DEPTHS OF HUBBLE ULTRA DEEP FIELD
ESO
Astronomers using the MUSE instrument on the Very Large Telescope in Chile
e have conducted the deepest spectroscopic survey ever. They focussed on
the Hubble Ultra-Deep Field, measuring distances and properties of 1600 very
faint galaxies including 72 galaxies that had never been detected before, even
by Hubble itself. The original HUDF images were pioneering deep-field observations
with the Hubble telescope, published in 2004. They probed more deeply than ever
before and revealed a menagerie of galaxies dating back to less than a billion years
after the Big Bang. The area was subsequently observed many times by Hubble
and other telescopes, resulting in the deepest view of the Universe to date. Now,
despite the depth of the Hubble observations, MUSE has -- among many other results --
revealed 72 galaxies never seen before in that tiny area of the sky. The MUSE data
provide a new view of dim, very distant galaxies, seen near the beginning of the Universe.
It has detected galaxies 100 times fainter than in previous surveys, adding to an already
richly observed field and deepening our understanding of galaxies across the ages. The
survey unearthed 72 candidate galaxies known as Lyman-alpha emitters, that shine only
in Lyman-alpha light. Current understanding of star formation cannot fully explain those
galaxies, which just seem to shine brightly at that one wavelength. Because MUSE
disperses the light into its component colours, those objects become apparent, but they
remain invisible in deep direct images such as those from Hubble. Another major
finding of this study was the systematic detection of luminous hydrogen
haloes around galaxies in the early Universe, giving astronomers a new and
promising way to study how material flows into and out of early galaxies.
GRAVITATIONAL WAVES SHED LIGHT ON BLACK HOLES
Brown University
A new study shows how scientists could use gravitational-wave experiments
to test the existence of primordial black holes, gravity wells formed just
moments after the Big Bang, that some scientists have posited could be an
explanation for dark matter. Scientists know very well that black holes can
be formed by the collapse of large stars, or as we have seen recently, the
merger of two neutron stars, but it has been hypothesized that there could
be black holes that formed in the very early Universe before stars existed
at all. The idea is that, shortly after the Big Bang, quantum-mechanical
fluctuations led to the density distribution of matter that we observe today
in the expanding Universe. It has been suggested that some of those density
fluctuations might have been large enough to result in black holes peppered
throughout the Universe. Such so-called primordial black holes were first
proposed in the early 1970s by Stephen Hawking and collaborators, but
have never been detected -- it is still not clear whether they exist at all. The
ability to detect gravitational waves, as demonstrated recently by the Laser
Interferometer Gravitational-Wave Observatory (LIGO), has the potential to
shed new light on the issue. Such experiments detect ripples in the fabric
of space-time associated with major astronomical events like the collision
of two black holes. LIGO has already detected several black-hole mergers,
and future experiments will be able to detect events that happened much
further back in time.
With future gravitational-wave experiments, cosmologists will be able to
look back to a time before the formation of the first stars. So if we see
black-hole merger events before stars existed, then we will know that those
black holes are not of stellar origin. For this study, researchers
calculated the redshift at which black hole mergers should no longer be
detected if they are only of stellar origin. They show that at a redshift
of 40, which equates to about 65 million years after the Big Bang, merger
events should be detected at a rate of no more than one per year, assuming
stellar origin. At redshifts greater than 40, events should disappear
altogether. In reality, merger events are expected to stop well before that
point, but a redshift of 40 or so is the absolute hardest bound or cutoff
point. A redshift of 40 should be within reach of several proposed
gravitational-wave experiments. If they detect merger events beyond that,
it means one of two things: either primordial black holes exist, or the
early Universe evolved in a way that is very different from the standard
cosmological model. Either would be very important discoveries, the
researchers say. For example, primordial black holes fall into a category
of entities known as MACHOs, or Massive Compact Halo Objects. Some
scientists have proposed that dark matter -- the unseen stuff that is
thought to comprise most of the mass of the universe -- may be made of
MACHOs in the form of primordial black holes. A detection of primordial
black holes would bolster that idea, while a non-detection would cast
additional doubt upon it. The only other possible explanation for black-hole
mergers at redshifts greater than 40 is that the Universe is 'non-Gaussian'.
