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Search Results: 1 - 10 of 812 matches for " Sangeeta Malhotra "
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High Altitude Molecular Clouds
Sangeeta Malhotra
Physics , 1994, DOI: 10.1086/174988
Abstract: A population of molecular clouds with a significantly greater scale height than that of Giant Molecular Clouds has been identified by examining maps of the latitude distribution of the $^{12}CO(1-0)$ emission in the first quadrant of the Galaxy. These clouds are found by identifying emission more than 2.6 times the scale-height away from the galactic midplane (centroid of CO emission) at the tangent points. Since the distance to the tangent points is known, we know the height and the sizes of these clouds. They are smaller and fainter than the GMCs and do not seem to be gravitationally bound. These clouds have properties similar to the high latitude clouds in the solar neighborhood. Although they lie outside the molecular cloud layer, the high altitude clouds are well within the HI layer in the Galaxy and coincide with distinct peaks in the HI distribution. These clouds represent a galaxy wide population of small molecular clouds having a larger scale height. They may be clouds in transition between molecular and atomic phases.
The Vertical Equilibrium of Molecular Gas in the Galactic Disk
Sangeeta Malhotra
Physics , 1994, DOI: 10.1086/174677
Abstract: We examine the vertical structure and equilibrium of the molecular gas layer in the galactic disk, measuring its scale height and velocity dispersion as a function of Galactic radius by modeling the CO emission at the tangent points. The model takes into account emission from a large path length along the line of sight, corresponding to an interval \Delta R ~ 200-400 pc; and is parametrized by the scale height of the gas, the centroid in z, the rotation velocity and the velocity dispersion. This model is then fit to the $^{12}CO$ survey of Knapp et al.(1985) to determine the best fit parameters. The reduced \chi^2 from fitting the models (considering only the photon statistics) range from 2 to 19. The main source of error is the `shot noise' due to the small number of clouds. Simulations using discrete molecular clouds are carried out to estimate the errors. The scale height of the molecular gas increases with radius. The velocity dispersion varies from ~ 2 km/s to 11 km/s with a typical uncertainty of ~3 km/s and shows a monotonic increase with galactic radius. The midplane mass density of the disk calculated from the scale-height and velocity dispersion is consistent with the local value \rho_0(R_0) of 0.2 \msun pc^{-3}.
The Vertical Distribution and Kinematics of HI and Mass Models of the Galactic Disk
Sangeeta Malhotra
Physics , 1994, DOI: 10.1086/175946
Abstract: We present full modeling of tangent point emission of HI as seen in the 21 cm transition in the inner Galaxy ($R \simeq 3-8 \kpc$). The model used takes into account emission from a large path length along the line of sight, corresponding to an interval ($\Delta R$) of typically $\le 1 \kpc$ in galactic radius; and is parametrized by the scale height of the gas, the centroid in z, the rotation velocity and the velocity dispersion. These parameters are assumed to be constant over the interval $\Delta R$. This modeling is carried out for the 21 cm surveys of Weaver \& Williams(1974), Bania \& Lockman (1984), and Kerr et al. (1986) to measure these parameters. The terminal velocity values are found to be in good agreement with previous measurements. The velocity dispersion is constant with radius at the theoretically expected value of $9 \pm 1 \kms$. The Gaussian scale height of the HI layer increases with Galactocentric radius. The centroid of the layer deviates significantly from $z=0$. Apart from small local variations, the velocity dispersion $\sigma_v$, scale height $\sigma_z$ and the centroid $z_0$ show similar variations in the first and the fourth quadrants. On balancing the turbulent pressure support of the layer against the disk gravitational potential, we confirm that additional support is needed for the HI layer. The radial profile of the reduced midplane mass density is an exponential with a scale length of $3.3 \pm 0.3 \kpc$. This picture is consistent with a constant mass-to-light ratio of the disk, and extra pressure support for the HI layer which is constant with radius in the inner Galaxy.
Detection of the 2175 angstrom dust feature in Mg II absorption systems
Sangeeta Malhotra
Physics , 1997, DOI: 10.1086/310934
Abstract: The broad absorption bump at 2175 angstrom due to dust, which is ubiquitous in the Galaxy and is seen in the Magellanic clouds, is also seen in a composite spectrum of Mg II absorbers. The composite absorber spectrum is obtained by taking the geometric mean of 92 quasar spectra after aligning them in the restframe of 96 absorbers. By aligning the spectra according to absorber redshifts we reinforce the spectral features of the absorbers, and smooth over possible bumps and wiggles in the emission spectra as well as small features in the flat fielding of the spectra. The width of the observed absorption feature is 200-300 angstrom (FWHM), or 0.4-0.6 (micron)^{-1} and the central wavelength is 2240 angstrom. These are somewhat different from the central wavelength of 2176 angstrqom and FWHM=0.8-1.25 (micron)^{-1} found in the Galaxy. Simulations show that this discrepancy between the properties of the 2175 angstrom feature in Mg II absorbers and Galactic ISM can be mostly explained by the different methods used to measure them.
Far Infrared Spectroscopy of star-forming galaxies: Expectations for the Herschel Space Observatory
Sangeeta Malhotra
Physics , 2001,
Abstract: ISO has enabled far-infrared spectroscopy of a variety of galaxies. Using the [CII] (158 micron) and [OI](63 micron) lines, we can characterize the physical conditions in the star-forming ISM. These observations also form the basis of our expectations for what the Herschel Space Observatory will observe for high redshift galaxies. While [CII] is suppressed in ULIRGs and normal galaxies with high dust temperatures, it is stronger than expected in metal poor galaxies by factors of a few. Young galaxies at high redshifts might be expected to be both metal poor and actively star-forming, leading to contrary expectations for the [CII] line strength. The best prediction for [CII] detection is derived by using the observed proportionality between [CII] and mid-IR emission from PAHs. Using the observed [CII]/7 micron ratio and number counts from ISO deep surveys we predict that HSO will be able to detect 100 sources/square-degree in the [CII] line.
