ALBERT

Description: The detection of planar structures within the satellite systems of both the Milky Way (MW) and Andromeda (M31) has been reported as being in stark contradiction to the predictions of the standard cosmological model ( cold dark matter – CDM). Given the ambiguity in defining a planar configuration, it is unclear how to interpret the low incidence of the MW and M31 planes in CDM. We investigate the prevalence of satellite planes around galactic mass haloes identified in high-resolution cosmological simulations. We find that planar structures are very common, and that ~10 per cent of CDM haloes have even more prominent planes than those present in the Local Group. While ubiquitous, the planes of satellite galaxies show a large diversity in their properties. This precludes using one or two systems as small-scale probes of cosmology, since a large sample of satellite systems is needed to obtain a good measure of the object-to-object variation. This very diversity has been misinterpreted as a discrepancy between the satellite planes observed in the Local Group and CDM predictions. In fact, ~10 per cent of CDM galactic haloes have planes of satellites that are as infrequent as the MW and M31 planes. The look-elsewhere effect plays an important role in assessing the detection significance of satellite planes and accounting for it leads to overestimating the significance level by a factor of 30 and 100 for the MW and M31 systems, respectively.

Description: We present densely sampled ultraviolet/optical photometric and low-resolution optical spectroscopic observations of the Type IIP supernova 2013ab in the nearby (~24 Mpc) galaxy NGC 5669, from 2 to 190 d after explosion. Continuous photometric observations, with the cadence of typically a day to one week, were acquired with the 1–2 m class telescopes in the Las Cumbres Observatory Global Telescope network, ARIES telescopes in India and various other telescopes around the globe. The light curve and spectra suggest that the supernova (SN) is a normal Type IIP event with a plateau duration of ~80 d with mid-plateau absolute visual magnitude of –16.7, although with a steeper decline during the plateau (0.92 mag 100 d –1 in V band) relative to other archetypal SNe of similar brightness. The velocity profile of SN 2013ab shows striking resemblance with those of SNe 1999em and 2012aw. Following the Rabinak & Waxman prescription, the initial temperature evolution of the SN emission allows us to estimate the progenitor radius to be ~800 R , indicating that the SN originated from a red supergiant star. The distance to the SN host galaxy is estimated to be 24.3 Mpc from expanding photosphere method. From our observations, we estimate that 0.064 M of 56 Ni was synthesized in the explosion. General relativistic, radiation hydrodynamical modelling of the SN infers an explosion energy of 0.35  x 10 51  erg, a progenitor mass (at the time of explosion) of ~9 M and an initial radius of ~600 R .

Description: The sterile neutrino is a viable dark matter candidate that can be produced in the early Universe via non-equilibrium processes, and would therefore possess a highly non-thermal spectrum of primordial velocities. In this paper we analyse the process of structure formation with this class of dark matter particles. To this end we construct primordial dark matter power spectra as a function of the lepton asymmetry, L 6 , that is present in the primordial plasma and leads to resonant sterile neutrino production. We compare these power spectra with those of thermally produced dark matter particles and show that resonantly produced sterile neutrinos are much colder than their thermal relic counterparts. We also demonstrate that the shape of these power spectra is not determined by the free-streaming scale alone. We then use the power spectra as an input for semi-analytic models of galaxy formation in order to predict the number of luminous satellite galaxies in a Milky Way-like halo. By assuming that the mass of the Milky Way halo must be no more than 2 x 10 12 M (the adopted upper bound based on current astronomical observations) we are able to constrain the value of L 6 for M s ≤ 8 keV. We also show that the range of L 6 that is in best agreement with the 3.5 keV line (if produced by decays of 7 keV sterile neutrino) requires that the Milky Way halo has a mass no smaller than 1.5 x 10 12 M . Finally, we compare the power spectra obtained by direct integration of the Boltzmann equations for a non-resonantly produced sterile neutrino with the fitting formula of Viel et al. and find that the latter significantly underestimates the power amplitude on scales relevant to satellite galaxies.

