arXiv subjects

Joseph Silk

Publications and source records attributed to Joseph Silk.

At least 307 records · Page 17Linked to original sources

arXiv2010

Forecast Constraints on Inflation from Combined CMB and Gravitational Wave Direct Detection Experiments

We study how direct detection of the inflationary gravitational wave background constrains inflationary parameters and complements CMB polarization measurements. The error ellipsoids calculated using the Fisher information matrix approach with Planck and the direct detection experiment, BBO (Big Bang Observer), show different directions of parameter degeneracy, and the degeneracy is broken when they are combined. For a slow-roll parameterization, we show that BBO could significantly improve the constraints on the tensor-to-scalar ratio compared with Planck alone. We also look at a quadratic and a natural inflation model. In both cases, if the temperature of reheating is also treated as a free parameter, then the addition of BBO can significantly improve the error bars. In the case of natural inflation, we find that the addition of BBO could even partially improve the error bars of a cosmic variance-limited CMB experiment.

astro-ph.CO↗

arXiv2010

Can the morphology of gamma-ray emission distinguish annihilating from decaying dark matter?

The recent results from the PAMELA, ATIC, FERMI and HESS experiments have focused attention on the possible existence of high energy cosmic ray e^+ e^- that may originate from dark matter (DM) annihilations or decays in the Milky Way. Here we examine the morphology of the gamma-ray emission after propagation of the electrons generated by both annihilating and decaying dark matter models. We focus on photon energies of 1 GeV, 10 GeV, 50 GeV (relevant for the FERMI satellite) and consider different propagation parameters. Our main conclusion is that distinguishing annihilating from decaying dark matter may only be possible if the propagation parameters correspond to the most optimistic diffusion models. In addition, we point to examples where morphology can lead to an erroneous interpretation of the source injection energy.

astro-ph.GA↗

arXiv2010

Environment and self-regulation in galaxy formation

The environment is known to affect the formation and evolution of galaxies considerably best visible through the well-known morphology-density relationship. In this paper we study the effect of environment on the evolution of early-type galaxies by analysing the stellar population properties of 3,360 galaxies morphologically selected by visual inspection from the SDSS in the redshift range 0.05<z<0.06. We find that the distribution of ages is bimodal with a strong peak at old ages and a secondary peak at young ages around ~ 2.5Gyr containing about 10 per cent of the objects. This is analogue to 'red sequence' and 'blue cloud' identified in galaxy populations usually containing both early and late type galaxies. The fraction of the young, rejuvenated galaxies increases with both decreasing galaxy mass and decreasing environmental density up to about 45 per cent. The rejuvenated galaxies have lower alpha/Fe ratios than the average and most of them show signs of ongoing star formation through their emission line spectra. All objects that host AGN in their centres without star formation are part of the red sequence population. We confirm and statistically strengthen earlier results that luminosity weighted ages, metallicities, and alpha/Fe element ratios of the red sequence population correlate well with velocity dispersion and galaxy mass. Most interestingly, however, these scaling relations are not sensitive to environmental densities and are only driven by galaxy mass. We infer that early-type galaxy formation has undergone a phase transition a few billion years ago around z~0.2. A self-regulated formation phase without environmental dependence has recently been superseded by a rejuvenation phase, in which the environment plays a decisive role possibly through galaxy interactions.

astro-ph.CO↗

arXiv2010

Identifying the progenitor set of present-day early-type galaxies: a view from the standard model

We present a comprehensive theoretical study, using a semi-analytical model within the standard LCDM framework, of the photometric properties of the progenitors of present-day early-type galaxies in the redshift range 0 0.7) spirals have 75-95% chance of being an early-type progenitor, while the corresponding probability for large blue spirals (M_B<-21.5, B-V<0.7) is 50-75%. The prescriptions developed here can be used to address, from the perspective of the standard model, the issue of `progenitor bias', whereby the exclusion of late-type progenitors in observational studies can lead to inaccurate conclusions regarding the evolution of the early-type population over cosmic time. (abridged)

astro-ph.CO↗

arXiv2010

Can the excess in the FeXXVI Ly gamma line from the Galactic Center provide evidence for 17 keV sterile neutrinos?

