Galaxies & Galaxy Clustering 

Joshua Barnes

Galaxy Collisions

Joshua Barnes uses N-body methods to simulate galactic collisions and other aspects of galactic dynamics. One area of ongoing effort is improving existing techniques for force calculation, construction of initial conditions, and simulation including star formation and recycling of interstellar material. A second area of emphasis is developing accurate models of well-observed interacting galaxies. Ultimately, one objective of this research is to test dark-matter models and prescriptions for star formation by comparing detailed models of specific interacting galaxies with observations..

Computer-generated model of NGC 4676 overlaid 
on maps of the actual HI and stellar distributions

The mass-metallicity relationship of star forming galaxies. Blue asterisks represent results from the analysis of individual supergiant stars in nearby galaxies out to 7 Mpc distance, whereas the black circles show stellar metallicities from an analysis of the integrated stellar population of galaxies at red shifts around z = 0.15. The red triangles display results from the study of red giant stars in nearby Local Group dwarf galaxies.


Rolf Kudritzki

The Chemical Evolution of Galaxies and the Mass-metallicity Relationship

When galaxies form and then evolve, the chemical composition of their stars and gas changes. The mass-metallicity relationship of galaxies is a key to understanding the physics of galaxy formation and evolution in an expanding universe dominated by dark matter and dark energ. Emeritus facuty member Rolf Kudritzki has pioneered a new approach, namely to use the Keck telescope on Mauna Kea and the ESO VLT in Chile to obtain low-resolution spectra of individual red and blue supergiant stars in external galaxies. These objects are the brightest stars in the Universe with absolute magnitudes in the range -9 to -11. More recently he and his colleagues have extended this technique to analyze the spectra of the integrated stellar population of star forming galaxies to determine accurate stellar metallicities.

Fabio Bresolin

Chemical Abundances of Nearby Galaxies

Fabio Bresolin is studying the chemical abundances of young, massive stars and HII regions in nearby galaxies. The comparison of the chemical composition derived independently from optical spectra of both stars and ionized gas allows us to test and constrain methods used to measure the chemical abundances of star-forming galaxies at low and high redshift. Recent focus has also been on the outer disks of spiral galaxies, where the star formation rate is about two orders of magnitude lower than in the inner, optically bright disks.

Location of the blue supergiant stars analyzed in the galaxy M83.

Images of the interacting galaxy Arp 84 taken with Pan-STARRS (top)
and the Herschel infrared space telescope (bottom).

Dave Sanders and Josh Barnes


Dave Sanders and Josh Barnes are members of GOALS“, which stands for “Great Observatory All-sky LIRG Survey”. The project is a comprehensive study of 200 of the most luminous infrared-selected galaxies in the local Universe.  The team uses a variety of space-based and ground-based instruments, including Spitzer, Herschel, Hubble, Chandra, GALEX and the VLA, as well as many of the large optical-infrared telescopes on Mauna Kea and Haleakala..

The galaxies under study include AGNs and starbursts in both merging and isolated systems, providing an unbiased picture of the processses responsible for enhanced infrared emission in the local Universe. They are excellent models for comparison with infrared and sub-millimeter selected galaxies at high-redshift.

Roberto Mendez

Extragalactic Planetary Nebulae

Planetary nebulae (PNs) are easy to de­tect in early-type galaxies at distances smaller than 25 Mpc. Once detected, the strong emission lines in PN spectra are well suited for accurate radial velocity measurements. PNs are valuable test particles for studying angular momentum content and dark matter existence and its distribution in elliptical galaxies, which are hard observational problems.