Description: We use a suite of cosmological simulations to study the mass–concentration–redshift relation, c ( M, z ), of dark matter haloes. Our simulations include standard -cold dark matter (CDM) models, and additional runs with truncated power spectra, consistent with a thermal warm dark matter (WDM) scenario. We find that the mass profiles of CDM and WDM haloes are self-similar and well approximated by the Einasto profile. The c ( M, z ) relation of CDM haloes is monotonic: concentrations decrease with increasing virial mass at fixed redshift, and decrease with increasing redshift at fixed mass. The mass accretion histories (MAHs) of CDM haloes are also scale-free, and can be used to infer concentrations directly. These results do not apply to WDM haloes: their MAHs are not scale-free because of the characteristic scale imposed by the power spectrum suppression. Further, the WDM c ( M, z ) relation is non-monotonic: concentrations peak at a mass scale dictated by the truncation scale, and decrease at higher and lower masses. We show that the assembly history of a halo can still be used to infer its concentration, provided that the total mass of its progenitors is considered (the ‘collapsed mass history’; CMH), rather than just that of its main ancestor. This exploits the scale-free nature of CMHs to derive a simple scaling that reproduces the mass–concentration–redshift relation of both CDM and WDM haloes over a vast range of halo masses and redshifts. Our model therefore provides a robust account of the mass, redshift, cosmology and power spectrum dependence of dark matter halo concentrations.

Description: We introduce Copernicus Complexio ( coco ), a high-resolution cosmological N -body simulation of structure formation in the CDM model. coco follows an approximately spherical region of radius ~17.4 h –1 Mpc embedded in a much larger periodic cube that is followed at lower resolution. The high-resolution volume has a particle mass of 1.135  x  10 5 h –1 M (60 times higher than the Millennium-II simulation). coco gives the dark matter halo mass function over eight orders of magnitude in halo mass; it forms ~60 haloes of galactic size, each resolved with about 10 million particles. We confirm the power-law character of the subhalo mass function, $\overline{N}( 〉 \mu )\propto \mu ^{-s}$ , down to a reduced subhalo mass M sub / M 200 μ = 10 –6 , with a best-fitting power-law index, s  = 0.94, for hosts of mass 〈 M 200 〉 = 10 12 h –1 M . The concentration–mass relation of coco haloes deviates from a single power law for masses M 200  〈 afew  x  10 8 h –1 M , where it flattens, in agreement with results by Sanchez-Conde et al. The host mass invariance of the reduced maximum circular velocity function of subhaloes, V max / V 200 , hinted at in previous simulations, is clearly demonstrated over five orders of magnitude in host mass. Similarly, we find that the average, normalized radial distribution of subhaloes is approximately universal (i.e. independent of subhalo mass), as previously suggested by the Aquarius simulations of individual haloes. Finally, we find that at fixed physical subhalo size, subhaloes in lower mass hosts typically have lower central densities than those in higher mass hosts.

Description: We present broad-band photometric and polarimetric observations of two Type II supernovae (SNe) 2013hj and 2014G. SN 2014G is a spectroscopically classified Type IIL event, which we also confirm photometrically because its light curve shows characteristic features – a plateau slope of 2.55 mag (100 d) –1 in the V band and a duration of ~77 d – of a generic Type IIL SN. However, SN 2013hj also shows a high plateau decline rate of 1.5 mag (100 d) –1 in the V band, similar to SNe IIL, but marginally lower than SNe IIL template light curves. Our high cadence photometric observations of SNe 2013hj and 2014G enables us to cover all characteristic phases up to the radioactive tail of optical light curves. Broad-band polarimetric observations reveal some polarization in SN 2013hj with subtle enhancement as the SN evolves towards the plateau end. However, the polarization angle remains constant throughout the evolution. This characteristic is consistent with the idea that the evolving SN with recombining hydrogen envelope is slowly revealing a more asymmetric central region of explosion. Modelling of the bolometric light curve yields a progenitor mass of ~11 M with a radius of ~700 R for SN 2013hj, while for the SN 2014G model estimated progenitor mass is ~9 M with a radius of ~630 R , both having a typical energy budget of ~2 x 10 51  erg.

