Towards direct observation of large samples of intergalactic filaments in the early universe

One quasar of the sample embedded into extended Lyman alpha emission (cyan), which reaches the edge of the circumgalactic medium of its host galaxy. The uncovered filamentary structure is stretched in the direction of the second quasar of the pair (not shown). Multiple further sources are visible in this field, which are not physically associated with the quasar pair; these lie between Earth and the observed quasar. © MPA

This plot shows the alignment of the Lyman alpha nebulae with the quasar pair direction. An angle of zero degrees corresponds to perfect alignment. In the sample studied, all large nebulae (extending into the circumgalactic medium by more than 200,000 light years) trace the quasar pair direction. This trend is not driven by the quasar luminosity (colour of the points) or the distance between the quasar pairs (size of the dots). This shows that the Lyman alpha nebulae indeed trace the cosmic web filaments. © MPA

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The distribution of matter in the universe is predicted by supercomputer simulations to occur in a network of filaments, known as the "cosmic web", where galaxies form and evolve. The vast majority of this intricate structure is in the form of diffuse hydrogen gas, so rarefied that it is extremely challenging to observe it directly. A collaboration led by MPA researchers has targeted the active supermassive black holes of galaxy pairs at close separations to reveal the connecting filamentary structures of the cosmic web in the early universe. The results are promising and unveil evidence for such structures stretching between the observed pairs, ultimately providing excellent targets for future ultra-deep observations.

Galaxies are embedded in large reservoirs of gas bound to them by gravity, the so-called 'circumgalactic medium'. Like all gas in the universe, it mainly consists of hydrogen and helium with traces of other elements that are produced in stars and ejected from the galaxy disks in bubbles of hot gas or fast winds expanding into the circumgalactic medium. In turn, cool gas is funneled back into the galaxy in streams and can form new stars or feed the supermassive black hole at the galactic centre. Galaxies are not hermits though: large filamentary gas structures connect galaxies to their neighbours. This overall skeleton is called 'cosmic web' (see Monthly Highlight of June 2024), and galaxies can accrete additional material from its filaments to rejuvenate and grow. While simulations have explored this process very well, observational evidence of the filamentary cosmic web is sparse and mainly indirect, e.g. inferred from the observed position of galaxies in the local universe or by how the cosmic web absorbs light from bright background sources.

The areas where multiple filaments of the cosmic web intersect are called 'nodes', typically inhabited by the most massive galaxies. In the early universe, 11.5 billion years ago, these massive galaxies are commonly pinpointed by quasars – a brief phase in these galaxies’ life cycle, when matter falling onto their central supermassive black holes powers exceptionally luminous events that easily outshine all stars in their host galaxy. Therefore quasars can act as powerful natural 'cosmic flashlights': Their radiation can reach far into the circumgalactic medium and the surrounding cosmic web, lighting up the hydrogen gas at a specific ultraviolet colour, the Lyman alpha wavelength.

Researchers from MPA have now observed a sample of quasar pairs, i.e. two massive active galaxies in direct vicinity to each other, to unveil the Lyman alpha emission in their circumgalactic medium and in-between the galaxies (commonly referred to as 'Lyman alpha nebulae'). Extended emission is detected in most targeted systems (see example in Fig. 1) and the emission is preferentially aligned with the pair direction (see Fig. 2). These results are in line with expectations, if a cosmic web filament connects the two quasars and cool gas gets funneled directly from the filament through the circumgalactic medium down to the galactic disk.

Compared to other massive galaxies at this epoch, quasar pairs are embedded in smaller reservoirs of cool gas. Their circumgalactic medium actually resembles that of galaxies at a cosmic time one billion years later. Such an accelerated evolution might be caused by the rich environment inhabited by quasar pairs and/or by highly energetic processes connected to the accreting supermassive black holes, which could heat up the gas surrounding the galaxy and counteract the gas accretion.

This sample of quasar pair observations is the largest to date and represents the best collection of promising targets for directly studying the emission of the cosmic web in the early universe with future ultra-deep observations. More and more observations of the intricate web of cosmic filaments will become available in the near future.

Source: Max Planck Institute for Astrophysics/News

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Author:

Eileen Herwig
PhD student
tel:2344
eherwig@mpa-garching.mpg.de

Original publication

Eileen Herwig
Arrigoni Battaia, Fabrizio;
González Lobos, Jay; et al.
QSO MUSEUM: II. Search for extended Lyα emission around eight z ∼ 3 quasar pairs
A&A, 691, A210 (2024)

Source | DOI

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Probing Cold Gas with the Resonance Doublet of Singly Ionized Magnesium

Figure 1: Schematic illustration of resonance doublets: the energy levels of resonance doublets are shown on the left, some example atoms and ions with one valence electron on the right. © MPA

Traditional studies of the gas around galaxies rely in particular on absorption and emission features of neutral hydrogen, the simplest and most abundant element in the universe. MPA researchers have now investigated alternative tracers, in particular the resonance doublet of singly ionized magnesium and found that analyzing this emission can lead to significant advances in studying the circum-galactic medium. They showed the potential of the magnesium doublet as an alternative to Lyman-alpha emission through a new radiative transfer code and suggest that the magnesium doublet ratio could even be used as a tracer of the Lyman-continuum escape.

