Professor James Binney FRS

Abstract:

Evidence is summarized that suggests that when a protogalaxy collapses, a fraction $f$ of its gas fails to heat to the virial temperature, where $f$ is large for haloes less massive than the value $M^*$ associated with $L^*$ galaxies. Stars and galaxies form only from the cool gas fraction. Hot gas is ejected from low-mass systems as in conventional semi-analytic models of galaxy formation. In high-mass systems it is retained but does not cool and form stars. Instead it builds up as a largely inert atmosphere, in which cooling is inhibited by an episodically active galactic nucleus. Cold gas frequently falls into galactic haloes. In the absence of a dense atmosphere of virial-temperature gas it builds up on nearly circular orbits and forms stars. When there is a sufficiently dense hot atmosphere, cold infalling gas tends to be ablated and absorbed by the hot atmosphere before it can form stars. The picture nicely explains away the surfeit of high-luminosity galaxies that has recently plagued semi-analytic models of galaxy formation, replacing them by systems of moderate luminosity from old stars and large X-ray luminosities from hot gas.

Abstract:

We have observed 54 nearby spiral galaxies with the Space Telescope Imaging Spectrograph (STIS) on the Hubble Space Telescope to obtain optical long-slit spectra of nuclear gas disks and STIS optical (∼R band) images of the central 5″ × 5′ of the galaxies. These spectra are being used to determine the velocity field of nuclear disks and hence to detect the presence of central massive black holes. Here we present the spectra for the successful observations. Dust obscuration can be significant at optical wavelengths, and so we also combine the STIS images with archival Near-Infrared Camera and Multi-Object Spectrometer H-band images to produce color maps to investigate the morphology of gas and dust in the central regions. We find a great variety in the different morphologies, from smooth distributions to well-defined nuclear spirals and dust lanes.

Abstract:

Active galactic nuclei are clearly heating gas in `cooling flows'. The effectiveness and spatial distribution of the heating are controversial. We use three-dimensional simulations on adaptive grids to study the impact on a cooling flow of weak, subrelativistic jets. The simulations show cavities and vortex rings as in the observations. The cavities are fast-expanding dynamical objects rather than buoyant bubbles as previously modelled, but shocks still remain extremely hard to detect with X-rays. At late times the cavities turn into overdensities that strongly excite the cluster's g-modes. These modes damp on a long timescale. Radial mixing is shown to be an important phenomenon, but the jets weaken the metallicity gradient only very near the centre. The central entropy density is modestly increased by the jets. We use a novel algorithm to impose the jets on the simulations.

Abstract:

We present Space Telescope Imaging Spectrograph spectra of the Sbc spiral galaxy NGC 4041, which were used to map the velocity field of the gas in its nuclear region. We detect the presence of a compact (r ≃ 0″.4 ≃ 40 pc), high surface brightness, rotating nuclear disk cospatial with a nuclear star cluster. The disk is characterized by a rotation curve with a peak-to-peak amplitude of ∼40 km s-1 and is systematically blueshifted by ∼10-20 km s-1 with respect to the galaxy systemic velocity. With the standard assumption of constant mass-to-light ratio and with the nuclear disk inclination taken from the outer disk, we find that a dark point mass of (1-0.7+0.6) × 107 M⊙ is needed to reproduce the observed rotation curve. However, the observed blueshift suggests the possibility that the nuclear disk could be dynamically decoupled. Following this line of reasoning, we relax the standard assumptions and find that the kinematical data can be accounted for by the stellar mass provided that either the central mass-to-light ratio is increased by a factor of ∼2 or the inclination is allowed to vary. This model results in a 3 σ upper limit of 6 × 106 M⊙ on the mass of any nuclear black hole (BH). Overall, our analysis only allows us to set an upper limit of 2 × 107 M⊙ on the mass of the nuclear BH. If this upper limit is taken in conjunction with an estimated bulge B magnitude of -17.7 and with a central stellar velocity dispersion of ≃95 km s-1, then these results are not inconsistent with both the MBH-Lsph and the MBH-σ* correlations. Constraints on BH masses in spiral galaxies of types as late as Sbc are still very scarce; therefore, the present result adds an important new data point to our understanding of BH demography.