ABSTRACT: No explanation exists so far for the observed dearth of dwarf galaxies in the local universe compared to the large number of dark matter halos predicted by ΛCDM. Although attempts have been made to attribute the discrepancy to observational systematics, this would require an extreme modification of the density profiles of haloes through baryonic processes. In this paper we perform a systematic evaluation of the uncertainties affecting the measurement of DM halo abundance using galaxy kinematics. Including observational systematics and modelling uncertainties, we derive the abundance of galaxies as a function of maximum circular velocity –a direct probe of mass– from the observed line-of-sight velocity function in the Local Volume. This provides a direct means of comparing the predictions of theoretical models and simulations (including nonstandard cosmologies and novel galaxy formation physics) to the observational constraints. The new “galactic Vmax” function is steeper than the line-of-sight velocity function but still shallower than the theoretical CDM VF, showing that some unaccounted physical process is necessary to reduce the abundance of galaxies and/or drastically modify their density profiles compared to CDM haloes. Using this new galactic Vmax function, we investigate the viability of baryonic solutions such as photoevaporation of gas from an ionising background as well as stellar feedback. However, we find that the observed relation between baryonic mass and Vmax places tight constraints on the maximum suppression from reionisation. Neither energetic feedback nor photoevaporation are effective enough to reconcile the disagreement. This might point to the need to modify cosmological predictions at small scales.
Denne type mørk stof kaldes på engelsk Fuzzy Dark Matter (FDM). Den væsentligste forskel til koldt mørk stof (CDM) er, at FDM ikke kan danne mørke haloer under en vis størrelse, som er bestemt ved bosonens masse. Hvis det mørke stof i Fornax-dværggalaksen består af CDM, ville de observerede kuglehobe forlængst være spiraleret til dens centrum på grund af dynamisk friktion. Dette problem er løst, hvis det mørke stof er FDM. Dynamisk friktion opstår, når et tungere legeme bevæger sig gennem et hav af lette partikler. De lette partiklers hastigheder afbøjes ved passagen af det tunge legeme, så hastighedskomponenten langs det tunge legemes bane forkortes svarende til, at der overføres impuls til det tunge legeme i modsat retning af bevægelsen.
ABSTRACT: An intriguing alternative to cold dark matter (CDM) is that the dark matter is a light ( ∼ 10-22 eV) boson having a de Broglie wavelength λ ∼ 1 kpc, often called fuzzy dark matter (FDM). We describe the arguments from particle physics that motivate FDM, review previous work on its astrophysical signatures, and analyze several unexplored aspects of its behavior. In particular, (i) FDM halos smaller than about 107 (m/10-22 eV)-3/2 M⊙ do not form. (ii) FDM halos are comprised of a core that is a stationary, minimum-energy configuration called a “soliton”, surrounded by an envelope that resembles a CDM halo. (iii) The transition between soliton and envelope is determined by a relaxation process analogous to two-body relaxation in gravitating systems, which proceeds as if the halo were composed of particles with mass ∼ ρλ3 where ρ is the halo density. (iv) Relaxation may have substantial effects on the stellar disk and bulge in the inner parts of disk galaxies. (v) Relaxation can produce FDM disks but an FDM disk in the solar neighborhood must have a half-thickness of at least 300 (m/10-22 eV)-2/3 pc. (vi) Solitonic FDM sub-halos evaporate by tunneling through the tidal radius and this limits the minimum sub-halo mass inside 30 kpc of the Milky Way to roughly 108 (m/10-22 eV)-3/2 M⊙. (vii) If the dark matter in the Fornax dwarf galaxy is composed of CDM, most of the globular clusters observed in that galaxy should have long ago spiraled to its center, and this problem is resolved if the dark matter is FDM.
ABSTRACT: We analyze the total and baryonic acceleration profiles of a set of well-resolved galaxies identified in the EAGLE suite of hydrodynamic simulations. Our runs start from the same initial conditions but adopt different subgrid models for stellar and AGN feedback, resulting in diverse populations of galaxies by the present day. Some of them reproduce observed galaxy scaling relations, while others do not. However, regardless of the feedback implementation, all of our galaxies follow closely a simple relationship between the total and baryonic acceleration profiles, consistent with recent observations of rotationally supported galaxies. The relation has small scatter: different feedback processes — which produce different galaxy populations — mainly shift galaxies along the relation, rather than perpendicular to it. Furthermore, galaxies exhibit a single characteristic acceleration, g†, above which baryons dominate the mass budget, as observed. These observations have been hailed as evidence for modified Newtonian dynamics but can be accommodated within the standard cold dark matter paradigm.
