Sangeeta Malhotra

A Robust Determination of the Time Delay in 0957+561A, B and a Measurement of the Global Value of Hubble's Constant

Continued photometric monitoring of the gravitational lens system 0957+561A, B in the g and r bands with the Apache Point Observatory (APO) 3.5 m telescope during 1996 shows a sharp g-band event in the trailing (B) image light curve at the precise time predicted in an earlier paper. The prediction was based on the observation of the event during 1995 in the leading (A) image and on a differential time delay of 415 days. This success confirms the so-called short delay, and the absence of any such feature at a delay near 540 days rejects the long delay for this system, thus resolving a long-standing controversy. A series of statistical analyses of our light-curve data yield a best-fit delay of 417 ? 3 days (95% confidence interval) and demonstrate that this result is quite robust against variations in the analysis technique, data subsamples, and assumed parametric relationship of the two light curves. Recent improvements in the modeling of the lens system (consisting of a galaxy plus a galaxy cluster) allow us to derive a value of the global value (at z = 0.36) of Hubbles constant H0 using Refsdals method, a simple and direct (single-step) distance determination based on experimentally verified and securely understood physics and geometry. The result is H0 = 64 ? 13 km s-1 Mpc-1 (for ? = 1), where this 95% confidence interval is dominantly due to remaining lens model uncertainties. However, it is reassuring that available observations of the lensing mass distribution overconstrain the model and thus provide an internal consistency check on its validity. We argue that this determination of the extragalactic distance scale (10% accurate at 1 ?) is now of comparable quality, in terms of both statistical and systematic uncertainties, to those based on more conventional techniques. Finally, we briefly discuss the prospects for improved H0 determinations using gravitational lenses, and some other possible implications and uses of the 0957+561A, B light curves.

Infrared Space Observatory Measurements of [C II] Line Variations in Galaxies

We report measurements of the [C II] fine-structure line at 157.714 ?m in 30 normal star-forming galaxies with the Long Wavelength Spectrometer (LWS) on the Infrared Space Observatory (ISO). The ratio of the line to total far-infrared (FIR) luminosity, LC II/LFIR, measures the ratio of the cooling of gas to that of dust, and thus the efficiency of the grain photoelectric heating process. This ratio varies by more than a factor of 40 in the current sample. About two-thirds of the galaxies have LC II/LFIR ratios in the narrow range of (2-7) ? 10 -->?3. The other one-third show trends of decreasing LC II/LFIR with increasing dust temperature, as measured by the flux ratio of infrared emission at 60 and 100 ?m, F?(60 ?m)/F?(100 ?m), and with increasing star formation activity, measured by the ratio of FIR and blue-band luminosity, LFIR/L -->B. We also find three FIR-bright galaxies that are deficient in the [C II] line, which is undetected with 3 ? upper limits of LC II/LFIR ?4. The trend in the LC II/LFIR ratio with the temperature of dust and with star formation activity may be due to decreased efficiency of photoelectric heating of gas at high UV radiation intensity as dust grains become positively charged, decreasing the yield and the energy of the photoelectrons. The three galaxies with no observed photodissociation region lines have among the highest LFIR/L -->B and F?(60 ?m)/F?(100 ?m) ratios. Their lack of [C II] lines may be due to a continuing trend of decreasing LC II/LFIR with increasing star formation activity and dust temperature seen in one-third of the sample with warm IRAS colors. In that case, the upper limits on LC II/LFIR imply a ratio of UV flux to gas density of G -->0/n>10 cm -->3 (where G -->0 is in units of the local average interstellar field). The low LC II/LFIR ratio could also be due to either weak [C II], owing to self-absorption, or a strong FIR continuum from regions weak in [C II], such as dense H II regions or plasma ionized by hard radiation of active galactic nuclei. The mid-infrared and radio images of these galaxies show that most of the emission comes from a compact nucleus. CO and H I are detected in these galaxies, with H I seen in absorption toward the nucleus.

