Double Power-law Shape Research Articles

First Light And Reionisation Epoch Simulations (FLARES) XII: The consequences of star-dust geometry on galaxies in the EoR

Abstract Using the First Light And Reionisation Epoch Simulations (Flares), a suite of hydrodynamical simulations we explore the consequences of a realistic model for star–dust geometry on the observed properties of galaxies. We find that the UV attenuation declines rapidly from the central regions of galaxies, and bright galaxies have spatially extended star formation that suffers less obscuration than their fainter counterparts, demonstrating a non-linear relationship between the UV luminosity and the UV attenuation, giving a double power-law shape to the UVLF. Spatially distinct stellar populations within galaxies experience a wide range of dust attenuation due to variations in the dust optical depth along their line-of-sight; which can range from completely dust obscured to being fully unobscured. The overall attenuation curve of a galaxy is then a complex combination of various lines-of-sight within the galaxy. We explore the manifestation of this effect to study the reliability of line ratios to infer galaxy properties, in particular the Balmer decrement and the BPT diagram. We find the Balmer decrement predicted Balmer line attenuation to be higher (factor of 1 to ≳ 10) than expected from commonly used attenuation curves. The observed BPT line ratios deviate from their intrinsic values (median difference of 0.08 (0.02) and standard deviation of 0.2 (0.05) for log10([N$\small {II}]\lambda 6585/$Hα) (log10([O iii]λ5008/Hβ)). Finally, we explore the variation in observed properties (UV attenuation, UV slope and Balmer decrement) with viewing angle, finding average differences of ∼0.3 magnitudes in the UV attenuation.

Stochastic Processes as the Origin of the Double Power-law Shape of the Quasar Luminosity Function

Abstract The quasar luminosity function (QLF) offers insight into the early coevolution of black holes and galaxies. It has been characterized observationally up to redshift z ∼ 6 with clear evidence of a double power-law shape, in contrast to the Schechter-like form of the underlying dark-matter halo mass function. We investigate a physical origin for the difference in these distributions by considering the impact of stochasticity induced by the processes that determine the quasar luminosity for a given host halo and redshift. We employ a conditional luminosity function and construct the relation between median quasar magnitude versus halo mass with log-normal in luminosity scatter Σ, and duty-cycle ϵ DC, and focus on high redshift z ≳ 4. We show that, in order to reproduce the observed QLF, the Σ = 0 abundance matching requires all of the brightest quasars to be hosted in the rarest most massive dark-matter halos (with an increasing in halo mass). Conversely, for Σ > 0 the brightest quasars can be overluminous outliers hosted in relatively common dark-matter halos. In this case, the median quasar magnitude versus halo mass relation, M UV,c, flattens at the high-end, as expected in self-regulated growth due to feedback. We sample the parameter space of Σ and ϵ DC and show that M UV,c flattens above for . Models with ϵ DC ∼ 1 instead require a high mass threshold close to . We investigate the impact of ϵ DC and Σ on measurements of clustering and find there is no luminosity dependence on clustering for Σ > 0.3, consistent with recent observations from Subaru HSC.

Charged-particle acceleration in a reconnecting current sheet including multiple magnetic islands and a nonuniform background magnetic field