Light, and in particular how it interacts with the atoms of various elements, plays a pivotal role in unveiling the secrets of the universe. At the heart of this interaction lies the resonance line, the transition between the ground state and the first excited state in an atom. This transition is significant and interesting for atoms and ions with just one electron in their outermost shell. Considering the fine structure of these atoms, the resonance line manifests as a doublet, the K and H lines. The most renowned example of such a resonance doublet is the hydrogen Lyman-alpha (Lyα) line at 1216 Å. Although the two lines of Lyman-alpha are not distinguishable due to a too small energy gap, other metal doublets are observed as separated doublets.

In astrophysical spectra, the resonance doublets stand out as prominent absorption lines since the abundant electrons in the ground state easily interact with photons near the line center. In addition to absorption features in astrophysical spectra, the doublets also appear as emission lines, acting as one of the main coolants of shock and ionized gas. The atomic physics of the resonance doublet dictates that the scattering cross-section of the K line is always two times higher than that of the H line. This also translates into the ratio of K and H emission from collisional excitation and recombination, which is generally two. Furthermore, due to their resonant nature, photons of the doublet emission suffer scattering with the electrons in the ground state, by which the physical properties of the gas are imprinted on their emission features. The study of resonance doublets, therefore, opens a window into the complexities of astrophysical environments, making it a cornerstone of astrophysical spectroscopy.

Figure 2: These diagrams show the magnesium (left) and Lyman-alpha (right) spectra for various column densities (different color hues). The black dashed line represents the intrinsic Gaussian profile. For magnesium, the doublet is clearly separated, while the energy gap is too small for hydrogen. With larger densities, the asymmetry of the lines becomes more pronounced. © MPA

Studying CGM with Resonance Doublets

The circumgalactic medium (CGM), the diffuse halo of multiphase gas that envelopes galaxies, is a key to understanding the mysteries of galaxy formation, evolution, and the flow of matter around galaxies. While traditional studies of the CGM have relied on analyzing the absorption features of the resonance lines observed in quasar spectra, this approach offers a view constrained by singular lines of sight. The evolution of observational technology, with instruments like MUSE on the VLT, KCWI on Keck, and HETDEX, has opened new windows into the CGM through the direct observation of spatially extended emissions such as Lyman-alpha and resonance doublets of heavier elements.

The Lyman-alpha emission is central to these advances and a powerful tool for probing the cold CGM (with temperatures up to 10000K) and studying the early universe (at redshift z from 2 to 5). However, observing Lyman-alpha faces significant challenges: it is obscured by Earth’s atmosphere at z < 2 and becomes difficult to detect beyond z > 6 due to the optically thick universe in the epoch of reionization. These limitations highlight the necessity for alternative tracers of cold gas within the CGM across different cosmic epochs.

Figure 3: The projected images of surface brightness in Mg II (top left) and Lyman-alpha (top right), the Mg II doublet ratio (bottom left), and the degree of polarization of Mg II (bottom right). All quantities are given for optically thin (left) and thick (right) environments in magnesium. © MPA

Resonance Doublet of Singly Ionized Magnesium as a New Tracer of Cold Gas

The resonance doublet of singly ionized magnesium (Mg II) at 2796 Å and 2803 Å presents such an alternative. Due to its resonance nature and with a similar ionization energy to atomic hydrogen, the Mg II emission can trace cold gas properties through scattering processes like Lyman-alpha. In this work, we developed a new 3D Monte Carlo radiative transfer code to investigate the escape of both emission lines through homogeneous and clumpy multiphase media. Our new code allows for exploring gas in arbitrary 3D geometries via both Lyman-alpha and metal resonance doublets, significantly enhancing our understanding of the cold gas environment surrounding galaxies.

One of the key findings of this research is the distinct behavior of Mg II emissions compared to Lyman-alpha, despite similarities in atomic physics. Lyman-alpha emission is more spatially extended via scattering than Mg II due to a small fraction of magnesium in the gas. Furthermore, the Mg II escape fraction generally exceeds that of Lyman-alpha, offering a clearer view through the cosmic dust that often obscures Lyman-alpha emissions. This makes Mg II an invaluable alternative for tracing cold gas, particularly in environments where Lyman-alpha emission is weak or unobservable.

