arXiv subjects
Publications and source records attributed to Sangeeta Malhotra.
The discovery of HDF~J123652+621227, a candidate gravitational lens in the HDF, is reported. This lens may be multiply imaging several optical sources at different redshifts. If follow-up spectroscopy of the lens and the brightest image confirms this hypothesis, observations of this system alone can be used to obtain an estimate of the redshift distribution at extremely faint flux levels.
Using the near infrared fluxes of local galaxies derived from COBE/DIRBE J(1.25 $μ$m) K (2.2 $μ$m) \& L (3.5 $μ$m) band maps and published Cepheid distances, we construct Tully-Fisher (TF) diagrams for the nearby galaxies. The measured dispersions in these luminosity-linewidth diagrams are remarkably small: $σ_J = 0.09$ magnitudes, $σ_K = 0.13$ magnitudes, and $σ_L = 0.20$ magnitudes. For the J and K bands, Monte Carlo simulations give a 95\% confidence interval upper limit on the true scatter in the T-F diagram of $σ_J \le 0.35$ and $σ_K \le 0.45$. We determine Milky Way's luminosity and place it in the T-F diagram by fitting a bar plus ``standard'' values of its size and circular speed (Sun-Galactic center distance $R_0 = 8.5 \kpc$ and $Θ_0 =220 \kms$), the Milky Way lies within $1.5 σ$ of the TF relations. We can use the TF relation and the Cepheid distances to nearby bright galaxies to constrain $R_0$ and $Θ_0$. Alternatively, we can fix the parameters of the Galaxy to their standard values, ignore the Cepheid zero-point, and use the Tully-Fisher relation to determine the Hubble Constant directly: $H_0 = 66 \pm 12$ km/s/Mpc.
We report the detection of a very red source coincident with the gravitational lens 1938+66.6 (Patnaik et al. 1992) in K' (2.12 micron), H (1.6 micron), J (1.25 micron), and Thuan-Gunn r (0.65 micron) bands. 1938+66.6 has previously been detected as a partial radio ring indicating lensing. We find K'=17.1 +- 0.1 and r = 23.9 +- 0.2, making it a very red source with (r-K')=6.8 +- 0.25. We also observed in Thuan-Gunn g band (0.49 micron) and found g>24.5 at the 90% confidence level. We interpret our observations as a reddened gravitational lens on the basis of its optical-IR color and positional coincidence with the radio source.
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 $Λ$ ($Λ=0.9, Ω=0.1$) and compare them with those deduced for a flat universe with zero cosmological constant ($Λ=0,Ω=1$). 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 $Λ$ 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 $Λ$ cosmology this mass is tied up in fewer but more massive black holes.
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 ($ΔR$) 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 $ΔR$. 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 $σ_v$, scale height $σ_z$ 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.
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 ΔR ~ 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 χ^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 ρ_0(R_0) of 0.2 \msun pc^{-3}.
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.