AGN feedback and triggering of star formation in galaxies

Feedback from the central black hole in active galactic nuclei (AGN) may be responsible for establishing the observed MBH-sigma relation and limiting the bulge stellar mass of the host galaxy. Here we explore the possibility of AGN feedback triggering star formation in the host galaxy. We consider a shell of dusty gas, driven outwards by radiation pressure, and analyse its escape/trapping condition in the galactic halo for different underlying dark matter potentials. In the isothermal potential, we obtain that the standard condition setting the observed MBH-sigma relation is not sufficient to clear gas out of the entire galaxy; whereas the same condition is formally sufficient in the case of the Hernquist and Navarro-Frenk-White profiles. The squeezing and compression of the inhomogeneous interstellar medium during the ejection process can trigger star formation within the feedback-driven shell. We estimate the resulting star formation rate and total additional stellar mass. In this picture, new stars are formed at increasingly larger radii and successively populate the outer regions of the host galaxy. This characteristic pattern may be compared with the observed 'inside-out' growth of massive galaxies.Comment: 8 pages, 8 figures, accepted for publication in MNRA

Variations on a theme of AGN-driven outflows: luminosity evolution and ambient density distribution

Galactic outflows are now commonly observed in starburst and active galactic nuclei (AGN) host galaxies. Yet, there is no clear consensus on their physical driving mechanism(s). We have previously shown that AGN radiative feedback, driven by radiation pressure on dust, can account for the observed dynamics and energetics of galactic outflows, provided that radiation trapping is taken into account. Here we generalise our model results by explicitly considering the temporal evolution of the central AGN luminosity, and the shell mass evolution in different ambient density distributions. In the case of fixed-mass shells, the high observed values of the momentum ratio ($\zeta = \dot{p}/(L/c)$) and energy ratio ($\epsilon_k = \dot{E}_{k}/L$) may be attributed to either radiation trapping or AGN luminosity decay. In contrast, for expanding shells sweeping up mass from the surrounding environment, a decay in AGN luminosity cannot account for the observed high energetics, and radiation trapping is necessarily required. Indeed, strong radiation trapping, e.g. due to high dust-to-gas ratios, can considerably boost the outflow energetics. We obtain a distinct radial dependence for the outflow energetics ($\zeta(r)$, $\epsilon_k(r)$) in the case of radiation trapping and luminosity decay, which may help discriminate between the two scenarios. In this framework, the recently discovered `fossil' outflows, with anomalously high values of the energetics, may be interpreted as relics of past AGN activity. The observed outflow properties may therefore provide useful constraints on the past history of AGN activity and/or the physical conditions of the outflow launch region.Comment: accepted for publication in MNRA

The connection between AGN-driven dusty outflows and the surrounding environment

Significant reservoirs of cool gas are observed in the circumgalactic medium (CGM) surrounding galaxies. The CGM is also found to contain substantial amounts of metals and dust, which require some transport mechanism. We consider AGN (active galactic nucleus) feedback-driven outflows based on radiation pressure on dust. Dusty gas is ejected when the central luminosity exceeds the effective Eddington luminosity for dust. We obtain that a higher dust-to-gas ratio leads to a lower critical luminosity, implying that the more dusty gas is more easily expelled. Dusty outflows can reach large radii with a range of velocities (depending on the outflowing shell configuration and the ambient density distribution) and may account for the observed CGM gas. In our picture, dust is required in order to drive AGN feedback, and the preferential expulsion of dusty gas in the outflows may naturally explain the presence of dust in the CGM. On the other hand, the most powerful AGN outflow events can potentially drive gas out of the local galaxy group. We further discuss the effects of radiation pressure of the central AGN on satellite galaxies. AGN radiative feedback may therefore have a significant impact on the evolution of the whole surrounding environment.Comment: accepted for publication in MNRA