Figure 4: The Mg II doublet ratio for various outflow/inflow velocities. While the doublet ratio is insensitive to velocities blow the separation velocity of the two lines (700 km/s), a clear distinction can be sees for larger velocities. © MPA

Magnesium Doublet Ratio

The escape of the Lyman continuum (LyC) or its leakage is particularly significant for understanding the mechanisms behind galaxy evolution and the reionization of the universe. The Mg II doublet ratio, which is the flux ratio of the two doublet lines, is one of the new promising indicators of the LyC leakage.

Our study investigates the Mg II doublet ratio in various environments. We found that the doublet ratio indicates a strong outflow/inflow. The double ratio of Mg II from stellar continuum becomes ~ 1 for high column densities of Mg II. In addition, we tested the doublet ratio as a leakage indicator of LyC and tracer of the LyC escape fraction.

The Mg II spectrum in the halo is composed of only scattered photons, and the physical properties of cold gas are clearly imprinted on it. We explored this and derived the analytic solution of LyC escape using the halo doublet ratio. These insights not only expand our methodologies in studying the CGM but also pave new pathways for future observation and theory.

Figure 5: Left: Schematic illustration for the relation between the Mg II doublet ratio and the Lyman-continuum (LyC) escape. Right: the Mg II doublet ratio in the halo, which is the flux ratio of Mg II K and H lines, as a function of Mg II column density. Both LyC escape fraction and the doublet ratio decrease with increasing Mg II column density. When the gas is optically thick for LyC, the Mg II doublet ratio in the halo, which is the flux ratio of Mg II K and H lines, is less than 2. On the other hand, the doublet ratio is higher than 2. Hence, the halo doublet ratio can be a tracer of LyC escape. © MPA

Potential of Metal Resonance Doublets

Moreover, our research opens the door to exploring the emission features of other metal resonance doublets as tracers of the CGM. The success of the Mg II doublet in providing new insights into cold gas properties and the escape of ionizing photons suggests the potential for similar analyses for other elements. For example, the C IV doublet can be a good indicator of the galactic wind. Other metal doublets, such as O VI, N V, and Si IV, share similar atomic physics and could trace the CGM at different temperatures. This avenue of research holds promise for broadening our understanding of the multiphase CGM, offering a richer, more nuanced view of the processes that govern galaxy evolution and the cosmic web.

Source: Max Planck Institute for Astrophysics

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Author:

Seok-Jun Chang
Postdoc
tel:2009
sjchang@mpa-garching.mpg.de

Original publication

Seok-Jun Chang & Max Gronke

Probing cold gas with Mg II and Lyα radiative transfer
submitted to MNRAS

Source

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A Fast Radio Burst Reveals Foreground Galaxy Clusters

Artist's impression of a fast radio burst traveling from its source in a distant galaxy to an observed on Earth. Along this path, the burst passes through the halo of another galaxy, which affects the radio signal. Credit: ESO/M. Kornmesser; CC BY 4.0

The repeating fast radio burst FRB 20190520B traveled through an unusually large amount of matter on its journey to Earth. Could unidentified galaxy clusters in the billions of light-years that separate us from the burst’s source explain why?

The signal from the first fast radio burst ever detected. The highest frequencies arrive first, and the lower frequencies follow. Credit: Wikipedia user Psr1909; CC BY-SA 4.0

An Astrophysical Mystery

Fast radio bursts are among the most mysterious events in the universe. Most of these powerful, milliseconds-long radio blips occur just once, each burst an astronomical flash in the pan that leaves researchers puzzling over its origin. In rare cases, fast radio bursts repeat, giving us a clue that at least some sources of these mysterious bursts survive the event.

Snapshot of an interactive figure showing the locations of the newly identified galaxy clusters relative to FRB 20190520B’s location.  You can interact with this figure here. Credit: Lee et al. 2023

Surveying a Superlative Burst

The dispersion measure of the repeating fast radio burst FRB 20190520B is more than twice as large as expected given its distance. This unusually high value caught the attention of a team led by Khee-Gan Lee (Kavli Institute for the Physics and Mathematics of the Universe), which is carrying out the Fast Radio Burst (FRB) Line-of-sight Ionization Measurement From Lightcone AAOmega Mapping survey, or FLIMFLAM. This survey aims to map the distribution of luminous matter in the universe by searching for galaxy groups that are revealed by fast radio bursts.

The team spectroscopically determined the distances to galaxies in the field of view surrounding FRB 20190520B’s location and used a group-finding algorithm to identify galaxy groups and clusters. They found multiple galaxy groups in the field of view, including two galaxy clusters that lie directly between us and FRB 20190520B. By using models to estimate the masses of these galaxies and their halos, Lee’s team determined how much these intervening galaxy clusters contributed to the burst’s dispersion measure.

A Revised Estimate

Based on FRB 20190520B’s extremely high dispersion measure, previous research estimated its host galaxy’s dispersion to be the highest of any known fast radio burst, a fact that has been difficult to reconcile with other observations of the galaxy. Now, with the new estimate of the foreground galaxies’ contribution, FRB 20190520B’s host galaxy has been assigned a more moderate value that aligns with its observational properties. This study demonstrates that even when focusing closely on a single fast radio burst, it’s still important to zoom out and consider the big picture!