The Mid-Infrared Spectra of Normal Galaxies

The mid-infrared spectra (2.5-5 and 5.7-11.6 µm) obtained by ISOPHOT reveal the interstellar medium emission from galaxies powered by star formation to be strongly dominated by the aromatic features at 6.2, 7.7, 8.6, and 11.3 µm. Additional emission appears in between the features, and an underlying continuum is clearly evident at 3-5 µm. This continuum would contribute about a third of the luminosity in the 3-13 µm range. The features together carry 5%-30% of the 40-120 µm far-infrared (FIR) luminosity. The relative fluxes in individual features depend very weakly on galaxy parameters such as the far-infrared colors, direct evidence that the emitting particles are not in thermal equilibrium. The dip at 10 µm is unlikely to result from silicate absorption since its shape is invariant among galaxies. The continuum component has a fnu~nu+0.65 shape between 3 and 5 µm and carries 1%-4% of the FIR luminosity; its extrapolation to longer wavelengths falls well below the spectrum in the 6-12 µm range. This continuum component is almost certainly of nonstellar origin and is probably due to fluctuating grains without aromatic features. The spectra reported here typify the integrated emission from the interstellar medium of the majority of star-forming galaxies and could thus be used to obtain redshifts of highly extincted galaxies up to z=3 with SIRTF.

ISO Mid-Infrared Observations of Normal Star-forming Galaxies: The Key Project Sample*

We present mid-infrared maps and preliminary analysis for 61 galaxies observed with the ISOCAM instrument aboard the Infrared Space Observatory. Many of the general features of galaxies observed at optical wavelengths?spiral arms, disks, rings, and bright knots of emission?are also seen in the mid-infrared, except the prominent optical bulges are absent at 6.75 and 15 ?m. In addition, the maps are quite similar at 6.75 and 15 ?m, except for a few cases where a central starburst leads to lower I?(6.75 ?m)/I?(15 ?m) ratios in the inner region. We also present infrared flux densities and mid-infrared sizes for these galaxies. The mid-infrared color I?(6.75 ?m)/I?(15 ?m) shows a distinct trend with the far-infrared color I?(60 ?m)/I?(100 ?m). The quiescent galaxies in our sample [I?(60 ?m)/I?(100 ?m) 0.6] show I?(6.75 ?m)/I?(15 ?m) near unity, whereas this ratio drops significantly for galaxies with higher global heating intensity levels. Azimuthally averaged surface brightness profiles indicate the extent to which the mid-infrared flux is centrally concentrated, and provide information on the radial dependence of mid-infrared colors. The galaxies are mostly well resolved in these maps: almost half of them have <10% of their flux in the central resolution element. A comparison of optical and mid-infrared isophotal profiles indicates that the flux at 4400 ? near the optical outskirts of the galaxies is approximately 8 (7) times that at 6.75 ?m (15 ?m), comparable to observations of the diffuse quiescent regions of the Milky Way.

Through a lens darkly: evidence for dusty gravitational lenses

Foreground galaxies that amplify the light from background quasars may also dim that light if the galaxies contain enough dust. Extinction by dust in lenses could hide the large number of lensed systems predicted for a flat universe with a large value of the cosmological constant A. We look for one signature of dust, namely reddening, by examining optical-infrared colours of gravitationally lensed images of quasars. We find that the lensed systems identified in radio and infrared searches have redder optical-infrared colours than optically selected ones. This could be due to a bias against selecting reddened (hence dimmed) quasars in the optical surveys, or due to the differences in the intrinsic colours of optical and radio quasars. Comparison of the radio-selected lensed and unlensed quasars shows that the lensed ones have redder colours. We therefore conclude that at least part of the colour difference between the two lens samples is due to dust. From the colour difference between lensed and unlensed radio quasars (and assuming the Galactic extinction law) we can reconcile a large cosmological constant (A = 0.9) with the number of lensed systems observed in flux-limited optical surveys. These results substantially weaken the strongest constraint on cosmological scenarios that invoke a non-zero cosmological constant to explain age discrepancy problems, satisfy predictions of inflationary models of the early Universe and playa role in large-scale structure formation models. They also raise the prospect of using gravitational lenses to study the interstellar medium in high-redshift galaxies.

