Abell 370: A Cluster with a Pronounced Triaxial Morphology

We here combine Sunyaev-Zel'dovich effect, X-ray observations and spectroscopic redshifts of member galaxies, to constrain the intrinsic three-dimensional shape of the galaxy cluster: Abell 370. The cluster turns out to be strongly elongated along the l.o.s., with two (or more) substructures in the process of merging. Spectroscopy further suggests that the process must be taking place at a small angle respect to the l.o.s.Comment: 7 pages, 2 figures. Contribution to the Proceedings of the COSPAR Scientific Assembly, E1.2 "Clusters of Galaxies: New Insights from XMM-Newton, Chandra and INTEGRAL", Paris (France), July 19-20, 2004. Accepted for publication in Advances in Space Researc

Small linearly equivalent GG-sets and a construction of Beaulieu

Two $G$-sets ($G$ a finite group) are called linearly equivalent over a commutative ring $k$ if the permutation representations $k[X]$ and $k[Y]$ are isomorphic as modules over the group algebra $kG$. Pairs of linearly equivalent non-isomorphic $G$-sets have applications in number theory and geometry. We characterize the groups $G$ for which such pairs exist for any field, and give a simple construction of these pairs. If $k$ is \Q, these are precisely the non-cyclic groups. For any non-cyclic group, we prove that there exist $G$-sets which are non-isomorphic and \lineq over \Q, of cardinality \leq 3(#G)/2. Also, we investigate a construction of P. Beaulieu which allows us to construct pairs of transitive linearly equivalent $S_n$-sets from arbitrary $G$-sets for an arbitrary group $G$. We show that this construction works over all fields and use it construct, for each finite set \mc P of primes, $S_n$-sets linearly equivalent over a field $k$ if and only if the characteristic of $k$ lies in \mc P.Comment: v2: fixed proof of Lemma 2.

Hydrodynamic simulations of the triaxial bulge of M31

The interstellar gas flow in the inner disk of M31 is modelled using a new, two dimensional, grid based, hydrodynamics code. The potential of the stellar bulge is derived from its surface brightness profile. The bulge is assumed to be triaxial and rotating in the same plane as the disk in order to explain the twisted nature of M31's central isophotes and the non circular gas velocities in the inner disk. Results are compared with CO observations and the bulge is found to be a fast rotator with a B-band mass-to-light ratio, Y = 6.5 +/- 0.8, and a ratio of co-rotation radius to bulge semi-major axis, R = 1.2 +/- 0.1, implying that any dark halo must have a low density core in contradiction to the predictions of CDM. These conclusions would be strengthened by further observations confirming the model's off axis CO velocity predictions.Comment: 11 pages, 9 figures, Accepted for A+

Keck Spectroscopy of 3<z<7 Faint Lyman Break Galaxies: The Importance of Nebular Emission in Understanding the Specific Star Formation Rate and Stellar Mass Density

The physical properties inferred from the SEDs of z>3 galaxies have been influential in shaping our understanding of early galaxy formation and the role galaxies may play in cosmic reionization. Of particular importance is the stellar mass density at early times which represents the integral of earlier star formation. An important puzzle arising from the measurements so far reported is that the specific star formation rates (sSFR) evolve far less rapidly than expected in most theoretical models. Yet the observations underpinning these results remain very uncertain, owing in part to the possible contamination of rest-optical broadband light from strong nebular emission lines. To quantify the contribution of nebular emission to broad-band fluxes, we investigate the SEDs of 92 spectroscopically-confirmed galaxies in the redshift range 3.8<z<5.0 chosen because the H-alpha line lies within the Spitzer/IRAC 3.6 um filter. We demonstrate that the 3.6 um flux is systematically in excess of that expected from stellar continuum, which we derive by fitting the SED with population synthesis models. No such excess is seen in a control sample at 3.1<z<3.6 in which there is no nebular contamination in the IRAC filters. From the distribution of our 3.6 um flux excesses, we derive an H-alpha equivalent width (EW) distribution. The mean rest-frame H-alpha EW we infer at 3.8<z<5.0 (270 A) indicates that nebular emission contributes at least 30% of the 3.6 um flux. Via our empirically-derived EW distribution we correct the available stellar mass densities and show that the sSFR evolves more rapidly at z>4 than previously thought, supporting up to a 5x increase between z~2 and 7. Such a trend is much closer to theoretical expectations. Given our findings, we discuss the prospects for verifying quantitatively the nebular emission line strengths prior to the launch of the James Webb Space Telescope.Comment: 16 pages, 9 figures, submitted to Ap

