maps,
Catalogues and Number Counts“. As Dr. Valiante told Universe Today
via email:
“Gas and dust are the main components
of stars: they collapse to form stars and they are ejected at the end of stars’
life. The interesting thing that has been discovered thanks to the Herschel
data is that the two phenomena are not in equilibrium. We knew this was true 10
billion years ago, but we expected, according to the current models, that some
equilibrium was reached at more recent times. Instead, the amount of dust in
galaxies 5 billion years ago was much larger than the amount we see in galaxies
today: this was unexpected.”
Until recently, such a survey would have been impossible due to
the fact that many of these infrared sources would have been invisible to
astronomers. The reason for this, which was revealed by the survey, was that
these galaxies were so dusty that they would have been virtually impossible to
detect with conventional optics. What’s more, their light would have been
gravitationally magnified by intervening galaxies.
The huge size of the survey has also meant that changes that
have occurred in galaxies – relatively recent in cosmic history – can be
studied for the first time. For instance, the survey showed that even only one
billion years in the past, a small fraction of the age of the universe,
galaxies were forming stars at a faster rate and contained more dust than they
do today.
Infrared images (like the one captured
by NASA’s Spitzer Space Telescope here) show countless stars and galaxies that
are obscured in visible-light by cosmic dust. Credit: NASA/JPL-Caltech
Dr. Nathan Bourne – from the University of Edinburgh
“We can think of
galaxies as big recycling machines. When they form, they accrete gas (mostly
hydrogen and helium, with traces of lithium and a couple of other elements)
from the universe around them, and they turn it into stars. As time goes on,
the stars pump this gas back out into the galaxy, into the interstellar medium.
Due to the nuclear processes within the stars, the gas is now enriched by heavy
elements (what we call metals, though they include both metals and non-metals),
and some of these form microscopic solid particles of dust, as a sort of
by-product.
“But there are still stars forming, and the next generations of
stars recycle this interstellar material, and now that it contains heavy
elements and dust, things are a bit different, and planets can also form around
the new stars, from accumulations of this heavy material. So, if you look at
the big picture, when the first galaxies started forming within the first
billion years after the Big Bang, they began using up the gas around them, and
then while they are active they fill their interstellar medium up with gas and
dust, but by the end of a galaxy’s lifecycle, it has used up all this gas and
dust, and you could say that it has cleaned itself.”
The catalogues and maps of the hidden universe are a triumph for
the Herschel team. Despite the fact that the last information obtained by the
Herschel observatory was back in 2013, the maps and catalogues produced from
its years of service have become vital to astronomers. In addition to showing
the Universe’s hidden energy, they are also laying the groundwork for future
research.
IR images of the entire sky take by the
WISE All-Sky Data Release (top), and a projection of the IR sky created by
images taken by the COBE spacecraft (bottom). Credit: NASA/JPL-Caltech/UCLA
(top), NASA/DIRBE Team/COBE/ (bottom)
“Now we need to explain why there is dust where we did not
expect to find it.” said Valiante. “And to explain this, we need to change our
theories about how the Universe evolves. Our data poses a challenge we have
accepted, but we haven’t overcome it yet!”
“[W]e understand a lot more about how galaxies evolve,” added
Bourne, “about when most of the stars formed, what happens to the gas and dust
as galaxies evolve, and how rapidly the star-forming activity in the Universe
as a whole has faded in the latter half of the Universe’s history. It’s fair to
say that this understanding comes from having a whole suite of different types
of instruments studying different aspects of galaxies in complementary ways,
but Herschel has certainly contributed a major part of that effort and will
have a lasting legacy.”
The implications of these findings are also likely to have a
far-reaching effect, ranging from cosmology and astronomy, to perhaps shedding
some light on that tricky Fermi paradox. Could it be intelligent life that
emerged billions of years ago didn’t venture to other star systems because they
couldn’t see them? Just a thought…
This data release from the H-ATLAS team was coordinated with
releases made in late June from the Herschel Extragalactic Project (HELP) team
and the Herschel Multi-tiered Extragalactic Survey (HerMES). H-ATLAS and HerMES are parts
of the EU Research Executive Agency’s HELP program, which brings together
various extragalactic surveys carried out by Herschel and combines them with
major surveys conducted by other observatories to give the Herschel mission a
lasting legacy.
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