An Etymological Dictionary of Astronomy and Astrophysics
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Having a relatively low temperature.

M.E., from O.E. cald, ceald "cold, cool" (cf. O.Fr. and O.Sax. kald, O.H.G. and Ger. kalt, Goth. kalds "cold"), from PIE root *gel-/*gol- "cold;" cf. L. gelare "to freeze," gelu "frost," glacies "ice;" Kurd. girsân, girsiân "to coagulate" (Cheung 2007).

Sard "cold, cool," afsordan, afsârdan "to congeal;" Mid.Pers. sard/sart "cold;" Av. sarəta- "cold;" cf. Skt. śiśira- "cold;" L. calidus "warm;" Lith. šaltas "cold;" Welsh clyd "warm;" PIE *keltos- "cool."

A broad → absorption feature observed in → X-ray spectra of → active galactic nuclei (AGN). It is caused by material associated with the → interstellar medium in our → Galaxy and/or the host galaxy of the AGN or cold material near the AGN. → Quasars commonly have their X-ray spectrum absorbed by cold gas between us and the quasar X-ray source (along our → line of sight). This is particularly common in less luminous quasars. See also → warm absorber.

→ cold; → absorber.

1) A type of → accretion flow by a → compact object such as a → black hole that consists of cool → optically thick gas and has a relatively high mass → accretion rate, in contrast to → hot accretion flows.
2) Gas accreting from the → intergalactic medium (IGM) onto → galactic haloes with sufficiently low velocities so that it will not be shocked to the → virial temperature of the halo, but will instead flow at a relatively low temperature (T ~ 10⁴ K). Galaxies grow by accreting gas from → cosmic filaments. Feedback from star formation and → active galactic nuclei returns a significant fraction of the → interstellar medium (ISM) to the halo and may even blow it out of the halo into the IGM. This "cold accretion" will happen if the cooling time of → virialized gas is too short to maintain a hot, → hydrostatic halo. The existence of such a cold accretion mode has been confirmed by simulations, which have furthermore demonstrated that cold mode accretion can also be important for halos sufficiently massive to contain hot, hydrostatic gas. Because gas accretes preferentially along the filaments of the cosmic web, the streams of infalling gas have relatively high gas densities and correspondingly low cooling times. This allows the cold streams to penetrate the hot, hydrostatic halos surrounding massive galaxies, particularly at → high redshifts (F. van de Voort et al., 2012, MNRAS 421, 2809).

→ cold; → accretion; → flow.

Any → hypothetical → non-baryonic  → dark matter that is → non-relativistic at the point of → decoupling in the → early Universe. CDM plays a key role in → cosmic structure formation. See also → CDM model, → lambda cold dark matter, → Meszaros effect, → missing satellites problem.

→ cold; → dark; → matter.

An accretion process whereby material coming from an → accretion disk settles onto the → protostellar surface through a geometrically thin layer or thin accretion columns. Heat brought into the protostar in the accretion flow radiates freely into space until the temperature attains the photospheric value. Most of the stellar surface is unaffected by the accretion flow (see, e.g., Hosokawa et al. 2010, ApJ 721, 478).

→ cold; → disk; → accretion.

The → standard model of → Big Bang that incorporates both → dark matter and → dark energy. See also → cold dark matter (CDM).

→ lambda, → cosmological constant; → cold; → dark; → matter; → model.