Models of quantum gravity predict that the very structure of spacetime on length scales comparable to the Planck length, l_P = 1.6 x 10-35 m, is `frothy' or `foamy' - spacetime should contain `bubbles' of uncertainty. The basic idea is that distance measurements come with some intrinsic uncertainty, and this uncertainty becomes magnified over very large distances. Consider a laser that is very far away, and emits light of precisely a single wavelength (i.e. colour). As this light travels through spacetime, it accumulates uncertainty, and the laser light would exhibit a larger spread in energy as it travels further.
Astronomers from the IMAGE group have used ESPRESSO Science Verification observations to provide a novel limit on the existence of spacetime foam. This was made possible because ESPRESSO is a very well-calibrated and stable instrument, allowing this team to directly measure the energy width of an intrinsically narrow Fe II absorption line. The width of this line places an upper limit on the amount of spacetime foam-induced broadening. This team concludes that, if photons take discrete steps through spacetime and accumulate a Planck-scale uncertainty at every step due to quantum foam, the ESPRESSO observations require that the photon step size must be at least >1013.2 lP. They have also identified targets that will allow us to improve this limit of quantum foam by more than an order of magnitude with future ESPRESSO observations.
Click here to read the paper.
A bound on the 12C/13C ratio in near-pristine gas with ESPRESSO
January 14, 2019
The first stars to form in the Universe (known as Population III stars) are yet to be observed. However, their properties can be investigated through studying the chemical composition of the gas that they enrich. Through observations made with ESO’s new instrument, ESPRESSO, members of the IMAGE group have conducted a search for the chemical signature of low mass Population III stars in a near-pristine environment. From simulations of stellar evolution, the carbon isotope ratio, 12C/13C, is expected to be sensitive to the yields of both intermediate mass Population II stars and low mass Population III stars. These ESPRESSO observations secured the first bound on the 12C/13C ratio of one of the least polluted reservoirs of neutral gas known in the Universe. For the Damped Lyman-alpha (DLA) system found towards J0035-0918, log 12C/13C > 0.37 (95 per cent confidence). This bound rules out the strong presence of 13C in this system. Using the abundance pattern of this DLA, and a stochastic chemical enrichment model, IMAGE members investigated whether low mass Population III stars have contributed to the enrichment of this system. By considering the lifetimes of the enriching stars, it was also possible to investigate the timescale on which this DLA has been enriched. This work suggests that, following the cosmic reionisation of hydrogen, this DLA experienced a hiatus in star formation for at least 1 Gyr. The most metal-poor DLAs may therefore be affected by reionisation quenching and could offer a new environment to study this process.
Click here to read the paper.
Determining the primordial helium abundance and UV background using fluorescent emission in star-free dark matter haloes
December 12, 2019
Most of the Universe's helium atoms were formed in the first few
minutes after Big Bang, through the process of Big Bang
nucleosynthesis (BBN). The conditions present at this early time can
be studied by measuring the abundance of helium relative to hydrogen
that results from BBN. Existing techniques for making this
measurement may be affected by a number of sources of uncertainty,
and the degree to which these affect the measurements can only be
tested by developing new ways to measure the helium abundance.
Members of the IMAGE group have demonstrated the potential of one
such technique, which involves an unusual type of object known as
Reionisation-Limited HI Clouds (RELHICs). RELHICs are dark matter
haloes which have too little mass to form luminous galaxies, but
instead retain a small core of neutral gas. The composition of this
gas has remained virtually unchanged since the period of BBN, making
it an ideal laboratory for measuring the primordial helium abundance.
Using a special-purpose code, the group members modelled the faint
emission produced by RELHICs as they interact with the ultraviolet
background (UVB). They showed that the relative intensity of hydrogen
and helium emission lines is tightly correlated to the underlying
helium abundance of the gas, and addtionally found that the
properties of the emission can be used to provide an entirely novel
constraint on the shape of the UVB spectrum.
These results are summarised in the figure, which shows the precision
with which the helium abundance and UVB spectral shape can be
measured, as a function of the size of the observational
uncertainties. With errors of approx. 3% or less, shown by the
second-from-outermost (blue) contour, the uncertainty in existing
determinations of the helium abundance could be overcome, permitting
a definitive measurement of this important quantity.
Click here to read the paper.
The environment of a near pristine gas cloud at z~3.5
October 29, 2019
Members of the IMAGE group have recently published the first results from the MUSE Analysis of Gas around Galaxies (MAGG) survey which uses the Extremely Large Telescope to study the connection between gas and galaxies in the early Universe. The survey maps the positions and properties of 100s of galaxies around gas clouds (known as Lyman-Limit systems; LLSs) which are detected via the absorption of light from bright background sources. As part of this survey, they observed one of the most metal poor LLSs yet known (shown by the black plus sign in the figure). The origin of this gas cloud is of particular interest as the lack of metals suggests that it could be a remnant of the very first stars to form in our Universe. Previous work proposed that the most likely scenario is that this LLS is isolated and has no nearby galaxies. However, the MAGG survey has shown that it actually lies within an overdensity of galaxies (shown by the squares and black crosses). Taken together with the small number of other similar systems studied to date, this indicates that there is a range of different environments around these structures. The full MAGG survey has observations of ~50 more gas clouds and will therefore enable a statistical analysis of the link between gas and galaxies.
Click here to read the paper.
