An Etymological Dictionary of Astronomy and Astrophysics
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A roughly spherical aggregation of → globular clusters, as well as the oldest stars and unseen mass that surrounds the Galaxy.

→ galactic, → halo.

In the → Galactic coordinate system, the angle between the line of sight to an object and the → Galactic equator. Galactic latitude, usually represented by the symbol b^II, ranges from +90 degrees to -90 degrees.

→ galactic; → latitude.

In the → Galactic coordinate system, the angle between the → Galactic Center and the projection of the object on the → Galactic plane. Galactic longitude, usually represented by the symbol l^II, ranges from 0 degrees to 360 degrees.

→ galactic; → longitude.

A concentration of stars and gas in the innermost region of a galaxy, sometimes extending over thousands of light-years from the center of the galaxy.

→ galactic; → nucleus.

→ galactic-scale outflow.

→ galactic; → outflow.

The plane in which the → disk of a → spiral galaxy, such as our → Milky Way, lies.

→ galactic; → plane.

The point on the sky, north or south, at which the galaxy's rotation axis would meet the celestial sphere.

→ galactic; → pole.

A diffuse radio signal that originates outside the solar system. It is strongest in the direction of the Galactic plane.

→ galactic; → radio; → noise.

The revolving of the gaseous and stellar content of a galaxy around its central nucleus. The rotation is not uniform, but differential. One revolution of the Sun within our own Galaxy takes about 220 million years, or one cosmic year.

→ galactic; → rotation.

The discrepancy between observed galaxy → rotation curves and the theoretical prediction, assuming a centrally dominated mass associated with the observed luminous material.

→ galactic; → rotation; → problem.

The global shape and the arrangement of the various parts or constituents of a galaxy.

→ galactic; → structure.

Same as → galactic coordinates.

→ galactic; → system.

An outflow of hot gas, analogous to the → solar wind, from a galaxy that has recently undergone a high → burst of star formation or has an → active galactic nucleus. Galactic winds are streams of high speed charged particles blowing out of galaxies with speeds of 300 to 3,000 km s^-1. In the case of starbursts, galactic winds are powered by → stellar winds driven by → massive stars and → supernova explosions. Galactic winds contain a mixture of extremely hot metal-enriched supernova ejecta and cooler entrained gas and dust. Outflowing material has been observed at great distances from galaxies (10 to 100 kpc). In some cases they escape the galaxy potential well and pollute the → intergalactic medium with → heavy elements. A prominent example is the → superwind of the starburst galaxy M82.

→ galactic; → wind.

The regions near the Galactic plane where there is low absorption of light by interstellar clouds so that some external galaxies may be seen through them.

→ galactic; → window.

The time taken for the Sun to revolve once around the center of the Milky Way, amounting to about 220 million years.

→ galactic; → year.

The enormous amounts of → mass and → energy released from active galaxies into the → intergalactic medium. → Supermassive black holes, believed to exist at the centres of active galaxies (→ active galaxy), → accrete matter and liberate huge quantities of energy. The energy output is often observed as → active galactic nuclei (AGN) outflows in a wide variety of forms, e.g. → collimated  → relativistic jets and/or huge overpressured cocoons in → radio, → blueshifted broad → absorption lines in the → ultraviolet and → optical, → warm absorbers and ultrafast outflows in → X-rays, and → molecular gas in → far infrared. Moreover, the processes of → star formation and → supernova explosions release mass/energy into the surroundings. This → stellar feedback heats up, ionizes and drives gas outward, often generating large-scale outflows/→ winds. Galactic outflows are observed at low redshifts reaching a velocity as large as 1000 km s^-1 and at high-z up to z ~ 5, sometimes extending over distances of 60-130 kpc. Galactic-scale outflows may be a primary driver of galaxy evolution through the removal of cool gas from star-forming regions to a galaxy's → halo or beyond.

→ galactic; → scale; → outflow.

Of, existing, or occurring in the space between galaxies. → intergalactic absorption, → intergalactic cloud, → intergalactic matter , → intergalactic medium , → warm-hot intergalactic medium.

→ inter-; → galactic.

→ Absorption features in the spectra of bright sources like → quasars attributed to a tenuous matter filling the medium between galaxies.

→ intergalactic; → absorption.

→ Intergalactic matter in the form of clouds.

→ intergalactic; → cloud.

Very low density material found indirectly in the space between galaxies. → intergalactic absorption.

→ intergalactic; → matter.