arXiv subjects

Steen H. Hansen

Publications and source records attributed to Steen H. Hansen.

70 records · Page 4Linked to original sources

arXiv2004

Cluster temperature profiles and Sunyaev-Zeldovich observations

Galaxy clusters are not iso-thermal, and the radial temperature dependence will affect the cluster parameters derived through the observation of the Sunyaev-Zeldovich (SZ) effect. We show that the derived peculiar velocity will be systematically shifted by 10-20%. For future all-sky surveys one cannot rely on the observationally expensive X-ray observations to remove this systematic error, but one should instead reach for sufficient angular resolution to perform a deprojection in the SZ spectra. The Compton weighted electron temperature is accurately derived through SZ observations.

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arXiv2004

Searching for the relativistic Sunyaev-Zeldovich effect with the PLANCK experiment

Populations of relativistic electrons in clusters of galaxies should imprint a characteristic spectral distortion signature on the Cosmic Microwave Background photons - the relativistic Sunyaev-Zeldovich (SZ) effect. We investigate how sensitive PLANCK and successor experiments will be in detecting or constraining the total optical depth tau of relativistic electrons. We find an expected sensitivity of PLANCK of sigma_tau ~ 2 10^-5 for an individual cluster, which is too insensitive to detect even optimistic scenarios, which predict tau ~ 10^-6 to 10^-7. However, by stacking the PLANCK-signal of all SZ detectable clusters the relativistic SZ effect can be statistically probed to a sensitivity level of sigma_tau ~ 10^-7. PLANCK successor experiments will be able to probe even conservative scenarios of relativistic electron populations in clusters of galaxies.

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arXiv2003

Constraining the helium abundance with CMB

We consider for the first time the ability of present-day cosmic microwave background (CMB) anisotropies data to determine the primordial helium mass fraction, Y_p. We find that CMB data alone gives the confidence interval 0.160 < Y_p < 0.501 (at 68% c.l.). We analyse the impact on the baryon abundance as measured by CMB and discuss the implications for big bang nucleosynthesis. We identify and discuss correlations between the helium mass fraction and both the redshift of reionization and the spectral index. We forecast the precision of future CMB observations, and find that Planck alone will measure Y_p with error-bars of 5%. We point out that the uncertainty in the determination of the helium fraction will have to be taken into account in order to correctly estimate the baryon density from Planck-quality CMB data.

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arXiv2003

Galactic hydraulic drop?

Galaxies and galaxy clusters are observed to have a rather non-trivial radial behaviour. The observations show that the radial profiles change from one power-law profile near the centre to another power-law profile in the outer region. We present a simple explanation for this complex behaviour by finding the analytical solutions to the governing hydrodynamic equations. We see that the origin of this complexity is the collisional nature of the baryonic plasma, possibly related to a turbulence-enhanced viscosity.

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arXiv2003

Extraction of cluster parameters from Sunyaev-Zeldovich effect observations with simulated annealing optimization

We present a user-friendly tool for the analysis of data from Sunyaev-Zeldovich effect observations. The tool is based on the stochastic method of simulated annealing, and allows the extraction of the central values and error-bars of the 3 SZ parameters, Comptonization parameter, y, peculiar velocity, v_p, and electron temperature, T_e. The f77-code SASZ will allow any number of observing frequencies and spectral band shapes. As an example we consider the SZ parameters for the COMA cluster.

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arXiv2003

Do We Need Stars to Reionize the Universe at High Redshifts? Early Reionization by Decaying Heavy Sterile Neutrinos

A remarkable result of the Wilkinson Microwave Anisotropy Probe (WMAP) observations is that the universe was significantly reionized at large redshifts. The standard explanation is that massive stars formed early and reionized the universe around redshift z=17. Here we explore an alternative possibility, in which the universe was reionized in two steps. An early boost of reionization is provided by a decaying sterile neutrino, whose decay products, relativistic electrons, result in partial ionization of the smooth gas. We demonstrate that a neutrino with a mass of m_nu ~ 200 MeV and a decay time of t ~ 4 * 10^{15} s can account for the electron scattering optical depth tau=0.16 measured by WMAP without violating existing astrophysical limits on the cosmic microwave and gamma ray backgrounds. Reionization is then completed by subsequent star formation at lower redshifts. This scenario alleviates constraints on structure formation models with reduced small-scale power, such as those with a running or tilted scalar index, or warm dark matter models.

