YOUNG VOLCANOES ON THE MOON
NASA
In 1971, Apollo 15 astronauts orbiting the Moon photographed what
looked like the aftermath of a volcanic eruption. There is nothing
unusual about volcanoes on the Moon, as much of its ancient surface is
covered with hardened lava. The main features of the 'Man in the
Moon' seen with the naked eye or minimal optical power are, in fact,
old basaltic lava flows deposited billions of years ago when the Moon
was wracked by violent eruptions. Planetary scientists have thought
that lunar vulcanism came to an end about a billion years ago, and
little has changed since. Yet the feature seen by Apollo 15 looked
remarkably fresh. For more than 30 years it remained a one-off oddity
that no one could explain. But recently a research team using the
Lunar Reconnaissance Orbiter has found 70 similar landscapes. They
call them 'Irregular Mare Patches' or IMPs for short. On the Moon, it
is possible to estimate the age of a landscape by counting its
craters. The Moon is pelted by a slow drizzle of meteoroids that
pepper its surface with impact scars. The older a landscape, the more
craters it contains.
Some of the IMPs the team found are very lightly cratered, suggesting
that they may be no more than 100 million years old. A hundred million
years sounds like a long time, but in selenological terms it is short.
The volcanic craters the LRO found may have been erupting during the
Cretaceous period of the Earth -- the heyday of dinosaurs. Some of the
features may be even younger, 50 million years old. IMPs are too
small to be seen from the Earth, averaging less 500 metres across in
their largest dimension. That is why they were not found previously.
Nevertheless, they appear to be widespread around the nearside of the
Moon. Not only are the IMPs striking landscapes, but also they tell
us something very important about the thermal evolution of the Moon --
the interior of the Moon may be hotter than has previously been
thought.
DOUBT AS TO HOW GLOBULAR CLUSTERS FORMED
ESA
The origin of globular star clusters -- large balls of stars that
orbit the centres of galaxies, but can lie very far from them -- is
still uncertain. They were once thought to consist of a single
population of stars that all formed together. However, research has
since shown that many of the Milky Way's globular clusters have more
complex formation histories and are made up of at least two distinct
populations of stars. Of those populations, around half the stars are
a single generation of normal stars that are thought to have formed first,
and the other half form a second generation of stars, which are
'polluted' with different chemical elements. In particular, the
polluted stars contain up to 50-100 times more nitrogen than the first
generation of stars. The proportion of polluted stars found in the
Milky Way's globular clusters is much higher than astronomers
expected, suggesting that a large proportion of the first-generation
star population is missing. A leading explanation for that is that
the clusters once contained many more stars but a large fraction of
the first-generation stars was ejected from the cluster at some time
in its past. That explanation makes some sense for globular clusters
in the Milky Way, where the ejected stars could easily hide among the
many similar old stars in the halo, but the new observations, which
look at globular clusters in a much smaller galaxy, call the idea into
question.
Astronomers used the Hubble telescope's Wide-Field Camera 3 (WFC3) to
observe four globular clusters in a small nearby galaxy known as the
Fornax Dwarf Spheroidal galaxy. Until now astronomers did not know
whether globular clusters in smaller galaxies had multiple generations
or not, but the new observations show clearly that they do. They too
contain a second polluted population of stars, so not only did they
form in a similar way to one another, but their formation process is
also similar to that of clusters in the Milky Way. The astronomers
measured the amount of nitrogen in the cluster stars, and found that
about half of the stars in each cluster are polluted at much the same
level as is seen in Milky Way clusters. Therefore it would seem that
the same process must have operated in the formation of the globular
clusters in the Fornax galaxy as in the Milky Way. The number of
polluted stars in the Fornax clusters seems to suggest that those
clusters would have to have been up to ten times more massive in the
past, before ejecting huge numbers of their first-generation stars and
reducing to their current size. But, unlike the Milky Way, the Fornax
galaxy has not got enough old stars to account for the huge number
that were supposedly banished from the clusters. There is nowhere
that Fornax could have hidden so many ejected stars, so it appears that
the clusters could not have been so much larger in the past. So *that*
theory on how mixed-generation globular clusters formed evidently does
not hold water and astronomers will have to think again.
HOW GALAXIES EVOLVE IN THE COSMIC WEB
University of California - Riverside
How do galaxies like our Milky Way form, and just how do they evolve?