Citation

“The FRB 20190520B Sight Line Intersects Foreground Galaxy Clusters,” Khee-Gan Lee et al 2023 ApJL 954 L7. doi:10.3847/2041-8213/acefb5

By Kerry Hensley

Source: American Astronomical Society/AAS-Nova

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Cool circumgalactic gas in galaxy clusters

Figure 1: Differential average surface mass density of MgII absorbers as a function of radius in and around DESI legacy galaxy clusters (purple), SDSS DR16 star-forming galaxies (blue) and SDSS luminous red galaxies (orange). The solid lines are best-fitting models described in Anand et al. 2022.© MPA

Figure 2: MgII emission surface brightness map around a star-forming galaxy simulated in TNG50 (Nelson et al. 2021). The extended emission from the galaxy out into the circumgalactic medium at distances of ∼ 10 − 20 kpc, with a complex morphology, can easily be seen. © MPA

Galaxy clusters are our universe's largest gravitationally bound systems, extending out to several million light-years and hosting up to 1000 galaxies. The matter permeating the clusters is known as the “intracluster medium” (ICM), a very hot and ionized gas (T~ 10-100 million K) emitting bright X-rays due to thermal bremsstrahlung. Scientists from MPA and the University of Heidelberg have discovered that the ICM also contains a significant amount of cool gas (10,000 K) up to large distances. The statistical connection between the haloes of cluster galaxies and absorption features points toward a complex origin of this cool gas where clouds are either associated with satellite galaxies or were previously stripped from their haloes.

Galaxies and the hot plasma around them (ICM) account for less than ~10 per cent of the total mass in galaxy clusters; dark matter makes up the other 90 per cent. Though astronomers have studied the hot ICM extensively with X-ray telescopes in space, such as Chandra and XMM-Newton, detailed studies of cool gas (T~10,000 K) are lacking as there are only few optically identified clusters. Quasar absorption lines at optical wavelengths are a powerful tool to study cool gas, where absorbers in intervening galaxy clusters leave traces in the spectra of a bright background source such as a quasar (see MPA Highlight of July 2021, Figure 1).

One of the most easily detectable absorbers in background quasars is singly-ionized magnesium (MgII), which exhibits a doublet profile, i.e. two absorption lines close together (with 2796 and 2803 Angstrom). Given the low ionization potential of magnesium, it traces cool gas (T ~ 10,000 K) in the circumgalactic (CGM) or intergalactic medium. Though the rest-frame wavelengths fall into the ultra-violet (UV) range, the lines shift to optical wavelengths at redshift z>0.4 and can be detected with ground-based telescopes. Previously, scientists at MPA constructed the most extensive absorber catalogue based on quasars detected in the Sloan Digital Sky Survey (SDSS) using a novel automated algorithm (see MPA Highlight of July 2021), which has allowed us to explore the nature of cool gas around galaxies in an unprecedented way.

The scientists now expanded this study to galaxy clusters, combining the latest absorber catalogue from the SDSS Data Release (DR16) and the Dark Energy Survey Instrument (DESI) legacy surveys cluster catalogue. The MgII-galaxy cluster cross-correlation from these large datasets provide us with an unprecedented opportunity to understand and constrain the nature of the cool gas in ICM. Furthermore, comparing our results with the studies explicitly performed for individual galaxies allows us to understand how the environment affects the nature of the cool gas round galaxies. We find a significant covering fraction relative to random sightlines, with the total Mg II mass within a cluster halo (estimated using surface mass density) being ~ ten times higher than for SDSS luminous red galaxies (see Figure 1). Our analysis also revealed that the covering fraction of cool gas in clusters decreases with increasing mass of the central galaxy.

Are the MgII absorbers detected in clusters associated with the CGM of its member galaxies? To investigate this question, we cross-correlated MgII absorption with cluster member galaxies from DESI and indeed found a statistically significant connection. Our analysis shows that the median projected distance between MgII absorbers and the nearest cluster member is ~200 kpc, compared to ~500 kpc in random mock samples with the same galaxy density profiles. However, we do not find a correlation between MgII strength and the star formation rate of the closest cluster neighbour. Combining our results with results from field galaxies suggests that cool gas in clusters, as traced by Mg II absorption, is: (i) associated with satellite galaxies, (ii) dominated by cold gas clouds in the intracluster medium, rather than by the interstellar medium of galaxies, and (iii) may originate in part from gas stripped from these cluster satellites in the past.