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arXiv2003

The origin of cores and density profiles of gaseous baryonic structures

We study the origin of cores and density profiles of gaseous baryonic structures in cosmology. By treating the baryons as a viscous gas, we find that both spheres and disks are possible solutions. We find analytically that the density profiles have inner and outer solutions, which in general are different. For disks we identify a central core, with density profile rho_d = constant, and the outer profile rho_d ~ r^{-3}. For spherical structures we find the profile rho_s ~ r^{-6}. In the presence of a dominating central black hole we find the inner profile rho ~ r^{-3/2}. When the mass is dominated by a dark matter component then the baryonic density profile will depend on the dark matter profile, and we point out how one can use this connection to infer the DM profile directly by observing the baryonic density profile.

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arXiv2003

Extraction of cluster parameters with future Sunyaev-Zel'dovich observations

The Sunyaev-Zel'dovich (SZ) effect of galaxy clusters is characterized by three parameters: Compton parameter, electron temperature and cluster peculiar velocity. In the present study we consider the problem of extracting these parameters using multi-frequency SZ observations only. We show that there exists a parameter degeneracy which can be broken with an appropriate choice of frequencies. As a result we discuss the optimal choice of observing frequencies from a theoretical point of view. Finally, we analyze the systematic errors (of the order micro K) on the SZ measurement introduced by finite bandwidths, and suggest a possible method of reducing these errors.

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arXiv2002

Interacting Dark Matter disguised as Warm Dark Matter

We explore some of the consequences of Dark Matter-photon interactions on structure formation, focusing on the evolution of cosmological perturbations and performing both an analytical and a numerical study. We compute the cosmic microwave background anisotropies and matter power spectrum in this class of models. We find, as the main result, that when Dark Matter and photons are coupled, Dark Matter perturbations can experience a new damping regime in addition to the usual collisional Silk damping effect. Such Dark Matter particles (having quite large photon interactions) behave like Cold Dark Matter or Warm Dark Matter as far as the cosmic microwave background anisotropies or matter power spectrum are concerned, respectively. These Dark Matter-photon interactions leave specific imprints at sufficiently small scales on both of these two spectra, which may allow to put new constraints on the acceptable photon-Dark Matter interactions. Under the conservative assumption that the abundance of 10^12 M_sol galaxies is correctly given by Cold Dark Matter, and without any knowledge of the abundance of smaller objects, we obtain the limit on the ratio of the Dark Matter-photon cross section to the Dark Matter mass sigma_{gamma-DM} / m_DM < 10^-6 sigma_Thomson / 100 GeV \sim 6 * 10^-33 cm^2 GeV^-1 .

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arXiv2002

Observational constraints on the inflaton potential combined with flow-equations in inflaton space

Direct observations provide constraints on the first two derivatives of the inflaton potential in slow roll models. We discuss how present day observations, combined with the flow equations in slow roll parameter space, provide a non-trivial constraint on the third derivative of the inflaton potential. We find a lower bound on the third derivative of the inflaton potential V'''/V > -0.2. We also show that unless the third derivative of the inflaton potential is unreasonably large, then one predicts the tensor to scalar ratio, r, to be bounded from below r > 3 * 10^{-6}.

hep-ph↗

arXiv2002

First measurement of cluster temperature using the thermal Sunyaev-Zeldovich effect

We discuss a new method of finding the cluster temperatures which is independent of distance and therefore very useful for distant clusters. The hot gas of electrons in clusters of galaxies scatters and distorts the cosmic microwave background radiation in a well determined way. This Sunyaev-Zel'dovich (SZ) effect is a useful tool for extracting information about clusters such as their peculiar radial velocity and optical depth. Here we show how the temperature of the cluster can be inferred from the SZ effect, in principle without use of X-ray data. We use recent millimetre observation of Abell 2163 to determine for the first time a cluster temperature using SZ observations only. The result T_e = 26^+34_-19 keV at 68% confidence level (at 95% c.l. we find T>1.5 keV) is in reasonable agreement with the X-ray results, T_e =12.4^+2.8_-1.9 keV.