The MUSE Ultra Deep Field (MUDF)
September 10, 2019
An ultra deep exposure of a sky region hosting a quasar pair at z~3.22 has been obtained with the MUSE integral field spectrograph at the ESO Very Large Telescope. Several galaxy groups at 0.5<z<1.5 have been found in this field. The superior quality of MUSE data, in combination with high-resolution spectroscopy of the quasars allowed researchers - including IMAGE group members - to detect cold gas around the galaxy groups using the absorption line spectroscopy technique. Five MgII absorbers have been identified in the quasar spectra, four of which are physically associated to a galaxy group, including one case of correlated absorption in both quasar sight-lines. The absorption strength associated with the groups is higher than what has been reported for more isolated galaxies of comparable mass and impact parameters, while no evidence of widespread gas has been found. The researchers conclude that gravitational interactions in the group environment strip gas from the galaxy haloes into the intragroup medium, boosting the cross section of cool gas and leading to the high fraction of strong MgII absorbers.
Click here to read the paper.
The origin of the metals in near-pristine gas clouds
June 4, 2019
The properties of the first stars remain elusive. However, we know that they formed in a primordial
environment (i.e. from gas made of hydrogen, helium, and trace amounts of lithium).
Elements heavier than lithium were first forged within the cores of the first stars.
These elements were later dispersed when some of the first stars ended their lives as
supernovae explosions. This altered the process of all subsequent star formation.
Members of the IMAGE group have created a model to investigate these first episodes
of chemical enrichment, with the hopes of detecting the unique chemical signature of
the first stars. This model is the first to consider the number of stars that have
contributed to a systems enrichment. This is considered alongside the mass distribution
of the enriching stars and the typical explosion energy of their supernovae. Members of
the group have used this model to investigate the chemical enrichment of near-pristine
gas that has been observed in the high-redshift Universe. The results show that these
gas clouds, known as Damped Lyman-alpha systems (DLAs), have likely been enriched by
< 72 massive stars (2 sigma confidence). With current data, it is unclear whether these
systems have been exclusively enriched by the first generation of stars. However, these
results can also be used to estimate the typical physical properties of the most
metal-poor DLAs, such as the total stellar mass and the total gas mass.
Click here to read the paper.
The environment of high-redshift gas clouds revealed
May 25, 2019
Members of the IMAGE group have used the MUSE instrument at the European Very Large Telescope to study the environment of Damped Lyman-alpha absorbers, which are distant gas clouds seen against bright background sources. DLAs are thought to contain the bulk of neutral hydrogen in the early Universe, and hence they are believed to play a critical role in the regulation of star formation. While previous attempts to relate DLAs to galaxies had mixed success, this work revealed the presence of several galaxies (blue regions marked with red IDs in the figure) in the surroundings of nearly all the gas clouds studied (in this figure, at the centre of the field, marked with Q). This work, published in the Monthly Notices of the Royal Astronomical Society, finally provides a novel way to characterise the connection between DLAs and galaxies, and shows the potential of instruments like MUSE to pin down the properties of gas-rich galaxies that formed 10 billion years ago.
Click here to read the paper.
Fluorescent rings in star-free dark matter haloes
May 4, 2019
Members of the IMAGE group have recently published a new radiative transfer calculation to explore the emission properties of low-mass dark-matter halos that cannot form stars and thus remain essentially ''dark''. Halos in a characteristic mass range, however, retain sufficient amount of neutral gas to emit fluorescent light, a faint glow that arises from hydrogen recombination following ionisation due to the extragalactic ultraviolet background (UVB).
As the halos are illuminated from the outside-in, a distinctive ring appears, as shown in the figure. By exploring a wide range of models, IMAGE members have shown how the detection of such ring-like emission, although challenging, would allow new measurements of the intensity of the UVB and of the dark matter properties of these halos.
Click here to read the paper.
A group of connected nebulae in the Muse Ultra Deep Field
March 10, 2019
Members of the IMAGE group have recently published the first results from 40 hours of MUSE observations at the Very Large Telescope in the MUSE Ultra Deep Field. These observations reveal the presence of a group of three extended nebulae associated with a bright quasar pair at a redshift of 3.2, when the Universe was only 2 billion years old. As shown in the figure, the alignment and shape of these nebulae offer a tantalising suggestion of the presence of large-scale filaments (orientation marked with a dotted line) connecting the two quasar host galaxies (marked with white crosses), in line with the expectation of simulations. Upcoming ultra-deep (up to 200 hours) MUSE observations in MUDF will enable a better view of the possible presence of the filament between the two quasars in the pair.
Click here to read the paper.
MUSE goes BIG
January 14, 2019
A group of galaxies falling into a massive galaxy cluster in the Local Universe (A1367) has been observed with the MUSE integral field spectrograph at the ESO Very Large Telescope. The group, dubbed the Blue Infalling Group (BIG), is filled by a spectacular complex of ionized gas filaments (red colour the figure, superposed to the stellar light emission). The superior quality of MUSE data allowed an international team of researchers led by IMAGE group members to derive the past history of the group galaxies. One galaxy is found to a clear result of a merger of two lower mass systems, while another galaxy is currently being stripped of its atomic gas by hydrodynamic interactions in the group and cluster environments. Atomic gas is removed from the galaxy potentials both by gravitational and hydrodynamic interactions and it is heated-up during these processes allowing observations of a warmer and ionised gas phase. The researchers concluded that the combined effect of galaxy interactions in the group environment and hydrodynamic interactions with the group system as a whole can reduce the star formation activity of galaxies, leading star forming systems to become quiescent even before they reach the harsher environment of a massive cluster. This phenomenon, called “pre-processing”, is believed to be even more effective in the distant Universe at the epoch of the formation of the first group-like systems.
