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Interacting & Merging Galaxies

The Universe does not exist in a static state.  Among the largest, most violent processes in the Universe is the gravitational interaction and merger between galaxies.  Galaxy mergers represent an ideal laboratory for studying a wide variety of astrophysical processes, including:  the formation, evolution and death of stars; the ability of gravity to reshape galaxies from spirals into ellipticals; the presence of shocks, plasmas, and ionized gas; gas inflows powered by the merging process and gas outflows powered by supernovae and AGN; the formation and destruction of dust grains of all sizes; and the formation and growth of super-massive black holes. Galaxy mergers are not limited to the local Universe, and thus allow us to study these and other astrophysical processes from the first few hundred million years of the Universe to the present day. They also provide a preview of our own Milky Way Galaxy’s fate, the inevitable collision with our biggest neighbor, the giant spiral galaxy of Andromeda. The research on galaxy mergers at AIP covers a variety of sub-topics.


Dynamics

The Toomre Merger Hypothesis posits that the merging of two spiral galaxies creates a new elliptical galaxy.  In order to test this, the distribution and motion of stars in the merger remnant must be mapped using deep imaging to construct stellar light profiles and spectroscopic observations to measure the velocity dispersions of the stellar absorption lines. These properties are then compared to "typical" elliptical galaxies.  The presence of gas in the progenitor spiral galaxies leads to the formation of new stars in the new elliptical.  The more cold gas the progenitors contain, the more stellar mass is added to the new galaxy.  Luminous and Ultraluminous Infrared Galaxies are known to contain almost a Milky Way's worth of mass in cold molecular hydrogen gas.  This gives them the potential to form massive ellipticals.  Recent work at the institute has demonstrated that ULIRGs are capable of forming the most massive elliptical galaxies in the Universe.

 


Stellar Populations

Studying the stellar populations in galaxy mergers is a much more complicated process than in typical galaxies due to the presence of at least two epochs of star-formation.  The progenitors each contribute an old stellar population to the new elliptical, but the various phases of the interaction (i.e. first and second passage, final coalescence) also trigger the formation of new stars.  Many, if not most of the stars are born from gas that dissipates to the barycenter of the new galaxy.  The angular momentum of the gas creates a gaseous disk which eventually transforms into a stellar disk ranging in size from a few hundred to several thousand parsecs in diameter.  The star formation also produces vast quantities of dust which can affect how we measure the properties of the proto-elliptical galaxy.  Dust blocks more light at optical (shorter) wavelengths than in the near-Infrared. Ongoing work at the institute has shown that when one measure the mass of galaxies in the near-IR the presence of young stars and a rotating stellar disk can confuse measurements, leading to an underestimation of the total dynamical mass of the galaxy.  In the optical, the presence of dust blocks the detection of the young central stellar disk, making it possible to correctly measure the true dynamical mass (known as the “Janus Effect.”)  However, measuring only the optical light makes it impossible to fully map the various stellar populations present.  Work is underway to catalog the properties of both the old and young stellar populations, including their mass, spatial distributions, metallicity and the efficiency at which stars are formed.



Active Galactic Nuclei

The subsequent funneling of gas into the barycenter of a galaxy merger produces not only an enormous burst in star-formation, but fuels the formation or further growth of a QSO in the heart of the new elliptical (forming an Active Galactic Nucleus or AGN). The evolution from (U)LIRG to QSO is an active area of research at the institute.  Ongoing work includes a comprehensive study of obscured QSOs, which may represent the last state of ULIRG mergers before they become bonafide elliptical galaxies; the detection and measurement of massive outflows of gas in ULIRGs which are likely powered by a QSO;  and mapping the emission lines present in both ULIRGs and AGNs to better understand the astrophysical processes occurring in the most powerful objects in the known Universe.

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