The effects of magnetic fields in cold clouds in cooling flows

Abstract

Large masses of absorbing material are inferred to exist in cooling flows in clusters of galaxies from the excess X-ray absorption in the spectra of some X-ray clusters. The absorbing material is probably in the form of cold clouds pressure-confined by the surrounding, hot, X-ray emitting gas. The cold clouds could remain relatively static until they are destroyed by evaporation or ablation, or give rise to star formation. If the final fate of the clouds is stars, the IMF of the stars formed over the whole cooling flow region ($r \sim 100$ kpc) should be biased to low masses, to avoid a very luminous, blue halo for the central galaxy of the cooling flow. However, there is evidence for bright star formation in the innermost (r < 10 kpc) regions of some cooling flows, and, therefore, the biasing of the IMF towards low masses should not occur or be less important at smaller radii. The consideration of magnetic fields may shed light on these two points. If magnetic fields are present, the magnetic critical mass should be considered, besides the Jeans mass, in establishing a natural mass scale for star formation. When this new mass scale is taken into account, we obtain the right variation of the biasing of the IMF with the radius in addition to inhibition of high-mass star formation at large radii. We also demonstrate that magnetic reconnection is a efficient than ambipolar diffusion in removing magnetic fields in cold clouds.Comment: 9 pages, 1 figure, accepted for publication in MNRA

Clues to the nature of damped Lyman α systems from chemical evolution models

“…However, assuming that G2 is at the redshift of the DLA, we can estimate its SFR from its magnitude, because, at z ∼ 3, the filter samples the rest‐frame far‐UV spectrum produced by recently formed O and B stars. Assuming that G2 has a typical DLA metallicity of [Fe/H]=−1.5 and that is undergoing continuous star formation for 10 8 yr, we obtain, following Friaça & Terlevich (1999), an SFR of 6.2 M ⊙ yr −1 . This corresponds to a model with M G = 10 9 M ⊙ and ν= 6 Gyr −1 (or M G = 2 × 10 9 M ⊙ and ν= 3 Gyr −1 ).…”

“…Our adopted cooling function is valid down to T = 1900 K, where the H 2 fraction is still unimportant. Any gas that cools below this temperature is removed from the fluid according to our prescriptions for thermal instabilities (Friaça et al 1993; Friaça & Jafelice 1999; Gonçalves & Friaça 1999), and is used in star formation. The identification of the H i gas to the neutral gas is supported by the fact that in DLAs N (H 2 ) ≪ N (H i ) (Petitjean, Srianand & Ledoux 2002).…”

“…Despite the high quality of their spectra, the large equivalent width of the saturated interstellar lines and the high velocity dispersions in the ISM of these galaxies make abundance determination in LBGs difficult. However, combining observational data and modelling allows one to infer typical LBG metallicities in the range of 0.3 Z ⊙ to Z ⊙ (Friaça & Terlevich 1999).…”

“…In order to investigate these questions, Fig. 19 compares metallicity versus N (H i ) trends obtained in this work for DLAs not only with predictions of the chemodynamical model for LBGs (Friaça & Terlevich 1999), but also with data of Lyα forest (LAF) clouds and LLSs. As we see from Fig.…”

“…A characteristic of these models is that several episodes of inflow and outflow occur simultaneously are different radii. The chemodynamical model for spheroids was used to investigate the relation between young elliptical galaxies and QSO activity (FT98), the absence of passively evolving elliptical galaxies in deep surveys (Jimenez et al 1999), LBGs (Friaça & Terlevich 1999), blue core spheroids (Friaça & Terlevich 2001) and the coupled spheroid and black hole formation (Archibald et al 2002).…”