Luminosity distributions within rich clusters — II. Demonstration and verification via simulation

We present detailed simulations of long-exposure CCD images. The simulations are used to explore the validity of the statistical method for reconstructing the luminosity distribution of galaxies within a rich cluster, i.e., by the subtraction of field number-counts from those of a sight-line through the cluster. In particular, we use the simulations to establish the reliability of our observational data to be presented in Paper III. Based on our intended CCD field-of-view (6.5 × 6.5 arcmin²) and a 1σ detection limit of 26 mag arcsec⁻², we conclude that the luminosity distribution can be robustly determined over a wide range of absolute magnitude (−23 <  M _R  < −16) provided: (a) the cluster has an Abell richness 1.5 or greater; (b) the redshift of the cluster lies in the range 0.1 <  z  < 0.3; (c) the seeing is better than FWHM 1.25 arcsec, and (d) the photometric zero-points are accurate to within Δ m  = ± 0.12. If these conditions are not met, then the recovered luminosity distribution is unreliable. Finally, although the method clearly has limitations, within these limitations the technique represents an extremely promising probe of galaxy evolution and environmental dependences.     

One-photon and two-photon annihilation of positronium in strong magnetic fields

In the presence of a strong magnetic field (such as those believed to be characteristic of neutron stars:B-10¹² Gauss) positronium may annihilate through the emission of a single photon, the magnetic field providing the photon momentum. We report on calculations of the one-photon and two-photon annihilation rates for the ground state of positronium, for magnetic fields in the range (1–44)×10¹² Gauss, and give, in the two-photon case, the minimum energy half-width of the emission line due to the momentum contributions from the magnetic field. We find that unless neutron stars have magnetic fields in excess of 10¹³ Gauss, it is unlikely that the one-photon process will be observable.Research supported in part by the National Research Council of Canada.     

The SCUBA HAlf Degree Extragalactic Survey – VI. 350-μm mapping of submillimetre galaxies

A follow-up survey using the Submillimetre High-Angular Resolution Camera (SHARC-II) at 350 μm has been carried out to map the regions around several 850-μm-selected sources from the Submillimetre HAlf Degree Extragalactic Survey (SHADES). These observations probe the infrared (IR) luminosities and hence star formation rates in the largest existing, most robust sample of submillimetre galaxies (SMGs). We measure 350-μm flux densities for 24 850-μm sources, seven of which are detected at ≥2.5σ within a 10 arcsec search radius of the 850-μm positions. When results from the literature are included the total number of 350-μm flux density constraints of SHADES SMGs is 31, with 15 detections. We fit a modified blackbody to the far-IR (FIR) photometry of each SMG, and confirm that typical SMGs are dust-rich  ( M _dust≃ 9 × 10⁸ M_⊙)  , luminous  ( L _FIR≃ 2 × 10¹² L_⊙)  star-forming galaxies with intrinsic dust temperatures of ≃35 K and star formation rates of  ≃400 M_⊙ yr⁻¹  . We have measured the temperature distribution of SMGs and find that the underlying distribution is slightly broader than implied by the error bars, and that most SMGs are at 28 K with a few hotter. We also place new constraints on the 350-μm source counts, N ₃₅₀(>25 mJy) ∼ 200–500 deg⁻².     

Hyperfast pulsars as the remnants of massive stars ejected from young star clusters

Recent proper motion and parallax measurements for the pulsar PSR B1508+55 indicate a transverse velocity of  ∼1100 km s⁻¹  , which exceeds earlier measurements for any neutron star. The spin-down characteristics of PSR B1508+55 are typical for a non-recycled pulsar, which implies that the velocity of the pulsar cannot have originated from the second supernova disruption of a massive binary system. The high velocity of PSR B1508+55 can be accounted for by assuming that it received a kick at birth or that the neutron star was accelerated after its formation in the supernova explosion. We propose an explanation for the origin of hyperfast neutron stars based on the hypothesis that they could be the remnants of a symmetric supernova explosion of a high-velocity massive star which attained its peculiar velocity (similar to that of the pulsar) in the course of a strong dynamical three- or four-body encounter in the core of dense young star cluster. To check this hypothesis, we investigated three dynamical processes involving close encounters between: (i) two hard massive binaries, (ii) a hard binary and an intermediate-mass black hole (IMBH) and (iii) a single stars and a hard binary IMBH. We find that main-sequence O-type stars cannot be ejected from young massive star clusters with peculiar velocities high enough to explain the origin of hyperfast neutron stars, but lower mass main-sequence stars or the stripped helium cores of massive stars could be accelerated to hypervelocities. Our explanation for the origin of hyperfast pulsars requires a very dense stellar environment of the order of  10⁶– 10⁷ stars pc⁻³  . Although such high densities may exist during the core collapse of young massive star clusters, we caution that they have never been observed.     

Understanding the halo-mass and galaxy-mass cross-correlation functions

We use the Millennium Simulation (MS) to measure the cross-correlation between halo centres and mass (or equivalently the average density profiles of dark haloes) in a Lambda cold dark matter (ΛCDM) cosmology. We present results for radii in the range  10  h ⁻¹ kpc < r < 30  h ⁻¹ Mpc  and for halo masses in the range  4 × 10¹⁰ < M ₂₀₀ < 4 × 10¹⁴  h ⁻¹ M_⊙  . Both at   z = 0  and at   z = 0.76  these cross-correlations are surprisingly well fitted if the inner region is approximated by a density profile of NFW or Einasto form, the outer region by a biased version of the linear mass autocorrelation function, and the maximum of the two is adopted where they are comparable. We use a simulation of galaxy formation within the MS to explore how these results are reflected in cross-correlations between galaxies and mass. These are directly observable through galaxy–galaxy lensing. Here also we find that simple models can represent the simulation results remarkably well, typically to ≲10 per cent. Such models can be used to extend our results to other redshifts, to cosmologies with other parameters, and to other assumptions about how galaxies populate dark haloes. Our galaxy formation simulation already reproduces current galaxy–galaxy lensing data quite well. The characteristic features predicted in the galaxy–galaxy lensing signal should provide a strong test of the ΛCDM cosmology as well as a route to understanding how galaxies form within it.     

Observation of an Extremely Large-Density Heliospheric Plasma Sheet Compressed by an Interplanetary Shock at 1 AU

At 11:46 UT on 9 September 2011, the Wind spacecraft encountered an interplanetary (IP) fast-forward shock. The shock was followed almost immediately by a short-duration (～?35 minutes) extremely dense pulse (with a peak ～?94 cm^?3). The pulse induced an extremely large positive impulse (SYM-H = 74 nT and Dst = 48 nT) on the ground. A close examination of other in situ parameters from Wind shows that the density pulse was associated with i) a spike in the plasma \(\upbeta\) (ratio of thermal to magnetic pressure), ii) multiple sign changes in the azimuthal component of the magnetic field (\(B_{\phi}\)), iii) a depressed magnetic field magnitude, iv) a small radial component of the magnetic field, and v) a large (>?90°) change in the suprathermal (～?255 eV) electron pitch angle across the density pulse. We conclude that the density pulse is associated with the heliospheric plasma sheet (HPS). The thickness of the HPS is estimated to be \({\sim}\,8.2\times10^{5}\ \mbox{km}\). The HPS density peak is about five times the value of a medium-sized density peak inside the HPS (～?18 cm^?3) at 1 AU. Our global three-dimensional magnetohydrodynamic simulation results (Wu et al. in J. Geophys. Res. 212, 1839, 2016) suggest that the extremely large density pulse may be the result of the compression of the HPS by an IP shock crossing or an interaction between an interplanetary shock and a corotating interaction region.