arXiv subjects

Priyamvada Natarajan

Publications and source records attributed to Priyamvada Natarajan.

247 records · Page 14Linked to original sources

arXiv1998

Sunyaev-Zeldovich decrements with no clusters?

Sunyaev-Zeldovich decrements in the temperature of the Cosmic Microwave Background (CMB) are produced by Thomson scattering of the CMB photons by the confined hot electrons in the Intra-Cluster Medium encountered along the line-of-sight. In this paper, we propose an alternate physical process that can produce detectable thermal and kinematic Sunyaev-Zeldovich decrements around quasars or quasar pairs in regions where no assembled, virialized clusters are detected either in the optical or X-ray wave-band. Invoking quasar outflows, we argue that both thermal and kinematic decrements can be produced by the baryons swept up by the mechanical luminosity of quasars. In contrast to the case of decrements produced by the hot electrons confined within the cluster potential, the magnitude of these quasar-outflow induced effects depend primarily on the luminosity of the QSO, and the physical scale of the outflow - which in turn depends both on the mean local density and the density gradient in the vicinity of the quasar. Quasar outflows produce (i) a frequency independent kinematic decrement ${ΔT/T} \sim {10^{-4}}$; (ii) a frequency dependent fluctuation in the intensity ${Δ{J_ν}/{J_ν}} \sim $ a 20% variation relative to ${ΔT/T}$, say at 15 GHz and 30 GHz; (iii) a temperature increment ${ΔT/T} \sim {10^{-4}}$ on the opposite side of the QSO; (iv) a linear polarization signal over 20'' at $\sim {10^{-7}}$ level; and therefore (v) patches of signal ${ΔT/T} \sim {10^{-4}}$ on degree scales.

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arXiv1998

Quasar outflows and the formation of dwarf galaxies

We propose a scenario for the formation of a population of baryon-rich, dark matter-deficient dwarf galaxies at high redshift that form from the mass swept out in the Intergalactic Medium (IGM) by energetic outflows from luminous quasars. We predict the intrinsic properties of these galaxies, and examine the prospects for their observational detection in the optical, X-ray and radio wavebands. Detectable thermal Sunyaev-Zeldovich decrements (cold spots) on arc-minute scales in the cosmic microwave background radiation maps are expected during the shock-heated expanding phase from these hot bubbles. We conclude that the optimal detection strategy for these dwarfs is via narrow-band Lyman-$α$ imaging of regions around high redshift quasars. An energetically scaled-down version of the same model is speculated upon as a possible mechanism for forming pre-galactic globular clusters.

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arXiv1998

The Mass/Light Ratio of Early-Type Galaxies: Constraints from Gravitational Lensing in the Rich Cluster AC114

We present a new wide field image of the distant cluster AC114 ($z=0.31$) obtained with Wide Field and Planetary Camera II onboard the Hubble Space Telescope. This image considerably extends our knowledge of the lensing properties of the cluster beyond those derived by Smail \et (1995a) from a single WF/PC-1 pointing. In conjunction with published ground-based spectroscopy, we utilise several newly-discovered multiple images to construct an improved mass model for the central regions of the cluster. Using this model, we apply the methodology introduced by Natarajan & Kneib (1997) to interpret local perturbations to the cluster shear field on small scales resulting from mass associated with individual cluster galaxies. We use the lensing signal to place new constraints on the average mass-to-light ratio and spatial extents of the dark matter halos associated with morphologically-classified early-type cluster members. We find that the total mass of a fiducial $L^*$ cluster spheroidal galaxy is largely contained within $\sim$ 15 kpc radius halo ($\sim$~8--10 $R_e$) with a mass-to-light ratio ${M/L_V} \sim {15^{+10}_{-4}} $ (90% c.l., $h=0.5$) in solar units within this radius. Comparisons with similar estimates for fieldgalaxies indicates that the cluster galaxies in AC114 may possess less extensive and less massive halos. We discuss the consequences of our result in the context of models for the dynamical evolution of cluster galaxies and the observational prospects for extending this analysis.