However, given the uncertainties in determining cluster membership for individual galaxies (with photometric redshifts), it is difficult to constrain the relative motion of absorbers and galaxies with our analysis. In the future, the upcoming cluster and active galactic nuclei (AGN) data from eROSITA and spectroscopic data of galaxies from DESI would allow us to perform a robust kinematic study of absorbers and cluster galaxies in more detail. This would put strong constraints on the motion of cool gas in cluster environments. Combining optical studies with the X-ray observations for clusters can also provide strong conditions on the nature of hot and cool gas in ICM.

On the other hand, one critical task would be comparing observational results such as ours with CGM simulations such as TNG50. Figure 2 (taken from Nelson et al. 2021) shows the MgII emission map around a galaxy and how it could be observed with a MUSE-like facility. These results are critical in constraining the physical models of ICM or CGM. An analysis such as ours or high resolution spectra observed with Keck like facility could provide the absorption clouds' size and mass, a key constraint that could then be compared with the theoretical models predicting the formation mechanism of cool gas clouds in such dense environments.

Authors:

Abhijeet Anand
PhD student
tel:2298
abhijeet@mpa-garching.mpg.de

Guinevere Kauffmann
Director
tel:2013
gamk@mpa-garching.mpg.de

Original publication

Anand, A., Kauffmann, G., Nelson, D.
Cool circumgalactic gas in galaxy clusters: connecting the DESI legacy imaging survey and SDSS DR16 MgII absorbers
MNRAS, 513, 3210
Source / DOI

Source: Max Planck Institute for Astrophysic/News

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Astronomers Catch Wind Rushing Out of Galaxy

Makani 360-degree rotation of KCWI spectral cube from Jim Geach on Vimeo

A volume rendering of the KCWI data cube revealing the structure of Makani. Credit: David Tree & Peter Richardson, Games and Visual Effects Research Lab, University of Hertfordshire

Researchers Directly Observe for the First Time a Huge Outflow of Gas Extending Far Beyond a Galaxy

Maunakea, Hawaii – Exploring the influence of galactic winds from a distant galaxy called Makani, University of California, San Diego’s Alison Coil, Rhodes College’s David Rupke and a group of collaborators from around the world made a novel discovery using W. M. Keck Observatory on Hawaii Island.

Published online today in the journal Nature, their study’s findings provide direct evidence for the first time of the role of galactic winds—ejections of gas from galaxies—in creating the circumgalactic medium (CGM). It exists in the regions around galaxies, and it plays an active role in their cosmic evolution. The unique composition of Makani—meaning ‘wind’ in Hawaiian—uniquely lent itself to the breakthrough findings.

“Makani is not a typical galaxy,” noted Coil, a physics professor at UC San Diego. “It’s what’s known as a late-stage major merger—two recently combined similarly massive galaxies, which came together because of the gravitational pull each felt from the other as they drew nearer. Galaxy mergers often lead to starburst events, when a substantial amount of gas present in the merging galaxies is compressed, resulting in a burst of new star births. Those new stars, in the case of Makani, likely caused the huge outflows—either in stellar winds or at the end of their lives when they exploded as supernovae.”

Coil explained that most of the gas in the universe inexplicably appears in the regions surrounding galaxies—not in the galaxies. Typically, when astronomers observe a galaxy, they are not witnessing it undergoing dramatic events—big mergers, the rearrangement of stars, the creation of multiple stars or driving huge, fast winds.

“While these events may occur at some point in a galaxy’s life, they’d be relatively brief,” noted Coil. “Here, we’re actually catching it all right as it’s happening through these huge outflows of gas and dust.”

Coil and Rupke, the paper’s first author, used data collected from one of Keck Observatory’s newest instruments – the Keck Cosmic Web Imager (KCWI) – combined with images from the Hubble Space Telescope and the Atacama Large Millimeter Array (ALMA), to draw their conclusions.

The KCWI data provided what the researchers call the “stunning detection” of the ionized oxygen gas to extremely large scales, well beyond the stars in the galaxy. It allowed them to distinguish a fast gaseous outflow launched from the galaxy a few million years ago, from a gas outflow launched hundreds of millions of years earlier that has since slowed significantly.

“The earlier outflow has flowed to large distances from the galaxy, while the fast, recent outflow has not had time to do so,” summarized Rupke, associate professor of physics at Rhodes College.

Figure 1: The giant galactic wind surrounding the massive, compact galaxy Makani. The colors and white contour lines show the amount of light emitted by the ionized gas from different parts of the oxygen nebula, from brightest (white) to faintest (purple). The middle part of the image (black) shows the full extent of the galaxy, though most of the galaxy is concentrated at the center (the tiny green circle). The axes show distance from the center of the galaxy in kiloparsecs. Figure by: Gene Leung (UC San Diego) 

From Hubble, the researchers procured images of Makani’s stars, showing it to be a massive, compact galaxy that resulted from a merger of two once separate galaxies. From ALMA, they could see that the outflow contains molecules as well as atoms. The data sets indicated that with a mixed population of old, middle-age and young stars, the galaxy might also contain a dust-obscured accreting supermassive black hole. This suggests to the scientists that Makani’s properties and timescales are consistent with theoretical models of galactic winds.

