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Shiv K. Sethi

Publications and source records attributed to Shiv K. Sethi.

61 records · Page 4Linked to original sources
arXiv1998

Contribution of Galaxies to the Background Hydrogen-Ionizing Flux

We estimate the evolution of the contribution of galaxies to the cosmic background flux at $912 Å$ by means of a semi-analytic model of galaxy formation and evolution. Such a modelling has been quite successful in reproducing the optical properties of galaxies. We assume hereafter the high-redshift damped Lyman-$α$ (DLA) systems to be the progenitors of present day galaxies, and we design a series of models which are consistent with the evolution of cosmic comoving emissivities in the available near infrared (NIR), optical, ultraviolet (UV), and far infrared (FIR) bands along with the evolution of the neutral hydrogen content and average metallicity of damped Lyman-$α$ systems (DLA). We use these models to compute the galactic contribution to the Lyman-limit emissivity and background flux for $0 \simeq z \le 4$. We take into account the absorption of Lyman-limit photons by HI and dust in the interstellar medium (ISM) of the galaxies. We find that the background Lyman-limit flux due to galaxies might dominate (or be comparable to) the contribution from quasars at almost all redshifts if the absorption by HI in the ISM is neglected. The ISM HI absorption results in a severe diminishing of this flux--by almost three orders of magnitude at high redshifts to between one and two orders at $z \simeq 0$. Though the resulting galaxy flux is completely negligible at high redshifts, it is comparable to the quasar flux at $z \simeq 0$.

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arXiv1998

Galactic dust polarized emission at high latitudes and CMB polarization

We estimate the dust polarized emission in our galaxy at high galactic latitudes, which is the dominant foreground for measuring CMB polarization using the high frequency instrument (HFI) aboard Planck surveyor. We compare it with the level of CMB polarization and conclude that, for angular scales $\le 1^{\circ}$, the scalar-induced CMB polarization and temperature-polarization cross-correlation are much larger than the foreground level at $ν\simeq 100 GHz$. The tensor-induced signals seem to be at best comparable to the foreground level.}

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arXiv1997

Decaying Neutrinos and Large Scale Structure Formation

We study the growth of density perturbations in a universe with unstable dark matter particles. The mass (m_ν) range 30 eV < m_ν< 10 keV with lifetimes ($t_d$) in the range 10^7 sec < t_d < 10^16 sec are considered. We calculate the COBE normalized matter power spectrum for these models. We find that it is possible to construct models consistent with observations for masses $m_ν> 50 eV by adjusting t_d so as to keep the quantity (m_ν(keV))^2 t_d(yr) constant at a value around 100. For m_ν< 1 keV the power spectrum has extra power at small scales which could result in an early epoch of galaxy formation. We do not find any value of t_d which gives a viable model in the mass range m_ν< 50 eV. We also consider the implications of radiatively decaying neutrinos---models in which a small fraction B << 1 of neutrinos decay into photons-which could possibly ionize the intergalactic medium (IGM) at high redshift. We show that the parameter space of decaying particles which satisfies the IGM observations does not give viable models of structure formation.

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arXiv1997

On The Source of Ionization of The Intergalactic Medium at $z \sim 2.4$

We use the recent detection of He II absorption at $z=2.2-2.6$ in the line of sight of the quasar HS1700+64 to put bounds on the sources of ionization. We find that given the uncertainty in $\tauhgp$ and the model of absorption in the intergalactic medium (IGM), a wide range of possible sources of ionization is still allowed by the observations. We show that a significant contribution from star forming galaxies is allowed and is consistent with the proximity effect. In the case of photoionization by the UV background radiation, the contribution of star forming galaxies to the intensity at $912 > Å$ can be within a range of $\sim 1 - 15$ times that of the quasars. We also investigate the case of a collisionally ionized intergalactic medium. We show that although collisional ionization can be a dominant source of ionization at $z \sim 2.4$ (taking into account the upper limit on Compton y-parameter of the microwave background radiation), the thermal state of the IGM cannot yet be determined. We also consider Sciama's model of radiatively decaying neutrinos and show that the model of decaying neutrino is consistent with the observations.

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arXiv1995

On the interpretation of the He II absorption in the line of sight of Q0302-003

We point out the peculiarities of the line of sight of Q0302-003 which was recently used to estimate the He II opacity at $z \sim 3.285$. This line of sight contains a large void in Lyman $α$ clouds, in the region used for the He II opacity observation, and the void is thought to be caused by the ionizing radiation from a foreground quasar (Dobrzycki and Bechtold 1991). If this is true, then the radiation from the foreground quasar is more relevant in the region used for the estimation of the He II opacity than the diffuse UV background radiation. We argue that in that case the observed opacity should not be used to put constraint on the diffuse UV background radiation. Using a typical spectral index of quasars, we estimate that most of the observed He II opacity must be due to Gunn-Peterson effect and suggest a H I Gunn-Peterson test along this line of sight. We further discuss the clumpiness of the intergalactic medium in the vicinity of Q0302-003 as suggested by observations.

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arXiv1995

A New Astrophysical Constraint on Radiatively Decaying Neutrinos

We calculate constraints on radiatively decaying neutrinos from the recent detection of singly ionized helium in the diffuse intergalactic medium (IGM) at $z \simeq 3.3$. We consider a model in which neutrinos predominantly decay into invisible relativistic particles with a rate $τ^{-1}$, and with a small branching ratio into the radiative mode. To satisfy the observation of singly ionized helium, which puts a lower bound on the number density of singly ionized helium in the IGM, we show that: for $τ\ga 10^{18} \,{\rm sec}(1 \,{\rm eV}/m_ν^2)$, transition moment of neutrinos $μ_{12}$ is constrained to be $\le 4 \hbox{--}8 \times 10^{-17} \,μ_{\sc B}$ for $ 110 \,{\rm eV} \la m_ν\la 10 \,{\rm keV}$. We compare this bound with other astrophysical and cosmological bounds on radiatively decaying neutrinos.

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