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arXiv1997

High-redshift galaxies, their active nuclei and central black holes

We demonstrate that the luminosity function of the recently detected population of star-forming galaxies and the QSO luminosity function at z=3 can be matched with the mass function of dark matter haloes predicted by hierarchical cosmogonies for lifetimes of optically bright QSOs in the range 10^6 to 10^8 yr. We suggest that the mass of supermassive black holes may be limited by the back-reaction of the emitted energy on the accretion flow in a self-gravitating disc. This would imply a relation of black hole to halo mass of the form M_bh ~(v_halo)^5 and a typical duration of the optically bright QSO phase of a few times 10^7 yr. The high integrated mass density of black holes inferred from recent black hole mass estimates in nearby galaxies may indicate that the overall efficiency of supermassive black holes for producing blue light is smaller than previously assumed. We discuss three possible accretion modes with low optical emission efficiency: (i) accretion at far above the Eddington rate, (ii) accretion obscured by dust, and (iii) accretion below the critical rate leading to an advection dominated accretion flow lasting for a Hubble time. We further argue that accretion with low optical efficiency might be closely related to the origin of the hard X-ray background and that the ionizing background might be progressively dominated by stars rather than QSOs at higher redshift.

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arXiv1997

Gamma-ray bursts from stellar remnants: probing the Universe at high redshift

A gamma-ray burst (GRB) releases an amount of energy similar to that of a supernova explosion, which combined with its rapid variability suggests an origin related to neutron stars or black holes. Since these compact stellar remnants form from the most massive stars not long after their birth, gamma-ray bursts should trace the star formation rate in the Universe; we show that the GRB flux distribution is consistent with this. Because of the strong evolution of the star formation rate with redshift, it follows that the dimmest known bursts have z ~ 6, much above the value usually quoted and beyond the most distant quasars. This explains the absence of bright galaxies in well-studied gamma-ray burst error boxes. The increased distances imply a peak luminosity of 8.3e51 erg/s and a rate density of 0.025 per million years per galaxy. These values are 20 times higher and 150 times lower, respectively, than are implied by fits with non-evolving GRB rates. This means that GRBs are either caused by a much rarer phenomenon than mergers of binary neutron stars, or their gamma-ray emission is often invisible to us due to beaming. Precise burst locations from optical transients will discriminate between the various models for GRBs from stellar deaths, because the distance between progenitor birth place and burst varies greatly among them. The dimmest GRBs are then the most distant known objects, and may probe the Universe at an age when the first stars were forming.

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arXiv1997

Do cluster galaxies have extended dark halos?

We present the results of the application of the methodology introduced by Natarajan & Kneib (1997) to interpret local perturbations to the cluster shear field resulting from mass associated with individual cluster galaxies. The lensing signal is used to place new constraints on the average mass-to-light ratio and spatial extents of the dark matter halos associated with morphologically-classified early-type cluster members in the Abell cluster AC114. The total mass of a fiducial $L^\ast$ cluster spheroidal galaxy is found to be largely contained within $\sim$ 15 kpc radius halo ($\sim$~8--10 $R_e$) with a mass-to-light ratio ${M/L_V} \sim {15^{+10}_{-4}} $ (90 % c.l.) in solar units within this radius. Comparisons with similar estimates for field galaxies suggests that the cluster galaxies in AC114 may possess less extensive and less massive halos. Additionally, there is some indication that, at a fixed luminosity, S0 galaxies are less extended than ellipticals, suggesting a difference in the efficiency of tidal stripping for different galaxy types. These results enable us to probe the variation of the mass-to-light ratio with scale via gravitational lensing methods. Therefore, the prospects for constraining the mass density of the Universe and understanding galaxy evolution using these gravitational lensing methods are very promising.

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arXiv1997

Estimating the mass density of neutral gas at $z < 1$

We use the relationships between galactic HI mass and B-band luminosity determined by Rao & Briggs to recalculate the mass density of neutral gas at the present epoch based on more recent measures of the galaxy luminosity function than were available to those authors. We find $Ω_{gas}(z=0) \simeq 5 \times 10^{-4}$ in good agreement with the original Rao & Briggs value, suggesting that this quantity is now reasonably secure. We then show that, if the scaling between H I mass and B-band luminosity has remained approximately constant since $z = 1$, the evolution of the luminosity function found by the Canada-France redshift survey translates to an increase of $Ω_{gas}$ by a factor of $\approx 3$ at $z = 0.5 - 1$ . A similar value is obtained quite independently from consideration of the luminosity function of Mg II absorbers at $z = 0.65$. By combining these new estimates with data from damped \lya systems at higher redshift, it is possible to assemble a rough sketch of the evolution of $Ω_{gas}$ over the last 90% of the age of the universe. The consumption of H I gas with time is in broad agreement with models of chemical evolution which include the effects of dust, although more extensive samples of damped \lya systems at low and intermediate redshift are required for a quantitative assessment of the dust bias.