“In terms of both their size and speed of travel, the two outflows are consistent with their creation by these past starburst events; they’re also consistent with theoretical models of how large and fast winds should be if created by starbursts. So observations and theory are agreeing well here,” noted Coil.

Rupke noticed that the hourglass shape of Makani’s nebula is strongly reminiscent of similar galactic winds in other galaxies, but that Makani’s wind is much larger than in other observed galaxies.

“This means that we can confirm it’s actually moving gas from the galaxy into the circumgalactic regions around it, as well as sweeping up more gas from its surroundings as it moves out,” Rupke explained. “And it’s moving a lot of it—at least one to 10 percent of the visible mass of the entire galaxy—at very high speeds, thousands of kilometers per second.” 

Rupke also noted that while astronomers are converging on the idea that galactic winds are important for feeding the CGM, most of the evidence has come from theoretical models or observations that don’t encompass the entire galaxy. 

“Here we have the whole spatial picture for one galaxy, which is a remarkable illustration of what people expected,” he said. “Makani’s existence provides one of the first direct windows into how a galaxy contributes to the ongoing formation and chemical enrichment of its CGM.”

Figure 2: The multiphase galactic wind: comparison of the ionized, neutral atomic and molecular gas. In the zoomed-in view of the inner 40 kiloparsecs at the upper right, molecular gas emission from carbon monoxide (green contours) is plotted on emission from magnesium atoms that trace neutral atomic gas (color, with white contours) in the same velocity range (-500 to +500 kilometers per second, where negative velocities are blueshifted and positive velocities redshifted with respect to the galaxy). The zoomed-in view at the lower left compares the emission from low-velocity molecules and ionized oxygen atoms, and the high-velocity molecular and ionized gas are shown at lower right. The molecules, neutral atoms and ionized gas all correspond well spatially, though the ionized gas extends far beyond the other two gas phases. Figure by: David Rupke (Rhodes College)

This study was supported by the National Science Foundation (collaborative grant AST-1814233, 1813365, 1814159 and 1813702), NASA (award SOF-06-0191, issued by USRA), Rhodes College and the Royal Society.

Source:  W.M. Keck Observatory/News

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About KCWI

The Keck Cosmic Web Imager (KCWI) is designed to provide visible band, integral field spectroscopy with moderate to high spectral resolution formats and excellent sky-subtraction. The astronomical seeing and large aperture of the telescope enables studies of the connection between galaxies and the gas in their dark matter halos, stellar relics, star clusters, and lensed galaxies. Support for this project was provided by the Heising-Simons Foundation. Learn more at www.heisingsimons.org.

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About W.M. Keck Observatory

The W. M. Keck Observatory telescopes are among the most scientifically productive on Earth. The two, 10-meter optical/infrared telescopes on the summit of Maunakea on the Island of Hawaii feature a suite of advanced instruments including imagers, multi-object spectrographs, high-resolution spectrographs, integral-field spectrometers, and world-leading laser guide star adaptive optics systems.

Some of the data presented herein were obtained at Keck Observatory, which is a private 501(c) 3 non-profit organization operated as a scientific partnership among the California Institute of Technology, the University of California, and the National Aeronautics and Space Administration. The Observatory was made possible by the generous financial support of the W. M. Keck Foundation.

The authors wish to recognize and acknowledge the very significant cultural role and reverence that the summit of Maunakea has always had within the Native Hawaiian community. We are most fortunate to have the opportunity to conduct observations from this mountain.

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The Circum-galactic Medium of Galaxies as Probe of Gas Accretion

Schematics of how the sightline from HST to a quasar goes through the extended gas halo of a foreground galaxy. The inset shows the quasar spectrum including the Lyman α forest. Credit: COS-HALOS survey

In collaboration with researchers from the USA, MPA scientists have mounted a series of ambitious experiments that use a combination of quasar absorption-line spectra, neutral hydrogen line data, and state-of-the-art cosmological hydrodynamical simulations to probe the interface between galaxies and their surrounding gaseous environment. The researchers found that galaxies with gas-rich disks are embedded within gas-rich halos and that the gas in these halos is distributed smoothly and relatively isotropically.

Galaxies need gas to fuel star formation; how galaxies acquire gas is therefore central to our understanding of galaxy evolution. In the standard paradigm, galaxies grow primarily through the accretion of gas that flows from the Inter-Galactic Medium (IGM), through the dark matter halo, and eventually settling onto the disk of the galaxy. Galaxies like or own Milky Way need a continuous supply of gas to fuel star formation, but little is known about the way in which gas cools and condenses into the disk due to difficulties in observationally mapping the disk/halo interface.

