1911.09720
Universality in the structure of dark matter haloes over twenty orders of magnitude in halo mass
Wang, et al
Dark matter haloes are the basic units of all cosmic structure. They grew by gravitational amplification of weak initial density fluctuations that are still visible on large scales in the cosmic microwave background radiation. Galaxies formed within relatively massive haloes as gas cooled and condensed at their centres, but many hypotheses for the nature of dark matter imply that the halo population should extend to masses many orders of magnitude below those where galaxies can form. Here, we use a novel, multi-zoom technique to create the first consistent simulation of the formation of present-day haloes over the full mass range populated when dark matter is aWeakly Interacting Massive Particle (WIMP) of mass ~100 GeV. The simulation has a dynamic range of 30 orders of magnitude in mass, resolving the internal structure of hundreds of Earth-mass haloes just as well as that of hundreds of rich galaxy clusters. Remarkably, halo density profiles are universal over the entire mass range and are well described by simple two-parameter fitting formulae. Halo mass and concentration are tightly related in a way which depends on cosmology and on the nature of the dark matter. At fixed mass, concentration is independent of local environment for haloes less massive than those of typical galaxies. These results are important for predicting annihilation radiation signals from dark matter, since these should be dominated by contributions from the smallest structures.
1911.10204
Astronomy and the new SI
Saha
In 2019 the International System of units (SI) conceptually re-invented itself. This was necessary because quantum-electronic devices had become so precise that the old SI could no longer calibrate them. The new system defines values of fundamental constants (including $c,h,k,e$ but not $G$) and allows units to be realized from the defined constants through any applicable equation of physics. In this new and more abstract SI, units can take on new guises --- for example, the kilogram is at present best implemented as a derived electrical unit. Relevant to astronomy, however, is that several formerly non-SI units, such as electron-volts, light-seconds, and what we may call "gravity seconds" $GM/c^3$, can now be interpreted not as themselves units, but as shorthand for volts and seconds being used with particular equations of physics. Moreover, the classical astronomical units have exact and rather convenient equivalents in the new SI: zero AB magnitude amounts to $\simeq5\times10^{10}$ photons $\rm m^{-2}\,s^{-1}$ per logarithmic frequency or wavelength interval, $\rm 1\,au\simeq 500$ light-seconds, $\rm 1\,pc\simeq 10^8$ light-seconds, while a solar mass $\simeq5$ gravity-seconds. As a result, the unit conversions ubiquitous in astrophysics can now be eliminated, without introducing other problems, as the old-style SI would have done. We review a variety of astrophysical processes illustrating the simplifications possible with the new-style SI, with special attention to gravitational dynamics, where care is needed to avoid propagating the uncertainty in $G$. Well-known systems (GPS satellites, GW170817, and the M87 black hole) are used as examples wherever possible.
1911.11735
Photometric asymmetry between galaxies with opposite spin patterns: a comparison of three telescopes
Shamir
The spin pattern of a spiral galaxy is a matter of the perspective of the observer, and therefore galaxies with clockwise spin patterns are expected to be identical in their characteristics to galaxies with counterclockwise spin patterns. However, observations of a large number of galaxies show clear photometric differences between clockwise and counterclockwise spiral galaxies. In this study the magnitude difference between clockwise and counterclockwise spiral galaxies imaged by the space-based COSMOS survey is compared to galaxies imaged by the Earth-based SDSS and PanSTARRS around the same field. The comparison shows that the same asymmetry was identified by all three telescopes, providing strong evidence that the rotation direction of the galaxy affects its magnitude as measured from Earth. Analysis of a large number of galaxies from different parts of the sky shows that the differences between clockwise and counterclockwise galaxies are oriented around an axis such that the photometric asymmetry in one hemisphere is inverse to the photometric asymmetry in the opposite hemisphere. Due to the provocative nature of the observation, it is difficult to identify an immediate explanation. A possible explanation could be related to the large-scale structure of the universe, which leads to violation of the cosmological homogeneity assumption. Another possible explanation that does not require the violation of the cosmological principle is that the observation is driven by galaxy rotation. Due to relativistic beaming, such difference is indeed expected to be identified and peak at the galactic pole, but it is expected to be far smaller than the differences observed by all three telescopes. Therefore, if the asymmetry is driven by galaxy rotation, it corresponds to a much higher velocity than the actual measured rotational velocity of galaxies.
1911.11947
Beyond Limber: Efficient computation of angular power spectra for galaxy clustering and weak lensing
Fang, Krause, Eifler, MacCrann
Angular two-point statistics of large-scale structure observables are important cosmological probes. To reach the high accuracy required by the statistical precision of future surveys, some of these statistics may need to be computed without the commonly employed Limber approximation; the exact computation however requires integration over Bessel functions, and a brute-force evaluation is slow to converge. We present a new method based on our generalized FFTLog algorithm for the efficient computation of angular power spectra beyond the Limber approximation. The new method significantly simplifies the calculation and improves the numerical speed and stability. It is easily extended to handle integrals involving derivatives of Bessel functions, making it equally applicable to numerically more challenging cases such as contributions from redshift-space distortions and Doppler effects. We implement our method for galaxy clustering and galaxy-galaxy lensing power spectra. We find that using the Limber approximation for galaxy clustering in future analyses like LSST Year 1 and DES Year 6 may cause significant biases in cosmological parameters, indicating that going beyond the Limber approximation is necessary for these analyses.
1911.12056
Angular redshift fluctuations: a new cosmological observable
Hernandez-Monteagudo, et al
We propose the use of angular fluctuations in the galaxy redshift field as a new way to extract cosmological information in the Universe. This new probe consists on the statistics of sky maps built by projecting redshifts under a Gaussian window of mean $z_{\rm obs}$ and width $\sigma_z$; $z(\hat{\mbox{n}}) = \bar{z}+\sum_{j\in \hat{\mbox{n}}} W_j (z_j-\bar{z}) / \langle \sum_i W_i \rangle= \bar{z} + \delta z (\hat{\mbox{n}})$, with $z_j$ and $W_j$ the redshift and the Gaussian weight, respectively, for the $j$-th galaxy falling on the pixel along sky direction $\hat{\mbox{n}}$, $\bar{z}=\sum_i W_i z_i / \sum_i W_i$ is the average redshift under the Gaussian shell, and the $\langle ... \rangle$ brackets denote an angular average over the entire footprint. We compute the angular power spectrum of the $\delta z (\hat{\mbox{n}})$ field in both numerical simulations and in linear perturbation theory. From these we find that the $\delta z (\hat{\mbox{n}})$ field: {\it (i)} is sensitive to the underlying density and peculiar velocity fields; {\it (ii)} is highly correlated, at the $\gtrsim 60\,\%$ level, to the line-of-sight projected peculiar velocity field; {\it (iii)} for narrow windows $(\sigma_z < 0.03$), it is almost completely uncorrelated to the projected galaxy angular density field under the same redshift window; and {\it (iv)} it is largely unaffected by multiplicative and additive systematic errors on the observed number of galaxies that are redshift-independent over $\sim\sigma_z$. We conclude that $\delta z (\hat{\mbox{n}})$ is a simple and robust tomographic measure of the cosmic density and velocity fields, complementary to angular clustering, that will contribute to a more complete exploitation of current and upcoming galaxy redshift surveys.
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