Are galaxies affected by their surrounding environment? An inter-
national team of researchers, led by astronomers at the University of
California, Riverside, has proposed some answers. The researchers
highlight the role of the 'cosmic web' -- a large-scale web-like
structure composed of galaxies -- on the evolution of galaxies that
took place in the distant Universe, a few billion years after the Big
Bang, and present observations showing that thread-like 'filaments' in
the cosmic web played an important role in that evolution. They think
that the cosmic web, dominated by dark matter, formed very early in
the history of the Universe, starting with small initial fluctua-
tions. Such a 'skeletal' Universe must have played, in principle, a
role in galaxy formation and evolution, but that was incredibly hard
to study and understand until recently. The distribution of galaxies
and matter in the Universe is non-random. Galaxies are organized,
even today, in a manner resembling an enormous network -- the cosmic
web. The web has dense regions made up of galaxy clusters and groups,
sparsely populated regions devoid of galaxies, as well as the fila-
ments that link over-dense regions. The filaments are like bridges
connecting the denser regions in the cosmic web. It is believed that
galaxies in less-dense regions have a higher probability of actively
forming stars (much like our Milky Way), while galaxies in denser
regions form stars at a much lower rate. But the role of intermediate
environments and, in particular, the role of filaments and the cosmic
web in the early Universe remained, until very recently, unknown.
What has greatly assisted the researchers is that a large section of
the cosmic web has been become apparent in two big cosmological
surveys, COSMOS and HiZELS. They considered also data from several
telescopes (Hubble, VLT, UKIRT and Subaru), and applied a new
computational method to identify the filaments, which, in turn, helped
them study the role of the cosmic web.
They found that galaxies in the cosmic web/filaments have a much
higher chance of actively forming stars. In other words, in the
distant Universe, galaxy evolution seems to have been accelerated in
the filaments. It is possible that such filaments 'pre-process'
galaxies, accelerating their evolution while also funnelling them
towards clusters, where they are fully processed by the dense
environment of clusters and are likely to end up as dead galaxies.
The results also show that such enhancement/acceleration may stem
from galaxy-galaxy interactions in the filaments. Because of the
complexities involved in quantifying the cosmic web, astronomers
usually limit the study of it to numerical simulations and
observations in our local Universe. However, in the new study, the
researchers focused their work on the distant Universe -- when the
Universe was approximately half its present age. Astronomers were
surprised by the crucial role the filaments seem to play in galaxy
formation and evolution. Star formation is enhanced in them. The
filaments probably increase the chance of gravitational interaction
between galaxies, which, in turn, results in star-formation
enhancement. There is evidence in our local Universe that that
process in filaments also continues up to the present time.
ALIGNMENTS OVER THE LARGEST STRUCTURES IN THE UNIVERSE
ESO
Quasars are galaxies with very active supermassive black holes at
their centres. The black holes are surrounded by spinning discs of
extremely hot material that is often ejected in jets along their axes
of rotation. Quasars can shine more brightly than all the stars in
the rest of their host galaxies put together. A European research
team using the Very Large Telescope (VLT) in Chile has found that the
rotation axes of the central black holes in a sample of quasars are
parallel to each other over distances of billions of light-years, and
that the rotation axes tend to be aligned with the structures in the
cosmic web in which they belong.
Astronomers used the VLT to study 93 quasars that were known to form
groupings spread over billions of light-years, seen at a time when the
Universe was about a third of its current age. They noticed that some
of the quasars' rotation axes were aligned with each other, despite
the quasars being separated by billions of light-years. The distri-
bution of galaxies on such scales is uneven. There is large-scale
structure -- a web of filaments and clumps, and voids where galaxies
are scarce. The new VLT results indicate that the rotation axes of
the quasars tend to be parallel to the large-scale structures in which
they find themselves. So, if the quasars are in a long filament then
the spins of the central black holes will point along the filament.
The researchers estimate that the probability that the alignments are
simply the result of chance is less than 1%. A correlation between
the orientations of quasars and the structures to which they belong is
an important prediction of numerical models of evolution of the
Universe. The new data provide the first observational confirmation
of such an effect, on scales much larger that any observed to date for
normal galaxies. The team could not see the rotation axes or the jets
of the quasars directly. Instead they measured the polarisation of
the light from each quasar and, for 19 of them, found a significantly
polarised signal. The direction of the polarisation, combined with
other information, could be used to deduce the angle of the accretion
disc and hence the direction of the spin axis of the quasar. The
alignments in the new data, on scales even bigger than predictions
from simulations, may hint that there is a missing ingredient in
current models of the cosmos.
Topic: Early December Astronomy Bulletin (Read 1661 times)