Click here to read the paper.
Big Bang fossil discovered
December 18, 2018
A relic cloud of gas, orphaned after the Big Bang, has been discovered in the distant Universe.
The cloud is unpolluted by stars even 1.5 billion years after the Big Bang and offers new information
about how the first galaxies formed. Gas in the Universe is usually polluted by waste from exploding
stars, but the newly discovered cloud appears to be pristine. Due to its lack of heavy elements, researchers - including IMAGE member Michele Fumagalli - have concluded that it must be a true relic of the Big Bang.
The fossil is one of only three such clouds ever found, following the discovery of two other similar relics in 2011. The researchers say that it is now possible to survey the skies for these fossil relics. This could tell us exactly how rare these gas clouds are and help us to understand how some gas formed stars and galaxies in the early Universe and why some did not. The figure shows simulation of galaxies and gas in the universe. Within the gas in the blue filaments connecting the orange galaxies lurk rare pockets of pristine gas. (Image credit: TNG Collaboration).
Click here to read the paper.
Towards an informed quest for accretion disc winds in quasars:
the intriguing case of Ton 28
November 25, 2018
IMAGE member Elisabeta Lusso, in partnership with international collaborators, has recently reported on the detection of a blueshifted absorption feature from highly ionized iron in two consecutive XMM–Newton observations (red and blue data points in the figure, where green crosses represent the combined data) of the luminous blue quasar Ton 28. The rest energy in the X-ray (marked in the left panel at 9.2 keV) implies the presence of an accretion disc wind with a bulk outflow velocity of approximately 0.3 times the speed of light, and kinetic power of at least a few per cent of the quasar bolometric luminosity. In the figure, the blue area highlights the wind absorption feature. To understand whether ongoing powerful X-ray winds leave any distinctive signatures imprinted at other wavelengths on the intrinsic quasar spectrum, the authors developed a backwards selection method to single out a quasar in its so-called ``X-ray blow-out'' evolutionary phase, which is believed to represent the initial stage of quasar feedback. Provided that the effectiveness of this selection method is confirmed with sources similar to Ton 28, this result could represent the first step towards the characterization of the fastest black-hole winds through multiwavelength indicators in the absence of high-quality X-ray spectra.
Click here to read the paper.
IMAGE members to lead deepest spectroscopic survey to date in the infrared
November 18, 2018
Members of the IMAGE group have been awarded 90 orbits on the Hubble Space Telescope
to complete the deepest spectroscopic survey to date in the near infrared.
This programme, co-led by scientists at the Space Telescope Science Institute
in the US, will deliver very deep spectroscopy using the WFC3 grism
to survey low-mass galaxies within the MUSE Ultra Deep Field, a region of the sky
that contains two bright quasars and is targeted by ultra-deep MUSE spectroscopy.
By combining the information encoded in these galaxy spectra with the absorption signatures present
in the two quasar spectra, this programme will provide an unprecedented view of the distribution of
gas and metals around low-mass galaxies at the peak of the cosmic star formation history (see figure above).
Combined with MUSE observations, this programme will also provide novel insight into the physics of
high redshift galaxies and their evolution in different environments.
A measurement of the primordial helium abundance of the Intergalactic Medium
October 15, 2018
Members of the IMAGE group have recently reported the first determination of the
primordial helium abundance based on a near-pristine intergalactic gas cloud that
is seen in absorption against the light of a background quasar. The figure above
shows the absorption profiles from the hydrogen Lyman Limit (top panel) and a
selection of neutral helium absorption lines (remaining panels). The red curves
show a model fit to the data (black histogram), and the tick marks above each
spectrum indicates the locations of the absorbing gas clouds that comprise the
partial Lyman Limit System (pLLS).
This pLLS, at a redshift z=1.724 (i.e. when the Universe was just one-third
of its present age), has a metal content 100 times less than the Sun, and contains
30 per cent fewer metals than the most metal-poor H II region currently
known where a determination of the primordial helium abundance is afforded.
Since the primordial helium abundance offers a sensitive test of new physics
beyond the Standard Model just a few minutes after the Big Bang, its determination
is of acute interest in cosmology. This study reports a 10 per cent measure
that agrees with the Standard Model value.
Click here to read the paper.
Quasar pairs trace the structure of the cosmic web
May 10, 2018
Members of the IMAGE group have conducted a survey to observe 54 closely projected quasar pairs at similar cosmological distances with the Wide Field Camera 3 on-board the Hubble Space Telescope. These quasar pairs act as double flashlights, piercing the tenuous environment in two sightlines, thus providing a 2-D map of both the large and small-scale structure of the cosmic web. The first results of this campaign reveal that these closely-projected quasar pairs show moderate excess of gas absorbing Lyman continuum photons compared to single quasars. By measuring the path of ionising photons and constructing a composite ultraviolet spectrum of pairs -- shown in figure -- IMAGE members found that the shape of the composite spectrum of pairs differ from the one of single quasars in the extreme ultraviolet. This is indicative of an excess of gas absorbing Lyman continuum photons, as also seen when measuring the mean free path (i.e. how far photons travel in the Universe before being absorbed) of ionising photons as a function of redshift. This excess of gas is thought to arise within large-scale structures or partially neutral regions inside the dark matter halos where the close pairs reside.
