Dark Matter Strikes Back

McGaugh og samarbejdspartnere har for en stor samling spiralgalakser fundet en tæt relation mellem den totale radiale acceleration g og dens komponent gb, som stammer fra fordelingen af baryonisk masse i form af stjerner og gas. Denne tætte relation mellem disse to størrelser betragtes af en del astronomer som en udfordring af ideen om galaktiske haloer bestående af mørkt stof. Paolo Salucci og samarbejdspartnere har udforsket denne sag ved at undersøge et stort antal galakser ved anvendelse af flere forskellige metoder. Deres resultater underbygger og forøger gyldigheden af McGaughs relation. De viser imidlertid, at en sådan relation også findes i scenarier, hvor galakserne dannes ud fra en blanding af baryonisk gas og kollisionsløse mørke partikler, idet den fremkommer ved, at det mørke stof er mindre koncentreret end det lysende stof, samt at det mørke stof er mere dominerende i de lyssvage galakser. Disse egenskaber skyldes velkendte astrofysiske effekter som gasdynamik, stjernedannelse og feed-back fra supernovaeksplosioner.

Dark Matter Strikes Back

ABSTRACT: Mc Gaugh et al. (2016) have found, by investigating a large sample of Spirals, a tight non linear relationship between the total radial acceleration, connected with the Dark Matter phenomenon, and its component which comes from the distribution of baryonic matter, as the stellar and HI disks. The strong link between these two quantities is considered by them and by other researchers, as challenging the scenario featuring the presence of DM halos in galaxies. Or, at least, to indicate the peculiar nature of the underlying dark matter particles. We have explored this issue by investigating a larger number of galaxies by means of several techniques of analysis. Our results support and even increase, both qualitatively and quantitatively, the validity of McGaugh et al. (2016) ‘s relationship. However, we prove that such relationship exists also in the scenario featuring dark matter halos + ordinary baryonic matter and that it arises by the fact the DM is less concentrated than the luminous matter and it is progressively more abundant in lower luminosity objects. These properties are due to well known astrophysical effects: the implications of this relationship for the properties of dark matter halos are nothing of new or of unexpected. The relationship, definitively, is not a portal to go beyond the standard picture of ΛCDM galaxy formation.

Paolo Salucci gør til slut opmærksom på, at der findes andre undersøgelser, som viser, at lysende og mørkt stof i galakser opfører sig som aktive partnere, samt at dette muligvis kræver ny fysik.

Universality of galactic surface densities within one dark halo scale-length

ABSTRACT: It was recently discovered that the mean dark matter surface density within one dark halo scale length – the radius within which the volume density profile of dark matter remains approximately flat – is constant across a wide range of galaxies. This scaling relation holds for galaxies spanning a luminosity range of 14 magnitudes and the whole Hubble sequence. Here we report that the luminous matter surface density is also constant within one scale length of the dark halo. This means that the gravitational acceleration generated by the luminous component in galaxies is always the same at this radius. Although the total luminous-to-dark matter ratio is not constant, within one halo scale length it is constant. Our finding can be interpreted as a close correlation between the enclosed surface densities of luminous and dark matter in galaxies.

Både middelfladetætheden og den radiale acceleration for baryoner inden for den mørke halos kerneradius er universielle over 14 magnituder i absolut lysstyrke.

Nye bånd på P9s bane og position

Constraints on Planet Nine’s Orbit and Sky Position within a Framework of Mean Motion Resonances

