Deep near-infrared imaging of W3 Main: constraints on stellar cluster formation

Embedded clusters like W3 Main are complex and dynamically evolving systems that represent an important phase of the star formation process. We aim at the characterization of the entire stellar content of W3 Main in a statistical sense to identify possible differences in evolutionary phase of the stellar populations and find clues about the formation mechanism of this massive embedded cluster. Methods. Deep JHKs imaging is used to derive the disk fraction, Ks-band luminosity functions and mass functions for several subregions in W3 Main. A two dimensional completeness analysis using artificial star experiments is applied as a crucial ingredient to assess realistic completeness limits for our photometry. We find an overall disk fraction of 7.7 $\pm$ 2.3%, radially varying from 9.4 $\pm$ 3.0 % in the central 1 pc to 5.6 $\pm$ 2.2 % in the outer parts of W3 Main. The mass functions derived for three subregions are consistent with a Kroupa and Chabrier mass function. The mass function of IRSN3 is complete down to 0.14 Msun and shows a break at M $\sim$ 0.5 Msun. We interpret the higher disk fraction in the center as evidence for a younger age of the cluster center. We find that the evolutionary sequence observed in the low-mass stellar population is consistent with the observed age spread among the massive stars. An analysis of the mass function variations does not show evidence for mass segregation. W3 Main is currently still actively forming stars, showing that the ionizing feedback of OB stars is confined to small areas ($\sim$ 0.5 pc). The FUV feedback might be influencing large regions of the cluster as suggested by the low overall disk fraction.Comment: 15 pages, 13 figures, accepted by A&

Svep1 stabilises developmental vascular anastomosis in reduced flow conditions

Molecular mechanisms controlling the formation, stabilization and maintenance of blood vessel connections remain poorly defined. Here we identify blood flow and the large extracellular protein Svep1 as co-modulators of vessel anastomosis during developmental angiogenesis in zebrafish embryos. Both loss of Svep1 and blood flow reduction contribute to defective anastomosis of intersegmental vessels. The reduced formation and lumenisation of the dorsal longitudinal anastomotic vessel (DLAV) is associated with a compensatory increase in Vegfa/Vegfr pERK signalling, concomittant expansion of apelin-positive tip cells, but reduced expression of klf2. Experimentally, further increasing Vegfa/Vegfr signalling can rescue the DLAV formation and lumenisation defects, while its inhibition dramatically exacerbates the loss of connectivity. Mechanistically, our results suggest that flow and Svep1 co-regulate the stabilization of vascular connections, in part by modulating the Vegfa/Vegfr signalling pathway

The impact of magnetic fields on the IMF in star-forming clouds near a supermassive black hole

Star formation in the centers of galaxies is thought to yield massive stars with a possibly top-heavy stellar mass distribution. It is likely that magnetic fields play a crucial role in the distribution of stellar masses inside star-forming molecular clouds. In this context, we explore the effects of magnetic fields, with a typical field strength of 38 microG, such as in RCW 38, and a field strength of 135 microG, similar to NGC 2024 and the infrared dark cloud G28.34+0.06, on the initial mass function (IMF) near (< 10 pc) a 10^7 solar mass black hole. Using these conditions, we perform a series of numerical simulations with the hydrodynamical code FLASH to elucidate the impact of magnetic fields on the IMF and the star-formation efficiency (SFE) emerging from an 800 solar mass cloud. We find that the collapse of a gravitationally unstable molecular cloud is slowed down with increasing magnetic field strength and that stars form along the field lines. The total number of stars formed during the simulations increases by a factor of 1.5-2 with magnetic fields. The main component of the IMF has a lognormal shape, with its peak shifted to sub-solar (< 0.3 M_sun) masses in the presence of magnetic fields, due to a decrease in the accretion rates from the gas reservoir. In addition, we see a top-heavy, nearly flat IMF above ~2 solar masses, from regions that were supported by magnetic pressure until high masses are reached. We also consider the effects of X-ray irradiation if the central black hole is active. X-ray feedback inhibits the formation of sub-solar masses and decreases the SFEs even further. Thus, the second contribution is no longer visible. We conclude that magnetic fields potentially change the SFE and the IMF both in active and inactive galaxies, and need to be taken into account in such calculations.Comment: 10 pages, 10 figures. Accepted for publication in Astronomy and Astrophysics. 2 more references adde

What controls star formation in the central 500 pc of the Galaxy?