Click here to read the paper.
IMAGE members to lead a new Large Programme on MUSE
December 21, 2017
Members of the IMAGE group have been awarded a new ESO Large Programme. In partnership with international collaborators, IMAGE members will use the MUSE instrument at the VLT to acquire the deepest spectroscopic observations (for up to 150 hours) ever collected in a field with multiple quasars. Building on a successful pilot programme that has revealed extended hydrogen Lyman-α emission near these quasars (red in the Figure showing an image reconstructed from our pilot MUSE data), the newly-awarded programme aims at acquiring a detailed view of the cosmic web that is expected to connect galaxy halos in the early universe. Thanks to the groundbreaking capabilities of MUSE, these data will also provide a wealth of information on the physics of gas near galaxies and in groups across 5 billion years of cosmic history, and will form one of the deepest spectroscopic surveys in blank fields.
A one per cent determination of the primordial deuterium abundance
November 1, 2017
IMAGE member Ryan Cooke has recently reported a one per cent determination of the
primordial deuterium abundance. Assuming the Standard Model of particle physics and
cosmology, this measurement corresponds to a Universal density of baryons in good
agreement with the recent measurement derived from the Cosmic Microwave Background
temperature fluctuations recorded by the Planck Satellite.
The figure above shows seven high precision measurements of the deuterium to hydrogen ratio
over a broad range of metallicity (blue symbols). The weighted mean of these seven
independent measurements is shown by the red horizontal lines. The grey horizontal
band shows the Planck measure of the baryon density, assuming the Standard Model.
Click here to read the paper.
IMAGE member awarded European Research Council Starting Grant
September 20, 2017
IMAGE member Michele Fumagalli has been awarded a prestigious Starting Grant, worth 1.5 million euros, from the European Research Council. The grant will fund several students and postdocs over the next 5 years to work within the IMAGE group on the project entitled "A new vantage point on how gas flows regulate the build-up of galaxies in the early universe". This project will focus on understanding the inflow and outflow of gas from galaxies and into the wider cosmic web — a process that regulates star-formation and the evolution of galaxies across cosmic time. The research will tackle the dual questions of how gas accretion regulates the build-up of galaxies and how efficiently outflows remove gas from star-forming regions within galaxies, using both state-of-the art simulations and new observational techniques.
IMAGE members get a taste of GALACSI at VLT
August 20, 2017
On the night of August 15, IMAGE members were the first visiting scientists to
turn on the four powerful lasers (shown here in a picture by IMAGE member Ruari Mackenzie)
of the recently-commissioned GALACSI system at VLT/UT4. With the aid of the
new adaptive-optics system, MUSE observations of a sky region known to host three physically-associated
quasars were collected with an exquisite image quality of less than 0.5 arcsec. These data will enable
a detailed study of the environment surrounding these bright quasars, as well as of the
intergalactic medium probed by multiple closely-spaced lines of sight.
Click here to watch a video by Ruari Mackenzie
of the UT4 lasers.
Large nebula uncovered next to a z~3 gas cloud
August 1, 2017
An extended nebula - one billion times larger than the distance between the Earth and the Sun - has been
uncovered by members of the IMAGE group next to a damped Lyman-α system (DLA), a dense gas cloud rich in
neutral hydrogen and heavy elements. Located at a large cosmological distance, DLAs are believed to trace the reservoir
of neutral hydrogen next to galaxies that are assembling a few billion years after the Big Bang.
This nebula (shown in the Figure above) is one of the largest known to date next to a DLA, and is believed to be
a signpost of an extended cosmic structure within which multiple galaxies are assembling. While commonly
predicted by cosmological simulations, these structures have been mostly elusive so far. However, equipped with the
MUSE instrument of the European Southern Observatory’s Very Large Telescope, IMAGE members are on the lookout
for even more examples of these nebulae near DLAs, which can provide new clues about how galaxies
form inside cosmic filaments by converting gas into stars.
Click here to read the discovery paper.
Most primitive galaxy known in our cosmic neighbourhood unveiled
July 4, 2017
A near-pristine galaxy (nicknamed the Little Cub, because it is
located in the Ursa Major, or Great Bear, constellation) has
recently been discovered by members of the IMAGE group at Durham
(Little Cub is located in the inner blue contour in the figure above).
Little Cub is among the most metal-poor star forming
galaxies currently known and is being cannibalised by a nearby
grand design spiral galaxy (NGC 3359 - the spiral galaxy
seen in the figure above). The discovery has enabled us to
learn about the processes that strip low mass galaxies of their gas and
stars. The discovery also allows us to study the genesis of the first
chemical elements (including hydrogen and helium), that were created
just minutes after the Big Bang.
Click here to find out more and
click here to read the discovery paper.
What Matter(s) Around Galaxies
June 30, 2017
Over 100 scientists from around the globe gathered at Durham University for the conference "What Matter(s) Around Galaxies: Resolving the physics of the circumgalactic medium", which took place between the 19th and 23rd of June 2017. Under the attentive supervision of IMAGE members involved in the local and scientific organisation, participants presented and discussed the latest developments in our understanding of the circumgalactic medium. Talks covered a wide range of topics, focusing on the use of different methodologies (analytic theory, numerical simulations, observations) and tracers (from radio to optical, to X-ray) to address key open questions on the nature of the circumgalactic medium.
