Decay of the vacuum energy into cosmic microwave background photons

We examine the possibility of the decay of the vacuum energy into a homogeneous distribution of a thermalized cosmic microwave background (CMB), which is characteristic of an adiabatic vacuum energy decay into photons. It is shown that observations of the primordial density fluctuation spectrum, obtained from CMB and galaxy distribution data, restrict the possible decay rate. When photon creation due to an adiabatic vacuum energy decay takes place, the standard linear temperature dependence   T ( z ) = T ₀(1 + z )  is modified, where T ₀ is the present CMB temperature, and can be parametrized by a modified CMB temperature dependence     . From the observed CMB and galaxy distribution data, a strong limit on the maximum value of the decay rate is obtained by placing a maximum value  β_max≃ 3.4 × 10⁻³  on the β parameter.     

Numerical action reconstruction of the dynamical history of dark matter haloes in N-body simulations

We test the ability of the numerical action method (NAM) to recover the individual orbit histories of mass tracers in an expanding universe, given the masses and redshift-space coordinates at the present epoch. The mass tracers are represented by dark matter (DM) haloes identified in a region of radius  26  h ⁻¹ Mpc  of a high-resolution N -body simulation of the standard Λ cold dark matter (CDM) cosmology. Since previous tests of NAM at this scale have traced the underlying distribution of DM particles rather than extended haloes, our study offers an assessment of the accuracy of NAM in a scenario which more closely approximates the complex dynamics of actual galaxy haloes. We show that NAM can recover the present-day halo distances with typical distance errors of less than 3 per cent and radial peculiar velocities with a dispersion of  ∼130 km s⁻¹  . The accuracy of individual orbit reconstructions was limited by the inability of NAM, in some instances, to correctly model the positions of haloes at early times solely on the basis of the redshifts, angular positions and masses of the haloes at the present epoch. Improvements in the quality of NAM reconstructions may be possible using the present-day three-dimensional halo velocities and distances to further constrain the dynamics. This velocity data is expected to become available for nearby galaxies in the coming generations of observations by Space Interferometry Mission ( SIM ) and Global Astrometric Interferometer for Astrophysics ( GAIA ).     

Absolute spectrophotometry and photometric evolution of Nova Scuti 2005 N.1 (≡ V476 Sct)

Our CCD photometry of Nova Scuti 2005 N.1 shows it to be a fast nova, characterized by   t ₂= 15  and   t ₃= 28d  , affected by an   E ( B − V ) ∼ 1.9  mag reddening, appearing at a position     (±0.04 arcsec)     (±0.09 arcsec, J2000) and peaking at   V ∼ 11.1  mag on ∼September 28.1 ut . Absolute spectrophotometry places it within the Fe  ii class. The profile of emission lines is characterized by a double peak with a velocity separation of 690 km s⁻¹ and a width at half intensity of 1200 km s⁻¹. The distance to the nova is  4 ± 1 kpc  , and its height above the Galactic plane is   z = 80 ± 20  pc. The highly crowded field affects a possible identification of the progenitor, whose pre-outburst magnitude should, however, have been  22 < V < 25  mag, thus below the limit of photographic surveys. A deep   B V R _C I _C  photometric sequence is provided to support continued observations of the advanced decline phases.     

A critical-density closed universe in BransDicke theory

In a BransDicke (BD) cosmological model, the energy density associated with some scalar field decreases as a ^{2[( o +1/2)/( _o +1)]} with the scalefactor a ( t ) of the universe, giving matter with an equation of state In this model, the universe could be closed but still have a non-relativistic matter density corresponding to its critical value, _o =1. Different cosmological expressions, such as luminosity distance, angular diameter, number count and ratio of the redshift thicknessangular size, are determined in terms of the redshift for this model.     

An SZ temperature decrement–X-ray luminosity relation for galaxy clusters

We present the observed relation between Δ T _SZ, the cosmic microwave background (CMB) temperature decrement due to the Sunyaev–Zeldovich (SZ) effect, and L , the X-ray luminosity of galaxy clusters. We discuss this relation in terms of the cluster properties, and show that the slope of the observed Δ T _SZ– L relation is in agreement with both the L – T _e relation based on numerical simulations and X-ray emission observations, and the M _gas– L relation based on observation. The slope of the Δ T _SZ– L relation is also consistent with the M _tot– L relation, where M _tot is the cluster total mass based on gravitational lensing observations. This agreement may be taken to imply a constant gas mass fraction within galaxy clusters, however, there are large uncertainties, dominated by observational errors, associated with these relations. Using the Δ T _SZ– L relation and the cluster X-ray luminosity function, we evaluate the local cluster contribution to arcmin-scale cosmic microwave background anisotropies. The Compton distortion y -parameter produced by galaxy clusters through the SZ effect is roughly two orders of magnitude lower than the current upper limit based on FIRAS observations.     

