mountain profile Institute for Astronomy University of Hawaii

Galaxies

Maintained by LG

Infrared Luminous Starbursts and Active Galactic Nuclei


The ultraluminous infrared galaxy Markarian 273, a major merger of two
gas-rich spiral galaxies. Left hand picture is a visible light image from HST:
right hand picture is 2.2 micron image from the Keck telescope.
Dave Sanders and Joshua Barnes are studying the multiwavelength properties of a complete sample of far-infrared selected galaxies in the local universe, as part of the Great Observatories All-Sky LIRGs Survey, in order to understand the origin and evolution of the most luminous infrared systems, with infrared luminosities 10–1000 times the total bolometric lumi­nosity of our Milky Way.

These “luminous infrared galaxies” (LIRGs) appear to be triggered through mergers of massive gas-rich spiral galaxies, an event which leads to powerful starbursts and the growth of supermassive black holes. The end stages of the merger process lead to qua­sarlike luminosities, including the final stage marked by binary active galaxy nucleus. The eventual merger of the two supermassive black holes is accompanied by a massive “blow-out” phase, ex­pelling as much as several billion solar masses of gas and dust into the intergalactic medium, leaving a massive gas-poor elliptical as the merger remnant. This exotic process of galaxy transformation, although relatively rare in the local universe, is now believed to be one of the dominant processes of galactic evolution in the early universe, when the space density of LIRGs was ~10,0000 times larger than observed locally, and coinciding with the peak epoch in the formation of quasars and superstarbursts.

Robert Joseph also studies interactions and mergers between galaxies. Using infrared photometry and spectroscopy for s sample of 50 galaxies he has found that

  1. Interactions are correlated with high-IR luminosities and mergers with very high IR luminosities.
  2. Starbursts are the dominant underlying energy source in interacting galaxies.
  3. The starburst initial mass function is biased against high-mass stars.
  4. While optical and 2 µm spectroscopy showed no evidence for buried AGNs in a sample of 30 LIRGs, mid-IR spectroscopy revealed high excitation coronal lines in ~20% of the sample.

Thus interactions and mergers trigger bursts of star for­mation with a “bottom-heavy” initial mass function,  and the starburst dominates the bolometric luminosity: the mergers are making elliptical galaxies.

 

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

 

 

 

 

 

Chemical Abundances of Normal, Nearby Spiral Galaxies

Fabio Bresolin is studying the chemical abundances of yourg stars and HII regions in spiral galaxies. Among the most interesting results is the measurement of the metallicity of HII regions in outer spiral disks, where the star formation rate is about 2 orders of magnitude lower than in the inner, optically bright disks. The spectroscopic investigation of the faint nebulae in these “extended” disks has been carried out with 8 m class telescopes on Mauna Kea and in Chile.

For the four cases studied to date he has found that the metallicity of the outer disks, rather than following an exponential decline with distance from the center (as is typically found for the inner disks), flattens out to a virtually constant value. Moreover, the metallicity measured in the outer disks is rather high (about 1/3 solar), contrary to the expectations for galactic regions that are considered to be unevolved relative to the inner regions.

The O/H abundance ratio in NGC 3621 as a function of distance from the galaxy's center.

 

The Mass-Metallicity Relationship for Galaxies

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 energy. Unfortunately, the standard technique to determine the metallicities of star-forming galaxies— using emission line spectra of HII regions — is subject to large systematic uncertainties that are poorly understood.

Rolf Kudritzki has pioneered an alternative approach, namely to use the Keck teleacope 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.  

 

Metallicities of individual supergiant stars (blue) as a function of galactocentric radius in the giant spiral galaxy M81.The red points are planetary nebulae. The lower metallicities of the planetary nebulae are probably accounted for by the fact that they were formed billions of years earlier than the blue supergant stars.

 

 

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.