No abstract

The discovery of quasars a few hundred megayears after the Big Bang represents a major challenge to our understanding of black holes as well as galaxy formation and evolution. Quasarsʼ luminosity is produced by extreme gas accretion onto black holes, which have already reached masses of M BH >10 9 M e by z∼6. Simultaneously, their host galaxies form hundreds of stars per year, using up gas in the process. To understand which environments are able to sustain the rapid formation of these extreme sources, we started a Very Large Telescope/Multi-Unit Spectroscopic Explorer (MUSE) effort aimed at characterizing the surroundings of a sample of 5.7<z<6.6 quasars, which we have dubbed the Reionization Epoch QUasar InvEstigation with MUSE (REQUIEM) survey. We here present results of our searches for extended Lyα halos around the first 31 targets observed as part of this program. Reaching 5σ surface brightness limits of 0.1-1.1×10 −17 erg s −1 cm −2 arcsec −2 over a 1 arcsec 2 aperture, we were able to unveil the presence of 12 Lyα nebulae, eight of which are newly discovered. The detected nebulae show a variety of emission properties and morphologies with luminosities ranging from 8×10 42 to 2×10 44 erg s −1 , FWHMs between 300 and 1700 km s −1 , sizes <30 pkpc, and redshifts consistent with those of the quasar host galaxies. As the first statistical and homogeneous investigation of the circumgalactic medium of massive galaxies at the end of the reionization epoch, the REQUIEM survey enables the study of the evolution of the cool gas surrounding quasars in the first 3 Gyr of the universe. A comparison with the extended Lyα emission observed around bright (M 1450 −25 mag) quasars at intermediate redshift indicates little variations on the properties of the cool gas from z∼6 to z∼3, followed by a decline in the average surface brightness down to z∼2.

We imaged, with ALMA and ARGOS/LUCI, the molecular gas and dust and stellar continuum in XID2028, which is an obscured quasi-stellar object (QSO) at z=1.593, where the presence of a massive outflow in the ionised gas component traced by the [O III]5007 emission has been resolved up to 10 kpc. This target represents a unique test case to study QSO feedback in action at the peak epoch of AGN-galaxy co-evolution. The QSO was detected in the CO(5-4) transition and in the 1.3 mm continuum at ∼30 and ∼20σ significance, respectively; both emissions are confined in the central (< 4 kpc) radius area. Our analysis suggests the presence of a fast rotating molecular disc (v∼ 400 km s −1 ) on very compact scales well inside the galaxy extent seen in the rest-frame optical light (∼ 10 kpc, as inferred from the LUCI data). Adding available measurements in additional two CO transitions, CO(2-1) and CO(3-2), we could derive a total gas mass of ∼ 10 10 M , thanks to a critical assessment of CO excitation and the comparison with the Rayleigh-Jeans continuum estimate. This translates into a very low gas fraction (< 5%) and depletion timescales of 40-75 Myr, reinforcing the result of atypical gas consumption conditions in XID2028, possibly because of feedback effects on the host galaxy. Finally, we also detect the presence of high velocity CO gas at ∼ 5σ, which we interpret as a signature of galaxy-scale molecular outflow that is spatially coincident with the ionised gas outflow. XID2028 therefore represents a unique case in which the measurement of total outflowing mass, of ∼ 500 − 800 M yr −1 including the molecular and atomic components in both the ionised and neutral phases, was attempted for a high-z QSO.

We observe the myosin V stepping mechanism by traveling wave tracking. This technique, associated with optical tweezers, allows one to follow a scattering particle in a two-dimensional plane, with nanometer accuracy and a temporal resolution in the microsecond range. We have observed that, at the millisecond time scale, the myosin V combines longitudinal and vertical motions during the step. Because at this time scale the steps appear heterogeneous, we deduce their general features by aligning and averaging a large number of them. Our data show that the 36-nm step occurs in three main stages. First, the myosin center of mass moves forward 5 nm; the duration of this short prestep depends on the ATP concentration. Second, the motor performs a fast motion over 23 nm; this motion is associated to a vertical movement of the myosin center of mass, whose distance from the actin filament increases by 6 nm. Third, the myosin head freely diffuses toward the next binding site and the vertical position is recovered. We propose a simple model to describe the step mechanism of the dimeric myosin V. molecular motor ͉ single molecule ͉ traveling wave tracking ͉ total internal reflection ͉ interference M olecular motors convert the chemical energy, obtained from ATP hydrolysis, into mechanical work in a very efficient way. Understanding this active system, which is constantly driven away from the thermodynamics equilibrium, has been a challenge for physicists, biologists, and chemists. Nowadays, the genetic approach, together with an increasing amount of structural information (1, 2) and single-molecule studies (3-10), has supplied a detailed description of molecular motor dynamics. To describe how those machines move and develop force, several hypothesis have been proposed. They range from purely mechanical models, in which a conformational change takes place during the chemical cycle and drives the motor to the final state (11-13), to stochastic descriptions based on thermal ratchets. In the latter models, the molecular motor thermally explores the energy landscape and the arrival to the final state triggers the chemical cycle and prevents the motor from stepping back (14-21).A detailed description of the mechanical cycle (existence of internal subcycles and their dynamics) is essential to discriminate among the various theoretical models. The myosin V-actin complex is naturally a good candidate for this study: at each chemical cycle the myosin V center of mass moves toward the plus-end of the actin filaments, by steps of 36 nm (5). This movement is among the widest steps observed in molecular motors, and it facilitates the observation and characterization of the mechanical substeps. In addition, myosin V, like many dimeric machines, moves along the filament in a hand-over-hand manner (9), which seems to be a very general feature of processive motors (22,23).The single-molecule approach has been indispensable to understand the myosin V dynamics: this motor steps back only occasionally at zero load, whereas the backward/forward...