9,704 research outputs found
The halo masses and galaxy environments of hyperluminous QSOs at z~2.7 in the Keck Baryonic Structure Survey
We present an analysis of the galaxy distribution surrounding 15 of the most
luminous (>10^{14} L_sun; M_1450 ~ -30) QSOs in the sky with z~2.7. Our data
are drawn from the Keck Baryonic Structure Survey (KBSS). In this work, we use
the positions and spectroscopic redshifts of 1558 galaxies that lie within ~3',
(4.2 h^{-1} comoving Mpc; cMpc) of the hyperluminous QSO (HLQSO) sightline in
one of 15 independent survey fields, together with new measurements of the
HLQSO systemic redshifts. We measure the galaxy-HLQSO cross-correlation
function, the galaxy-galaxy autocorrelation function, and the characteristic
scale of galaxy overdensities surrounding the sites of exceedingly rare,
extremely rapid, black hole accretion. On average, the HLQSOs lie within
significant galaxy overdensities, characterized by a velocity dispersion
sigma_v ~ 200 km s^{-1} and a transverse angular scale of ~25", (~200 physical
kpc). We argue that such scales are expected for small groups with
log(M_h/M_sun)~13. The galaxy-HLQSO cross-correlation function has a best-fit
correlation length r_0_GQ = (7.3 \pm 1.3) h^{-1} cMpc, while the galaxy
autocorrelation measured from the spectroscopic galaxy sample in the same
fields has r_0_GG = (6.0 \pm 0.5) h^{-1} cMpc. Based on a comparison with
simulations evaluated at z ~ 2.6, these values imply that a typical galaxy
lives in a host halo with log(M_h/M_sun) = 11.9\pm0.1, while HLQSOs inhabit
host halos of log(M_h/M_sun) = 12.3\pm0.5. In spite of the extremely large
black hole masses implied by their observed luminosities [log(M_BH/M_sun) >
9.7], it appears that HLQSOs do not require environments very different from
their much less luminous QSO counterparts. Evidently, the exceedingly low space
density of HLQSOs (< 10^{-9} cMpc^{-3}) results from a one-in-a-million event
on scales << 1 Mpc, and not from being hosted by rare dark matter halos.Comment: 15 pages, 6 figures. Accepted for publication in Ap
Extreme Walrasian Dynamics: The Gale Example in the Lab
We study the classic Gale (1963) economy using laboratory markets. Tatonnement theory
predicts prices will diverge from an equitable interior equilibrium towards infinity or zero
depending only on initial prices. The inequitable equilibria determined by these dynamics
give all gains from exchange to one side of the market. Our results show surprisingly strong
support for these predictions. In most sessions one side of the market eventually outgains the
other by more than twenty times, leaving the disadvantaged side to trade for mere pennies.
We also find preliminary evidence that these dynamics are sticky, resisting exogenous
interventions designed to reverse their trajectories
Driving safety: enhancing communication between clients, constructors and designers
This paper, which stems from qualitative research undertaken by the CRC for Construction Innovation in the context of the development of a Guide to Best Practice for Safer Construction in the Australian construction industry, investigates the communication relationship between the client, designer and constructor, and identifies the conditions under which effective communication takes place. Previous research has made little headway with respect to putting into practice strategies that have the potential to improve communication between the client, designer and constructor. This paper seeks to address this ongoing problem. From analysis of client, designer and constructor interviews that form part of industry-selected case studies reflecting excellence in OHS, best-practice tools that have the potential to enhance multi-party communication between the client, designer and constructor are presented. This research also informs the development of workable implementation strategies
Graduate teaching assistants use different criteria when grading introductory physics vs. quantum mechanics problems
Physics graduate teaching assistants (TAs) are often responsible for grading.
Physics education research suggests that grading practices that place the
burden of proof for explicating the problem solving process on students can
help them develop problem solving skills and learn physics. However, TAs may
not have developed effective grading practices and may grade student solutions
in introductory and advanced courses differently. In the context of a TA
professional development course, we asked TAs to grade student solutions to
introductory physics and quantum mechanics problems and explain why their
grading approaches were different or similar in the two contexts. TAs expected
and rewarded reasoning more frequently in the QM context. Our findings suggest
that these differences may at least partly be due to the TAs not realizing that
grading can serve as a formative assessment tool and also not thinking about
the difficulty of an introductory physics problem from an introductory physics
student's perspective
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