The effects of a hot gaseous halo on disc thickening in galaxy minor mergers

We employ hydrodynamical simulations to study the effects of dissipational gas physics on the vertical heating and thickening of disc galaxies during minor mergers. For the first time we present a suite of simulations that includes a diffuse, rotating, cooling, hot gaseous halo, as predicted by cosmological hydrodynamical simulations as well as models of galaxy formation. We study the effect of this new gaseous component on the vertical structure of a Milky Way-like stellar disc during 1:10 and 1:5 mergers. For 1:10 mergers we find no increased final thin disc scale height compared to the isolated simulation, leading to the conclusion that thin discs can be present even after a 1:10 merger if a reasonable amount of hot gas is present. The reason for this is the accretion of new cold gas, leading to the formation of a massive new thin stellar disc that dominates the surface brightness profile. In a previous study, in which we included only cold gas in the disk, we showed that the presence of cold gas decreased the thickening by a minor merger relative to the no-gas case. Here, we show that the evolution of the scale height in the presences of a cooling hot halo is dominated by the formation of the new stellar disc. In this scenario, the thick disc is the old stellar disc that has been thickened in a minor merger at z>1, while the thin disc is the new stellar disc that reforms after this merger. In addition, we study the evolution of the scale height during a 1:5 merger and find that a thin disc can be present even after this merger, provided enough hot gas is available. The final scale height in our simulations depends on the mass of the hot gaseous halo, the efficiency of the winds and the merger mass ratio. We find post-merger values in the range 0.5<z0<1.0 kpc in good agreement with observational constraints by local galaxies.Comment: 14 pages, 10 figures, 2 tables, submitted to MNRA

Structure in phase space associated with spiral and bar density waves in an N-body galactic disk

An N-body hybrid simulation, integrating both massive and tracer particles, of a Galactic disk is used to study the stellar phase space distribution or velocity distributions in different local neighborhoods. Pattern speeds identified in Fourier spectrograms suggest that two-armed and three-armed spiral density waves, a bar and a lopsided motion are coupled in this simulation, with resonances of one pattern lying near resonances of other patterns. We construct radial and tangential (uv) velocity distributions from particles in different local neighborhoods. More than one clump is common in these local velocity distributions regardless of the position in the disk. Features in the velocity distribution observed at one galactic radius are also seen in nearby neighborhoods (at larger and smaller radii) but with shifted mean v values. This is expected if the v velocity component of a clump sets the mean orbital galactic radius of its stars. We find that gaps in the velocity distribution are associated with the radii of kinks or discontinuities in the spiral arms. These gaps also seem to be associated with Lindblad resonances with spiral density waves and so denote boundaries between different dominant patterns in the disk. We discuss implications for interpretations of the Milky Way disk based on local velocity distributions. Velocity distributions created from regions just outside the bar's Outer Lindblad resonance and with the bar oriented at 45 degrees from the Sun-Galactic center line more closely resemble that seen in the solar neighborhood (triangular in shape at lower uv and with a Hercules like stream) when there is a strong nearby spiral arm, consistent with the observed Centaurus Arm tangent, just interior to the solar neighborhood.Comment: accepted for publication in MNRA

Properties of the intracluster medium in an ensemble of nearby galaxy clusters

We present a systematic analysis of the intracluster medium (ICM) in an X-ray flux limited sample of 45 galaxy clusters. Using archival ROSAT Position-Sensitive Proportional Counter (PSPC) data and published ICM temperatures, we present best-fit double and single beta model profiles, and extract ICM central densities and radial distributions. We use the data and an ensemble of numerical cluster simulations to quantify sources of uncertainty for all reported parameters. We examine the ensemble properties within the context of models of structure formation and feedback from galactic winds. We present best-fit ICM mass-temperature M-ICM-[T-X] relations for M-ICM calculated within r(500) and 1 h(50)(-1) Mpc. These relations exhibit small scatter (17%), providing evidence of regularity in large, X-ray flux limited cluster ensembles. Interestingly, the slope of the M-ICM-[T-X] relation (at limiting radius r(500)) is steeper than the self-similar expectation by 4.3 sigma. We show that there is a mild dependence of ICM mass fraction f(ICM) on [T-X]; the clusters with ICM temperatures below 5 keV have a mean ICM mass fraction [f(ICM)] = 0.160 +/- 0.008, which is significantly lower than that of the hotter clusters [f(ICM)] = 0.212 +/- 0.006 (90% confidence intervals). In apparent contradiction with previously published analyses, our large, X-ray flux limited cluster sample provides no evidence for a more extended radial ICM distribution in low-[T-X] clusters down to the sample limit of 2.4 keV. By analyzing simulated clusters we find that density variations enhance the cluster X-ray emission and cause M-ICM and f(ICM) to be overestimated by similar to 12%. Additionally, we use the simulations to estimate an f(ICM) depletion factor at r(500). We use the bias corrected mean f(ICM) within the hotter cluster subsample as a lower limit on the cluster baryon fraction. In combination with nucleosynthesis constraints this measure provides a firm upper limit on the cosmological density parameter for clustered matter Omega(M) less than or equal to (0.36 +/- 0.01) h(50)(-1/2).Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/60582/1/Mohr1999Properties.pd

Differential limit on the extremely-high-energy cosmic neutrino flux in the presence of astrophysical background from nine years of IceCube data

We report a quasi-differential upper limit on the extremely-high-energy (EHE) neutrino flux above $5\times 10^{6}$ GeV based on an analysis of nine years of IceCube data. The astrophysical neutrino flux measured by IceCube extends to PeV energies, and it is a background flux when searching for an independent signal flux at higher energies, such as the cosmogenic neutrino signal. We have developed a new method to place robust limits on the EHE neutrino flux in the presence of an astrophysical background, whose spectrum has yet to be understood with high precision at PeV energies. A distinct event with a deposited energy above $10^{6}$ GeV was found in the new two-year sample, in addition to the one event previously found in the seven-year EHE neutrino search. These two events represent a neutrino flux that is incompatible with predictions for a cosmogenic neutrino flux and are considered to be an astrophysical background in the current study. The obtained limit is the most stringent to date in the energy range between $5 \times 10^{6}$ and $5 \times 10^{10}$ GeV. This result constrains neutrino models predicting a three-flavor neutrino flux of $E_\nu^2\phi_{\nu_e+\nu_\mu+\nu_\tau}\simeq2\times 10^{-8}\ {\rm GeV}/{\rm cm}^2\ \sec\ {\rm sr}$at$10^9\ {\rm GeV}$. A significant part of the parameter-space for EHE neutrino production scenarios assuming a proton-dominated composition of ultra-high-energy cosmic rays is excluded.Comment: The version accepted for publication in Physical Review