Interactions, star formation and AGN activity

It has long been known that galaxy interactions are associated with enhanced star formation. In a companion paper, we explored this connection by applying a variety of statistics to Sloan Digital Sky Survey (SDSS) data. In particular, we showed that specific star formation rates of galaxies are higher if they have close neighbours. Here, we apply exactly the same techniques to active galactic nuclei (AGN) in the survey, showing that close neighbours are not associated with any similar enhancement of nuclear activity. Star formation is enhanced in AGN with close neighbours in exactly the same way as in inactive galaxies, but the accretion rate on to the black hole, as estimated from the extinction-corrected [O  iii ] luminosity, is not influenced by the presence or absence of companions. Previous work has shown that galaxies with more strongly accreting black holes contain more young stars in their inner regions. This leads us to conclude that star formation induced by a close companion and star formation associated with black hole accretion are distinct events. These events may be part of the same physical process, for example a merger, provided they are separated in time. In this case, accretion on to the black hole and its associated star formation would occur only after the two interacting galaxies have merged. The major caveat in this work is our assumption that the extinction-corrected [O  iii ] luminosity is a robust indicator of the bolometric luminosity of the central black hole. It is thus important to check our results using indicators of AGN activity at other wavelengths.     

Relativist ic Corrections to the Thermal Sunyaev-Zel''''dovich Power Spectrum

We present a quantitative estimate of the relativistic corrections to the thermal SZ power spectrum produced by the energetic electrons in massive clusters. The corrections are well within 10% for current experiments with working frequencies below v < 100 GHz, but become non-negligible at high frequencies v > 350 GHz. Moreover, the corrections appear to be slightly smaller at higher l or smaller angular scales. We conclude that there is no need to include the relativistic corrections in the theoretical study of the SZ power spectrum especially at low frequencies unless the SZ power spectrum is used for precision cosmology.     

Interaction-induced star formation in a complete sample of 105 nearby star-forming galaxies

We investigate the clustering properties of a complete sample of 10⁵ star-forming galaxies drawn from the data release 4 (DR4) of the Sloan Digital Sky Survey. On scales less than 100 kpc, the amplitude of the correlation function exhibits a strong dependence on the specific star formation rate (SSFR) of the galaxy. We interpret this as the signature of enhanced star formation induced by tidal interactions. We then explore how the average star formation rate (SFR) in a galaxy is enhanced as the projected separation r _p between the galaxy and its companions decreases. We find that the enhancement strongly depends on r _p, but very weakly on the relative luminosity of the companions. The enhancement is also stronger in low-mass galaxies than in high-mass galaxies. In order to explore whether a tidal interaction is not only sufficient, but also necessary to trigger enhanced star formation in a galaxy, we compute background subtracted neighbour counts for the galaxies in our sample. The average number of close neighbours around galaxies with low to average values of SFR/ M _* is close to zero. At the highest SSFRs, however, more than 40 per cent of the galaxies in our sample have a companion within a projected radius of 100 kpc. Visual inspection of the highest SFR/ M _* galaxies without companions reveals that more than 50 per cent of these are clear interacting or merging systems. We conclude that tidal interactions are the dominant trigger of enhanced star formation in the most strongly star-forming systems. Finally, we find clear evidence that tidal interactions not only lead to enhanced star formation in galaxies, but also cause structural changes such as an increase in concentration.     

COBE and global topology: an example of the application of the identified circles principle

The significance to which the cosmic microwave background (CMB) observations by the satellite COBE can be used to refute a specific observationally based hypothesis for the global topology (3-manifold) of the Universe is investigated, by a new method of applying the principle of matched circle pairs.
Moreover, it is shown that this can be done without assuming Gaussian distributions for the density perturbation spectrum.
The Universe is assumed to correspond to a flat Friedmann–Lemaître model with a zero value of the cosmological constant. The 3-manifold is hypothesized to be a 2-torus in two directions, with a third axis larger than the horizon diameter. The positions and lengths of the axes are determined by the relative positions of the galaxy clusters Coma, RX J1347.5−1145 and CL 09104+4109, assumed to be multiple topological images of a single, physical cluster.
If the following two assumptions are valid: (i) that the error estimates in the COBE DMR data are accurate estimates of the total random plus systematic error; and (ii) that the temperature fluctuations are dominated by the naı¨ve Sachs–Wolfe effect; then the distribution of the temperature differences between multiply imaged pixels is significantly wider than the uncertainty in the differences, and the candidate is rejected at the 94 per cent level.
This result is valid for either the 'subtracted' or 'combined' Analysed Science Data Sets, for either 10° or 20° smoothing, and is slightly strengthened if suspected contaminated regions from the galactic centre and the Ophiuchus and Orion complexes are removed.     

The effect of reionization on the COBE normalization

We point out that the effect of reionization on the microwave anisotropy power spectrum is not necessarily negligible on the scales probed by COBE . It can lead to an upward shift of the COBE normalization by more than the 1 σ error quoted, ignoring reionization. We provide a fitting function to incorporate reionization into the normalization of the matter power spectrum.     

