Topic: Late April Astronomy Bulletin

[Clive]

MARS MIGHT HAVE SALTY LIQUID WATER
University of Copenhagen -- Niels Bohr Institute

Researchers have long known that there is water in the form of ice on
Mars. Now, new research from the Mars rover Curiosity shows that it
is possible that there is liquid water close to the surface of Mars.
The explanation is that calcium perchlorate has been found in the
soil, which lowers the freezing point, so the water does not freeze
into ice, but is liquid and present as very salty salt water -- a
brine. Under the right conditions, calcium perchlorate absorbs water
vapour from the atmosphere. Measurements from the Curiosity rover's
weather monitor show that those conditions exist at night and just
after sunrise in the winter. On the basis of measurements of humidity
and temperature at a height of 1.6 metres and at the surface of the
planet, an estimate can be made of the amount of water that is
absorbed. When night falls, some of the water vapour in the atmo-
sphere condenses on the surface; it would ordinarily be frost, but the
calcium perchlorate in the soil is very absorbent and it forms a brine
with the water, so the freezing point is lowered and the condensate
remains liquid. The soil is porous, so the water seeps down through
the soil. Over time, other salts may also dissolve in the soil, and
since they are in a liquid, they can move and precipitate elsewhere
under the surface.

Observations by the Mars probe's stereo camera have previously shown
areas characteristic of old river beds with rounded pebbles that
clearly show that a long time ago there was flowing water with a depth
of up to one metre. Now the new close-up images taken by the rover
all the way en route to Mount Sharp show that there are expanses of
sedimentary deposits, lying as 'plates' one above the other and
leaning a bit toward Mount Sharp. Such deposits are formed when large
amounts of water flow down the slopes of the crater and the streams of
water meet the stagnant water in the form of a lake. When the stream
meets the surface, the solid material carried by the stream falls down
and is deposited in the lake just at the lakeshore. Gradually, a
slightly inclined slope is built up just below the surface of the
water, and traces of such slanting deposits were found during the
entire trip to Mount Sharp. Very-fine-grained sediments, which slowly
fell down through the water, were deposited right at the very bottom
of the crater lake. The sediment plates on the bottom are level, so
everything indicates that the entire Gale Crater may have been a large
lake. About 4.5 billion years ago, Mars had 6 times as much water as
it does now and a thicker atmosphere. But most of the water has
disappeared out into space, and the reason is that Mars no longer has
global magnetic fields, as we have on Earth. Currents of liquid iron
in the Earth's interior generate the magnetic fields and they act as a
shield that protects us from cosmic radiation. The magnetic field
protects the Earth's atmosphere against degradation from energetic
particles shot out from the Sun. But Mars no longer has a global
magnetic field and its atmosphere is not protected in that way from
radiation from the Sun, so the solar particles (protons) 'shoot' the
atmosphere out into space little by little. Even though liquid water
has now been found, it is not likely that life will be found on Mars
-- it is too dry, too cold and the cosmic radiation is so powerful
that it penetrates at least a metre into the surface and would kill
all life -- at least life as we know it on Earth.

SUN FORMED LATER THAN MOST STARS IN MILKY WAY
Space Telescope Science Institute (STScI)

In one of the most comprehensive multi-observatory galaxy surveys yet,
astronomers find that galaxies like our Milky Way had a period of
intense star formation, producing stars about 30 times faster than
today. Our Sun, however, is a late arrival. The Milky Way's great
star-formation period peaked 10 billion years ago, but the Sun did not
form until roughly 5 billion years ago. By that time the star-
formation rate in our Galaxy had fallen to a trickle. The Sun's late
appearance may actually have fostered the growth of the Solar System's
planets. Elements heavier than hydrogen and helium were more abundant
later on, as more massive stars ended their lives early and enriched
the Galaxy with material that served as the building blocks of planets
and even of life on Earth. Of course, astronomers have not got any
pictures of the Milky Way's formative years to trace the history of
stellar growth. Instead, they compile the story from studies of other
galaxies similar in mass to the Milky Way, found in deep surveys of
the Universe. The farther into the Universe astronomers look, the
further back in time they are seeing, because starlight from long ago
is just arriving at Earth now. From those surveys, stretching back in
time more than 10 billion years, researchers assembled an album of
images containing nearly 2,000 snapshots of Milky-Way-like galaxies.
The new census provides the most complete picture yet of how galaxies
like the Milky Way grew over the past 10 billion years into today's
majestic spiral galaxies. The multi-wavelength study spans from
ultraviolet to far-infrared light, combining observations from the
Hubble, Spitzer and Herschel space telescopes and ground-based
telescopes.

