Galaxies

Spitzer will conduct extensive imaging and spectroscopic studies of galaxies, ranging from the local neighborhood of our Milky Way Galaxy to the far reaches of the observable universe.

Arp 220 NASA/HST/R. Thompson

Ultraluminous Infrared Galaxies (ULIRGs)

The IRAS satellite revealed a class of ulraluminous galaxies that radiate more than 90 percent of their light in the infrared. Combined infrared and optical studies reveal that the majority of these objects are found in merging and interacting galaxy systems. Understanding the nature of ULIRGs and their relationship to active galaxies (see paragraph below) remains one of the most compelling problems in astrophysics. Spitzer will study their properties and evolution of ULIRGs out to great cosmological distances, and determine the relationship between these galaxies and AGN discovered by other techniques. Spectroscopic measurements will determine the physical conditions in the optically obscured interiors of these galaxies, providing insight into the ultimate power source for these bright beacons.

Centaurus A AAO/David Malin

Active Galaxies / Active Galactic Nuclei

An active galaxy is characterized by extremely intense non-thermal emission, typically at radio and x-ray wavelengths, the signature that a high-energy process is at the core of the intense radiation (the active galactic nuclei, or AGN). The source of the incredible energies released by such a galaxy may be a giant black hole in the galaxy nucleus. Quasars, radio galaxies and Seyfert galaxies are all active galaxies. Many of these objects are enshrouded in dust, in which case they emit large amount of infrared light also. Spitzer will observe AGN at great distances in an effort to understand the relationship between various AGN types, the evolution of their observable properties over time, and the physical processes underlying their energy outputs.

Messier 82 Robert Gendler

Starburst Galaxies

Starburst galaxies exhibit unusually high rates of star formation on large scales, and are therefore dominated by infrared and visible-light emission from young and massive stars. The starburst phenomenon may be localized near the galaxy center, or may appear throughout the galactic disk. The enhanced star formation is thought to be the result of galaxy mergers or a gravitational encounter with an interacting galaxy. Spitzer will take advantage of the fact that starburst galaxies emit huge amounts of infrared radiation to conduct comprehensive studies of their distribution and evolution over cosmological timescales.

Normal Galaxies

Most of the extragalactic (external to our galaxy) stellar systems are normal galaxies, which display none of the unusual characteristics of active galaxies. Astronomers classify such galaxies by their geometrical appearance: spiral, elliptical, or irregular. The unprecedented sensitivity and spatial resolution of Spitzer at mid- and far-infrared wavelengths will allow astronomers to conduct extensive studies of normal galaxies, extending from the local environs of the Milky Way to the distant and early Universe. Systematic measurements of the dust, gas and stellar content of galaxies within a few tens of millions of light years will provide astronomers with valuable templates for interpreting Spitzer observations of galaxies that are far more distant (and hence smaller in angular extent). Other programs will seek to detect the faint halos that are inferred from dynamical studies of spiral galaxies, map the distribution of molecular hydrogen gas within galaxy disks, and study how the formation and evolution of stars depends on the metallicity of a galaxy.

Messier 31 - Andromeda Galaxy Jason Ware	Messier 104 - Sombrero Galaxy AAO/David Malin

Galaxy Clusters

Galaxies are often found in large gravitationally bound clusters that are many times larger than their analogous stellar cousins. A rich cluster contains thousands of galaxies and a hot, clumpy intracluster gas detected in x-rays. Spitzer will investigate the extent to which cooling flows of gas, gravitationally attracted to the cluster center, produces low-mass stars and brown dwarfs. In a bizarre confirmation of Einstein's general theory of relativity, massive galaxy clusters are observed to act as gravitational lenses, whereby background (more distant) objects are magnified by the mass of the intervening cluster. Spitzer will utilize such fortuitous alignments to image distant galaxies that would otherwise be too faint to be seen.

Virgo Cluster SEDS/Univ. Arizona	Gravitational Lens in Abell 2218 NASA/HST/W. Couch

NGC 3603 NASA/HST/W. Brandner et al.

ISM in External Galaxies

Astronomers know that galaxies contain much more than stars. An interstellar medium (ISM) of dust and both atomic and molecular gas permeates the regions between stars. It is this ISM that provides the seeds for future star formation. Spitzer will not only conduct exhaustive studies of the ISM in our Milky Way Galaxy, but will also obtain valuable data from external galaxies. Measurements of H II regions in nearby galaxies will yield important data on how star formation is related to the general properties of the host galaxy.

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