The role of thermal evaporation in galaxy formation

In colour-magnitude diagrams most galaxies fall in either the ``blue cloud'' or the ``red sequence'', with the red sequence extending to significantly brighter magnitudes than the blue cloud. The bright-end of the red sequence comprises elliptical galaxies (Es) with boxy isophotes and luminosity profiles with shallow central cores, while fainter Es have disky isophotes and power-law inner surface-brightness (SB) profiles. An analysis of published data reveals that the centres of galaxies with power-law central SB profiles have younger stellar populations than the centres of cored galaxies. We argue that thermal evaporation of cold gas by virial-temperature gas plays an important role in determining these phenomena. In less massive galaxies, thermal evaporation is not very efficient, so significant amounts of cold gas can reach the galaxy centre and fill a central core with newly formed stars, consistent with the young stellar ages of the cusps of Es with power-law SB profiles. In more massive galaxies, cold gas is evaporated within a dynamical time, so star formation is inhibited, and a core in the stellar density profile produced by dissipationless dynamics cannot be refilled. The different observed properties of AGN in higher-mass and lower-mass ellipticals are also explained because in the former the central black holes invariably accrete hot gas, while in the latter they typically accrete cold gas. An important consequence of our results is that at the present time there cannot be blue, star-forming galaxies in the most massive galactic halos, consistent with the observed truncation of the blue cloud at L*. [abridged]Comment: MNRAS, accepted. Added discussion and references, conclusions unchanged. 14 pages, 6 figures (2 color

Is Galactic Structure Compatible with Microlensing Data?

We generalize to elliptical models the argument of Kuijken (1997), which connects the microlensing optical depth towards the Galactic bulge to the Galactic rotation curve. When applied to the latest value from the MACHO collaboration for the optical depth for microlensing of bulge sources, the argument implies that the Galactic bar cannot plausibly reconcile the measured values of the optical depth, the rotation curve and the local mass density. Either there is a problem with the interpretation of the microlensing data, or our line of sight to the Galactic centre is highly atypical in that it passes through a massive structure that wraps only a small distance around the Galactic centre.Comment: Submitted to ApJ Letters. 8 pages LaTeX, 3 figures. Corrected error in description of microlensing observation

The uncertainty in Galactic parameters

We reanalyse the measurements of parallax, proper motion, and line-of-sight velocity for 18 masers in high mass star-forming regions presented by Reid et al. (2009). We use a likelihood analysis to investigate the distance of the Sun from the Galactic centre, R_0, the rotational speed of the local standard of rest, v_0, and the peculiar velocity of the Sun, vsol, for various models of the rotation curve, and models which allow for a typical peculiar motion of the high mass star-forming regions. We find that these data are best fit by models with non-standard values for vsol or a net peculiar motion of the high mass star-forming regions. We argue that a correction to vsol is much more likely, and that these data support the conclusion of Binney (2009) that V_sol should be revised upwards from 5.2 km/s to 11 km/s. We find that the values of R_0 and v_0 that we determine are heavily dependent on the model we use for the rotation curve, with model-dependent estimates of R_0 ranging from 6.7 \pm 0.5kpc to 8.9 \pm 0.9kpc, and those of v_0 ranging from 200 \pm 20 km/s to 279 \pm 33 km/s. We argue that these data cannot be thought of as implying any particular values of R_0 or v_0. However, we find that v_0/R_0 is better constrained, lying in the range 29.9-31.6 km/s/kpc for all models but one.Comment: 8 pages. MNRAS accepted. Revised to reflect final versio

Analysing surveys of our Galaxy I: basic astrometric data

We consider what is the best way to extract science from large surveys of the Milky Way galaxy. The diversity of data gathered in these surveys, together with our position within the Galaxy, imply that science must be extracted by fitting dynamical models to the data in the space of the observables. Models based on orbital tori promise to be superior for this task than traditional types of models, such as N-body models and Schwarzschild models. A formalism that allows such models to be fitted to data is developed and tested on pseudodata of varying richness.Comment: 15 pages, 6 figures, MNRAS accepted, changed to reflect final versio

Modeling the flyby anomalies with dark matter scattering

We continue our exploration of whether the flyby anomalies can be explained by scattering of spacecraft nucleons from dark matter gravitationally bound to the earth. We formulate and analyze a simple model in which inelastic and elastic scatterers populate shells generated by the precession of circular orbits with normals tilted with respect to the earth's axis. Good fits to the data published by Anderson et al. are obtained.Comment: Latex, 20 pages; revised version has moved derivations to Appendices and gives further numerical results in Sec. III and in added Tables VI and VI

Dark Matter Problem in Disk Galaxies

In the generic CDM cosmogony, dark-matter halos emerge too lumpy and centrally concentrated to host observed galactic disks. Moreover, disks are predicted to be smaller than those observed. We argue that the resolution of these problems may lie with a combination of the effects of protogalactic disks, which would have had a mass comparable to that of the inner dark halo and be plausibly non-axisymmetric, and of massive galactic winds, which at early times may have carried off as many baryons as a galaxy now contains. A host of observational phenomena, from quasar absorption lines and intracluster gas through the G-dwarf problem point to the existence of such winds. Dynamical interactions will homogenize and smooth the inner halo, and the observed disk will be the relic of a massive outflow. The inner halo expanded after absorbing energy and angular momentum from the ejected material. Observed disks formed at the very end of the galaxy formation process, after the halo had been reduced to a minor contributor to the central mass budget and strong radial streaming of the gas had died down.Comment: 5 pages; submitted to MNRA

Feedback Heating with Slow Jets in Cooling Flow Clusters

We propose a scenario in which a large fraction, or even most, of the gas cooling to low temperatures of T<10^4 K in cooling flow clusters, directly gains energy from the central black hole. Most of the cool gas is accelerated to non-relativistic high velocities, v ~ 10^3-10^4 km/sec, after flowing through, or close to, an accretion disk around the central black hole. A poorly collimated wind (or double not-well collimated opposite jets) is formed. According to the proposed scenario, this gas inflates some of the X-ray deficient bubbles, such that the average gas temperature inside these bubbles (cavities) in cooling flow clusters is kT_b ~< 100 keV. A large fraction of these bubbles will be very faint, or not detectable, in the radio. The bright rims of these weak smaller bubbles will appear as ripples. We suggest that the X-ray ripples observed in the Perseus cluster, for example, are not sound waves, but rather the rims of radio-faint weak bubbles which are only slightly hotter than their environment. This scenario is incorporated into the moderate cooling flow model; although not a necessary ingredient in that model, it brings it to better agreement with observations. In the moderate cooling flow model a cooling flow does exist, but the mass cooling rate is ~<10% of that in old versions of cooling flow models.Comment: The Astrophysical Journal, in pres