Selecting ultra-faint dwarf candidate progenitors in cosmological N-body simulations at high redshifts [GA]

The smallest satellites of the Milky Way ceased forming stars during the epoch of reionization and thus provide archaeological access to galaxy formation at $z>6$. Numerical studies of these ultra-faint dwarf galaxies (UFDs) require expensive cosmological simulations with high mass resolution that are carried out down to $z=0$. However, if we are able to statistically identify UFD host progenitors at high redshifts with relatively high probabilities, we can avoid this high computational cost. To find such candidates, we analyze the merger trees of Milky Way type halos from the high-resolution Caterpillar suite of dark matter only simulations. Satellite UFD hosts at $z=0$ are identified based on four different abundance matching techniques. All the halos at high redshifts are traced forward in time in order to compute the probability of surviving as satellite UFDs today. Our results show that selecting potential UFD progenitors based solely on their mass at z=12 (8) results in a 10\% (20\%) chance of obtaining a surviving UFD at $z=0$. We find that the progenitors of surviving satellite UFDs have lower virial ratios ($\eta$), and are preferentially located at large distances from the main MW progenitor, while they show no correlation with concentration parameter. Halos with favorable locations and virial ratios are $\approx 3$ times more likely to survive as satellite UFD candidates at $z=0.$

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M. Safarzadeh, A. Ji, G. Dooley, et. al.
Wed, 13 Dec 17

Comments: 12 pages, 7 figures, submitted to MNRAS

Isentropic thermal instability in atomic surface layers of photodissociation regions [GA]

We consider the evolution of an isentropic thermal instability in the atomic zone of a photodissociation region (PDR). In this zone, gas heating and cooling are associated mainly with photoelectric emission from dust grains and fine-structure lines ([\ion{C}{ii}] 158, [\ion{O}{i}] 63, and [\ion{O}{i}] 146 {\micron}), respectively. The instability criterion has a multi-parametric dependence on the conditions of the interstellar medium. We found that instability occurs when the intensity of the incident far-ultraviolet field $G_0$ and gas density $n$ are high. For example, we have $3\times10^3<G_0<10^6$ and $4.5\times10^4<n<10^6$ {\cmc} at temperatures $360 <T<10^4$ K for typical carbon and oxygen abundances $\xi_{\rm C}=1.4\times10^{-4}$ and $\xi_{\rm O}=3.2\times10^{-4}$. The instability criterion depends on the relation between $\xi_{\rm C}$ and $\xi_{\rm O}$ abundances and line opacities. We also give examples of observed PDRs where instability could occur. For these PDRs, the characteristic perturbation growth time is $t_{\rm inst}\sim10^3$ — $10^4$ yr and the distance characterizing the formation of secondary waves is $L\sim10^{-3}$ — $5\times10^{-2}$ pc. For objects that are older than $t_{\rm inst}$ and have sizes of the atomic zone larger than $L$, we expect that instability influences the PDR structure significantly. The presence of multiple shock waves, turbulent velocities of several kilometers per second and inhomogeneities with higher density and temperature than the surrounding medium can characterize isentropic thermal instability in PDRs.

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K. Krasnobaev and R. Tagirova
Wed, 13 Dec 17

Comments: 12 pages, 6 figures, 2 tables

Null Geodesics and Wave Front Singularities in the Godel Space-time [CL]

We explore wave fronts of null geodesics in the Godel metric emitted from point sources both at, and away from, the origin. For constant time wave fronts emitted by sources away from the origin, we find cusp ridges as well as blue sky metamorphoses where spatially disconnected portions of the wave front appear, connect to the main wave front, and then later break free and vanish. These blue sky metamorphoses in the constant time wave fronts highlight the non-causal features of the Godel metric. We introduce a concept of physical distance along the null geodesics, and show that for wave fronts of constant physical distance, the reorganization of the points making up the wave front leads to the removal of cusp ridges.

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T. Kling, K. Roebuck and E. Grotzke
Wed, 13 Dec 17

Comments: N/A

Conformal gravity: light deflection revisited and the galactic rotation curve failure [CEA]

