Nature
Acceleration of petaelectronvolt protons in the Galactic Centre
HESS collaboration
Galactic CRs reach energies of at least a few PeV (~1e15 eV). This implies that the Galaxy contains PeV accelerators ('PeVatrons'), but all proposed models of Galactic CR accelerators encounter difficulties at exactly these energies. Dozens of Galactic accelerators capable of accelerating particles to energies of tens of TeVs (of the order of 1e13 eV) were inferred from recent gamma-ray observations. However, none of the currently known accelerators -- not even the handful of shell-type SN remnants commonly believed to supply most Galactic CRs -- has shown the characteristic tracers of PeV parties, namely, power-law spectra of gamma-rays extending without a cut-off or a spectral break to tens of TeVs. Here, report deep gamma-ray observations with arc minute angular resolution of the region surrounding the Galactic Centre, which show the expected tracer of the presence of PeV protons within the central 10 pcs of the Galaxy. Propose that the SMBH Sgr A* is linked to this PeVatron. Sagittarius A* went through active phases in the past, as demonstrated by X-ray outbursts and an outflow from the Galactic Centre. Although its current rate of particle acceleration is not sufficient to provide a substantial contribution to Galactic CRs, Sgr A* could have plausibly been more active over the last 1e6-7 years, and therefore should be considered as a viable alternative to SN remnants as a source of PeV Galactic CRs.
1603.04855
Organized chaos: scatter in the relation between stellar mass and halo mass in small galaxies
Garrison-Kimmel, Bullock, Moylan-Kolchin, Bardwell
Use LG galaxy counts together with the ELVIS N-body sims to jointly constrain the scatter and slope in the stellar mass vs halo mass relation at low masses, M*~1e5-8 Msun. Assuming log-normal scatter about a median relation of the form M*~M_halo^alpha, the preferred log-slope steepens from alpha~1.8 in the limit of zero scatter to alpha~2.6 in the case of 2 dex off scatter in M* at fixed halo mass. Provide fitting functions for the best-fit relations as a function of scatter, including cases where the relation becomes increasingly stochastic with decreasing mass. Show that if the scatter at fixed halo mass is large enough (>~1 dex) and if the median relation is steep enough (alpha>~2), then the "too-big-to-fail" problem seen in the LG can be self-consistently eliminated in about ~5-10% of realizations. This scenario requires that the most massive sub halos host unobservable ultra-faint dwarfs fairly often; discuss potentially observable signatures of these systems. Compare the derived constraints to recent high-resolution simulations of dwarf galaxy formation in the literature. Through sim-to-sim scatter in M* at fixed M_halo is large among separate authors (~2 dex), individual codes produce relations with much less scatter and usually give relations that would over-produce local galaxy counts.
1603.05040
Cosmology constraints from shear peak statistics in Dark Energy Survey Science Verification data
Kacprzak, Kirk, .. et al
Shear peak statistics has gained a log of attention recently as a practical alternative to the 2pt statistics for containing cosmological parameters. Perform a shear peak statistics analysis of the DES SV data, using WL measurements from a 139 deg2 field. Measure the abundance of peaks identified in aperture mass maps, as a function of their S/N ratio, in the S/N ranges 0<S/N<4. To predict the peak counts as a function of cosmo parameters, use a suite of N-body sims spanning 158 models with varying Omega_m and sigma_8, fixing w=-1, Omega_b=0.04, h=0.7 and n_s=1, to which the DES SV mask and z distribution is applied. In the fiducial analysis, measure sigma_8(Omega_m/0.3)^0.6=0.77±0.07, after marginalizing over the shear multpicative bias and the error on the mean redshift of the galaxy sample. Introduce models of IAs, blending, and source contamination by cluster members. These models indicate that peaks with S/N>4 would require significant corrections, which is why the are not included in the analysis. Compare the results to the cosmo constraints from the 2pt analysis on the SV field and find them to be in good agreement in both the central value and its uncertainty. Discuss prospects for future peak statistics analysis with upcoming DES data.
1603.05184
Redshift-space distortions around voids
Cai, Taylor, Peacock, Padilla
Derive estimators for the linear growth rate of density fluctuations using the cross-correlation function of voids and haloes in z-space, both directly and in Fourier form. In linear theory, this cross-correlation contains only monopole and quadrupole terms. At scales grater than the void radius, linear theory is a good match to voids traced out by haloes in N-body simulations; small-scale random velocities are unimportant at these radii, only tending to cause small and often negligible elongation of the redshift-space cross-correlation function near its origin. By extracting the monopole and quadrupole from the cross-correlation function, measure the linear growth rate without prior knowledge of the void profile or velocity dispersion. Recover the linear growth parameter beta to 9% precision from an effective volume of 3(Gpc/h)^3 using voids with radius greater than 25 Mpc/h. Smaller voids are predominantly sub-voids, which may be more sensitive to the random velocity dispersion; they introduce noise and do not help to improve the measurement. Adding velocity dispersion as a free parameter allows use of information at radii as small as half of the void radius. The precision on beta is reduced to approximately 5%. Contrary to the simple z-space distortion patter in over densities, voids show diverse shapes in z-space, and can appear either enlarged or flattened along the LoS. This can be explained by the competing amplitudes of the local entity contrast, plus the radial velocity profile and its gradient, with the latter two factors being determined by the cumulative density profile of voids. The distortion pattern is therefore determined solely by the void profile and is different for void-in-cloud and void-in-void. This diversity of z-space void morphology complicates measurements of the Alcock-Paczynski effect using voids.
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