In the standard cosmological model, matter fluctuations in the early
Universe are described by a Gaussian probability distribution. A merger
detection could mean that matter fluctuations deviate from a Gaussian
distribution. The rate at which detections are made past a redshift of 40
-- if indeed such detections are made -- should indicate whether they are
a sign of primordial black holes or evidence for non-Gaussianity. But a
non-detection would present a strong challenge to those ideas.
MASSIVE PRIMORDIAL GALAXIES
National Radio Astronomy Observatory
Astronomers expect that the first galaxies, those that formed just a few
hundred million years after the Big Bang, would share many similarities
with some of the dwarf galaxies that we see in the nearby Universe today.
Those early agglomerations of a few thousand million stars would then
become the building blocks of the larger galaxies that came to dominate the
Universe after the first few thousand million years. Ongoing observations with
ALMA, however, have discovered surprising examples of massive, star-filled
galaxies seen when the cosmos was less than a thousand million years old.
That suggests that smaller galactic building blocks were able to assemble into
large galaxies quite quickly. The latest ALMA observations push back that epoch
of massive-galaxy formation even further by identifying two giant galaxies seen
when the Universe was 'only' 780 million years old, or about 5 per cent its current
age. ALMA also revealed that those uncommonly large galaxies are nestled inside
an even-more-massive cosmic structure, a halo of dark matter several billion times
as massive as the Sun. The two galaxies are in such close proximity -- less than the
distance from the Earth to the centre of our Galaxy -- that they will shortly merge to
form the largest galaxy ever observed at that period in cosmic history. This discovery
provides new details about the emergence of large galaxies and the role that dark
matter plays in assembling the most massive structures in the Universe.
The galaxies that researchers have studied, collectively known as
SPT0311-58, were originally identified as a single source by the South Pole
Telescope. The first observations indicated that that object was very
distant and glowing brightly in infrared light, meaning that it was
extremely dusty and likely to be going through a burst of star formation.
Subsequent observations with ALMA revealed the distance and dual nature
of the object, clearly resolving the pair of interacting galaxies. To make
that observation, ALMA had some help from a gravitational lens, which
provided an observing boost to the telescope. Gravitational lenses form
when an intervening massive object, like a galaxy or galaxy cluster, bends
the light from more distant galaxies. They do, however, distort the
appearance of the object being studied, requiring sophisticated computer
models to reconstruct the image as it would appear in its unaltered state.
That 'de-lensing' process provided intriguing details about the galaxies,
showing that the larger of the two is forming stars at a rate of 2,900 solar
masses per year. It also contains about 270 billion times the mass of
our Sun in gas and nearly 3 billion times the mass of our Sun in dust.
The astronomers determined that that galaxy's rapid star formation was
probably triggered by a close encounter with its slightly smaller companion,
which already hosts about 35 billion solar masses of stars and is
increasing its rate of starburst at the breakneck pace of 540 solar masses
per year. The researchers note that galaxies of that era are 'messier' than
the ones we see in the nearby Universe. Their more jumbled shapes would
be due to the vast stores of gas raining down on them and their ongoing
interactions and mergers with their neighbours. The new observations also
allowed the researchers to infer the presence of a truly massive dark-matter
halo surrounding both galaxies. Dark matter provides the pull of gravity
that causes the Universe to collapse into structures (galaxies, groups and
clusters of galaxies, etc.). By comparing their calculations with current
cosmological predictions, the researchers found that this halo is one of the
most massive that should exist at that time.
Topic: Mid December Astronomy Bulletin Part 2 (Read 1657 times)