The study confirms that galaxies have put much of their mass into
stars over the past 10 billion years; most of that happened within the
first 5 billion years. The new analysis reinforces earlier research
that showed Milky Way-like galaxies began as small clumps of stars.
The diminutive galaxies built themselves up by swallowing large
amounts of gas that ignited a firestorm of star birth. The study
reveals a strong correlation between the galaxies' star formation and
their growth in stellar mass. Observations indicated that as the
star-formation slowed down, the galaxies' growth decreased as well.
Evidence suggests that we can account for the majority of the build-up
of a galaxy like our Milky Way through its star formation. When we
calculate the star-formation rate of a Milky Way galaxy and add up all
the stars it would have produced, it is pretty consistent with the
mass growth we expected. That means we are able to understand the
growth of the 'average' galaxy with the mass of the Milky Way. The
astronomers selected the Milky-Way-like progenitors by sifting through
more than 24,000 galaxies in catalogues made with the Hubble and
Magellan telescopes.


COMPLEX MOLECULES IN INFANT STAR SYSTEM
ESO

For the first time, astronomers have detected the presence of complex
organic (i.e. carbon-containing) molecules, the building blocks of
life, in a protoplanetary disc surrounding a young star. The
discovery, made with the Atacama Large Millimetre/sub-mm Array (ALMA),
reaffirms that the conditions that spawned the Earth and Sun are not
unique in the Universe. The new ALMA observations show that the
proto-planetary disc surrounding the young star MWC 480 contains large
amounts of methyl cyanide (CH3CN), a carbon-based molecule. There is
enough methyl cyanide around MWC 480 to fill all the Earth's oceans.
Both that molecule and its simpler cousin hydrogen cyanide (HCN) were
found in the cold outer reaches of the star's newly formed disc, in a
region that astronomers believe is analogous to the Kuiper Belt -- the
realm of icy planetesimals and comets in the Solar System beyond
Neptune. Comets retain a pristine record of the early chemistry of
the Solar System, from the period of planet formation. Comets and
asteroids from the outer Solar System are thought to have seeded the
young Earth with water and organic molecules, helping to set the stage
for the development of primordial life. Studies of comets and
asteroids show that the solar nebula that formed the Sun and planets
was rich in water and complex organic compounds. We now have even
better evidence that that same chemistry exists elsewhere in the
Universe, in regions that could form planetary systems not unlike our
own. Indeed, molecules found in MWC 480 are also found in similar
concentrations in the Solar System's comets.

The star MWC 480, which is about twice the mass of the Sun, is about
150 parsecs away in the Taurus star-forming region. Its surrounding
disc is in an early stage of development, having recently coalesced
out of a cold, dark nebula of dust and gas. Studies with ALMA and
other telescopes have yet to detect any obvious signs of planet form-
ation in it. Astronomers have known for some time that cold, dark
interstellar clouds are very efficient at making complex organic
molecules, including a group of molecules known as cyanides. Those,
especially methyl cyanide, are important because they contain carbon-
nitrogen bonds, which are essential for the formation of amino acids,
the foundation of proteins and the building blocks of life. Until
now, however, it has not been known whether such complex organic
molecules could form and survive in the energetic environment of a
newly forming solar system, where shocks and radiation can easily
break chemical bonds. Astronomers can see from the latest observa-
tions that such molecules do survive. Importantly, the molecules
that ALMA detected are much more abundant than would be found in
interstellar clouds. That suggests that protoplanetary discs are very
efficient at forming complex organic molecules and that they are able
to form them on relatively short time-scales.

