1510.00004
Search for extended gamma-ray emission from the Virgo galaxy cluster with Fermi-LAT
Ackermann et al
Galaxy clusters are one of the prime sites to search for DM annihilation signals. Depending on the substructure of the DM halo of a galaxy cluster and the cross sections for DM annihilation channels, these signals might be detectable by the latest generation of gamma-ray telescopes. Use 3 years of Fermi LAT data, which are the most suitable for searching for very extended emission in the vicinity of nearby Virgo galaxy cluster. Analysis reveals statistically significant extended emission which can be well characterized by a uniformly emitting disk profile with a radius of 3 deg that moreover is offset from the cluster center. Demonstrate that the significance of this extended emission strongly depends on the adopted interstellar emission model (IEM) and is most likely an artifact of the incomplete description of the IEM in this region. Also search for and find new point source candidates in the region. Then derive conservative upper limits on the velocity-averaged DM pair annihilation cross section from Virgo. Take into account the potential gamma-ray flux enhancement due to DM sub-haloes and its complex morphology as a merging cluster. For DM annihilating into bb-bar, assuming a conservative sub-halo model setup, find limits that are between 1 and 1.5 orders of magnitude above the expectation from the thermal cross section for m_DM<100 GeV. In a more optimistic scenario, exclude <sigma nu>~3e-26 cm^3/s for m_DM<40 GeV for the same channel. Finally, derive upper limits on the gamma-rya flux produced by hadronic cosmic-ray interactions in the ICM. Find that the volume-averaged cosmic-ray-to-thermal pressure ratio is less than ~6%.
1510.01359
Probing star formation in the dense environments of z~1 lensing halos aligned with dusty star-forming galaxies detected with the South Pole Telescope
Welikala, et al
Probe SF in the environments of massive ~1e13 Msun DM haloes at z~1. This SF is linked to a sub-millimeter clustering signal which is detected in maps of Planck HFI that are stacked at the positions of a sample of high-z (>2) strongly-lensed dusty SF galaxies (DSFGs) selected from the SPT 2500 deg2 survey. The clustering signal has sub-millimeter colors which are consistent wit the mean redshift of the foreground lensing haloes (z~1). Report a mean excess of SFR compared to the field, of (2700±700) Msun/yr from all galaxies contributing to this clustering signal within a radius of 3.5' from the SPT DSFGs. The magnitude of the Planck excess is in broad agreement with predictions of a current model of the CIB. The model predicts that 80% of the excess emission measured by Planck originates from galaxies lying in the neighboring haloes of the lensing halo. Using Herschel maps of the same fields, find a clear excess, relative to the field, of individual sources which contribute to the Planck excess. The mean excess SFR compared to the field is measured to be (370±40) Msun/yr per resolved, clustered source. The findings suggest that the environments around these massive z~1 lensing haloes host intense SF out to about 2Mpc. The flux enhancement due to clustering should also be considered when measuring flux densities of galaxies in Planck data.
1510.05040
The necessity of feedback physics in setting the peak of the initial mass function
Guszejnov, Krumholz, Hopkins
A popular theory of SF is gravitoturbulent fragmentation, in which self-gravitating structures are created by turbulence-driven density fluctuations. Simple theories of isothermal fragmentation successfully reproduce the core mass function (CMF) which has a very similar shape to the IMF of stars. However, numerical sims of isothermal turbulent fragmentation thus far have not succeeded in identifying a fragment mass scale that is independent of the simulation resolution. Moreover, the fluid equations for magnetized, self-gravitating, isothermal turbulence are scale-free, and do not predict any characteristic mass. In this paper, show that although an isothermal self-gravitating flow does produce a CMF with a mass scale imposed by the ICs, this scale changes as the parent cloud evolves. In addition, the cores that form undergo further fragmentation and after sufficient time forget about their ICs, yielding a scale-free pure power-law distribution dN/dM~M^-2 for the stellar IMF. Show that this problem can be alleviated by introducing a simple model for stellar radiation feedback. Radiative heating, powered by accretion onto forming stars, arrests the fragmentation cascade and imposes a characteristic mass scale that is nearly independent of the time-evolution or ICs in the SF cloud, and that agrees well wit the peak of the observed IMF. In contrast, models that introduce a stiff equation of state for denser clouds but that do not explicitly include the effects of feedback do not yield an invariant IMF.
1510.05388
Penetrating gas streams generate unrelaxed, non-cool-core clusters of galaxies
Zinger, Dekel, Birnboim, Kravtsov, Nagai
Utilize cosmo sims of 16 galaxy clusters at z=0 and 0.6 to study the effect of inflowing streams on the properties of the inner ICM. Find that the mass accretion occurs predominantly along streams that originate from the cosmic web and consist of heated gas. Clusters that are unrelaxed in terms of their X-ray morphology are characterized by higher mass inflow rates and deeper penetration of the streams, typically in the inner third of the viral radius. The penetrating streams generate elevated random motions, bulk flows, cold fronts and metal mixing, thus producing non-cool-core clusters. The degree of penetration of the streams may change over time such that clusters can switch from being unrelated to relaxed over a time-scale of several Gyrs. The stream properties thus help us understand the distinction between cool-core and non-cool-core clusters.
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