Roberto Mendez has been using the Subaru telescope on Mauna Kea to discover and measure the velocities of more than one thousand PNs in galaxies like NGC 4697, NGC 821, and NGC 4649. The figure shows radial velocities of PNs in the flattened, almost edge-on elliptical NGC 4697, plotted as a function of their coordinates along the major axis of the galaxy. The slight asymmetry in the distribution is because of the rotation of the PN system, which is significant inside, but becomes undetectable in the outskirts. The marked outward decrease in the velocity dispersion can be interpreted either as a relative lack of dark matter in the halo of NGC 4697, or as the consequence of radial anisotropy in the PN velocity distribution.

Location of the blue supergiant stars analyzed in the galaxy M83.

Images of the interacting galaxy Arp 84 taken with Pan-STARRS (top)
and the Herschel infrared space telescope (bottom).

Harald Ebeling

Galaxy Evolution

There is now considerable evidence that many, if not all, elliptical galaxies in clusters were originally spirals.

Harald Ebeling’s research investigates this topic by studying the colors, spectra, and morphologies of galaxies in massive clusters as a function of their environment. At present, ram-pressure stripping appears to be the most probable physical cause: as spirals from the field fall into the dense cluster core, the collision of the cold molecular gas within them with the hot intra-cluster gas causes first a period of intense star formation and then the removal of all gas from the infalling galaxy.

Alan Stockton

Massive Quiescent Compact Galaxies

Emeritus faculty member Alan Stockton and his collaborators are investigating the nature of the massive quiescent compact galaxies that seem to have been common when the universe was only about 20% of its present age. These were apparently the first massive galaxies to form, but they are extremely rare at the present epoch. While it is possible to study the morphologies and estimate the ages of the stars for this high- redshift sample, detailed spectroscopy of them is almost impossible with currently available instruments because of their faintness and lack of emission lines.

One option for learning more about these interesting and important galaxies is to attempt to find the extremely rare examples of those from this population that have survived closer to the present, intact and essentially unscathed. The figure shows an image of one of about a half-dozen cases, from a survey of ~2400 square degrees of sky, that closely mimic the properties of the more extreme examples from those observed in the early universe. Close study of these relatively nearby galaxies should give us a better understanding of these very first massive galaxies and how they were formed.

Fitting stellar population models to the spectrum of this galaxy indicate that the great majority of the total stellar mass formed very early in the history of the Universe, about 13 Gigayears ago. The top panel shows a Keck adaptive-optics image of the galaxy SDSS J014355.21+133451.4, at z = 0.487, along with (in the middle panel) the subtraction of the best 2-component model for the galaxy. The bottom image shows the model without convolving with the instrumental and atmospheric point- spread function, which should give the best global indication of the true shape of the galaxy. Each panel is 3 arc-seconds on a side.

Location of the blue supergiant stars analyzed in the galaxy M83.

Dave Sanders, Istvan Szapudi, Josh Barnes and Ken Chambers

Hawaii Two-0

A team of UHIfA astronomers comprised of Dave Sanders, Istvan SzapudiJosh Barnes and Ken Chambers are undertaking a detailed study of the galaxies in two 10 square degree fields using the Subaru and Keck telescopes.  The fields correspond with those which will be studied in depth by the EUCLID satellite, a near infrared survey instrument due for launch in 2021 with the aim of measuring the acceleration of our expanding Universe. These gound-based observations should be complete by the time EUCLID is launched.

Galaxies are not randomly distributed in space. There are major concentrations of galaxies we refer to as clusters, nearly empty areas that we refer to as voids, and more complicated distributions such as filaments and sheets.

The projects on this page show some of the ways IfA astronomers study the universe by focusing their attention on galaxy clusters and voids.

The outlines of the structure of 100,000 galaxies that we live in has been identified and given the name Laniakea Supercluster.

Brent Tully

Galaxy Clustering: Laniakea

Brent Tully has been leading an international team in a program called Cosmicflows.  The measurement of accurate distances to galaxies permits a differentiation from the distances they would have if they just participate in the mean cosmic expansion at their observed velocities.  That difference translates as a “peculiar velocity” caused by the cumulative distribution of matter in the vicinity.  With 18,000 individual distance measurements in the third release of the program a detailed 3-dimensional map can be created of the clusters, filaments, sheets, and voids across a region that extends to 5% of the edge of the observable universe.

