A Note on the Viability of Gaseous Ionization in Active Galaxies by Fast Shocks

Observational evidence suggest that shocks may affect the spatial and velocity distributions of gas in the NLR/ENLR of some active galaxies. It thus seemed plausible that shocks may also energize the NLR. The observed emission line ratios strongly favor photoionization as the heating source, but it is not clear whether the ionizing radiation is generated in the NLR by "photoionizing shocks" or whether it originates at the central continuum source. Here I point out that shocks are highly inefficient in producing line emission. Shocks in the NLR can convert at most 10^{-6} of the rest mass to ionizing radiation, compared with a maximum conversion efficiency of ~0.1 for the central continuum source. The required mass flow rate through shocks in the NLR is thus a few orders of magnitude higher than the mass accretion rate required to power the NLR by the central continuum source. Since gravity appears to dominate the NLR cloud dynamics, shocks must lead to an inflow, and the implied high inflow rates can be ruled out in most active galaxies. NLR dynamics driven by a thermal wind or by some jet configurations may produce the mass flux through shocks required for photoionizing shocks to be viable, but the mass flux inward from the NLR must be kept ~100-1000 times smaller. Photoionizing shocks are a viable mechanism in very low luminosity active galaxies if they are highly sub-Eddington (<~10^{-4}) and if they convert mass to radiation with a very low efficiency (<~10^{-4}).Comment: 6 pages, aas2pp4.sty, accepted for publication in ApJ Letter

Short Employment Spells in Italy, Germany and the UK: Testing the Port of Entry Hypothesis

This paper looks at short employment spells in three European countries: Great Britain, whose labour market is considered the most flexible in the EU; Italy, regarded as the least flexible; and Germany, tightly regulated, but characterised by a deservedly famous apprenticeship system. In particular, it aims to assess whether young people in short-lived jobs stand a better chance of finding a 'good job' compared to their older colleagues. The increasingly held belief that - in modern economies - a 'bad job' at the beginning of one's career is the 'port-of-entry' to stable employment and to upward mobility, makes this assessment particularly relevant; ie it matters greatly if short-duration jobs are entry ports into better employment or become long term-traps. The lack of accepted benchmarks makes it difficult to reach strong conclusions in regard to the 'efficiency' of labour markets, however, this study should help to highlight the effect of different labour market institutions on mobility and on the soundness of the 'port-of-entry' hypothesis.

Signatures of Cool Gas Fueling a Star-Forming Galaxy at Redshift 2.3

Galaxies are thought to be fed by the continuous accretion of intergalactic gas, but direct observational evidence has been elusive. The accreted gas is expected to orbit about the galaxy's halo, delivering not just fuel for star-formation but also angular momentum to the galaxy, leading to distinct kinematic signatures. Here we report observations showing these distinct signatures near a typical distant star-forming galaxy where the gas is detected using a background quasar passing 26 kpc from the host. Our observations indicate that gas accretion plays a major role in galaxy growth since the estimated accretion rate is comparable to the star-formation rate.Comment: 33 pages, 8 figures, version matching the proofed tex

Manufacturing polymeric porous capsules

Polymeric porous capsules represent hugely promising systems that allow a size-selective through-shell material exchange with their surroundings. They have vast potential in applications ranging from drug delivery and chemical microreactors to artificial cell science and synthetic biology. Due to their porous core-shell structure, polymeric porous capsules possess an enhanced permeability that enables the exchange of small molecules while retaining larger compounds and macromolecules. The cross-capsule transfer of material is regulated by their pore size cut-off, which depends on the molecular composition and adopted fabrication method. This review outlines the main strategies adopted for manufacturing polymeric porous capsules to provide some practical guidance for designing polymeric capsules with controlled pore size

The VIMOS VLT Deep Survey. The different assembly history of passive and star-forming L_B >= L*_B galaxies in the group environment at z < 1