ABSTRACT: A number of authors have proposed that the statistically significant orbital alignment of the most distant Kuiper Belt Objects (KBOs) is evidence of an as-yet undetected planet in the outer solar system, now referred to colloquially a “Planet Nine”. Dynamical simulations by Batygin & Brown (2016) have provided constraints on the range of the planet’s possible orbits and sky locations. We extend these investigations by exploring the suggestion of Malhotra et al. (2016) that Planet Nine is in small integer ratio mean-motion resonances (MMRs) with several of the most distant KBOs. We show that the observed KBO semi-major axes present a set of commensurabilities with an unseen planet at ∼ 654 AU (P ∼ 16,725 yr) that has a greater than 98% chance of stemming from a sequence of MMRs rather than from a random distribution. We describe and implement a Monte-Carlo optimization scheme that drives billion-year dynamical integrations of the outer solar system to pinpoint the orbital properties of perturbers that are capable of maintaining the KBOs’ apsidal alignment. This optimization exercise suggests that the unseen planet is most consistently represented with mass, m ∼ 6-12 M, semi-major axis, a ∼ 654 AU, eccentricity, e ∼ 0.45, inclination, i ∼ 30, argument of periastron, ω ∼ 150, longitude of ascending node, Ω ∼ 50, and mean anomaly, M ∼ 180. A range of sky locations relative to this fiducial ephemeris are possible. We find that the region 30 ≲ RA ≲ 50, -20 ≲ Dec ≲ 20 is promising.

 

Slugte KIC8462852 en planet?

Secular Dimming of KIC 8462852 Following its Consumption of a Planet

ABSTRACT: The Kepler-field star KIC 8462852, an otherwise apparently ordinary F3 main-sequence star, showed several highly unusual dimming events of variable depth and duration. Adding to the mystery was the discovery that KIC 8462852 faded by 14% from 1890 to 1989, as well as by another 3% over the 4 year Kepler mission. Following an initial suggestion by Wright & Sigurdsson, we propose that the secular dimming behavior is the result of the inspiral of a planetary body or bodies into KIC 8462852, which took place ~10 to 1e4 years ago (depending on the planet mass). Gravitational energy released as the body inspirals into the outer layers of the star caused a temporary and unobserved brightening, from which the stellar flux is now returning to the quiescent state. The transient dimming events could then be due to obscuration by planetary debris from an earlier partial disruption of the same inspiraling bodies, or due to evaporation and out-gassing from a tidally detached moon system. Alternatively, the dimming events could arise from a large number of bodies comet- or planetesimal-mass bodies placed onto high eccentricity orbits by the same mechanism (e.g. Lidov-Kozai oscillations due to the outer M-dwarf companion) responsible for driving the more massive planets into KIC 8462852. The required high occurrence rate of KIC 8462852-like systems which have undergone recent major planet inspiral event(s) is the greatest challenge to the model, placing large lower limits on the mass of planetary systems surrounding F stars and/or requiring an unlikely probability to catch KIC 8462852 in its current state.

 

KIC8462852: En mulig forklaring

De fire hovedformørkelser observeret med KEPLER. BKJD = Kepler barycentrisk juliansk dag.

Mysterious eclipses in the light-curve of KIC8462852: a possible explanation

ABSTRACT: Apart from thousands of ‘regular’ exoplanet candidates, Kepler satellite has discovered a few stars exhibiting peculiar eclipse-like events. They are most probably caused by disintegrating bodies transiting in front of the star. However, the nature of the bodies and obscuration events, such as those observed in KIC8462852, remain mysterious. Swarm of comets or artificial alien mega-structures have been proposed as an explanation for the latter object.
We explore the possibility that such eclipses are caused by the dust clouds associated with massive parent bodies orbiting the host star.
We assume a massive object and a simple model of the dust cloud surrounding the object. Then, we use the numerical integration to simulate the evolution of the cloud, its parent body, and resulting light-curves as they orbit and transit the star.
It is found that it is possible to reproduce the basic features in the light-curve of KIC8462852 with only four objects enshrouded in dust clouds. The fact that they are all on similar orbits and that such models require only a handful of free parameters provide additional support for this hypothesis.
This model provides an alternative to the comet scenario. With such physical models at hand, at present, there is no need to invoke alien mega-structures into the explanation of these light-curves.

 

The Greenland Telescope

The Greenland Telescope: Antenna Retrofit Status and Future Plans

ABSTRACT: Since the ALMA North America Prototype Antenna was awarded to the Smithsonian Astrophysical Observatory (SAO), SAO and the Academia Sinica Institute of Astronomy & Astrophysics (ASIAA) are working jointly to relocate the antenna to Greenland. This paper shows the status of the antenna retrofit and the work carried out after the recommissioning and subsequent disassembly of the antenna at the VLA has taken place. The next coming months will see the start of the antenna reassembly at Thule Air Base. These activities are expected to last until the fall of 2017 when commissioning should take place. In parallel, design, fabrication and testing of the last components are taking place in Taiwan.

