special topics (ApJ)
New constraints on quantum gravity from X-ray and gamma-ray observations
Perlman et al 2015, ApJ
One aspect of the quantum nature of spacetime is its "foaminess" at very small scales. Many models for spacetime foam are defined by the accumulation power alpha, which parameterizes the rate at which Planck-scale spatial uncertainties (and the phase shifts they produce) may accumulate over large path lengths. Here alpha is defined by the expression for the path-length fluctuations, delta ell, of a source at distance ell, wherein delta ell ~ ell^(1alpha) ell_P^(alpha) with ell_P being the Planck length. Reassess previous proposals to use astronomical observations of distant quasars and AGN to test models of spacetime foam. Show explicitly how wavefront distortions on small scales cause the image intensity to decay to the point where distant objects become undetectable when the path-length fluctuations become comparable to the wavelength of the radiation. Use X-ray observations from Chandra to set the constraint alpha>0.58, which rules out the random-walk model (with alpha=1/2). Much firmer constraints can be set by utilizing detections of quasars at GeV energies with Fermi and at TeV energies with ground-based Cherenkov telescopes: alpha>0.67 and alpha>0.72, respectively. These limits on alpha seem to rule out alpha=2/3, the model of some physical interest.
1506.00044
Finding gas-rich dwarf galaxies betrayed by their ultraviolet emission
Meyer, Peek, Putman, Grcevich
Present UV follow-up of a sample of potential dwarf galaxy candidates selected for their HI properties, taking advantage of the low UV BG seen by the GALEX satellite and its large and publicly available imaging footprint. The HI clouds, which are drawn from published GALFA-HI and ALFALFA HI survey compact cloud catalogs, are selected to be galaxy candidates based on their spatial compactness and non-association with known high-velocity cloud complexes or Galactic HI emission. Based on a comparison of their UV characteristics to those of known dwarf galaxies, half (48%) of the compact HI clouds have at least one potential stellar counterpart with UV properties similar to those of nearby dwarf galaxies. If galaxies, the star formation rates, HI masses, and star formation efficiencies of these systems follow the trends seen for much larger galaxies, the presence of UV emission is an efficient method to identify the best targets for spectroscopic follow-up, which is necessary to prove that the stars are associated with the compact HI. Further, searches of this nature help to refine the salient HI properties of likely dwarfs (even beyond the Local Group). In particular, HI compact clouds considered to be velocity outliers relative to their neighbor HI clouds have the most significant detection rate of single, appropriate UV counterparts. Correcting for the sky coverage of the two all-Arecibo sky surveys yielding the compact HI clouds, these results may imply the presence of potentially hundreds of new tiny galaxies across the entire sky.
1506.00169
The Tully$-$Fisher and Mass$-$Size relations from halo abundance matching
Desmond, Wechsler
The TFR expresses the connection between rotating galaxies and the DM haloes they inhabit, and therefore contains a wealth of information about galaxy formation. Construct a general framework to investigate whether models based on halo abundance matching are able to reproduce the observed stellar mass TFR and mass-size relation (MSR), and use the data to constrain galaxy formation parameters. The model tests a range of plausible scenarios, differing in the response of haloes to disc formation, the relative angular momentum of baryons and DM, the impact of selection effects, and the abundance matching parameters. Show that agreement with the observed TFR puts and upper limit on the scatter between galaxy and halo properties, requires weak or reversed halo contraction, and favors selection effects that preferentially eliminate fast-rotation galaxies. The MSR constrains the ratio of the disc to halo specific angular momentum to be approximately in the range 0.6-1.2. Identify and quantify two problems that models of this nature face: 1) they predict too large and intrinsic scatter for the MSR, and 2) they predict too strong an anti correlation between the TFR and MSR residuals. Argue that resolving these problems requires introducing a correlation between stellar surface density and enclosed DM mass, which could be achieved by correlating abundance matching scatter with halo spin, or by performing a second abundance match between galaxy size and halo mass or concentration. Finally, explore the expected difference between the TFRs of central and satellite galaxies, and find that in the favoured models, this difference should be detectable in a sample of ~700 galaxies.
1506.00384
The difficulty of measuring the local dark matter density
Hessman
The analysis of the vertical velocity dispersion of disc stars is the most direct astronomical means of estimating the local DM density, rho_DM. Current estimates based on the mid-plane dynamic density use a local baryonic correction that ignores the non-local effects of spiral structure and significantly underestimates the amount of dynamically relevant gas; the additional gas plus the remaining uncertainties make it practically impossible to measure rho_DM from mid-plane kinematics alone. The sampling of inhomogeneous tracer populations with different scale-heights and scale-lengths results in a systematic increase in the observed dispersion gradients and changes in the nominal density distributions that, if not properly considered, can be misinterpreted as a sign of more DM. If the disc gravity is modeled using an infinite disc, the local variation in the vertical gravity due to the globally exponential disc components results in an underestimation of the baryonic contribution by as much as ~40%. Given only the assumptions of stationarity, and axially and vertically symmetric disc, double exponential tracer and mass-component density profiles, a phenomenologically justified model for the cross-dispersion component sigma_Rz, and a realistic model for g_z, it is possible to solve the full vertical Jeans equation analytically for the vertical dispersion sigma_z(z) and hence test the robustness of previous attempts at measuring rho_DM. When the model parameters are estimated from SEGUE G dwarf star data, it is still not possible to explain the difference in behavior seen in the simple thick- and thin-disc datasets reported by Buedenbender et al. Rather than being a fundamental problem with the kinematic model, this effect appears to be a further sign of the difficulty of defining and handling kinematically homogeneous tracer populations.
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