Game Changer: The Topology of Creativity

This article examines the sociological factors that explain why some creative teams are able to produce game changers—cultural products that stand out as distinctive while also being critically recognized as outstanding. The authors build on work pointing to structural folding—the network property of a cohesive group whose membership overlaps with that of another cohesive group. They hypothesize that the effects of structural folding on game changing success are especially strong when overlapping groups are cognitively distant. Measuring social distance separately from cognitive distance and distinctiveness independently from critical acclaim, the authors test their hypothesis about structural folding and cognitive diversity by analyzing team reassembly for 12,422 video games and the career histories of 139,727 video game developers. When combined with cognitive distance, structural folding channels and mobilizes a productive tension of rules, roles, and codes that promotes successful innovation. In addition to serving as pipes and prisms, network ties are also the source of tools and tensions

Disruptive Diversity and Recurring Cohesion: Assembling Creative Teams in the Video Game Industry, 1979-2009

To test the proposition that a high level of recurring cohesion and a high level of stylistic diversity can combine for successful team performance, this study constructs a dataset of the careers of 139,727 individuals who participated in project teams producing 16,507 video games between 1979 and 2009. Findings indicate that teams with more dissimilar stylistic experiences outperform teams with more homogenous backgrounds, but only for higher levels of recurring cohesion. Teams with high diversity and high social cohesion are better able to harmonize the noisy cacophony of an (otherwise) excessive plurality of voices, thereby exploiting the potential beneficial effects of cognitive diversity

Origin of Radially Increasing Stellar Scaleheight in a Galactic Disk

For the past twenty years, it has been accepted that the vertical scaleheight of the stellar disk in spiral galaxies is constant with radius. However, there is no clear physical explanation for this in the literature. Here we calculate the vertical stellar scaleheight for a self-gravitating stellar disk including the additional gravitational force of the HI and H_2 gas and the dark matter halo. We apply our model to two edge-on galaxies, NGC 891 and NGC 4565, and find that the resulting scaleheight shows a linear increase of nearly a factor of two within the optical disk for both these galaxies. Interestingly, we show that the observed data when looked at closely, do not imply a constant scaleheight but actually support this moderate flaring in scaleheight.Comment: 8 pages, 4 .EPS figures, Astron. & Astrophys Letters, In press (Vol 390, L35 - L38

Proliferation of anomalous symmetries in colloidal monolayers subjected to quasiperiodic light fields

Quasicrystals provide a fascinating class of materials with intriguing properties. Despite a strong potential for numerous technical applications, the conditions under which quasicrystals form are still poorly understood. Currently, it is not clear why most quasicrystals hold 5- or 10-fold symmetry but no single example with 7 or 9-fold symmetry has ever been observed. Here we report on geometrical constraints which impede the formation of quasicrystals with certain symmetries in a colloidal model system. Experimentally, colloidal quasicrystals are created by subjecting micron-sized particles to two-dimensional quasiperiodic potential landscapes created by n=5 or seven laser beams. Our results clearly demonstrate that quasicrystalline order is much easier established for n = 5 compared to n = 7. With increasing laser intensity we observe that the colloids first adopt quasiperiodic order at local areas which then laterally grow until an extended quasicrystalline layer forms. As nucleation sites where quasiperiodicity originates, we identify highly symmetric motifs in the laser pattern. We find that their density strongly varies with n and surprisingly is smallest exactly for those quasicrystalline symmetries which have never been observed in atomic systems. Since such high symmetry motifs also exist in atomic quasicrystals where they act as preferential adsorption sites, this suggests that it is indeed the deficiency of such motifs which accounts for the absence of materials with e.g. 7-fold symmetry