The standard model of particle physics assumes that neutrinos are massless, although adding non-zeros is required by the experimentally established phenomenon of neutrino oscillations requires neutrinos to have non-zero mass. Sterile neutrinos (or right-handed neutrinos) are a good warm dark matter candidate. We find that the excess of the intensity in the 8.7 keV line (at the energy of the FeXXVI Ly$γ$ line) in the spectrum of the Galactic center observed by the Suzaku X-ray mission cannot be explained by standard ionization and recombination processes. We suggest that the origin of this excess is via decays of sterile neutrinos with a mass of 17.4 keV and estimate the value of the mixing angle. The estimated value of the mixing angle $\sin^2(2θ)=(4.1 \pm 2.2)\times10^{-12}$ lies in the allowed region of the mixing angle of dark matter sterile neutrino with a mass of 17-18 keV.

astro-ph.HE↗

arXiv2009

Exploring the Star Formation History of Elliptical Galaxies: Beyond Simple Stellar Populations with a New Estimator of Line Strengths

(Abridged) We study the stellar populations of 14 elliptical galaxies in the Virgo cluster. We propose an alternative approach to the standard side-band method to measure equivalent widths (EWs). Our Boosted Median Continuum maps the EWs more robustly than the side-band method, minimising the effect from neighbouring absorption lines and reducing the age-metallicity degeneracy. We concentrate on Balmer lines (Hbeta,Hgamma,Hdelta), the G band and the 4000A break as age-sensitive indicators, and on the combination [MgFe] as the main metallicity indicator. We go beyond the standard comparison of the observations with simple stellar populations (SSP) and consider various models to describe the star formation histories, either with a continuous star formation rate or with a mixture of two different SSPs. Composite models are found to give more consistent fits among individual line strengths and agree with an independent estimate using the spectral energy distribution. Our age and metallicity estimates correlate well with stellar mass or velocity dispersion, with a significant threshold around 5E10 Msun above which galaxies are uniformly old and metal rich. In a more speculative way, our models suggest that it is formation **epoch** and not formation timescale what drives the Mass-Age relationship of elliptical galaxies.

astro-ph↗

arXiv2009

Cosmic microwave background anomalies viewed via Gumbel Statistics

We describe and discuss the application of Gumbel statistics, which model extreme events, to WMAP 5-year measurements of the cosmic microwave background. We find that temperature extrema of the CMB are well modelled by the Gumbel formalism and describe tests for Gaussianity that the approach can provide. Comparison to simulations reveals Gumbel statistics to have only weak discriminatory power for the conventional statistic: $f_{NL}<1000$, though it may probe other regimes of non-Gaussianity. Tests based on hemispheric cuts reveal interesting alignment with other reported CMB anomalies. The approach has the advantage of model independence and may find further utility with smaller scale data.

astro-ph.CO↗

arXiv2009

Kerr Black Holes as Particle Accelerators to Arbitrarily High Energy

We show that intermediate mass black holes conjectured to be the early precursors of supermassive black holes and surrounded by relic cold dark matter density spikes can act as particle accelerators with collisions, in principle, at arbitrarily high centre of mass energies in the case of Kerr black holes. While the ejecta from such interactions will be highly redshifted, we may anticipate the possibility of a unique probe of Planck-scale physics.

hep-ph↗

arXiv2009

Can the WIMP annihilation boost factor be boosted by the Sommerfeld enhancement?

