Cooling flow

Abstract: Not the way one might have thought. In hydrodynamic simulations of galaxy formation, some gas follows the traditionally envisioned route, shock heating to the halo virial temperature before cooling to the much lower temperature of the neutral ISM. But most gas enters galaxies without ever heating close to the virial temperature, gaining thermal energy from weak shocks and adiabatic compression, and radiating it just as quickly. This “cold mode” accretion is channeled along filaments, while the conventional, “hot mode” accretion is quasi-spherical. Cold mode accretion dominates high redshift growth by a substantial factor, while at z < 1 the overall accretion rate declines and hot mode accretion has greater relative importance. The decline of the cosmic star formation rate at low z is driven largely by geometry, as the typical cross section of filaments begins to exceed that of the galaxies at their intersections.

Abstract: The cooling time in the dense gas within 50 – 300 kpc of the central galaxy in most clusters is found from X-ray images to be less than about 1010 yr. The weight of the overlying gas then causes a net inflow which is called a cooling flow. X-ray spectra confirm that the gas is cooling and loses at least 90 per cent of its thermal energy. The rate at which the gas cools ranges from ~ 10 – 500 M⊙ yr−1 . The soft X-ray absorption now discovered in cooling flows suggests that the cooled gas accumulates as very cold, small, gas clouds. Any large-scale star formation must be biased to low mass objects, except in the centres of some flows where some massive star may form, possibly from larger clouds assembled from cloud collisions and aggregation.

Abstract: We study the formation of galaxies in a large volume (50 h −1 Mpc, 2 × 288 3 particles) cosmological simulation, evolved using the entropy and energy-conserving smoothed particle hydrodynamics (SPH) code GADGET-2. Most of the baryonic mass in galaxies of all masses is originally acquired through filamentary ‘cold mode’ accretion of gas that was never shock heated to its halo virial temperature, confirming the key feature of our earlier results obtained with a different SPH code. Atmospheres of hot, virialized gas develop in haloes above 2–3 × 10 11 M � , a transition mass that is nearly constant from z = 3 to 0. Cold accretion persists in haloes above the transition mass, especially at z ≥ 2. It dominates the growth of galaxies in low-mass haloes at all times, and it is the main driver of the cosmic star formation history. Our results suggest that the cooling of shock-heated virialized gas, which has been the focus of many analytic models of galaxy growth spanning more than three decades, might be a relatively minor element of galaxy formation. At high redshifts, satellite galaxies have gas accretion rates similar to central galaxies of the same baryonic mass, but at z < 1t he accretion rates of low-mass satellites are well below those of comparable central galaxies. Relative to our earlier simulations, the GADGET-2 simulations predict much lower rates of ‘hot mode’ accretion from the virialized gas component. Hot accretion rates compete with cold accretion rates near the transition mass, but only at z ≤ 1. Hot accretion is inefficient in haloes

Abstract: The morphology of the X-ray- and radio-emitting features in the central ~50 kpc region around the galaxy M87 strongly suggests that buoyant bubbles of cosmic rays (inflated by an earlier nuclear active phase of the galaxy) rise through the cooling gas at roughly half the sound speed. In the absence of strong surface tension, initially spherical bubbles will transform into tori as they rise through an external medium. Such structures can be identified in the radio images of the halo of M87. During their rise, bubbles will uplift relatively cool X-ray-emitting gas from the central regions of the cooling flow to larger distances. This gas is colder than the ambient gas and has a higher volume emissivity. As a result, rising "radio" bubbles may be trailed by elongated X-ray features, as indeed is observed in M87. We performed simple hydrodynamic simulations to illustrate qualitatively the evolution of buoyant bubbles in the M87 environment.