NOT ALL TYPE Ia SUPERNOVAE ARE THE SAME
University of Arizona

A team of astronomers has discovered that certain types of supernovae
are more diverse than previously thought. The results have
implications for big cosmological questions, such as how fast the
Universe has been expanding since the Big Bang. Most importantly, the
findings hint at the possibility that the acceleration of the
expansion of the Universe might not be quite as fast as textbooks say.
The team discovered that type-Ia supernovae, which have been
considered so uniform that cosmologists have used them as 'standard
candles' to plumb the depths of the Universe, actually fall into
different populations. The differences are not random, but lead to
separating Ia supernovae into two groups, where the group that is in
the minority near us are in the majority at large distances -- and
thus when the Universe was younger. The discovery casts new light on
the currently accepted view of the Universe expanding at a faster and
faster rate, pulled apart by a hypothetical entity called dark energy.
In 2011 astronomers discovered that many supernovae appeared fainter
than predicted because they had moved farther away from us than they
should have done if the Universe expanded at the same rate. That
indicated that the rate at which stars and galaxies move away from
each other is increasing; in other words, something has been pushing
the Universe apart faster and faster. The idea behind that reasoning
was that type-Ia supernovae would be the same brightness -- they would
all end up pretty similar when they explode. Once people knew why,
they started using them as mileposts for the far side of the Universe.
The faraway supernovae should be like the ones nearby because they
look like them; but because actually they are fainter than expected,
it led people to conclude that they are further away than expected,
and that in turn led to the conclusion that the Universe is expanding
faster than it did in the past.

The team observed a large sample of type-Ia supernovae in ultraviolet
and visible light. For their study, they combined observations made
by the Hubble telescope with those made by the Swift satellite. The
data collected with Swift were crucial because the differences between
the populations -- slight shifts toward the red or the blue spectrum
-- are subtle in visible light, which had been used to detect type-Ia
supernovae previously, but became obvious only through Swift's
dedicated follow-up observations in the ultraviolet. The realization
that there were two groups of type-Ia supernovae started with Swift
data, and then researchers went through all the other data sets.
As we go back in time, we see a change in the supernova population.
The explosion has something different about it, something that doesn't
jump out at you when you look at it in optical light, but does in the
ultraviolet. Since nobody realized that before, all the supernovae
were supposed to be the same. But if you were to look at 10 of them
nearby, they would be found to be redder on average than a sample of
10 supernovae far away. The authors conclude that some of the
reported acceleration of the Universe can be explained by colour
differences between the two groups of supernovae, leaving less
acceleration than initially reported. That would, in turn, require
less 'dark energy' than currently assumed. The authors pointed out
that more data have to be collected before scientists can understand
the impact on current measures of dark energy.


GIANT GALAXIES DIE FROM INSIDE OUT
RAS

There has been a question as to how massive, quiescent elliptical
galaxies, common in the modern Universe, quenched their once-furious
rates of star formation. Such colossal galaxies, often also called
spheroidal because of their shape, typically pack stars ten times as
densely in the central regions as in our home Galaxy, the Milky Way,
and have about ten times its mass. Astronomers refer to those big
galaxies as red and dead because they exhibit an ample abundance of
ancient red stars, but lack young blue stars and show no evidence of
new star-formation. The estimated ages of the red stars suggest that
their host galaxies ceased to make new stars about ten billion years
ago. The shutdown began right at the peak of star formation in the
Universe, when many galaxies were still giving birth to stars at a
pace about twenty times faster than nowadays. Massive dead spheroids
contain about half of all the stars that the Universe has produced
during its entire existence, and astronomers cannot claim to
understand how the Universe evolved and became as we see it today
unless they understand how those galaxies came to be. Researchers
observed a total of 22 galaxies, spanning a range of masses, from an
era about three billion years after the Big Bang. According to the
new data, the most massive galaxies in the sample kept up a steady
production of new stars in their peripheries. In their bulging,
densely packed centres, however, star formation had already stopped.
A leading theory is that star-making materials are scattered by
torrents of energy released by a galaxy's central super-massive black
hole as it sloppily devours matter. Another idea is that fresh gas
stops flowing into a galaxy, starving it of fuel for new stars and
transforming it into a red and dead spheroid.