Context. Charged particles are accelerated to high energies in solar flares. Although we know that magnetic reconnection is an efficient mechanism in generating energetic particles, the detailed role it plays in accelerating particles is still unknown. Aims. We investigate particle acceleration by magnetic reconnection in the current sheet, including multiple islands and a guide field.Methods. The long current sheet produced by the disruption in the corona magnetic field is usually not stable to various plasma instabilities, among which the tearing mode is the most important, and magnetic islands start to form in the current sheet when these instabilities develop. Two reverse processes are typically observed in the sheet: cascading of large islands to smaller ones, and merging of small islands into larger ones. Coalescent reconnection consequently takes place between two adjacent islands when merging occurs. The electric field induced by the coalescent reconnection is opposite to the electric field of the primary large-scale reconnection. We studied particle acceleration in such a current sheet and examined in detail the dynamic properties of electrons and protons in the current sheet through test particle approach.Results. We found that some particles can be accelerated to high energies in a very short time, and some particles (near the coalescence reconnection site) are accelerated and decelerated back and forth by the primary and secondary electric fields. Particle motions show two distinct types along different trajectories: some particles are trapped around magnetic islands, and some escape from the current sheet mainly along open field lines. With the presence of a guide field, protons and electrons are found to eventually move in different directions. The energy spectra for both species follow a double power-law shape. The softer components of the power-law spectrum are due to the particles that are trapped and circulate around magnetic islands, while the particles that escape and are partly trapped contribute to the harder component of the spectrum. Finally, we also investigated the influences of the parameters for the reconnection process on the spectral feature.

The spectral slope and escape fraction of bright quasars atz∼ 3.8: the contribution to the cosmic UV background

We use a sample of 1669 QSOs ($r<20.15$, $3.6<z<4.0$) from the BOSS survey to study the intrinsic shape of their continuum and the Lyman continuum photon escape fraction (f$_{esc}$), estimated as the ratio between the observed flux and the expected intrinsic flux (corrected for the intergalactic medium absorption) in the wavelength range 865-885 \AA\ rest-frame. Modelling the intrinsic QSO continuum shape with a power-law, $F_{\lambda}\propto\lambda^{-\gamma}$, we find a median $\gamma=1.30$ (with a dispersion of $0.38$, no dependence on the redshift and a mild intrinsic luminosity dependence) and a mean f$_{esc}=0.75$ (independent of the QSO luminosity and/or redshift). The f$_{esc}$ distribution shows a peak around zero and a long tail of higher values, with a resulting dispersion of $0.7$. If we assume for the QSO continuum a double power-law shape (also compatible with the data) with a break located at $\lambda_{\rm br}=1000$ \AA\ and a softening $\Delta\gamma=0.72 $ at wavelengths shorter than $\lambda_{\rm br}$, the mean f$_{esc}$ rises to $=0.82$. Combining our $\gamma$ and f$_{esc}$ estimates with the observed evolution of the AGN luminosity function (LF) we compute the AGN contribution to the UV ionizing background (UVB) as a function of redshift. AGN brighter than one tenth of the characteristic luminosity of the LF are able to produce most of it up $z\sim 3$, if the present sample is representative of their properties. At higher redshifts a contribution of the galaxy population is required. Assuming an escape fraction of Lyman continuum photons from galaxies between $5.5$ and $7.6\%$, independent of the galaxy luminosity and/or redshift, a remarkably good fit to the observational UVB data up to $z\sim 6$ is obtained. At lower redshift the extrapolation of our empirical estimate agrees well with recent UVB observations, dispelling the so-called Photon Underproduction Crisis.

GAMA/WiggleZ: the 1.4 GHz radio luminosity functions of high- and low-excitation radio galaxies and their redshift evolution toz= 0.75

We present radio Active Galactic Nuclei (AGN) luminosity functions over the redshift range 0.005 < z < 0.75. The sample from which the luminosity functions are constructed is an optical spectroscopic survey of radio galaxies, identified from matched Faint Images of the Radio Sky at Twenty-cm survey (FIRST) sources and Sloan Digital Sky Survey (SDSS) images.The radio AGN are separated into Low Excitation Radio Galaxies (LERGs) and High Excitation Radio Galaxies (HERGs) using the optical spectra. We derive radio luminosity functions for LERGs and HERGs separately in the three redshift bins (0.005 < z < 0.3, 0.3 < z < 0.5 and 0.5 < z <0.75). The radio luminosity functions can be well described by a double power-law. Assuming this double power-law shape the LERG population displays little or no evolution over this redshift range evolving as ~$(1+z)^{0.06}$ assuming pure density evolution or ~ $(1+z)^{0.46}$ assuming pure luminosity evolution. In contrast, the HERG population evolves more rapidly, best fitted by ~$(1+z)^{2.93}$ assuming a double power-law shape and pure density evolution. If a pure luminosity model is assumed the best fitting HERG evolution is parameterised by ~$(1+z)^{7.41}$. The characteristic break in the radio luminosity function occurs at a significantly higher power (~1 dex) for the HERG population in comparison to the LERGs. This is consistent with the two populations representing fundamentally different accretion modes.