 

Meteorer detekteret med infralyd

Jeg gengiver i det følgene nogle af figurerne fra artikel I:

Artiklen inddeler lydimpulserne fra meteorer i disse 4 klasser.
Lydkildens højde er her plottet mod ankomsthastighed. Gruppen med hastigheder over 40 km/s stammer sandsynligvis fra kometer.
Venstre: Den dominerende frekvens som funktion af hastighed. Højre: Kildens højde som funktion af den dominerende frekvens.

Optical Observations of Meteors Generating Infrasound – I: Acoustic Signal Identification and Phenomenology

ABSTRACT: We analyze infrasound signals from 71 bright meteors simultaneously detected by video to investigate the phenomenology and characteristics of meteor-generated near-field infrasound and shock production. A taxonomy for meteor generated infrasound signal classification has been developed using the time-pressure signal of the infrasound arrivals. Based on the location along the meteor trail where the infrasound signal originates, we find most signals are associated with cylindrical shocks, with about a quarter of events evidencing spherical shocks associated with fragmentation events and optical flares. The video data indicate that all events with ray launch angles >117 deg from the trajectory heading are most likely generated by a spherical shock, while infrasound produced by the meteors with ray launch angles <117 deg can be attributed to both a cylindrical line source and a spherical shock. We find that meteors preferentially produce infrasound toward the end of their trails with a smaller number showing a preference for mid-trail production. Meteors producing multiple infrasound arrivals show a strong infrasound source height skewness to the end of trails and are much more likely to be associated with optical flares. While a significant fraction of our meteors producing infrasound (~1/4 of single arrivals) are produced by fragmentation events, we find no instances where acoustic radiation is detectable more than about 60 deg beyond the ballistic regime at our meteoroid sizes (grams to tens of kg) emphasizing the strong anisotropy in acoustic radiation for meteors which are dominated by cylindrical line source geometry, even in the presence of fragmentation.

Optical Observations of Meteors Generating Infrasound – II: Weak Shock Theory and Validation

ABSTRACT: We have recorded a dataset of 24 centimeter-sized meteoroids detected simultaneously by video and infrasound to critically examine the ReVelle [1974] weak shock meteor infrasound model. We find that the effect of gravity wave perturbations to the wind field and updated absorption coefficients in the linear regime on the initial value of the blast radius (R0), which is the strongly non-linear zone of shock propagation near the body and corresponds to energy deposition per path length, is relatively small. Using optical photometry for ground-truth for energy deposition, we find that the ReVelle model accurately predicts blast radii from infrasound periods (τ), but systematically under-predicts R0 using pressure amplitude. If the weak shock to linear propagation distortion distance is adjusted as part of the modelling process we are able to self-consistently fit a single blast radius value for amplitude and period. In this case, the distortion distance is always much less (usually just a few percent) than the value of 10 percent assumed in the ReVelle model. Our study shows that fragmentation is an important process even for centimeter sized meteoroids, implying that R0, while a good measure of energy deposition by the meteoroid, is not a reliable means of obtaining the meteoroid mass. We derived an empirical period-blast radius relation appropriate to cm sized meteoroids. Our observations suggest that meteors having blast radii as small as 1m are detectable infrasonically at the ground, an order of magnitude smaller than previously considered.

 

Binære sorte huller som standardsirener

Late time cosmology with LISA: probing the cosmic expansion with massive black hole binary mergers as standard sirens

ABSTRACT: This paper summarises the potential of the LISA mission to constrain the expansion history of the universe using massive black hole binary mergers as gravitational wave standard sirens. After briefly reviewing the concept of standard siren, the analysis and methodologies of Ref. [1] are briefly outlined to show how LISA can be used as a cosmological probe, while a selection of results taken from Refs. [1,2] is presented in order to estimate the power of LISA in constraining cosmological parameters.