ABSTRACT: The recent detections of two transit events attributed to the super-Earth candidate K2-18b have provided the unprecedented prospect of spectroscopically studying a habitable-zone planet outside the Solar System. Orbiting a nearby M2.5 dwarf and receiving virtually the same stellar insolation as Earth, K2-18b would be a prime candidate for the first detailed atmospheric characterization of a habitable-zone exoplanet using HST and JWST. Here, we report the detection of a third transit of K2-18b near the predicted transit time using the Spitzer Space Telescope. The Spitzer detection demonstrates the periodic nature of the two transit events discovered by K2, confirming that K2-18 is indeed orbited by a super-Earth in a 33-day orbit and ruling out the alternative scenario of two similarly-sized, long-period planets transiting only once within the 75-day K2 observation. We also find, however, that the transit event detected by Spitzer occurred 1.85 hours (7-sigma) before the predicted transit time. Our joint analysis of the Spitzer and K2 photometry reveals that this early occurrence of the transit is not caused by transit timing variations (TTVs), but the result of an inaccurate K2 ephemeris due to a previously undetected data anomaly in the K2 photometry likely caused by a cosmic ray hit. We refit the ephemeris and find that K2-18b would have been lost for future atmospheric characterizations with HST and JWST if we had not secured its ephemeris shortly after the discovery. We caution that immediate follow-up observations as presented here will also be critical in confirming and securing future planets discovered by TESS, in particular if only two transit events are covered by the relatively short 27-day TESS campaigns.
ABSTRACT: The equation of motion in the generally covariant modified gravity (MOG) theory leads for weak gravitational fields and the non-relativistic limit to a modification of the Newtonian gravitational acceleration law, expressed in terms of two parameters α and μ. The parameter α determines the strength of the gravitational field and μ is the effective mass of the vector field φμ, coupled with gravitational strength to baryonic matter. The MOG acceleration law for weak field gravitation and non-relativistic particles has been demonstrated to fit a wide range of galaxies, galaxy clusters and the Bullet Cluster and Train Wreck Cluster mergers. We demonstrate that the MOG acceleration law for a point mass source is in agreement with the McGaugh et al., correlation between the radial acceleration traced by galaxy rotation curves and the distribution of baryonic matter for the SPARC sample of 153 rotationally supported spiral and irregular galaxies.
ABSTRACT: A powerful outburst of X-ray radiation from the supermassive black hole Sgr A* at the center of the Milky Way is believed to be responsible for the illumination of molecular clouds in the central ~100 pc of the Galaxy. The reflected/reprocessed radiation comes to us with a delay corresponding to the light propagation time that depends on the 3D position of molecular clouds with respect to Sgr A*. We suggest a novel way of determining the age of the outburst and positions of the clouds by studying characteristic imprints left by the outburst in the spatial and time variations of the reflected emission. We estimated the age of the outburst that illuminates the Sgr A molecular complex to be ~110 yr. This estimate implies that we see the gas located ~10 pc further away from us than Sgr A*. If the Sgr B2 complex is also illuminated by the same outburst, then it is located ~130 pc closer than our Galactic Center. The outburst was short (less than a few years) and the total amount of emitted energy in X-rays is ∼ 1048/ρ3 erg, where ρ3 is the mean hydrogen density of the cloud complex in units of 103/cm3. Energetically, such fluence can be provided by a partial tidal disruption event or even by a capture of a planet. Further progress in more accurate positioning and timing of the outburst should be possible with future X-ray polarimetric observations and long-term systematic observations with Chandra and XMM-Newton. A few hundred-years long X-ray observations would provide a detailed 3D map of the gas density distribution in the central ∼100 pc region.
ABSTRACT: One of the fundamental results used in observational cosmology is the distance duality relation (DDR), which relates the luminosity distance, DL, with angular diameter distance, DA, at a given redshift z. We suggest to employ the observed limits of this relation to constrain the coupling of axion like particles (ALPs) with photons. With available data we are able to provide improved mixing limit. The method can provide very stringent constraint on ALPs mixing with future improved DDR observations. Also any deviation in DDR can be conventionally explained as photons decaying to axions or vice-versa.
Denne korte artikel gør opmærksom på, at McGaughs relation mellem den målte radiale acceleration i skivegalakser og den beregnede acceleration ud fra stjerner og gas kan forklares som et resultat af dissipativ gasdynamik i en standard ΛCDM-kosmologi; men dette bliver sikkert ikke afslutningen på denne sag.
ABSTRACT: Recent analysis (McGaugh et al. 2016) of the SPARC galaxy sample found a surprisingly tight relation between the radial acceleration inferred from the rotation curves, and the acceleration due to the baryonic components of the disc. It has been suggested that this relation may be evidence for new physics, beyond ΛCDM . In this letter we show that the 18 galaxies from the MUGS2 match the SPARC acceleration relation. These cosmological simulations of star forming, rotationally supported discs were simulated with a WMAP3 ΛCDM cosmology, and match the SPARC acceleration relation with less scatter than the observational data. These results show that this acceleration law is a consequence of dissipative collapse of baryons, rather than being evidence for exotic dark-sector physics or new dynamical laws.