Large Equivalent width Lyman-alpha line emission at z=4.5: young galaxies in a young universe
Sangeeta Malhotra,James Rhoads
Physics , 2001, DOI: 10.1086/338980
Abstract: The Large Area Lyman Alpha survey has found ~ 150 Lyman-alpha emitters at z=4.5. While stellar models predict a maximum Lyman-alpha equivalent width (EW) of 240 angstrom, 60% of the Lyman-alpha emitters have EWs exceeding this value. We attempt to model the observed EW distribution by combining stellar population models with an extrapolation of Lyman break galaxy luminosity function at z=4, incorporating observational selection effects and Malmquist bias. To reproduce the high EWs seen in the sample we need to postulate a stellar initial mass function (IMF) with extreme slope alpha = 0.5 (instead of 2.35); zero metallicity stars; or narrow-lined active galactic nuclei. The models also reveal that only 7.5-15% of galaxies need show Lyman-alpha emission to explain the observed number counts. This raises the possibility that either star-formation in high redshift galaxies is episodic or the Lyman-alpha galaxies we are seeing are the youngest 7.5-15% and that Lyman-alpha is strongly quenched by dust at about 10 Mega-years of age.
Quasar Populations in a Cosmological Constant Dominated Flat Universe
Sangeeta Malhotra,Edwin L Turner
Physics , 1994, DOI: 10.1086/175720
Abstract: Most physical properties derived for quasars, as single entities or as a population, depend upon the cosmology assumed. In this paper, we calculate the quasar luminosity function and some related quantities for a flat universe dominated by a cosmological constant $\Lambda$ ($\Lambda=0.9, \Omega=0.1$) and compare them with those deduced for a flat universe with zero cosmological constant ($\Lambda=0,\Omega=1$). We use the AAT quasar survey data (Boyle et al. 1990) as input in both cases. The data are fit well by a pure luminosity evolution model for both the cosmologies, but with different evolutionary parameters. From the luminosity function, we predict (extrapolate) a greater number of quasars at faint apparent magnitudes (twice the number at B=24, $z < 2.2$) for the $\Lambda$ dominated universe. This population of faint quasars at high redshift would result in a higher incidence of gravitational lensing. The total luminosity of the quasar population and the total mass tied up in black hole remnants of quasars is not sensitive to the cosmology. However, for a $\Lambda$ cosmology this mass is tied up in fewer but more massive black holes.
Microlensing of Globular Clusters as a Probe of Galactic Structure
James E. Rhoads,Sangeeta Malhotra
Physics , 1997, DOI: 10.1086/311211
Abstract: The spatial distribution of compact dark matter in our Galaxy can be determined in a few years of monitoring Galactic globular clusters for microlensing. Globular clusters are the only dense fields of stars distributed throughout the three-dimensional halo and hence are uniquely suited to probe its structure. The microlensing optical depths towards different clusters have varying contributions from the thin disk, thick disk, bulge, and halo of the Galaxy. Although measuring individual optical depths to all the clusters is a daunting task, we show that interesting Galactic structure information can be extracted with as few as $40$--$120$ events in total for the entire globular cluster system (observable with 2--5 years of monitoring). The globular cluster microlensing is particularly sensitive to the core radius of the halo mass distribution and to the scale length, surface mass density, and radial scale height variations of the thin disk.
Lyman Alpha Emitters at Redshift z=5.7
James E. Rhoads,Sangeeta Malhotra
Physics , 2001, DOI: 10.1086/338477
Abstract: Lyman alpha galaxies at high redshifts offer a powerful probe of both the formation of galaxies and the reionization of the intergalactic medium. Lyman alpha line emission is an efficient tool for identifying young galaxies at high redshift, because it is strong in systems with young stars and little or no dust-- properties expected in galaxies undergoing their first burst of star-formation. Lyman alpha galaxies also provide a robust test of the reionization epoch that is independent of Gunn-Peterson trough observations in quasar spectra and is better able to distinguish line center optical depths tau=5 from tau=10^5. This is because neutral gas scatters Lyman alpha photons, dramatically ``blurring'' images of Lyman alpha galaxies embedded in a neutral intergalactic medium and rendering them undetectable. We present a photometrically selected sample of z=5.7 Lyman alpha emitters derived from the Large Area Lyman Alpha survey. The presence of these low-luminosity Lyman alpha sources at z=5.7 immediately implies that the reionization redshift was > 5.7. Comparing these objects to our earlier z=4.5 sample, we find that the number of z=5.7 emitters at fixed line luminosity marginally exceeds the no-evolution expectation, but falls well short of published model predictions. The equivalent width distribution is similar at the two redshifts. The large equivalent widths of the Lyman alpha line indicate young galaxies undergoing their first star formation.
International Journal of Engineering Science and Technology , 2010,
Abstract: A number of application level multicast protocols have been proposed for core selection and core migration in Mobile Ad Hoc Networks. Core migration is necessary to minimize any disruptions on the transmission of data due to the changes in tree structure and to achieve improvement in the delivery of media streams in multicastgroup. In this paper, the broad review of literature on application level of core selection and core migration is discussed. Further, the performance of various techniques viz. centroid based core selection and random based core selection have investigated. On the performance comparison of the core selection techniques, various techniques are proposed for the migration of core if needed.
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