 

Shapiroforsinkelse til krabbepulsaren

I. I. Shapiro gjorde i 1964 opmærksom på, at en elektromagnetisk impuls reflekteret fra solsystemets indre planeter får en ekstra forsinkelse som følge af Solens gravitationsfelt. Einsteins ækvivalensprincip medfører, at lokale ure går langsommere i et dybt gravitationspotential sammenlignet med ure uden for gravitationsfeltet. Shapiroforsinkelsen er en direkte følge af ækvivalensprincippet. Effekten er proportional med (γ+1)/2. γ antager værdien 1 i Einsteins generelle relativitetsteori. Shapiro og medarbejdere anvendte i 1977 Viking-projektets 4 rumfartøjer til at bestemme γ=1.000±0.002 ved passage af Mars bag Solen.
Denne artikel anvender korte (< 0.4 ns) kæmpemæssige radioimpulser fra Krabbepulsaren sammenlignet med en beregnet total forsinkelse på 3.84 døgn til at finde Δγ < 1.41×10-15 for radioområdet. Tidsforskellen mellem radio og optiske bølgelængder er Δt = 255±21 μs. Den øverste grænse for γ-variationer mellem radio og optiske bølgelængder er derfor Δγ<1.54×10-9. Einsteins ækvivalensprincip kræver, at alle elektromagnetiske impulser følger samme bane i rumtiden, hvorfor der må gælde Δγ = 1. Enhver alternativ teori til Einsteins generelle relativitetsteori, må kunne forklare, hvorfor Δγ er så lille.

Det viser sig, at Δγ(radio – optisk) er baseret på tidstagning af de “normale” impulser og ikke de sjældne meget korte kæmpeimpulser. De normale impulsers tider kan ikke bestemmes med tilnærmelsesvis nanosekund-nøjagtighed. En anden artikel har bestemt Δγ(radio – optisk), Δγ(radio – røntgen) og Δγ(radio – gamma) for de normale impulser. Man kan ikke bestemme den absolutte værdi for γ, som må bestemmes ved lysafbøjning i solsystemet. Man kan så anvende de relative Shapiro-forsinkelser til at bestemme den absolutte γ-parameter helt op til gammastråler.

Galactic Shapiro Delay to the Crab Pulsar and limit on Einstein’s Equivalence Principle

ABSTRACT: We calculate the total galactic Shapiro delay to the Crab pulsar by including the contributions from the dark matter as well as baryonic matter along the line of sight. The total delay due to dark matter potential is about 3.4 days. For baryonic matter, we included the contributions from both the bulge and the disk, which are approximately 0.12 and 0.32 days respectively. The total delay from all the matter distribution is therefore 3.84 days. We also calculate the limit on violations of Einstein’s equivalence principle by using observations of “nano-shot” giant pulses from the Crab pulsar with time-delay <0.4 ns as well as using time differences between radio and optical photons observed from this pulsar. Using the former, we obtain a limit on violation of Einstein’s equivalence principle in terms of the PPN parameter Δγ < 2.41×10-15. From the time-difference between simultaneous optical and radio observations, we get Δγ < 1.54×10-9.

Testing the Einstein’s Equivalent Principle with the timing of the Crab Pulsar

ABSTRACT: The Einstein’s Equivalent Principle can be tested through the parameterized post-Newtonian parameters, such as the parameter γ, denoting how much space curvature is produced by unit rest mass. The parameter γ has been constrained by comparing time delays of correlated particles or photons from extragalactic transient sources, like supernova, gamma-ray bursts, etc. The best constraint of γ has been obtained by the time delay between two radio bands, which gives γ(1.23GHz)-γ(1.45GHz) < 4.36×10-9. In this letter, γ is constrained by the timing of the Crab Pulsar from radio, optical, X-ray and γ-ray bands as follows: γradio – γoptical < 2.63×10-9, γradio – γX-ray < 4.01×10-9, and γradio – γγ-ray < 3.28×10-9. The new result sets the most stringent constraint on γ up to date. The Einstein’s Equivalent Principle is thus tested at a region much closer to us than those transient sources, through the timing of a stable source having been observed extensively.