mountain profile Institute for Astronomy University of Hawaii

Galaxies

Maintained by WW

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 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.


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.

 

M83Chemical 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.

 

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.

 

Galaxy Evolution

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

 

This HST image of a field galaxy falling into a massive MACS cluster at z=0.43 shows ram pressure in action. As the spiral morphology of the arriving galaxy is destroyed by the galaxy's infall (indicated by the arrow) toward ever-denser intra-cluster gas, the developing shock front (red arc) compresses cool molecular gas within the galaxy, triggering burst of star formation and creating a debris field of young stars in the wake of the collision.


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.

Active Galaxy Nuclei

Andreea Petric studies nearby AGNs  with Maunakea facilities and with archival data from NASA's Spitzer and Herschel space missions. Her projects now include: 

  • Using CFHT’s wide field imaging Fourier transform spectrograph SITELLE, to studythe properties of ionized gas in merging luminous infrared galaxies.
  • Using Keck’s OSIRIS to measure the impact galaxy-wide radio jets in radio loud AGN have on the molecular gas.  
  • Analyzing more than 2000 mid-infrared spectra of active galaxies in the Spitzer archive to determine if and how accreting black holes at the centers of galaxies impact the ISM of their hosts.
  • Combining MegaCam public archived images (with average seeing of 0.7”) with other archives to study the morphologies of AGN hosts.

The Halpha emission from the active galaxy IIZw96