GENERALIZED CONTINUITY EQUATION SOLUTIONS FOR THE QSO LUMINOSITY FUNCTION

ABSTRACT We present a generalized continuity equation that describes the relationship between the statistical and physical evolution of populations of astronomical objects. This equation allows us to parameterize the luminosity function (LF) in terms of physically meaningful quantities, such as creation timescale, τ s , object evolutionary timescale, τ g , and lifetime, a m . The shape and evolution of the LF are shown to be sensitive to these physical parameters, with large regions of the parameter space producing relatively simple evolutionary scenarios such as density evolution (DE) or luminosity evolution (LE). Regions of parameter space where τ s ≲ 0.3t H and τ g ≲ 0.5t H, where t H is the Hubble time, may be characterized by more complex evolution including the natural formation of a double power-law shape and mixed density and luminosity evolution (ME). This result has important consequences for the interpretation of the quasi-stellar object (QSO) LF, implying that the timescales for creation and physical evolution may fall near the above range. A fit to QSO survey data for redshifts 0.68 &lt; z &lt; 4 implies that τ s ∼ 0.2t H and τ g ∼ 0.05t H with QSOs having a maximum lifetime of a m ∼ 0.25t H.

Energy Spectra, Composition, and Other Properties of Ground-Level Events During Solar Cycle 23

We report spacecraft measurements of the energy spectra of solar protons and other solar energetic particle properties during the 16 Ground Level Events (GLEs) of Solar Cycle 23. The measurements were made by eight instruments on the ACE, GOES, SAMPEX, and STEREO spacecraft and extend from ∼0.1 to ∼500–700 MeV. All of the proton spectra exhibit spectral breaks at energies ranging from ∼2 to ∼46 MeV and all are well fit by a double power-law shape. A comparison of GLE events with a larger sample of other solar energetic particle (SEP) events shows that the typical spectral indices are harder in GLE events, with a mean slope of −3.18 at >40 MeV/nuc. In the energy range 45 to 80 MeV/nucleon about ∼50 % of GLE events have properties in common with impulsive ^(3)He-rich SEP events, including enrichments in Ne/O, Fe/O, ^(22)Ne/^(20)Ne, and elevated mean charge states of Fe. These ^(3)He-rich events contribute to the seed population accelerated by CME-driven shocks. An analysis is presented of whether highly-ionized Fe ions observed in five events could be due to electron stripping during shock acceleration in the low corona. Making use of stripping calculations by others and a coronal density model, we can account for events with mean Fe charge states of ≈+20 if the acceleration starts at ∼1.24–1.6 solar radii, consistent with recent comparisons of CME trajectories and type-II radio bursts. In addition, we suggest that gradual stripping of remnant ions from earlier large SEP events may also contribute a highly-ionized suprathermal seed population. We also discuss how observed SEP spectral slopes relate to the energetics of particle acceleration in GLE and other large SEP events.