We demonstrate that the Sommerfeld correction to CDM annihilations can be appreciable if even a small component of the dark matter is extremely cold. Subhalo substructure provides such a possibility given that the smallest clumps are relatively cold and contain even colder substructure due to incomplete phase space mixing. Leptonic channels can be enhanced for plausible models and the solar neighbourhood boost required to account for PAMELA/ATIC data is plausibly obtained, especially in the case of a few TeV mass neutralino for which the Sommerfeld-corrected boost is found to be $\sim10^4-10^5.$ Saturation of the Sommerfeld effect is shown to occur below $β\sim 10^{-4},$ thereby constraining the range of contributing substructures to be above $\sim 10^5\rm M_\odot.$ We find that the associated diffuse gamma ray signal from annihilations would exceed EGRET constraints unless the channels annihilating to heavy quarks or to gauge bosons are suppressed. The lepton channel gamma rays are potentially detectable by the FERMI satellite, not from the inner galaxy where substructures are tidally disrupted, but rather as a quasi-isotropic background from the outer halo, unless the outer substructures are much less concentrated than the inner substructures and/or the CDM density profile out to the virial radius steepens significantly.

astro-ph↗

arXiv2009

Global star formation revisited

A general treatment of disk star formation is developed from a dissipative multi-phase model, with the dominant dissipation due to cloud collisions. The Schmidt-Kennicutt law emerges naturally for star-forming disks and starbursts. We predict that there should be an inverse correlation between Tully-Fisher law and Schmidt-Kennicutt law residuals. The model is extended to include a multi-phase treatment of supernova feedback that leads to a turbulent pressure-regulated generalization of the star formation law and is applicable to gas-rich starbursts. Enhanced pressure, as expected in merger-induced star formation, enhances star formation efficiency. An upper limit is derived for the disk star formation rate in starbursts that depends on the ratio of global ISM to cloud pressures. We extend these considerations to the case where the interstellar gas pressure in the inner galaxy is dominated by outflows from a central AGN. During massive spheroid formation, AGN-driven winds trigger star formation, resulting in enhanced supernova feedback and outflows. The outflows are comparable to the AGN-boosted star formation rate and saturate in the super-Eddington limit. Downsizing of both SMBH and spheroids is a consequence of AGN-driven positive feedback. Bondi accretion feeds the central black hole with a specific accretion rate that is proportional to the black hole mass. AGN-enhanced star formation is mediated by turbulent pressure and relates spheroid star formation rate to black hole accretion rate. The relation between black hole mass and spheroid velocity dispersion has a coefficient (Salpeter time to gas consumption time ratio) that provides an arrow of time. Highly efficient, AGN-boosted star formation can occur at high redshift.

astro-ph.CO↗

arXiv2009

Starburst--driven galactic outflows

We propose a model of starburst--driven galactic outflows whose dynamics depends on both radiation and thermal pressure. Standard models of thermal pressure--driven winds fail to explain some key observations of outflows at low and high redshift galaxies. We discuss a scenario in which radiation pressure from massive stars in a stellar population drive a shell of gas and dust. Subsequent supernova (SN) explosions from the most massive stars in this population then drive stellar ejecta outward in a rarefied medium, making it collide with the radiation pressure driven shell. The collision imparts renewed momentum to the shell, and the resulting re-acceleration makes the shell Rayleigh-Taylor unstable, fragmenting the shell. We show that the speed of these ballistic fragments can explain some recently observed correlations in Lyman break galaxies between wind speed, reddening and star formation rate.