Visit the conference website for more information and for the PDFs of the talks.
First detection of fluorescence induced by ultraviolet background
March 20, 2017
Fluorescent emission - a red glow that appears when ultraviolet (UV) light interacts with gas -
has been observed near the edge of the UGC 7321 galaxy at 30 million light years from Earth. Members of the IMAGE group
in collaboration with European and US scientists, pointed the MUSE instrument of the European Southern Observatory’s
Very Large Telescope to map the red light produced by the UV radiation illuminating the gas in UGC 7321.
By measuring the intensity of this very faint glow - shown in an artistic impression in the above Figure -
it has been possible to infer the presence of the elusive UV background. This study paves the ground for new ways to map the
temperature of the cosmos and to understand why there are so few small galaxies in the Universe.
Click here to find out more and
click here to read the paper.
Regular events
IMAGE group meeting
First Friday of each month, 11 am. Room: OCW116
Group discussion of the latest science highlights from IMAGE. See calendar of events
for the schedule of the upcoming meetings.
IGM Journal Club
Fortnightly. Room: R5
In-depth discussion of recent papers on topics of interest to the
IMAGE group. All are welcome to join. See calendar of events for the times of the
upcoming meetings together with title of the papers for discussion.
Models of quantum gravity predict that the very structure of spacetime on length scales comparable to the Planck length, l_P = 1.6 x 10-35 m, is `frothy' or `foamy' - spacetime should contain `bubbles' of uncertainty. The basic idea is that distance measurements come with some intrinsic uncertainty, and this uncertainty becomes magnified over very large distances. Consider a laser that is very far away, and emits light of precisely a single wavelength (i.e. colour). As this light travels through spacetime, it accumulates uncertainty, and the laser light would exhibit a larger spread in energy as it travels further.
Astronomers from the IMAGE group have used ESPRESSO Science Verification observations to provide a novel limit on the existence of spacetime foam. This was made possible because ESPRESSO is a very well-calibrated and stable instrument, allowing this team to directly measure the energy width of an intrinsically narrow Fe II absorption line. The width of this line places an upper limit on the amount of spacetime foam-induced broadening. This team concludes that, if photons take discrete steps through spacetime and accumulate a Planck-scale uncertainty at every step due to quantum foam, the ESPRESSO observations require that the photon step size must be at least >1013.2 lP. They have also identified targets that will allow us to improve this limit of quantum foam by more than an order of magnitude with future ESPRESSO observations.
Click here to read the paper.
The first stars to form in the Universe (known as Population III stars) are yet to be observed. However, their properties can be investigated through studying the chemical composition of the gas that they enrich. Through observations made with ESO’s new instrument, ESPRESSO, members of the IMAGE group have conducted a search for the chemical signature of low mass Population III stars in a near-pristine environment. From simulations of stellar evolution, the carbon isotope ratio, 12C/13C, is expected to be sensitive to the yields of both intermediate mass Population II stars and low mass Population III stars. These ESPRESSO observations secured the first bound on the 12C/13C ratio of one of the least polluted reservoirs of neutral gas known in the Universe. For the Damped Lyman-alpha (DLA) system found towards J0035-0918, log 12C/13C > 0.37 (95 per cent confidence). This bound rules out the strong presence of 13C in this system. Using the abundance pattern of this DLA, and a stochastic chemical enrichment model, IMAGE members investigated whether low mass Population III stars have contributed to the enrichment of this system. By considering the lifetimes of the enriching stars, it was also possible to investigate the timescale on which this DLA has been enriched. This work suggests that, following the cosmic reionisation of hydrogen, this DLA experienced a hiatus in star formation for at least 1 Gyr. The most metal-poor DLAs may therefore be affected by reionisation quenching and could offer a new environment to study this process.
Click here to read the paper.
Most of the Universe's helium atoms were formed in the first few
minutes after Big Bang, through the process of Big Bang
nucleosynthesis (BBN). The conditions present at this early time can
be studied by measuring the abundance of helium relative to hydrogen
that results from BBN. Existing techniques for making this
measurement may be affected by a number of sources of uncertainty,
and the degree to which these affect the measurements can only be
tested by developing new ways to measure the helium abundance.
Members of the IMAGE group have demonstrated the potential of one
such technique, which involves an unusual type of object known as
Reionisation-Limited HI Clouds (RELHICs). RELHICs are dark matter
haloes which have too little mass to form luminous galaxies, but
instead retain a small core of neutral gas. The composition of this
gas has remained virtually unchanged since the period of BBN, making
it an ideal laboratory for measuring the primordial helium abundance.
Using a special-purpose code, the group members modelled the faint
emission produced by RELHICs as they interact with the ultraviolet
background (UVB). They showed that the relative intensity of hydrogen
and helium emission lines is tightly correlated to the underlying
helium abundance of the gas, and addtionally found that the
properties of the emission can be used to provide an entirely novel
constraint on the shape of the UVB spectrum.
These results are summarised in the figure, which shows the precision
with which the helium abundance and UVB spectral shape can be
measured, as a function of the size of the observational
uncertainties. With errors of approx. 3% or less, shown by the
second-from-outermost (blue) contour, the uncertainty in existing
determinations of the helium abundance could be overcome, permitting
a definitive measurement of this important quantity.
