624 research outputs found
Toy Models for Galaxy Formation versus Simulations
We describe simple useful toy models for key processes of galaxy formation in
its most active phase, at z > 1, and test the approximate expressions against
the typical behaviour in a suite of high-resolution hydro-cosmological
simulations of massive galaxies at z = 4-1. We address in particular the
evolution of (a) the total mass inflow rate from the cosmic web into galactic
haloes based on the EPS approximation, (b) the penetration of baryonic streams
into the inner galaxy, (c) the disc size, (d) the implied steady-state gas
content and star-formation rate (SFR) in the galaxy subject to mass
conservation and a universal star-formation law, (e) the inflow rate within the
disc to a central bulge and black hole as derived using energy conservation and
self-regulated Q ~ 1 violent disc instability (VDI), and (f) the implied steady
state in the disc and bulge. The toy models provide useful approximations for
the behaviour of the simulated galaxies. We find that (a) the inflow rate is
proportional to mass and to (1+z)^5/2, (b) the penetration to the inner halo is
~50% at z = 4-2, (c) the disc radius is ~5% of the virial radius, (d) the
galaxies reach a steady state with the SFR following the accretion rate into
the galaxy, (e) there is an intense gas inflow through the disc, comparable to
the SFR, following the predictions of VDI, and (f) the galaxies approach a
steady state with the bulge mass comparable to the disc mass, where the
draining of gas by SFR, outflows and disc inflows is replenished by fresh
accretion. Given the agreement with simulations, these toy models are useful
for understanding the complex phenomena in simple terms and for
back-of-the-envelope predictions.Comment: Resubmitted to MNRAS after responding to referee's comments; Revised
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Press shaping of arched components by means of a mobile tool
The best tool motion in a press is considered, when producing U-shaped components from sheet. The elastoplastic properties of the deformed material are taken into account. © 2013 Allerton Press, Inc
Superconducting Single-photon Detectors Made of Ultra-thin VN Films
We optimized technology of thin VN films deposition in order to study VN-based superconducting single-photon detectors. Investigation of the main VN film parameters showed that this material has lower resistivity compared to commonly used NbN. Fabricated from obtained films devices showed 100% intrinsic detection efficiency at 900 nm, at the temperature of 1.7 K starting with the bias current of 0.7·
Particle tagging and its implications for stellar population dynamics
We establish a controlled comparison between the properties of galactic stellar haloes obtained with hydrodynamical simulations and with ‘particle tagging’. Tagging is a fast way to obtain stellar population dynamics: instead of tracking gas and star formation, it ‘paints’ stars directly on to a suitably defined subset of dark matter particles in a collisionless, dark-matter-only simulation. Our study shows that ‘live’ particle tagging schemes, where stellar masses are painted on to the dark matter particles dynamically throughout the simulation, can generate good fits to the hydrodynamical stellar density profiles of a central Milky Way-like galaxy and its most prominent substructure. Energy diffusion processes are crucial to reshaping the distribution of stars in infalling spheroidal systems and hence the final stellar halo. We conclude that the success of any particular tagging scheme hinges on this diffusion being taken into account, and discuss the role of different subgrid feedback prescriptions in driving this diffusion
The Kinematic Properties of BHB and RR Lyrae stars towards the Anticentre and the North Galactic Pole: The Transition between the Inner and the Outer Halo
We identify 51 blue horizontal branch (BHB) stars, 12 possible BHB stars and
58 RR Lyrae stars in Anticentre fields. Their selection does not depend on
their kinematics. Light curves and ephemerides are given for 7 previously
unknown RR Lyrae stars. All but 4 of the RR Lyrae stars are of Oosterhoff type
I. Our selection criteria for BHB stars give results that agree with those used
by Smith et al. (2010) and Ruhland et al. (2011). We use 5 methods to determine
distances for the BHB stars and 3 methods for the RR Lyrae stars to get
distances on a uniform scale. Absolute proper motions (largely derived from the
GSCII and SDSS (DR7) databases) are given for these stars; radial velocities
are given for 31 of the BHB stars and 37 of the RR Lyrae stars. Combining these
data for BHB and RR Lyrae stars with those previously found in fields at the
North Galactic Pole, we find that retrograde orbits dominate for galactocentric
distances greater than 12.5 kpc. The majority of metal-poor stars in the solar
neighbourhood are known to be concentrated in a Lperp vs. Lz angular momentum
plot. We show that the ratio of the number of outliers to the number in the
main concentration increases with galactocentric distance. The location of
these outliers with Lperp and Lz shows that the halo BHB and RR Lyrae stars
have more retrograde orbits and a more spherical distribution with increasing
galactocentric distance. Six RR Lyrae stars are identified in the H99 group of
outliers; the small spread in their [Fe/H] suggests that they could have come
from a single globular cluster. Another group of outliers contains two pairs of
RR Lyrae stars; the stars in each pair have similar properties.Comment: 40 pages, 19 figures, to be published in MNRA
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