The star formation rate (SFR) in the Central Molecular Zone (CMZ, i.e. the central 500 pc) of the Milky Way is lower by a factor of ≥10 than expected for the substantial amount of dense gas it contains, which challenges current star formation theories. In this paper, we quantify which physical mechanisms could be responsible. On scales larger than the disc scaleheight, the low SFR is found to be consistent with episodic star formation due to secular instabilities or possibly variations of the gas inflow along the Galactic bar. The CMZ is marginally Toomre-stable when including gas and stars, but highly Toomre-stable when only accounting for the gas, indicating a low condensation rate of self-gravitating clouds. On small scales, we find that the SFR in the CMZ may be caused by an elevated critical density for star formation due to the high turbulent pressure. The existence of a universal density threshold for star formation is ruled out. The H I–H2 phase transition of hydrogen, the tidal field, a possible underproduction of massive stars due to a bottom-heavy initial mass function, magnetic fields, and cosmic ray or radiation pressure feedback also cannot individually explain the low SFR. We propose a self-consistent cycle of star formation in the CMZ, in which the effects of several different processes combine to inhibit star formation. The rate-limiting factor is the slow evolution of the gas towards collapse – once star formation is initiated it proceeds at a normal rate. The ubiquity of star formation inhibitors suggests that a lowered central SFR should be a common phenomenon in other galaxies. We discuss the implications for galactic-scale star formation and supermassive black hole growth, and relate our results to the star formation conditions in other extreme environments

Non-Thermal Insights on Mass and Energy Flows Through the Galactic Centre and into the Fermi Bubbles

We construct a simple model of the star-formation- (and resultant supernova-) driven mass and energy flows through the inner ~200 pc (in diameter) of the Galaxy. Our modelling is constrained, in particular, by the non-thermal radio continuum and {\gamma}-ray signals detected from the region. The modelling points to a current star-formation rate of 0.04 - 0.12 M\msun/year at 2{\sigma} confidence within the region with best-fit value in the range 0.08 - 0.12 M\msun/year which - if sustained over 10 Gyr - would fill out the ~ 10^9 M\msun stellar population of the nuclear bulge. Mass is being accreted on to the Galactic centre (GC) region at a rate ~0.3M\msun/year. The region's star-formation activity drives an outflow of plasma, cosmic rays, and entrained, cooler gas. Neither the plasma nor the entrained gas reaches the gravitational escape speed, however, and all this material fountains back on to the inner Galaxy. The system we model can naturally account for the recently-observed ~> 10^6 'halo' of molecular gas surrounding the Central Molecular Zone out to 100-200 pc heights. The injection of cooler, high-metallicity material into the Galactic halo above the GC may catalyse the subsequent cooling and condensation of hot plasma out of this region and explain the presence of relatively pristine, nuclear-unprocessed gas in the GC. The plasma outflow from the GC reaches a height of a few kpc and is compellingly related to the recently-discovered Fermi Bubbles. Our modelling demonstrates that ~ 10^9 M\msun of hot gas is processed through the GC over 10 Gyr. We speculate that the continual star-formation in the GC over the age of the Milky Way has kept the SMBH in a quiescent state thus preventing it from significantly heating the coronal gas, allowing for the continual accretion of gas on to the disk and the sustenance of star formation on much wider scales in the Galaxy [abridged].Comment: 30 pages, 35 figures. Accepted for publication in MNRAS (20/04/2012). Minor textual revision