Click here to read the paper.
Members of the IMAGE group have recently published the first results from the MUSE Analysis of Gas around Galaxies (MAGG) survey which uses the Extremely Large Telescope to study the connection between gas and galaxies in the early Universe. The survey maps the positions and properties of 100s of galaxies around gas clouds (known as Lyman-Limit systems; LLSs) which are detected via the absorption of light from bright background sources. As part of this survey, they observed one of the most metal poor LLSs yet known (shown by the black plus sign in the figure). The origin of this gas cloud is of particular interest as the lack of metals suggests that it could be a remnant of the very first stars to form in our Universe. Previous work proposed that the most likely scenario is that this LLS is isolated and has no nearby galaxies. However, the MAGG survey has shown that it actually lies within an overdensity of galaxies (shown by the squares and black crosses). Taken together with the small number of other similar systems studied to date, this indicates that there is a range of different environments around these structures. The full MAGG survey has observations of ~50 more gas clouds and will therefore enable a statistical analysis of the link between gas and galaxies.
Click here to read the paper.
An ultra deep exposure of a sky region hosting a quasar pair at z~3.22 has been obtained with the MUSE integral field spectrograph at the ESO Very Large Telescope. Several galaxy groups at 0.5<z<1.5 have been found in this field. The superior quality of MUSE data, in combination with high-resolution spectroscopy of the quasars allowed researchers - including IMAGE group members - to detect cold gas around the galaxy groups using the absorption line spectroscopy technique. Five MgII absorbers have been identified in the quasar spectra, four of which are physically associated to a galaxy group, including one case of correlated absorption in both quasar sight-lines. The absorption strength associated with the groups is higher than what has been reported for more isolated galaxies of comparable mass and impact parameters, while no evidence of widespread gas has been found. The researchers conclude that gravitational interactions in the group environment strip gas from the galaxy haloes into the intragroup medium, boosting the cross section of cool gas and leading to the high fraction of strong MgII absorbers.
Click here to read the paper.
The properties of the first stars remain elusive. However, we know that they formed in a primordial
environment (i.e. from gas made of hydrogen, helium, and trace amounts of lithium).
Elements heavier than lithium were first forged within the cores of the first stars.
These elements were later dispersed when some of the first stars ended their lives as
supernovae explosions. This altered the process of all subsequent star formation.
Members of the IMAGE group have created a model to investigate these first episodes
of chemical enrichment, with the hopes of detecting the unique chemical signature of
the first stars. This model is the first to consider the number of stars that have
contributed to a systems enrichment. This is considered alongside the mass distribution
of the enriching stars and the typical explosion energy of their supernovae. Members of
the group have used this model to investigate the chemical enrichment of near-pristine
gas that has been observed in the high-redshift Universe. The results show that these
gas clouds, known as Damped Lyman-alpha systems (DLAs), have likely been enriched by
< 72 massive stars (2 sigma confidence). With current data, it is unclear whether these
systems have been exclusively enriched by the first generation of stars. However, these
results can also be used to estimate the typical physical properties of the most
metal-poor DLAs, such as the total stellar mass and the total gas mass.
Click here to read the paper.
Members of the IMAGE group have used the MUSE instrument at the European Very Large Telescope to study the environment of Damped Lyman-alpha absorbers, which are distant gas clouds seen against bright background sources. DLAs are thought to contain the bulk of neutral hydrogen in the early Universe, and hence they are believed to play a critical role in the regulation of star formation. While previous attempts to relate DLAs to galaxies had mixed success, this work revealed the presence of several galaxies (blue regions marked with red IDs in the figure) in the surroundings of nearly all the gas clouds studied (in this figure, at the centre of the field, marked with Q). This work, published in the Monthly Notices of the Royal Astronomical Society, finally provides a novel way to characterise the connection between DLAs and galaxies, and shows the potential of instruments like MUSE to pin down the properties of gas-rich galaxies that formed 10 billion years ago.
Click here to read the paper.
Members of the IMAGE group have recently published a new radiative transfer calculation to explore the emission properties of low-mass dark-matter halos that cannot form stars and thus remain essentially ''dark''. Halos in a characteristic mass range, however, retain sufficient amount of neutral gas to emit fluorescent light, a faint glow that arises from hydrogen recombination following ionisation due to the extragalactic ultraviolet background (UVB).
As the halos are illuminated from the outside-in, a distinctive ring appears, as shown in the figure. By exploring a wide range of models, IMAGE members have shown how the detection of such ring-like emission, although challenging, would allow new measurements of the intensity of the UVB and of the dark matter properties of these halos.
Click here to read the paper.
Members of the IMAGE group have recently published the first results from 40 hours of MUSE observations at the Very Large Telescope in the MUSE Ultra Deep Field. These observations reveal the presence of a group of three extended nebulae associated with a bright quasar pair at a redshift of 3.2, when the Universe was only 2 billion years old. As shown in the figure, the alignment and shape of these nebulae offer a tantalising suggestion of the presence of large-scale filaments (orientation marked with a dotted line) connecting the two quasar host galaxies (marked with white crosses), in line with the expectation of simulations. Upcoming ultra-deep (up to 200 hours) MUSE observations in MUDF will enable a better view of the possible presence of the filament between the two quasars in the pair.
Click here to read the paper.