Spin-orbit coupling and anisotropic exchange in two-electron double quantum dots

The influence of the spin-orbit interactions on the energy spectrum of two-electron laterally coupled quantum dots is investigated. The effective Hamiltonian for a spin qubit pair proposed in F. Baruffa et al., Phys. Rev. Lett. 104, 126401 (2010) is confronted with exact numerical results in single and double quantum dots in zero and finite magnetic field. The anisotropic exchange Hamiltonian is found quantitatively reliable in double dots in general. There are two findings of particular practical importance: i) The model stays valid even for maximal possible interdot coupling (a single dot), due to the absence of a coupling to the nearest excited level, a fact following from the dot symmetry. ii) In a weak coupling regime, the Heitler-London approximation gives quantitatively correct anisotropic exchange parameters even in a finite magnetic field, although this method is known to fail for the isotropic exchange. The small discrepancy between the analytical model (which employes the linear Dresselhaus and Bychkov-Rashba spin-orbit terms) and the numerical data for GaAs quantum dots is found to be mostly due to the cubic Dresselhaus term.Comment: 15 pages, 11 figure

The relationship between cooling flows and metallicity measurements for X-ray luminous clusters

We explore the relationship between the metallicity of the intracluster gas in clusters of galaxies, determined by X-ray spectroscopy, and the presence of cooling flows. Using ASCA spectra and ROSAT images, we demonstrate a clear segregation between the metallicities of clusters with and without cooling flows. On average, cooling-flow clusters have an emission-weighted metallicity a factor ~ 1.8 times higher than that of non-cooling flow systems. We suggest this to be due to the presence of metallicity gradients in the cooling flow clusters, coupled with the sharply peaked X-ray surface brightness profiles of these systems. Non-cooling flow clusters have much flatter X-ray surface brightness distributions and are thought to have undergone recent merger events which may have mixed the central high-metallicity gas with the surrounding less metal-rich material. We find no evidence for evolution in the emission-weighted metallicities of clusters within z~0.3.Comment: Submitted to MNRAS letters (December 1997). 6 pages, 2 figures in MNRAS LaTex style. Minor revision

ROSAT PSPC detection of soft X-ray absorption in GB 1428+4217: The most distant matter yet probed with X-ray spectroscopy

We report on a ROSAT PSPC observation of the highly-luminous z = 4.72 radio-loud quasar GB 1428+4217 obtained between 1998 December 11 and 17, the final days of the ROSAT satellite. The low-energy sensitivity of the PSPC detector was employed to constrain the intrinsic X-ray absorption of the currently most distant X-ray detected object. Here we present the detection of significant soft X-ray absorption towards GB 1428+4217, making the absorbing material the most distant matter yet probed with X-ray spectroscopy. X-ray variability by 25+-8 per cent is detected on a timescale of 6500 s in the rest frame. The X-ray variation requires an unusually high radiative efficiency of at least 4.2, further supporting the blazar nature of the source.Comment: 6 pages incl. 6 figures, accepted for publication in Monthly Notice

Chandra observations of the galaxy cluster Abell 1835

We present the analysis of 30 ksec of Chandra observations of the galaxy cluster Abell 1835. Overall, the X-ray image shows a relaxed morphology, although we detect substructure in in the inner 30 kpc radius. Spectral analysis shows a steep drop in the X-ray gas temperature from ~12 keV in the outer regions of the cluster to ~4 keV in the core. The Chandra data provide tight constraints on the gravitational potential of the cluster which can be parameterized by a Navarro, Frenk & White (1997) model. The X-ray data allow us to measure the X-ray gas mass fraction as a function of radius, leading to a determination of the cosmic matter density of \Omega_m=0.40+-0.09 h_50^-0.5. The projected mass within a radius of ~150 kpc implied by the presence of gravitationally lensed arcs in the cluster is in good agreement with the mass models preferred by the Chandra data. We find a radiative cooling time of the X-ray gas in the centre of Abell 1835 of about 3x10^8 yr. Cooling flow model fits to the Chandra spectrum and a deprojection analysis of the Chandra image both indicate the presence of a young cooling flow (~6x10^8 yr) with an integrated mass deposition rate of 230^+80_-50 M_o yr^-1 within a radius of 30 kpc. We discuss the implications of our results in the light of recent RGS observations of Abell 1835 with XMM-Newton.Comment: 15 pages, 15 figures, accepted by MNRA