Brent Tully

The Local Void

Brent Tully is working to understand the origin of the 600 km/s motion of our Galaxy with respect to the cosmic microwave background. He has found that the reference frame is increasingly understood to be made up of several parts. A significant contributor, at the level of 260 km/s, is a motion away from the Local Void, a nearby region of space that is almost devoid of galaxies. The compelling evidence comes from a discontinuity in velocities just beyond the structure we live in, the Local Sheet. Galaxies within the Local Sheet are moving coherently with a tiny dispersion, while galaxies in the adjacent structures are moving with their own coherent but quite distinct flow. The nature of the motions makes it clear that our Local Sheet is part of the wall of the Local Void and experiencing the expansion of the void. The substantial expansion velocity implies that the Local Void is impressively large and empty.


Each spot in this figure is a galaxy, with the Milky Way at the origin. The arrow shows the motion of the Milky Way away from the Local Void.

The snake-like feature in this figure is the highly magnified and distorted image of a distant background galaxy, created by the bending of light as it traverses the massive cluster MACSJ1206.2-0847 which acts like a giant lens..

Harald Ebeling

Gravitational Lensing

Gravitational lensing is the bending of light from a distant background source by a mass concentration between this source and the observer. In the strong-lensing regime (near high mass concentrations) this effect can lead to dramatically magnified and distorted images of faint background objects.

Harald Ebeling uses massive X-ray selected galaxy clusters as gravitational telescopes (a) to constrain the mass distribution within the cluster (most of which consists of dark matter that cannot be detected by other means), and (b) to find and characterize distant background galaxies that would be beyond the reach of even the largest present-day telescopes without amplification by the cluster “lens”.

Harald Ebeling

Massive Galaxy Clusters

Harald Ebeling has been leading several all-sky searches for the most massive galaxy clusters, among them the MAssive Cluster Survey (MACS) which discovered the majority of the targets studied in depth by the Hubble Frontier Fields initiative and, more recently, the extended MAssive Cluster Survey (eMACS) which focuses on yet more distant systems at redshifts beyond z=0.5. Observing these extreme mass concentrations across the electromagnetic spectrum, from the radio through the optical to the X-ray regime, reveals the physical mechanisms at work in the formation and evolution of structure over a huge range of spatial scales, from galaxies to large-scale filaments. Specifically cluster collisions offer rare opportunities to study and quantify the dynamic properties of both the luminous and dark matter that make up galaxy clusters.

On yet larger scales, Ebeling’s team played a central role in the discovery of the Dark Flow, a large-scale motion of galaxy clusters across the entire observable universe detected via measurements of distortions in the cosmic microwave background data.

The massive galaxy cluster MACS J0717.5+3745 a complex merger of at least four separate galaxy clusters. The diffuse X-ray emission from the hot intra-clusters gas is color-coded to show different gas temperatures.

Pat Henry

Galaxy Clustering at z = 1.7

Emeritus faculty member Pat Henry and others have been studying an X-ray-selected large-scale structure comprised of a diffuse X-ray source flanked by two galaxy over densities of redshifts 1.68 and 1.75 with a bridge in between. This structure is among the highest redshift examples known. The quiescent galaxies in it formed about 3.5 Gyr before the epoch of observation. It is not known whether the structure consists of one, two or three clusters or one, two or three proto-clusters, or some combination of clusters and proto-clusters

Dots are galaxies with photometric redshifts between 1.65–1.80; the contours are another representation of these same galaxies. Blue and red dots are galaxies with spectroscopic redshifts of 1.660–1.695 and 1.744–1.755, respectively. The diffuse X-ray source, LH146, is centered on the red dot in the middle of the frame. Squares mark red sequence quiescent galaxies.

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