TEMPORAL POWER SPECTRA OF THE HORIZONTAL VELOCITY OF THE SOLAR PHOTOSPHERE

We have derived the temporal power spectra of the horizontal velocity of the solar photosphere. The data sets for 14 quiet regions observed with the Gband filter of Hinode/SOT are analyzed to measure the temporal fluctuation of the horizontal velocity by using the local correlation tracking (LCT) method. Among the high resolution (~0.2") and seeing-free data sets of Hinode/SOT, we selected the observations whose duration is longer than 70 minutes and cadence is about 30 s. The so-called k-{\omega} diagrams of the photospheric horizontal velocity are derived for the first time to investigate the temporal evolution of convection. The power spectra derived from k-omega diagrams typically have a double power law shape bent over at a frequency of 4.7 mHz. The power law index in the high frequency range is -2.4 while the power law index in the low frequency range is -0.6. The root mean square of the horizontal speed is about 1.1 km/s when we use a tracer size of 0.4" in LCT method. Autocorrelation functions of intensity fluctuation, horizontal velocity, and its spatial derivatives are also derived in order to measure the correlation time of the stochastic photospheric motion. Since one of possible energy sources of the coronal heating is the photospheric convection, the power spectra derived in the present study will be of high value to quantitatively justify various coronal heating models.

High spectral resolution measurements of a solar flare hard X-ray burst

Observations are reported of an intense solar flare hard X-ray burst on June 27, 1980, made with a balloon-borne array of liquid nitrogen-cooled Ge detector which provided unprecedented spectral resolution (no more than 1 keV FWHM). The hard X-ray spectra throughout the impulsive phase burst fitted well to a double power-law form, and emission from an isothermal 0.1-1 billion K plasma can be specifically excluded. The temporal variations of the spectrum indicate that the hard X-ray burst is made up of two superposed components: individual spikes lasting about 3-15 sec, which have a hard spectrum and a break energy of 30-65 keV; and a slowly varying component characterized by a soft spectrum with a constant low-energy slope and a break energy which increases from 25 kev to at least 100 keV through the event. The double power-law shape indicates that DC electric field acceleration, similar to that occurring in the earth's auroral zone, may be the source of the energetic electrons which produce the hard X-ray emission.

Electron acceleration in solar flares

We present observations of an intense solar flare hard X-ray burst on 1980 June 27, made with a balloon-borne array of liquid nitrogen-cooled germanium detectors which provided unprecedented spectral resolution (≲1 keV FWHM). The hard X-ray spectra throughout the impulsive phase burst fitted well to a double power-law form, and emission from an isothermal 108–109K plasma can be specifically excluded. The temporal variations of the spectrum indicate that the hard X-ray burst is made up of two superposed components: individual spikes lasting ∼3–15 s, whch have a hard spectrum and a break energy of 30–65 keV; and a slowly varying component characterized by a soft spectrum with a constant low-energy slope and a break energy which increases from 25 keV to ≳100 keV through the event. The double power-law shape indicates that acceleration by DC electric fields parallel to the magnetic field, similar to that occurring in the Earth's auroral zone, may be the source of the energetic electrons which produce the hard X-ray emission. The total potential drop required for flares is typically ∼102 kV compared to ∼10 kV for auroral substorms.

Energetic solar electrons in the interplanetary medium

ISEE-3 measurements extending down to 2 keV energy have provided a new perspective on energetic solar electrons in the interplanetary medium. Impulsive solar electron events are observed, on average, several times a day near solar maximum, with ∼40% detected only below ∼ 15 keV. The electron energy spectra have a nearly power-law shape extending smoothly down to 2 keV, indicating that the origin of these events is high in the corona. These coronal flare-like events often produced 3He-rich particle events. In large solar flares which accelerate electrons and ions to relativistic energies, the electron spectrum appears to be modified by a second acceleration which results in a double power-law shape above ∼ 10 keV with a break near 100 keV and flattening from ∼10–100 keV. Large flares result in long-lived (many days) streams of outflowing electrons which dominate the interplanetary fluxes at low energies. Even in the absence of solar activity, significant fluxes of low energy electrons flow out from the Sun. Solar type-III radio bursts are produced by the escaping 2–102 keV electrons through a beam-plasma instability. The detailed ISEE-3 measurements show that electron plasma waves are generated by the bump-on-tail distribution created by the faster electrons running ahead of the slower ones. These plasma waves appear to be converted into radio emission by nonlinear wave-wave interactions.