A group of galaxies falling into a massive galaxy cluster in the Local Universe (A1367) has been observed with the MUSE integral field spectrograph at the ESO Very Large Telescope. The group, dubbed the Blue Infalling Group (BIG), is filled by a spectacular complex of ionized gas filaments (red colour the figure, superposed to the stellar light emission). The superior quality of MUSE data allowed an international team of researchers led by IMAGE group members to derive the past history of the group galaxies. One galaxy is found to a clear result of a merger of two lower mass systems, while another galaxy is currently being stripped of its atomic gas by hydrodynamic interactions in the group and cluster environments. Atomic gas is removed from the galaxy potentials both by gravitational and hydrodynamic interactions and it is heated-up during these processes allowing observations of a warmer and ionised gas phase. The researchers concluded that the combined effect of galaxy interactions in the group environment and hydrodynamic interactions with the group system as a whole can reduce the star formation activity of galaxies, leading star forming systems to become quiescent even before they reach the harsher environment of a massive cluster. This phenomenon, called “pre-processing”, is believed to be even more effective in the distant Universe at the epoch of the formation of the first group-like systems.
Click here to read the paper.
A relic cloud of gas, orphaned after the Big Bang, has been discovered in the distant Universe.
The cloud is unpolluted by stars even 1.5 billion years after the Big Bang and offers new information
about how the first galaxies formed. Gas in the Universe is usually polluted by waste from exploding
stars, but the newly discovered cloud appears to be pristine. Due to its lack of heavy elements, researchers - including IMAGE member Michele Fumagalli - have concluded that it must be a true relic of the Big Bang.
The fossil is one of only three such clouds ever found, following the discovery of two other similar relics in 2011. The researchers say that it is now possible to survey the skies for these fossil relics. This could tell us exactly how rare these gas clouds are and help us to understand how some gas formed stars and galaxies in the early Universe and why some did not. The figure shows simulation of galaxies and gas in the universe. Within the gas in the blue filaments connecting the orange galaxies lurk rare pockets of pristine gas. (Image credit: TNG Collaboration).
Click here to read the paper.
IMAGE member Elisabeta Lusso, in partnership with international collaborators, has recently reported on the detection of a blueshifted absorption feature from highly ionized iron in two consecutive XMM–Newton observations (red and blue data points in the figure, where green crosses represent the combined data) of the luminous blue quasar Ton 28. The rest energy in the X-ray (marked in the left panel at 9.2 keV) implies the presence of an accretion disc wind with a bulk outflow velocity of approximately 0.3 times the speed of light, and kinetic power of at least a few per cent of the quasar bolometric luminosity. In the figure, the blue area highlights the wind absorption feature. To understand whether ongoing powerful X-ray winds leave any distinctive signatures imprinted at other wavelengths on the intrinsic quasar spectrum, the authors developed a backwards selection method to single out a quasar in its so-called ``X-ray blow-out'' evolutionary phase, which is believed to represent the initial stage of quasar feedback. Provided that the effectiveness of this selection method is confirmed with sources similar to Ton 28, this result could represent the first step towards the characterization of the fastest black-hole winds through multiwavelength indicators in the absence of high-quality X-ray spectra.
Click here to read the paper.
Members of the IMAGE group have been awarded 90 orbits on the Hubble Space Telescope
to complete the deepest spectroscopic survey to date in the near infrared.
This programme, co-led by scientists at the Space Telescope Science Institute
in the US, will deliver very deep spectroscopy using the WFC3 grism
to survey low-mass galaxies within the MUSE Ultra Deep Field, a region of the sky
that contains two bright quasars and is targeted by ultra-deep MUSE spectroscopy.
By combining the information encoded in these galaxy spectra with the absorption signatures present
in the two quasar spectra, this programme will provide an unprecedented view of the distribution of
gas and metals around low-mass galaxies at the peak of the cosmic star formation history (see figure above).
Combined with MUSE observations, this programme will also provide novel insight into the physics of
high redshift galaxies and their evolution in different environments.
Members of the IMAGE group have recently reported the first determination of the
primordial helium abundance based on a near-pristine intergalactic gas cloud that
is seen in absorption against the light of a background quasar. The figure above
shows the absorption profiles from the hydrogen Lyman Limit (top panel) and a
selection of neutral helium absorption lines (remaining panels). The red curves
show a model fit to the data (black histogram), and the tick marks above each
spectrum indicates the locations of the absorbing gas clouds that comprise the
partial Lyman Limit System (pLLS).
This pLLS, at a redshift z=1.724 (i.e. when the Universe was just one-third
of its present age), has a metal content 100 times less than the Sun, and contains
30 per cent fewer metals than the most metal-poor H II region currently
known where a determination of the primordial helium abundance is afforded.
Since the primordial helium abundance offers a sensitive test of new physics
beyond the Standard Model just a few minutes after the Big Bang, its determination
is of acute interest in cosmology. This study reports a 10 per cent measure
that agrees with the Standard Model value.
Click here to read the paper.
Members of the IMAGE group have conducted a survey to observe 54 closely projected quasar pairs at similar cosmological distances with the Wide Field Camera 3 on-board the Hubble Space Telescope. These quasar pairs act as double flashlights, piercing the tenuous environment in two sightlines, thus providing a 2-D map of both the large and small-scale structure of the cosmic web. The first results of this campaign reveal that these closely-projected quasar pairs show moderate excess of gas absorbing Lyman continuum photons compared to single quasars. By measuring the path of ionising photons and constructing a composite ultraviolet spectrum of pairs -- shown in figure -- IMAGE members found that the shape of the composite spectrum of pairs differ from the one of single quasars in the extreme ultraviolet. This is indicative of an excess of gas absorbing Lyman continuum photons, as also seen when measuring the mean free path (i.e. how far photons travel in the Universe before being absorbed) of ionising photons as a function of redshift. This excess of gas is thought to arise within large-scale structures or partially neutral regions inside the dark matter halos where the close pairs reside.