astro-ph.CO↗

arXiv2009

The impact of TP-AGB stars on hierarchical galaxy formation models

The spectro-photometric properties of galaxies in galaxy formation models are obtained by combining the predicted history of star formation and mass accretion with the physics of stellar evolution through stellar population models. In the recent literature, significant differences have emerged regarding the implementation of the Thermally-Pulsing Asymptotic Giant Branch phase of stellar evolution. The emission in the TP-AGB phase dominates the bolometric and near-IR spectrum of intermediate-age (~1 Gyr) stellar populations, hence it is crucial for the correct modeling of the galaxy luminosities and colours. In this paper for the first time, we incorporate a full prescription of the TP-AGB phase in a semi-analytic model of galaxy formation. We find that the inclusion of the TP-AGB in the model spectra dramatically alters the predicted colour-magnitude relation and its evolution with redshift. When the TP-AGB phase is active, the rest-frame V-K galaxy colours are redder by almost 2 magnitudes in the redshift range z~2-3 and by 1 magnitude at z~1. Very red colours are produced in disk galaxies, so that the V-K colour distributions of disk and spheroids are virtually undistinguishable at low redshifts. We also find that the galaxy K-band emission is more than 1 magnitude higher in the range z~1-3. This may alleviate the difficulties met by the hierarchical clustering scenario in predicting the red galaxy population at high redshifts. The comparison between simulations and observations have to be revisited in the light of our results.

astro-ph↗

arXiv2009

Probing the Primordial Power Spectrum with Cluster Number Counts

We investigate how well galaxy cluster number counts can constrain the primordial power spectrum. Measurements of the primary anisotropies in the cosmic microwave background (CMB) may be limited, by the presence of foregrounds from secondary sources, to probing the primordial power spectrum at wave numbers less than about 0.30 h Mpc^{-1}. We break up the primordial power spectrum into a number of nodes and interpolate linearly between each node. This allows us to show that cluster number counts could then extend the constraints on the form of the primordial power spectrum up to wave numbers of about 0.45 h Mpc^{-1}. We estimate combinations of constraints from PLANCK and SPT primary CMB and their respective SZ surveys. We find that their constraining ability is limited by uncertainties in the mass scaling relations. We also estimate the constraint from clusters detected from a SNAP like gravitational lensing survey. As there is an unambiguous and simple relationship between the filtered shear of the lensing survey and the cluster mass, it may be possible to obtain much tighter constraints on the primordial power spectrum in this case.

astro-ph↗

arXiv2009

Constraining the Sommerfeld enhancement with Cherenkov telescope observations of dwarf galaxies

The presence of dark matter in the halo of our galaxy could be revealed through indirect detection of annihilation products. Dark matter annihilation is one of the possible interpretations of the recent measured excesses in positron and electron fluxes, once boost factors of the order of 10^3 or more are taken into account. Such boost factors are actually achievable through the velocity-dependent Sommerfeld enhancement of the annihilation cross-section. Here we study the expected gamma-ray flux from two local dwarf galaxies for which air Cerenkov measurements are available, namely Draco and Sagittarius. We use velocity dispersion measurements to model the dark matter halos of the dwarfs, and the results of numerical simulations to model the presence of an associated population of subhalos. We incorporate the Sommerfeld enhancement of the annihilation cross-section. We compare our predictions with observations of Draco and Sagittarius performed by MAGIC and HESS, respectively. We also compare our results with the sensitivities of Fermi and of the future Cherenkov Telescope Array. We find that the boost factor due to the Sommerfeld enhancement is already constrained by the MAGIC and HESS data, with enhancements greater than 5 x 10^4 being excluded. While Fermi will not be able to detect gamma-rays from the dwarf galaxies s even with the most optimistic Sommerfeld effect, we show that the Cherenkov Telescope Array will be able to test enhancements greater than 1.5 x 10^3.

astro-ph.HE↗

arXiv2009

Destruction of Molecular Gas Reservoirs in Early-Type Galaxies by Active Galactic Nucleus Feedback

Residual star formation at late times in early-type galaxies and their progenitors must be suppressed in order to explain the population of red, passively evolving systems we see today. Likewise, residual or newly accreted reservoirs of molecular gas that are fuelling star formation must be destroyed. This suppression of star formation in early-type galaxies is now commonly attributed to AGN feedback wherein the reservoir of gas is heated and expelled during a phase of accretion onto the central supermassive black hole. However, direct observational evidence for a link between the destruction of this molecular gas and an AGN phase has been missing so far. We present new mm-wavelength observations from the IRAM 30m telescope of a sample of low redshift SDSS early-type galaxies currently undergoing this process of quenching of late-time star formation. Our observations show that the disappearance of the molecular gas coincides within less than 100 Myr with the onset of accretion onto the black hole and is too rapid to be due to star formation alone. Since our sample galaxies are not associated to powerful quasar activity or radio jets, we conclude that low-luminosity AGN episodes are sufficient to suppress residual star formation in early-type galaxies. This `suppression mode' of AGN feedback is very different from the `truncation mode' linked to powerful quasar activity during early phases of galaxy formation.