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arXiv2002

Constraining the window on sterile neutrinos as warm dark matter

Sterile neutrinos may be one of the best Warm Dark Matter candidates we have today. Both lower and upper bounds on the mass of the sterile neutrino come from astronomical observations. We show that the proper inclusion of the neutrino momentum distribution reduces the allowed region to be 2.6 keV < m < 5 keV for the simplest models. A search for a spectral line with E=m/2 is thus more interesting than ever before.

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arXiv2002

Constraining the Dark Universe

We combine complementary datasets to constrain dark energy. Using standard Big Bang Nucleosynthesis and the observed abundances of primordial nuclides to put constraints on $Ω_Q$ at temperatures near $T \sim 1MeV$, we find the strong constraint $Ω_Q({MeV}) < 0.045$ at $2σ$ c.l.. Under the assumption of flatness, using results from Cosmic Microwave Background (CMB) anisotropy measurements, high redshift supernovae (SN-Ia) observations and data from local cluster abundances we put a new constraint on the equation of state parameter $w_Q < -0.85$ at 68% c.l..

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arXiv2001

Helioseismology as a New Constraint on SUSY Dark Matter

The presence of dark matter in the solar neighbourhood can be tested in the framework of stellar evolution theory by using the new results of helioseismology. If weakly interacting massive particles accumulate in the center of the Sun, they can provide an additional mechanism for transferring energy from the solar core. The presence of these particles produces a change in the local luminosity of the Sun of the order of 0.1%, which is now within the reach of seismic solar experiments. We find that typical WIMPs with a mass of 60 GeV, annihilation cross-section of 10^{-32} cm^3/sec, and scalar scattering interaction between 10^{-36} cm^2 and 10^{-41} cm^2, the lower end of this range being that suggested by the DAMA direct detection experiment, modify the radial profile of the square of the sound speed in the solar core by \~ 0.1 %. This level of change is strongly constrained by the most recent helioseismological results. Significantly more massive WIMPs have a much smaller effect on the solar core temperature profile. However future helioseismological experiments have the potential exploring most, if not all, of the WIMP parameter space accessible to current and planned direct detection experiments.

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arXiv2001

Early-universe constraints on a Primordial Scaling Field

In the past years 'quintessence' models have been considered which can produce the accelerated expansion in the universe suggested by recent astronomical observations. One of the key differences between quintessence and a cosmological constant is that the energy density in quintessence, $Ω_ϕ$, could be a significant fraction of the overall energy even in the early universe, while the cosmological constant will be dynamically relevant only at late times. We use standard Big Bang Nucleosynthesis and the observed abundances of primordial nuclides to put constraints on $Ω_ϕ$ at temperatures near $T \sim 1MeV$. We point out that current experimental data does not support the presence of such a field, providing the strong constraint $Ω_ϕ(MeV) < 0.045$ at $2σ$ C.L. and strengthening previous results. We also consider the effect a scaling field has on CMB anisotropies using the recent data from Boomerang and DASI, providing the CMB constraint $Ω_ϕ\le 0.39$ at $2σ$ during the radiation dominated epoch.

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arXiv1997

On the possibility of distinguishing between Majorana and Dirac neutrinos

We clarify that one cannot distinguish between Majorana and Dirac neutrinos in the limit of vanishing neutrino mass. In particular we show, that the forward-backward asymmetry in the reaction e^+ e^- -> ννis the same for ν_M and ν_D, in contrast to recent claims made in the literature.

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