Star Formation Activity in Normal Galaxies

Tracers of Star Formation

Star formation and the excitation of the ISM by the young stars, the explosion of stars and the subsequent acceleration of energetic particles leading to nonthermal radioemission, lead to the well established correlation between the nonthermal radioemission, the far-infrared dust emission and the X-ray emission from strongly star forming regions in galaxies and entire galaxies.

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Although this general correlation is well understood, the detailed physical steps leading not only to a correlation but also to a tight connection are not finally clear.

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Tracers of star formation are thus molecules and atoms in excited states tracing out high-density regions in clouds (emission,

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Absorption and Masers:

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Far-Infrared Dust Emission from dust particles heated by massive young stars:

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infrared and optical line emission from dense gas excited by young stars, and the nonthermal and thermal radio continuum emission from the hot phase of the ISM). These arguments seem to hold to a fair degree in normal galaxies as well as irregular and blue compact galaxies.

Star Formation in the Nuclear Regions of Observed Galaxies

Star formation is usually concentrated in the inner region of a galaxy, and manifests itself in a centrally peaked distribution of all the indicators of star formation.

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Early-type galaxies often show a central hole in star formation. Most interesting is the finding that many galaxies show evidence of central rings on which star formation is rather intense.

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The origin of such a ring can be traced to the forcing of a putative bar:

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often known to be present, to a turnover in the rotation curve and the pile up from normal accretion throughout the disk on galactic time scales.

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Such a ring is also found often in AGN galaxies, where the star formation activity in such a nuclear ring can dominate the emission at some wavelengths through the starburst-driven wind (e.g., NGC 1068).

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This nuclear ring has to be considered as the inner edge of a disk in molecular gas.

Disk components are often visible in the stellar distribution, as dust lanes, emission lines and, in some cases, as molecular gas, also in elliptical galaxies.

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Since such dust lanes are indicative of cool molecular clouds, it can be expected that stars form and that thus the stellar population changes and evolves.

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Theoretical Interpretation

Galaxy-wide accretion after the disk has been formed is now seen as a prime mechanism to produce the exponential disk in the gravitational potential given by the dark matter as well as the spheroidal component of the stellar population. The gradual increase in primary nucleosynthesis elements is now well documented, with the initial increase with time often dominant. During mergers new globular clusters can form, hence leading to a number of globular clusters which increases faster than linear with stellar luminosity. It is now well recognized that an AGN at the center of a galaxy is being fed from the surrounding regions, usually accompanied with strong star formation. Even many radio galaxies, usually elliptical galaxies, have prominent dust lanes usually perpendicular to their radio symmetry axis, which contain star forming regions (the best known example is Cen A).

The advent of data from the satellites COBE (Cosmic Background Explorer), then the Hubble Space Telescope (HST), the Infrared Space Observatory (ISO), and now Hipparcos have modified our view of the Universe considerably. In ground-based observations, the micro-lensing observations of the multitude of stars in the Large Magellanic Cloud as well as the central region of our Galaxy above the disk have shown that there is an otherwise invisible population of compact objects, quite possibly a population of very faint stars. These space and ground-based observations all have deep implications for our understanding of the evolution of the universe and its constituents. Although we have not yet converged to a clear understanding of how galaxies form and evolve, now we can see many more facets of this process.


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