Bright quasars at large distances from the observer act as cosmic light beacons. As the light from distant quasars travels through the Universe, it encounters gas clouds containing mainly hydrogen. These clouds absorb and scatter ultraviolet photons, leading to characteristic dips (or absorption lines) in the spectrum of the quasar, the so-called "Lyman α forest". By choosing quasars that happen to be positioned in such a way that their light will pass within a short distance (a few hundred kiloparsec) of a foreground galaxy, we are able to probe the gas in the so-called "circum-galactic medium" surrounding these systems.

Two large programmes to investigate the circum-galactic medium around nearby galaxies have now received a total allocation of 200 orbits of observation time with the Hubble Space Telescope (HST). The first of these, COS-GASS, used the Cosmic Origins Spectrograph (COS) on board HST to probe neutral hydrogen around nearby galaxies out to the outer radius of their surrounding dark matter halos.

Distribution of sight lines as a function of impact parameter and orientation of the target galaxy. The red and blue areas correspond to the HI and the optical disk. The yellow region corresponds to the extended disk region. The quasar sightlines included as part of the COS-GASS programme are shown in purple and the sightlines from the COS-DISK programme in green.© MPA

The COS-GASS programme found a highly significant correlation (at 99.5% confidence) between the strength of the Lyman α absorption lines, which are tracing neutral hydrogen in the surrounding halo, with the ratio of gas mass to stellar mass within the disk. This means that galaxies with gas-rich disks are embedded within gas-rich halos.

The Lyman α signature was detected in nearly every quasar spectrum and the average strength of the Lyman α lines decreased gradually as a function of distance from the galaxy. Finally, the strength of the Lyman α lines seems to be independent of the orientation of the disk. This means that the gas in the surrounding gas halos is distributed smoothly and relatively isotropically.

The quasar spectra obtained as part of the COS-GASS programme mainly probed sightlines well outside the disk of the galaxy. In 2015, the follow-on, large programme COS-DISK was approved to probe gas at the interface between disk and circum-galactic medium. While reduction, processing and analysis of the HST data is being carried out at Johns Hopkins University in Baltimore, MPA scientists are closely involved in using state-of-the-art cosmological hydro-dynamical simulations to interpret the observational data.

Most of the work so far has focused on the Illustris simulations. The simulation includes thousands of galaxies with masses in the range of the galaxies in the COS-GASS and COS-DISK samples, making it ideal for studying how the disk, circum-galactic medium and disk/halo interface properties vary as a function of the stellar mass of the galaxy, morphological type, star formation rate, and gas mass fraction.

An example from the Illustris simulations: the predicted distribution and kinematics of neutral hydrogen surrounding a simulated galaxy (with the same mass as the Milky Way). The image on the left shows the HI column density at the scale of the virial radius (white circle), the middle and right columns show edge on and face on projections of the HI column density (top) and the line of sight velocity (bottom). © MPA 

A first comparison with the observational data shows that the observed correlation between the gas content of the halo and the gas content of the disk is well reproduced, as is the isotropic geometry of the neutral gas at large radii. However, the simulations do not match very well the observational result that almost all sightlines intercept a neutral gas cloud. The simulations incorporate various kinds of gas heating processes and these are clearly too effective at heating and destroying large pockets of neutral hydrogen far out into the halo.

Future work, motivated by new data from COS-DISK, will examine gas closer to the disk in more detail. The simulations predict that the gas in the inner circum-galactic medium should be co-rotating with the galaxy and our new observations will allow us to test this hypothesis. In addition, several large Illustris follow-up simulations will significantly improve upon the physical models used in the original Illustris simulations. Ongoing comparisons between new data and improved models will significantly improve our understanding of how galaxies grow by gas accretion.

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Authors :

Guinevere Kauffmann  

Director

Phone: 2013

Email:gkauffmann@mpa-garching.mpg.de

Dylan Nelson

Postdoc

Phone: 2251

Fax: 2235

Email: dnelson@mpa-garching.mpg.de

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Original Publications

1. Borthakur et al.

Connection between the Circumgalactic Medium and the Interstellar Medium of Galaxies: Results from the COS-GASS Survey
ApJ, 813, 46B, 2015
Source , DOI

2. Nelson et al.
The illustris simulation: Public data release
A&C, 13, 12N, 2015
Source , DOI 

3. Kauffmann, Borthakur & Nelson
The morphology and kinematics of neutral hydrogen in the vicinity of z = 0 galaxies with Milky Way masses - a study with the Illustris simulation
MNRAS, 462, 3751K, 2016
Source ,  DOI  

 Source: Max Planck Institute for Astrophysics

Studying diffuse, warm gas in the outskirts of galaxies

An optical image of galaxy M82 with the ionized gas of hydrogen (Hα) shown in pink flowing out of the galaxy. 