Description: The cold dark matter (CDM) cosmological model unambiguously predicts that a large number of haloes should survive as subhaloes when they are accreted into a larger halo. The CDM model would be ruled out if such substructures were shown not to exist. By contrast, if the dark matter consists of Warm Dark Matter (WDM) particles, then below a threshold mass that depends on the particle mass far fewer substructures would be present. Finding subhaloes below a certain mass would then rule out warm particle masses below some value. Strong gravitational lensing provides a clean method to measure the subhalo mass function through distortions in the structure of Einstein rings and giant arcs. Using mock lensing observations constructed from high-resolution N -body simulations, we show that measurements of approximately 100 strong lens systems with a detection limit of M low  = 10 7 h –1 M would clearly distinguish CDM from WDM in the case where this consists of 7 keV sterile neutrinos such as those that might be responsible for the 3.5 keV X-ray emission line recently detected in galaxies and clusters.

Description: We use the Copernicus Complexio ( coco ) high-resolution N -body simulations to investigate differences in the properties of small-scale structures in the standard cold dark matter (CDM) model and in a model with a cutoff in the initial power spectrum of density fluctuations consistent with both a thermally produced warm dark matter (WDM) particle with a rest mass of 3.3 keV and a sterile neutrino with mass 7 keV and leptogenesis parameter L 6  = 8.7. The latter corresponds to the ‘coldest’ model with this sterile neutrino mass compatible with the identification of the recently detected 3.5 keV X-ray line as resulting from particle decay. CDM and WDM predict very different number densities of subhaloes with mass 10 9 h –1 M although they predict similar, nearly universal, normalized subhalo radial density distributions. Haloes and subhaloes in both models have cuspy Navarro-Frenk-White profiles, but WDM subhaloes below the cut-off scale in the power spectrum (corresponding to maximum circular velocities V max z = 0 ≤ 50 kms – 1 ) are less concentrated than their CDM counterparts. We make predictions for observable properties using the galform semi-analytic model of Galaxy formation. Both models predict Milky Way satellite luminosity functions consistent with observations, although the WDM model predicts fewer very faint satellites. This model, however, predicts slightly more UV bright galaxies at redshift z  〉 7 than CDM, but both are consistent with observations. Gravitational lensing offers the best prospect of distinguishing between the models.

Description: Theoretical models of galaxy formation based on the cold dark matter cosmogony typically require strong feedback from supernova (SN) explosions in order to reproduce the Milky Way satellite galaxy luminosity function and the faint end of the field galaxy luminosity function. However, too strong a SN feedback also leads to the universe reionizing too late, and the metallicities of Milky Way satellites being too low. The combination of these four observations therefore places tight constraints on SN feedback. We investigate these constraints using the semi-analytical galaxy formation model galform . We find that these observations favour a SN feedback model in which the feedback strength evolves with redshift. We find that, for our best-fitting model, half of the ionizing photons are emitted by galaxies with rest-frame far-UV absolute magnitudes M AB (1500Å) 〈 –17.5, which implies that already observed galaxy populations contribute about half of the photons responsible for reionization. The z = 0 descendants of these galaxies are mainly galaxies with stellar mass M * 〉 10 10 M and preferentially inhabit haloes with mass M halo 〉 10 13 M .

Description: Self-consistent N -body simulations of modified gravity models are a key ingredient to obtain rigorous constraints on deviations from general relativity using large-scale structure observations. This paper provides the first detailed comparison of the results of different N -body codes for the f ( R ), Dvali–Gabadadze–Porrati and Symmetron models, starting from the same initial conditions. We find that the fractional deviation of the matter power spectrum from cold dark matter agrees to better than 1 per cent up to k ~ 5–10 h Mpc –1 between the different codes. These codes are thus able to meet the stringent accuracy requirements of upcoming observational surveys. All codes are also in good agreement in their results for the velocity divergence power spectrum, halo abundances and halo profiles. We also test the quasi-static limit, which is employed in most modified gravity N -body codes, for the Symmetron model for which the most significant non-static effects among the models considered are expected. We conclude that this limit is a very good approximation for all of the observables considered here.