1.8-BILLION-LIGHT-YEAR SUPER-VOID IS LARGEST STRUCTURE IN UNIVERSE
RAS

In 2004, astronomers examining a map of the radiation left over from
the Big Bang (the cosmic microwave background, or CMB) discovered the
Cold Spot, a larger-than-expected unusually cold area of the sky. The
physics surrounding the Big Bang theory predicts warmer and cooler
spots of various sizes in the infant Universe, but a spot so large and
so cold was unexpected. Now, a team of astronomers may have found an
explanation for the existence of the Cold Spot, which may be the
largest individual structure ever identified in the Universe. If the
Cold Spot originated from the Big Bang itself, it could be a rare sign
of exotic physics that the standard cosmology (basically, the Big Bang
theory and related physics) does not explain. If, however, it is
caused by a foreground structure between us and the CMB, it would be a
sign that there is an extremely rare large-scale structure in the mass
distribution of the Universe. Using data from the Pan-STARRS1 (PS1)
telescope at Haleakala on Maui, and the Wide Field Survey Explorer
(WISE) satellite, the team discovered a large super-void, a vast
region 1.8 billion light-years across, in which the density of
galaxies is much lower than usual in the known Universe. The void was
found by combining observations taken by PS1 at optical wavelengths
with observations taken by WISE at infrared wavelengths to estimate
the distance to and position of each galaxy in that part of the sky.

Earlier studies observed a much smaller area in the direction of the
Cold Spot, but they could establish only that there is no very distant
structure in that part of the sky. Paradoxically, identifying nearby
large structures is harder than finding distant ones, since we must
map larger portions of the sky to see the closer structures. The
super-void is 'only' about 3 billion light-years away from us, a
relatively short distance in the cosmic scheme of things. While the
existence of the super-void and its expected effect on the CMB do not
fully explain the Cold Spot, it is very unlikely that the super-void
and the Cold Spot are at the same location just by coincidence. The
team will continue its work using improved data from PS1, and from the
Dark Energy Survey being conducted with a telescope in Chile to study
the Cold Spot and super-void, as well as another large void located
near the constellation Draco.


DARK MATTER IS NOT COMPLETELY DARK
RAS

Astronomers believe that they might have observed the first potential
signs of dark matter interacting with a force other than gravity.
They made the discovery by using the Hubble telescope to view the
simultaneous collision of four distant galaxies at the centre of a
galaxy cluster 1.3 billion light-years away. The researchers said one
dark-matter clump appeared to be lagging 5,000 light-years behind the
galaxy it surrounds. Such an offset is predicted during collisions if
dark matter interacts, even very slightly, with forces other than
gravity. Computer simulations show that the extra friction from the
collision would make the dark matter slow down, and eventually lag
behind. Scientists believe that all galaxies exist inside clumps of
dark matter -- called dark because it is thought to interact only with
gravity, therefore making it invisible. Nobody knows what dark matter
is, but it is believed in some quarters to make up about 85% of the
Universe's mass. Without the constraining effect of its extra
gravity, galaxies like our Milky Way would fling themselves apart as
they spin. In the latest study, the researchers were able to 'see'
the dark-matter clump because of the distorting effect its mass has on
the light from background galaxies -- an effect called gravitational
lensing. The researchers added that their finding potentially rules
out the standard theory of Cold Dark Matter, where dark matter
interacts only with gravity.

If the dark matter really slowed down during the collision, that could
be the first dynamical evidence that dark matter interacts with the
world around it. The researchers note that while they appear to have
observed the offsetting of dark matter, more investigation will be
needed into other potential effects that could also produce a lag
between the dark matter and the galaxy it hosts. Similar observations
of more galaxies and computer simulations of galaxy collisions are
under way to confirm the interpretation. Previous observations showed
that dark matter interacted very little during 72 collisions between
galaxy clusters (each containing up to 1,000 galaxies). The latest
research concerns the motion of individual galaxies. Researchers say
that the collision between the galaxies could have lasted longer than
the collisions observed in the previous study, allowing even a small
frictional force to build up over time. The main uncertainty in the
result is the time span for the collision: the friction that slowed
the dark matter could have been a very weak force over acting over
about a billion years, or a relatively stronger force acting for
'only' 100 million years. Taken together, the two results may bracket
the behaviour of dark matter for the first time -- it interacts 'more
than this, but less than that'.