Image Credit: NASA, ESA, The Hubble Heritage Team, (STScI/AURA)

The diffuse gas around galaxies is hard to detect, but shows properties which are quite different to the star-forming gas inside a galaxy. Scientists at MPA have used observations from the recent MaNGA survey to study how the ionized gas changes with distance from the center of the galaxy. They have demonstrated the usefulness of adding spectra from multiple galaxies in order to analyze the gas in the outskirts of galaxies. Their study shows that the brightness of the gas decreases, while its temperature increases the further the gas is located from the center of the galaxy. The differences between star-forming and circumgalactic gas also seem to correlate with the star-formation rate and stellar mass of the galaxies.

Understanding gas in and around galaxies is crucial to understanding star formation. The gas within a galaxy is the main ingredient for forming stars, and these stars, in turn, enrich the gas with heavy elements, or “metals”. Continuous star formation needs a constant supply of gas, and most likely this comes from a reservoir of gas surrounding the galaxy in its outskirts, or halo, called the circum-galactic medium (CGM). Additionally, enriched gas flows out of the galaxies through supernova explosions, galactic winds, active galactic nuclei, etc. (see Fig 1 for an example of gas outflows). By studying the gas in the CGM and near the disk-halo boundary we can better understand these regulatory processes, gas properties and flows. 

Gas in the halo is difficult to study because it is very faint and diffuse. Cold neutral gas can be seen by looking for neutral hydrogen (HI), and through HI surveys it is known that most galaxies have large reservoirs of gas surrounding the galaxies. Warm ionized gas with temperatures around 1000 K can be detected with optical emission lines and in the outskirts of galaxies this is called extra-planar, diffuse ionized gas (eDIG). Most previous work has been done with long exposures of individual nearby galaxies, including our own Milky Way. 

With optical spectroscopy, only a few handfuls of galaxies have been studied, as it is difficult to obtain exposures deep enough to detect and analyze the diffuse gas. These studies find that the eDIG has different properties compared to gas in star-forming regions. Both the eDIG and star-forming gas are ionized mostly by energy from massive OB stars. As these stars are located in the disk of the galaxy, many of the differences arise because the eDIG is farther away from the OB stars than the gas in star-forming regions. Some other differences are not so easy to explain and vary from galaxy to galaxy. In some galaxies an additional source of energy may be needed to explain the properties of the eDIG, such as turbulence or shocks in the gas, or hot evolved stars in the outskirts of galaxies. 

An example of one of the MaNGA galaxies. The left panel is an SDSS image with the MaNGA field of view overlaid.The middle panel shows a map of the brightness of the galaxy seen with MaNGA and the right panel shows a map of the ionized gas of hydrogen (Hα). The color bars are in logarithmic units. For an individual galaxy, the gas can barely be detected in the outskirts. Thus, for scientific analysis, spectra from many galaxies have to be added to increase the signal far enough above the noise level. © MPA. Hi-res image

With a new dataset from the survey Mapping Nearby Galaxies at APO (MaNGA), which is part of the Sloan Digital Sky Survey (SDSS) IV, a group of MPA scientists addressed these differences and questions about the eDIG. As an Integral Field Unit survey, MaNGA takes spectra at multiple spatial locations. The eDIG is faint and diffuse and in Fig 2 we show an example for the MaNGA observations of one particular galaxy. Adding multiple spectra taken at similar locations from similar edge-on, late-type galaxies, we can study the faint diffuse gas. 

The first year of MaNGA data includes a sample of 49 galaxies that are suitable for this study. We add the spectra from these 49 galaxies from 7 different locations off the disk of the galaxies to find how the eDIG varies with distance from the center of the galaxy. Our analysis shows that the brightness of the eDIG decreases logarithmically with distance and that most likely the temperature of the gas increases with distance from the center of the galaxies. 

For a more detailed analysis, e.g. to figure out which type of galaxies need an additional energy source and what type of source, we to split the sample by different properties of the galaxies, such as stellar mass or star formation. With the first year of data we split the full sample in half and find that in galaxies with a higher star formation rate, the eDIG is more similar to the star-forming gas inside the galaxies compared to low star-forming galaxies where the eDIG is markedly different. Moreover, galaxies with higher stellar mass have a steeper temperature gradient compared to those with lower stellar mass. In the future, with more data, we will be able to split the sample even further to better understand these questions.

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Author:

Jones, Amy

Postdoc

Phone: 2215

Email: ymamay@mpa-garching.mpg.de

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Original Publication

1. A. Jones, G. Kauffmann, R. D'Souza, D. Bizyaev, D. Law, L. Haffner, Y. Bahe, B. Andrews, M. Bershady, J. Brownstein, B. Cherinka, A.Diamond-Stanic, N. Drory, R. A. Riffel, S. F. Sanchez, D. Thomas, D. Wake, R. Yan, K. Zhang.    

SDSS IV MaNGA: Deep observations of extra-planar, diffuse ionized gas around late-type galaxies from stacked IFU spectra

2016, submitted to A&A

Source: Max Planck Institute for Astrophysics