Detection of the 2175 Å Dust Feature in Mg II Absorption Systems

The broad absorption bump at 2175 A 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 rest frame 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 A (FWHM), or 0.4-0.6 μm-1, and the central wavelength is 2240 A. These are somewhat different from the central wavelength of 2176 A and FWHM = 0.8-1.25 μm-1 found in the Galaxy. Simulations show that this discrepancy between the properties of the 2175 A feature in Mg II absorbers and the Galactic interstellar medium can be mostly explained by the different methods used to measure them.

A candidate gravitational lens in the Hubble Deep Field

The discovery of HDF J123652+621227, a candidate gravitational lens in the Hubble Deep Field, 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.

Microlensing of Globular Clusters as a Probe of Galactic Structure

The spatial distribution of compact dark matter in the Galaxy can be determined in a few years by monitoring Galactic globular clusters for microlensing. Globular clusters are the only dense fields of stars distributed throughout the three-dimensional halo, and hence, they are uniquely suited to probe its structure. The microlensing optical depths toward 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 yr of monitoring). This experiment is particularly sensitive to the core radius of the halo mass distribution and to the parameters of the thin disk.

ATLAS probe for the study of galaxy evolution with 300,000,000 galaxy spectra

ATLAS (Astrophysics Telescope for Large Area Spectroscopy) Probe is a mission concept for a NASA probe-class space mission with primary science goal the definitive study of galaxy evolution through the capture of 300,000,000 galaxy spectra up to z=7. It is made of a 1.5-m Ritchey-Chretien telescope with a field of view of solid angle 0.4 deg2. The wavelength range is at least 1 μm to 4 μm with a goal of 0.9 μm to 5 μm. Average resolution is 600 but with a possible trade-off to get 1000 at the longer wavelengths. The ATLAS Probe instrument is made of 4 identical spectrographs each using a Digital Micro-mirror Device (DMD) as a multi-object mask. It builds on the work done for the ESA SPACE and Phase-A EUCLID projects. Three-mirror fore-optics re-image each sub-field on its DMD which has 2048 x 1080 mirrors 13.6 μm wide with 2 possible tilts, one sending light to the spectrograph, the other to a light dump. The ATLAS Probe spectrographs use prisms as dispersive elements because of their higher and more uniform transmission, their larger bandwidth, and the ability to control the resolution slope with the choice of glasses. Each spectrograph has 2 cameras. While the collimator is made of 4 mirrors, each camera is made of only one mirror which reduces the total number of optics. All mirrors are aspheric but with a relatively small P-V with respect to their best fit sphere making them easily manufacturable. For imaging, a simple mirror to replace the prism is not an option because the aberrations are globally corrected by the collimator and camera together which gives large aberrations when the mirror is inserted. An achromatic grism is used instead. There are many variations of the design that permit very different packaging of the optics. ATLAS Probe will enable ground-breaking science in all areas of astrophysics. It will (1) revolutionize galaxy evolution studies by tracing the relation between galaxies and dark matter from the local group to cosmic voids and filaments, from the epoch of reionization through the peak era of galaxy assembly; (2) open a new window into the dark universe by mapping the dark matter filaments to unveil the nature of the dark Universe using 3D weak lensing with spectroscopic redshifts, and obtaining definitive measurements of dark energy and modification of gravity using cosmic large-scale structure; (3) probe the Milky Ways dust-shrouded regions, reaching the far side of our Galaxy; and (4) characterize asteroids and other objects in the outer solar systems.

LAE Galaxies at High Redshift: Formation Sites for Low-Metal Globular Clusters

Lyman-α emitting (LAE) galaxies observed at intermediate to high redshift have the correct size, mass, star formation rate, metallicity, and space density to have been the formation sites of metal-poor globular clusters. LAEs are typically small galaxies with transient starbursts. They should accrete onto spiral and elliptical galaxies over time, delivering metal-poor clusters into the larger galaxies’ halos as they themselves get dispersed by tidal forces. The galaxy WLM is a good example of a dwarf remnant from a very early starburst that contains a metal-poor globular cluster but failed to get incorporated into the Milky Way or M31 because of its remote location in the local group.