astro-ph↗

arXiv2008

The role of minor mergers in the recent star formation history of early-type galaxies

We demonstrate that the large scatter in the ultra-violet (UV) colours of intermediate-mass early-type galaxies in the local Universe and the inferred low-level recent star formation in these objects can be reproduced by minor mergers in the standard LCDM cosmology. Numerical simulations of mergers with mass ratios less than or equal to 1:4, with reasonable assumptions for the ages, metallicities and dust properties of the merger progenitors, produce good agreement to the observed UV colours of the early-type population, if the infalling satellites are assumed to have (cold) gas fractions greater than 20%. Early-types that satisfy (NUV-r) < 3.8 are likely to have experienced mergers with mass ratios between 1:4 and 1:6 within the last ~1.5 Gyrs, while those that satisfy 3.8<(NUV-r)<5.5 are consistent with either recent mergers with mass ratios < 1:6 or mergers with higher mass ratios that occurred more than ~1.5 Gyrs in the past. We demonstrate that the early-type colour-magnitude relations and colour distributions in both the UV and optical spectral ranges are consistent with the expected frequency of minor merging activity in the standard LCDM cosmology at low redshift. We present a strong plausibility argument for minor mergers to be the principal mechanism behind the large UV scatter and associated low-level recent star formation observed in early-type galaxies in the nearby Universe.

astro-ph↗

arXiv2008

Delayed Recombination and Standard Rulers

Measurements of Baryonic Acoustic Oscillations in galaxy surveys have been recognized as a powerful tool for constraining dark energy. However, this method relies on the knowledge of the size of the acoustic horizon at recombination derived from Cosmic Microwave Background Anisotropy measurements. This estimate is typically derived assuming a standard recombination scheme; additional radiation sources can delay recombination altering the cosmic ionization history and the cosmological inferences drawn from CMB and BAO data. In this paper we quantify the effect of delayed recombination on the determination of dark energy parameters from future BAO surveys such as BOSS and WFMOS. We find the impact to be small but still not negligible. In particular, if recombination is non-standard (to a level still allowed by CMB data), but this is ignored, future surveys may incorrectly suggest the presence of a redshift dependent dark energy component. On the other hand, in the case of delayed recombination, adding to the analysis one extra parameter describing deviations from standard recombination, does not significantly degrade the error-bars on dark energy parameters and yields unbiased estimates.

astro-ph↗

arXiv2008

On the Origin of the 511 keV Emission in the Galactic Centre

Diffuse 511 keV line emission, from the annihilation of cold positrons, has been observed in the direction of the Galactic Centre for more than 30 years. The latest high-resolution maps of this emission produced by the SPI instrument on INTEGRAL suggest at least one component of the emission is spatially coincident with the distribution of ~70 luminous, low-mass X-ray binaries detected in the soft gamma-ray band. The X-ray band, however, is generally a more sensitive probe of X-ray binary populations. Recent X-ray surveys of the Galactic Centre have discovered a much larger population (>4000) of faint, hard X-ray point sources. We investigate the possibility that the positrons observed in the direction of the Galactic Centre originate in pair-dominated jets generated by this population of fainter accretion-powered X-ray binaries. We also consider briefly whether such sources could account for unexplained diffuse emission associated with the Galactic Centre in the microwave (the WMAP `haze') and at other wavelengths. Finally, we point out several unresolved problems in associating Galactic Centre 511 keV emission with the brightest X-ray binaries.

astro-ph↗