2002.09494
Understanding the large inferred Einstein radii of observed low-mass galaxy clusters
Robertson, Massey, Eke
We assess a claim that observed galaxy clusters with mass ~10^14 Msun are more centrally concentrated than predicted in LCDM. We generate mock strong gravitational lensing observations, taking the lenses from a cosmological hydrodynamical simulation, and analyse them in the same way as the real Universe. The observed and simulated lensing arcs are consistent with one another, with three main effects responsible for the previously claimed inconsistency. First, galaxy clusters containing baryonic matter have higher central densities than their counterparts simulated with only dark matter. Second, a sample of clusters selected because of the presence of pronounced gravitational lensing arcs preferentially finds centrally concentrated clusters with large Einstein radii. Third, lensed arcs are usually straighter than critical curves, and the chosen image analysis method (fitting circles through the arcs) overestimates the Einstein radii. After accounting for these three effects, LCDM predicts that galaxy clusters should produce giant lensing arcs that match those in the observed Universe.
2002.09497
The Edge of the Galaxy
Deason, et al
We use cosmological simulations of isolated Milky Way-mass galaxies, as well as Local Group analogues, to define the "edge'' --- a caustic manifested in a drop in density or radial velocity --- of Galactic-sized haloes, both in dark matter and in stars. In the dark matter, we typically identify two caustics: the outermost caustic located at ~1.4r_200m corresponding to the "splashback'' radius, and a second caustic located at ~0.6r_200m which likely corresponds to the edge of the virialized material which has completed at least two pericentric passages. The splashback radius is ill defined in Local Group type environments where the halos of the two galaxies overlap. However, the second caustic is less affected by the presence of a companion, and seems a more natural definition for the boundary of the Milky Way halo. Curiously, the stellar distribution also has a clearly defined caustic, which, in most cases, coincides with the second caustic of the dark matter. This can be identified in both radial density and radial velocity profiles, and should be measurable in future observational programmes. Finally, we show that the second caustic can also be identified in the phase-space distribution of dwarf galaxies in the Local Group. Using the current dwarf galaxy population, we predict the edge of the Milky Way halo to be 292 +/- 61 kpc.
2002.09502
Illuminating black hole cusp populations in young star clusters
Kaaz, et al
There is increasing evidence that globular clusters retain sizeable black hole populations at present day. This is supported by dynamical simulations of cluster evolution, which have unveiled the spatial distribution and mass spectrum of black holes in clusters across cosmic age. However, the black hole populations of young, high metallicity clusters remain unconstrained. The black holes hosted by these clusters are expected to rapidly mass segregate early in their evolutionary history, forming central cusps of hundreds to thousands of black holes. Here, we argue that the host young cluster can accumulate gas from its dense surroundings, from which the black hole cusp members can accrete at highly enhanced rates. Their collective accretion luminosity can be substantial and provides a novel observational constraint for nearby young massive clusters. We test this hypothesis by performing 3D hydrodynamic simulations where we embed discretized potentials, representing our black holes, within the core potential of a massive cluster. This system moves supersonically with respect to a gaseous medium from which it accretes. We study the accretion of this black hole cusp for different cusp populations and determine the integrated accretion luminosity of the black hole cusp. We apply our results to the young massive clusters of the Antennae Galaxies and find that a typical cusp accretion luminosity should be in excess of $\approx 10^{40}\,{\rm ergs\,\,s^{-1}}$. We argue that no strong candidates of this luminous signal have been observed, and constrain the cusp population of a typical cluster in the Antennae Galaxies to $\lesssim10-2\times10^2$ $10\,M_\odot$ black holes. This is in conflict with predictions of dynamical simulations, which suggest that these systems should harbor several hundred to thousands of black holes, and we explore the reasons for this discrepancy in our conclusions.
2002.09909
Multiple measurements of quasars acting as standard probes: exploring the cosmic distance duality relation at higher redshift
Zheng, et al
General relativity reproduces main current cosmological observations, assuming the validity of cosmic distance duality relation (CDDR) at all scales and epochs. However, CDDR is poorly tested in the redshift interval between the farthest observed Type Ia supernovae (SN Ia) and that of the Cosmic Microwave background (CMB). We present a new idea of testing the validity of CDDR, through the multiple measurements of high-redshift quasars. Luminosity distances are derived from the relation between the UV and X-ray luminosities of quasars, while angular diameter distances are obtained from the compact structure in radio quasars. This will create a valuable opportunity where two different cosmological distances from the same kind of objects at high redshifts are compared. Our constraints are more stringent than other currently available results based on different observational data and show no evidence for the deviation from CDDR at $z\sim 3$. Such accurate model-independent test of fundamental cosmological principles can become a milestone in precision cosmology.
2002.10024
Spacecraft tracking applications of the Square Kilometre Array
Bij de Vaate, et al
The Square Kilometre Array (SKA) is the next generation radio telescope distinguished by a superb sensitivity due to its large aperture (about one square kilometre) and advanced instrumentation. It will cover a broad range of observing bands including those used for tracking of and communications to deep space missions. While spacecraft tracking is not a main application defining the technical specifications of the SKA, this facility might play a role in tracking deep space probes as a backup to the ``dedicated'' deep space tracking networks. This paper presents possible applications of the SKA as a deep space tracking facility and major related technical specifications of various concepts of the SKA. It was presented at the 3rd International Workshop on Tracking, Telemetry and Command Systems for Space Applications, ESA-ESOC, Darmstadt, Germany, 7-9 September 2004. Over the past years, the SKA concept has developed to a much higher level of detalisation and is currently at the implementation phase. A number of specific considerations in this presentation no longer correspond to the actual status of the SKA project. However, the overall concept of the SKA applications for communication and tracking of interplanetary spacecraft remain topical, and some approaches presented here remain of interest for prospective deep space missions.
2002.10287
Inequalities in access to permanent positions in astronomy in France
Berné, Hilaire
We investigate inequalities in access to permanent positions in professional astronomy in France, focusing on the hiring stage. We use results from a national survey conducted on behalf of the French society of astronomy and astrophysics (SF2A) aimed at young astronomers holding a PhD obtained in France, and answered by over 300 researchers. We find that women are nearly two times less likely than men to be selected by the (national or local) committees attributing permanent positions ($p=0.06$). We also find that applicants who did their undergraduate studies in an elite school ("Grande \'Ecole"), where women are largely under-represented, rather than in a university, are nearly three times more likely to succeed in obtaining a position ($p=0.0026$). Our analysis suggests the existence of two biases in committees attributing permanent positions in astronomy in France: a gender bias, and a form of elitism. These biases against women in their professional life impacts their personal life as our survey shows that a larger fraction of them declare that having children can have a negative effect on their careers. They are half as many as men having children in the sample.
2002.10370
Extrasolar enigmas: from disintegrating exoplanets to exoasteroids
Budaj, et al
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