Cosmic microwave background constraints on the epoch of reionization

We use a compilation of cosmic microwave anisotropy data to constrain the epoch of reionization in the Universe, as a function of cosmological parameters. We consider spatially flat cosmologies, varying the matter density Ω₀ (the flatness being restored by a cosmological constant), the Hubble parameter h and the spectral index n of the primordial power spectrum. Our results are quoted both in terms of the maximum permitted optical depth to the last-scattering surface, and in terms of the highest allowed reionization redshift assuming instantaneous reionization. For critical-density models, significantly tilted power spectra are excluded as they cannot fit the current data for any amount of reionization, and even scale-invariant models must have an optical depth to last scattering of below 0.3. For the currently favoured low-density model with Ω₀=0.3 and a cosmological constant, the earliest reionization permitted to occur is at around redshift 35, which roughly coincides with the highest estimate in the literature. We provide general fitting functions for the maximum permitted optical depth, as a function of cosmological parameters. We do not consider the inclusion of tensor perturbations, but if present they would strengthen the upper limits that we quote.     

Estimating β from redshift-space distortions in the 2dF galaxy survey

Given the failure of existing models for redshift-space distortions to provide a highly accurate measure of the β -parameter, and the ability of forthcoming surveys to obtain data with very low random errors, it becomes necessary to develop better models for these distortions. Here we review the failures of the commonly used velocity dispersion models and present an empirical method for extracting β from the quadrupole statistic that has little systematic offset over a wide range of β and cosmologies. This empirical model is then applied to an ensemble of mock 2dF southern strip surveys, to illustrate the technique and see how accurately we can recover the true value of β . We compare this treatment with the error we expect to find caused only by the finite volume of the survey. We find that non-linear effects reduce the range of scales over which β can be fitted, and introduce covariances between nearby modes in addition to those introduced by the convolution with the survey window function. The result is that we are only able to constrain β to a 1 σ accuracy of 25 per cent ( β =0.55±0.14 for the cosmological model considered). We explore one possible means of reducing this error, that of cluster collapse, and show that accurate application of this method can greatly reduce the effect of non-linearities, improving the determination of β . We conclude by demonstrating that, when the contaminating effects of clusters are dealt with, this simple analysis of the full 2dF survey yields β =0.55±0.04. For this model, this represents a determination of β to an accuracy of 8 per cent and hence an important constraint on the cosmological density parameter Ω₀.     

Microwave polarization in the direction of galaxy clusters induced by the CMB quadrupole anisotropy

Electron scattering induces a polarization in the cosmic microwave background (CMB) signal measured in the direction of a galaxy cluster owing to the presence of a quadrupole component in the CMB temperature distribution. Measuring the polarization towards distant clusters provides the unique opportunity to observe the evolution of the CMB quadrupole at moderate redshifts, z ∼0.5–3. We demonstrate that for the local cluster population the polarization degree will depend on the cluster celestial position. There are two extended regions in the sky, which are opposite to each other, where the polarization is maximal, ∼0.1( τ /0.02) μK in the Rayleigh–Jeans part of the CMB spectrum ( τ being the Thomson optical depth across the cluster). This value exceeds the polarization introduced by the cluster transverse peculiar motion if v _t<1300 km s⁻¹. One can hope to detect this small signal by measuring a large number of clusters, thereby effectively removing the systematic contribution from other polarization components produced in clusters. These polarization effects, which are of the order of ( v _t c )² τ , ( v _t c ) τ ² and ( kT _e m _e c ²) τ ², as well as the polarization owing to the CMB quadrupole, were previously given by Sunyaev and Zel'dovich for the Rayleigh–Jeans part of the spectrum. We fully confirm their earlier results and present exact frequency dependences for all these effects. The polarization degree is considerably higher in the Wien region.     

The time evolution of cosmological redshift as a test of dark energy

The variation of the expansion rate of the Universe with time produces an evolution in the cosmological redshift of distant sources (e.g. quasar Lyman α absorption lines) that might be directly observed by future ultrastable, high-resolution spectrographs (such as the COsmic Dynamics Experiment) coupled to extremely large telescopes (such as the European Southern Observatory's Extremely Large Telescope). This would open a new window to explore the physical mechanism responsible for the current acceleration of the Universe. We investigate the evolution of cosmological redshift from a variety of dark energy models, and compare it with simulated data. We perform a Fisher matrix analysis and discuss the prospects for constraining the parameters of these models and for discriminating among competing candidates. We find that, because of parameter degeneracies, and the inherent technical difficulties involved in this kind of observations, the uncertainties on parameter reconstruction can be rather large unless strong external priors are assumed. However, the method could be a valuable complementary cosmological tool, and give important insights on the dynamics of dark energy, not obtainable using other probes.