We use the VIMOS VLT Deep Survey to study the close environment of galaxies in groups at 0.2 = L*_B galaxies (Me_B = M_B + 1.1z <= -20) are identified with Me_B <= -18.25 and within a relative distance 5h^-1 kpc <= rp <= 100h^-1 kpc and relative velocity Delta v <= 500 km/s . The richness N of a group is defined as the number of Me_B <= -18.25 galaxies belonging to that group. We split our principal sample into red, passive galaxies with NUV - r >= 4.25 and blue, star-forming galaxies with NUV - r < 4.25. We find that blue galaxies with a close companion are primarily located in poor groups, while the red ones are in rich groups. The number of close neighbours per red galaxy increases with N, with n_red being proportional to 0.11N, while that of blue galaxies does not depend on N and is roughly constant. In addition, these trends are found to be independent of redshift, and only the average n_blue evolves, decreasing with cosmic time. Our results support the following assembly history of L_B >= L*_B galaxies in the group environment: red, massive galaxies were formed in or accreted by the dark matter halo of the group at early times (z >= 1), therefore their number of neighbours provides a fossil record of the stellar mass assembly of groups, traced by their richness N. On the other hand, blue, less massive galaxies have recently been accreted by the group potential and are still in their parent dark matter halo, having the same number of neighbours irrespective of N. As time goes by, these blue galaxies settle in the group potential and turn red and/or fainter, thus becoming satellite galaxies in the group. With a toy quenching model, we estimate an infall rate of field galaxies into the group environment of R_infall = 0.9 - 1.5 x 10^-4 Mpc^-3 Gyr^-1 at z ~ 0.7.Comment: Astronomy and Astrophysics, in press. 11 pages, 11 figures, 4 tables. Minor changes with respect to the first versio

Evolution of the mass, size, and star formation rate in high-redshift merging galaxies MIRAGE - A new sample of simulations with detailed stellar feedback

We aim at addressing the questions related to galaxy mass assembly through major and minor wet merging processes in the redshift range 1<z<2. A consequent fraction of Milky Way like galaxies are thought to have undergone an unstable clumpy phase at this early stage. Using the adaptive mesh refinement code RAMSES, with a recent physically-motivated implementation of stellar feedback, we build the Merging and Isolated high-Redshift Adaptive mesh refinement Galaxies (MIRAGE) sample. It is composed of 20 mergers and 3 isolated idealized disks simulations with global physical properties in accordance with the 1<z<2 mass complete sample MASSIV. The numerical hydrodynamical resolution reaches 7 parsecs in the smallest Eulerian cells. Our simulations include: star formation, metal line cooling, metallicity advection, and a recent implementation of stellar feedback which encompasses OB-type stars radiative pressure, photo-ionization heating, and supernovae. The initial conditions are set to match the z~2 observations, thanks to a new public code DICE. The numerical resolution allows us to follow the formation and evolution of giant clumps formed in-situ from Jeans instabilities triggered by high initial gas fraction. The star formation history of isolated disks shows stochastic star formation rate, which proceeds from the complex behavior of the giant clumps. Our minor and major gas-rich merger simulations do not trigger starbursts, suggesting a saturation of the star formation in a turbulent and clumpy interstellar medium fed by substantial accretion from the circum-galactic medium. Our simulations are close to the normal regime of the disk-like star formation on a Schmidt-Kennicutt diagram. The mass-size relation and its rate of evolution matches observations, suggesting that the inside-out growth mechanisms of the stellar disk do not necessarily require to be achieved through a cold accretion.Comment: 18 pages, 12 figures. Accepted in A&

Enzyme-driven chemotactic synthetic vesicles

Directional locomotion or taxis is possibly one of the most important evolutionary milestones, since it has allowed many living organisms to outperform their non-motile competitors. In particular, chemotaxis is one of the most elaborated targeting processes present in Nature. By sensing a chemical gradient, uni or multicellular organisms are able to move toward or away from favourable/unfavourable stimuli, adapting to changes in environmental conditions. This phenomenon normally involves the presence of a specific chemical gradient of signaling molecules that guides cells in their orientation and movement. Chemotaxis is therefore a potent long-range directional process, extending over length scales that are several orders of magnitude larger than the motilesystem itself. Creating an artificial self-propelled object, similar to many biological micro and nano-motors present in Nature, is one of the main challenges in nanotechnology. The chemotaxis applied to an artificial nanovector could be used for a wide range of applications, including drug delivery systems and nanoreactors. Combining natural enzymes with synthetic vesicles, we propose an efficient chemotactic nano-system driven by enzymatic conversion of small water-soluble molecules. We achieve this by encapsulating enzymes into nanoscopic polymer vesicles (known as polymersomes) whose membranes are designed to contain permeable domains within an impermeable matrix. The asymmetric distribution of the permeable domains enables the localised expulsion of the entrapped enzyme reaction products. This, in turn, allows propulsion in a specific direction that is controlled by the signaling molecule concentration. We demonstrate this concept, using physiologically relevant hydrogen peroxide and glucose coupled with catalase, glucose peroxidase and their combination loaded within asymmetric polymersomes. We show that the combination of membrane topology and enzyme encapsulation produces propulsion and chemotaxis without requiring chemical modification. Finally, we propose a new diffusion mechanism whereby selective permeability across nanoscopic membrane compartments is exploited to generate locomotion