Click here to read the paper.
Members of the IMAGE group have been awarded a new ESO Large Programme. In partnership with international collaborators, IMAGE members will use the MUSE instrument at the VLT to acquire the deepest spectroscopic observations (for up to 150 hours) ever collected in a field with multiple quasars. Building on a successful pilot programme that has revealed extended hydrogen Lyman-α emission near these quasars (red in the Figure showing an image reconstructed from our pilot MUSE data), the newly-awarded programme aims at acquiring a detailed view of the cosmic web that is expected to connect galaxy halos in the early universe. Thanks to the groundbreaking capabilities of MUSE, these data will also provide a wealth of information on the physics of gas near galaxies and in groups across 5 billion years of cosmic history, and will form one of the deepest spectroscopic surveys in blank fields.
IMAGE member Ryan Cooke has recently reported a one per cent determination of the
primordial deuterium abundance. Assuming the Standard Model of particle physics and
cosmology, this measurement corresponds to a Universal density of baryons in good
agreement with the recent measurement derived from the Cosmic Microwave Background
temperature fluctuations recorded by the Planck Satellite.
The figure above shows seven high precision measurements of the deuterium to hydrogen ratio
over a broad range of metallicity (blue symbols). The weighted mean of these seven
independent measurements is shown by the red horizontal lines. The grey horizontal
band shows the Planck measure of the baryon density, assuming the Standard Model.
Click here to read the paper.
On the night of August 15, IMAGE members were the first visiting scientists to
turn on the four powerful lasers (shown here in a picture by IMAGE member Ruari Mackenzie)
of the recently-commissioned GALACSI system at VLT/UT4. With the aid of the
new adaptive-optics system, MUSE observations of a sky region known to host three physically-associated
quasars were collected with an exquisite image quality of less than 0.5 arcsec. These data will enable
a detailed study of the environment surrounding these bright quasars, as well as of the
intergalactic medium probed by multiple closely-spaced lines of sight.
Click here to watch a video by Ruari Mackenzie
of the UT4 lasers.
An extended nebula - one billion times larger than the distance between the Earth and the Sun - has been
uncovered by members of the IMAGE group next to a damped Lyman-α system (DLA), a dense gas cloud rich in
neutral hydrogen and heavy elements. Located at a large cosmological distance, DLAs are believed to trace the reservoir
of neutral hydrogen next to galaxies that are assembling a few billion years after the Big Bang.
This nebula (shown in the Figure above) is one of the largest known to date next to a DLA, and is believed to be
a signpost of an extended cosmic structure within which multiple galaxies are assembling. While commonly
predicted by cosmological simulations, these structures have been mostly elusive so far. However, equipped with the
MUSE instrument of the European Southern Observatory’s Very Large Telescope, IMAGE members are on the lookout
for even more examples of these nebulae near DLAs, which can provide new clues about how galaxies
form inside cosmic filaments by converting gas into stars.
Click here to read the discovery paper.
A near-pristine galaxy (nicknamed the Little Cub, because it is
located in the Ursa Major, or Great Bear, constellation) has
recently been discovered by members of the IMAGE group at Durham
(Little Cub is located in the inner blue contour in the figure above).
Little Cub is among the most metal-poor star forming
galaxies currently known and is being cannibalised by a nearby
grand design spiral galaxy (NGC 3359 - the spiral galaxy
seen in the figure above). The discovery has enabled us to
learn about the processes that strip low mass galaxies of their gas and
stars. The discovery also allows us to study the genesis of the first
chemical elements (including hydrogen and helium), that were created
just minutes after the Big Bang.
Click here to find out more and
click here to read the discovery paper.
Over 100 scientists from around the globe gathered at Durham University for the conference "What Matter(s) Around Galaxies: Resolving the physics of the circumgalactic medium", which took place between the 19th and 23rd of June 2017. Under the attentive supervision of IMAGE members involved in the local and scientific organisation, participants presented and discussed the latest developments in our understanding of the circumgalactic medium. Talks covered a wide range of topics, focusing on the use of different methodologies (analytic theory, numerical simulations, observations) and tracers (from radio to optical, to X-ray) to address key open questions on the nature of the circumgalactic medium.
Visit the conference website for more information and for the PDFs of the talks.
Fluorescent emission - a red glow that appears when ultraviolet (UV) light interacts with gas -
has been observed near the edge of the UGC 7321 galaxy at 30 million light years from Earth. Members of the IMAGE group
in collaboration with European and US scientists, pointed the MUSE instrument of the European Southern Observatory’s
Very Large Telescope to map the red light produced by the UV radiation illuminating the gas in UGC 7321.
By measuring the intensity of this very faint glow - shown in an artistic impression in the above Figure -
it has been possible to infer the presence of the elusive UV background. This study paves the ground for new ways to map the
temperature of the cosmos and to understand why there are so few small galaxies in the Universe.
Click here to find out more and
click here to read the paper.