We prove that Conformal Gravity (CG) is unable to describe galactic rotation curves without the aid of dark matter as suggested in the literature: if we interpret CG as a gauge natural theory, we can derive conservation laws and their associated superpotentials without ambiguities. We consider the light deflection of a point-like lens in CG and impose that the two Schwarzschild-like metrics with and without the lens at the origin of the reference frame are identical at infinite distances. The energy conservation law implies that the free parameter $\gamma$ appearing in the linear term of the metric has to vanish. This linear term is responsible for mimicing the role of dark matter in the standard model and also appears in numerous previous investigations of gravitational lensing. Our result thus shows that the possibility of removing the presence of dark matter with CG is untenable. We also illustrate why the results of previous investigations of gravitational lensing in CG largely disagree. These discrepancies derive from the erroneous use of the deflection angle definition adopted in General Relativity, where the vacuum solution is asymptotically flat, unlike CG. In addition, the lens mass is identified with various combinations of the metric parameters. However, these identifications are arbitrary, because the mass is not a conformally invariant quantity, unlike the conserved charge associated to the energy conservation law. Based on this conservation law, the energy difference between the metric with the point-like lens and the metric without it, which implies $\gamma=0$, also defines a conformally invariant quantity that can in principle be used for (1) a proper derivation of light deflection in CG, and (2) the identification of the lens mass with a function of the parameters $\beta$ and $k$ of the Schwarzschild-like metric.

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M. Campigotto, A. Diaferio and L. Fatibene
Wed, 13 Dec 17

Comments: 13 pages, 1 figure, comments are welcome, prepared for submission to JCAP

Search for the Signatures of a New-Born Black Hole from the Collapse of a Supra-massive Millisecond Magnetar in Short GRB Light Curves [HEAP]

`Internal plateau’ followed by a sharp decay is commonly seen in short gamma-ray burst (GRB) light curves. The plateau component is usually interpreted as the dipole emission from a supra-massive magnetar, and the sharp decay may imply the collapse of the magnetar to a black hole (BH). Fall-back accretion onto the new-born BH could produce long-lasting activities via the Blandford-Znajek (BZ) process. The magnetic flux accumulated near the BH would be confined by the accretion disks for a period of time. As the accretion rate decreases, the magnetic flux is strong enough to obstruct gas infall, leading to a magnetically-arrested disk (MAD). Within this scenario, we show that the BZ process could produce two types of typical X-ray light curves: type I exhibits a long-lasting plateau, followed by a power-law decay with slopes ranging from 5/3 to 40/9; type II shows roughly a single power-law decay with slope of 5/3. The former requires low magnetic filed strength, while the latter corresponds to relatively high values. We search for such signatures of the new-born BH from a sample of short GRBs with an internal plateau, and find two candidates: GRB 101219A and GRB 160821B, corresponding to type II and type I light curve, respectively. It is shown that our model can explain the data very well.

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Q. Zhang, W. Lei, B. Zhang, et. al.
Wed, 13 Dec 17

Comments: 12 pages, 4 figures, accepted for publication in MNRAS

Gravitational entropy and the cosmological no-hair conjecture [CL]

The gravitational entropy and no-hair conjectures seems to predict contradictory future states of our Universe. The growth of the gravitational entropy is associated with the growth of inhomogeneity, while the no-hair conjecture argues that a universe dominated by dark energy should asymptotically approach a homogeneous and isotropic de Sitter state. The aim of this paper is to study these two conjectures. The investigation is based on the Simsilun simulation, which simulates the universe using the approximation of the Silent Universe. The Silent Universe is a solution to the Einstein equations that assumes irrotational, non-viscous, and insulated dust, with vanishing magnetic part of the Weyl curvature. The initial conditions for the Simsilun simulation are sourced from the Millennium simulation, which results with a realistically appearing but relativistic at origin simulation of a universe. The Simsilun simulation is evolved from the early universe (t = 25 Myr) till far future (t = 1000 Gyr). The results of this investigation show that both conjectures are correct. On global scales, a universe with a positive cosmological constant and non-positive spatial curvature does indeed approach the de Sitter state. At the same time it keeps generating the gravitational entropy.

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K. Bolejko
Wed, 13 Dec 17

Comments: 19 pages, 3 figures

Can gravitational microlensing detect extragalactic exoplanets? Self-lensing models of the Small Magellanic Cloud [EPA]

We use three-dimensional distributions of classical Cepheids and RR Lyrae stars in the Small Magellanic Cloud (SMC) to model the stellar density distribution of a young and old stellar population in that galaxy. We use these models to estimate the microlensing self-lensing optical depth to the SMC, which is in excellent agreement with the observations. Thus, we estimate the total stellar mass of the SMC of about 1.0 x 10^9 MSun under assumption that all microlensing events toward this galaxy are caused by self-lensing. We also calculate the expected event rates and estimate that future large-scale surveys, like the Large Synoptic Survey Telescope (LSST), will be able to detect up to a few dozen microlensing events in the SMC annually. If the planet frequency in the SMC is similar to that in the Milky Way, a few extragalactic planets can be detected over the course of the LSST survey, provided significant changes in the SMC observing strategy are devised. A relatively small investment of LSST resources can give us a unique probe of the population of extragalactic exoplanets.

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P. Mroz and R. Poleski
Wed, 13 Dec 17

Comments: submitted to ApJ