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arXiv1997

Lensing by galaxy halos in clusters of galaxies

Weak shear maps of the outer regions of clusters have been successfully used to map the distribution of mass at large radii from the cluster center. The typical smoothing lengths employed thus far preclude the systematic study of the effects of galactic-scale substructure on the measured weak lensing signal. In this paper, we present two methods to infer the possible existence and extent of dark halos around bright cluster galaxies by quantifying the `local' weak lensing induced by them. The proposed methods are: direct radial averaging of the shear field in the vicinity of bright cluster members and a maximum-likelihood method to extract fiducial parameters characterizing galaxy halos. We demonstrate using simulations that these observed local weak-shear effects on galaxy scales within the cluster can be used to statistically constrain the mean mass-to-light ratio, and fiducial parameters like the halo size, velocity dispersion and hence mass of cluster galaxies. We compare the two methods and investigate their relative drawbacks and merits in the context of feasibility of application to HST cluster data, whereby we find that the prospects are promising for detection on stacking a minimum of 20 WFPC2 deep cluster fields.

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arXiv1996

An analytic approximation to the isothermal sphere

We present a useful analytic approximation to the solution of the Lane-Emden equation for infinite polytropic index - the isothermal sphere. The optimized expression obtained for the density profile can be accurate to within 0.04% within 5 core-radii and to 0.1% within 10 core-radii.

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arXiv1996

Distribution functions for clusters of galaxies from N-body simulations

We present the results of an attempt to adapt the distribution function formalism to characterize large-scale structures like clusters of galaxies that form in a cosmological N-body simulation. While galaxy clusters are systems that are not strictly in equilibrium, we show that their evolution can nevertheless be studied using a physically motivated extension of the language of equilibrium stellar dynamics. Restricting our analysis to the virialized region, a prescription to limit the accessible phase-space is presented, which permits the construction of both the isotropic and the anisotropic distribution functions $f(E)$ and $f(E,L)$. The method is applied to models extracted from a catalogue of simulated clusters. Clusters evolved in open and flat background cosmologies are followed during the course of their evolution, and are found to transit through a sequence of what we define as `quasi-equilibrium' states. An interesting feature is that the computed $f(E)$ is well fit by an exponential form. We conclude that the dynamical evolution of a cluster, undergoing relaxation punctuated by interactions and violent mergers with consequent energy-exchange, can be studied both in a qualitative and quantitative fashion by following the time evolution of $f(E)$.

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arXiv1996

Probing the dynamics of cluster-lenses

We propose a new approach to study the dynamical implications of mass models of clusters for the velocity structure of galaxies in the core. Strong and weak lensing data are used to construct the total mass profile of the cluster, which is used in conjunction with the optical galaxy data to solve in detail for the nature of galaxy orbits and the velocity anisotropy in the central regions. We also examine other observationally and physically motivated mass models, specifically those obtained from X-ray observations and N-body simulations. The aim of this analysis is to understand qualitatively the structure of the core and test some of the key assumptions of the standard picture of cluster formation regarding relaxation, virialization and equilibrium. This technique is applied to the cluster Abell 2218, where we find evidence for an anisotropic core, which we interpret to indicate the existence of a dynamically disturbed central region. We find that the requirement of physically meaningful solutions for the velocity anisotropy places stringent bounds on the slope of cluster density profiles in the inner regions.

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arXiv1995

Effect of Sub-Structure in Clusters on the Local Weak-Shear Field

Weak shear maps of the outer regions of clusters have been successfully used to map the distribution of mass at large radii. The effects of substructure in clusters on such reconstructions of the total mass have not been systematically studied. We propose a new method to study the effect of perturbers (bright cluster galaxies or sub-groups within the cluster) on the weak shear field. We present some analytic results below.

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arXiv1995

Study of the dynamics of the core of A2218

We report the results of an attempt to study the velocity structure of the core of the cluster A2218 in order to probe its dynamical state. The aim is to understand if the core is indeed virialized? relaxed? or in hydrodynamic equilibrium? We use cluster lensing data in conjunction with the optical galaxy data to solve in detail for the nature of galaxy orbits and the velocity anisotropy in the core. We present our preliminary results below.

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