Ground-based Near-infrared Observations Research Articles

Scattering from dust in molecular clouds: Constraining the dust grain size distribution through near-infrared cloudshine and infrared coreshine

Context. The largest grains (0.5-1 micron) in the interstellar size distribution are efficient in scattering near- and mid-infrared radiation. These wavelengths are therefore particularly well suited to probe the still uncertain high-end of the size distribution. Aims. We investigate the change in appearance of a typical low-mass molecular core from the Ks (2.2 micron) band to the Spitzer IRAC 3.6 and 8 micron bands, and compare with model calculations, which include variations of the grain size distribution. Methods. We combine Spitzer IRAC and ground-based near-infrared observations to characterize the scattered light observed at the near- and mid-infrared wavelengths from the core L260. Using a spherical symmetric model core, we perform radiative transfer calculations to study the impact of various dust size distributions on the intensity profiles across the core. Results. The observed scattered light patterns in the Ks and 3.6 micron bands are found to be similar. By comparison with radiative transfer models the two profiles place constraints on the relative abundance of small and large (more than 0.25 micron) dust grains. The scattered light profiles are found to be inconsistent with an interstellar silicate grain distribution extending only to 0.25 micron and large grains are needed to reach the observed fluxes and the flux ratios. The shape of the Ks band surface brightness profile limits the largest grains to 1-1.5 micron. Conclusions. In addition to observing coreshine in the Spitzer IRAC channels, the combination with ground-based near-infrared observations are suited to constrain the properties of large grains in cores.

Ground-based near-infrared observations of water vapour in the Venus troposphere

We present a study of water vapour in the Venus troposphere obtained by modelling specific water vapour absorption bands within the 1.18μm window. We compare the results with the normal technique of obtaining the abundance by matching the peak of the 1.18μm window. Ground-based infrared imaging spectroscopy of the night side of Venus was obtained with the Anglo-Australian Telescope and IRIS2 instrument with a spectral resolving power of R∼2400. The spectra have been fitted with modelled spectra simulated using the radiative transfer model VSTAR. We find a best fit abundance of 31ppmv (−6 +9ppmv), which is in agreement with recent results by Bézard et al. (Bézard, B., Fedorova, A., Bertaux, J.-L., Rodin, A., Korablev, O. [2011]. Icarus, 216, 173–183) using VEX/SPICAV (R∼1700) and contrary to prior results by Bézard et al. (Bézard, B., de Bergh, C., Crisp, D., Maillard, J.P. [1990]. Nature, 345, 508–511) of 44ppmv (±9ppmv) using VEX/VIRTIS-M (R∼200) data analyses. Comparison studies are made between water vapour abundances determined from the peak of the 1.18μm window and abundances determined from different water vapour absorption features within the near infrared window. We find that water vapour abundances determined over the peak of the 1. 18μm window results in plots with less scatter than those of the individual water vapour features and that analyses conducted over some individual water vapour features are more sensitive to variation in water vapour than those over the peak of the 1. 18μm window. No evidence for horizontal spatial variations across the night side of the disk are found within the limits of our data with the exception of a possible small decrease in water vapour from the equator to the north pole. We present spectral ratios that show water vapour absorption from within the lowest 4km of the Venus atmosphere only, and discuss the possible existence of a decreasing water vapour concentration towards the surface.

The stellar and hot gas content of low-mass galaxy clusters

We analyse the stellar and hot gas content of 18 nearby, low-mass galaxy clusters, detected in redshift space and selected to have a dynamical mass 3E14<M/Msun<6E14, as measured from the 2dF Galaxy Redshift Survey. We combine X-ray measurements from both Chandra and XMM with ground-based near-infrared observations from CTIO, AAT and CFHT to compare the mass in hot gas and stars to the dynamical mass and state of the clusters. Only 13 of the clusters are detected in X-ray emission, and for these systems we find that a range of 7-20 per cent of their baryonic mass, and <3 per cent of their dynamical mass, is detected in starlight, similar to what is observed in more massive clusters. In contrast, the five undetected clusters are underluminous in X-ray emission, by up to a factor 10, given their stellar mass. Although the velocity distribution of cluster members in these systems is indistinguishable from a Gaussian, all show subtle signs of being unrelaxed: either they lack a central, dominant galaxy, or the bright galaxy distribution is less concentrated and/or more elongated than the rest of the sample. Thus we conclude that low-mass clusters and groups selected from the velocity distribution of their galaxies exhibit a dichotomy in their hot gas properties. Either they are detected in X-ray, in which case they generally lie on the usual scaling relations, or they are completely undetected in X-ray emission. The non-detections may be partly related to the apparently young dynamical state of the clusters, but it remains a distinct possibility that some of these systems are exceptionally devoid of hot emitting gas as the result of its expulsion or rarefaction.

The nature of late-type spiral galaxies: structural parameters, optical and near-infrared colour profiles and dust extinction

We analyse V- and H-band surface photometry of a sample of 18 Sb–Sd galaxies. Combining high-resolution Hubble Space Telescope (HST) images with ground-based near-infrared observations, we extract photometric profiles, which cover the whole disc and provide the highest possible resolution. This is the first photometric study of late-type spirals for which the stellar kinematics have been measured. For 10 out of the 18 galaxies, HST data in both F160W (H) and F606W (V) are available, and, for those, we present colour maps and radial colour profiles at the resolution of the HST. Colours vary significantly from galaxy to galaxy, but tend to be highly homogeneous within each galaxy, with smooth and flat colour profiles. Some of the colour maps show jumps in the inner regions, likely due to dust. We determine extinction maps in an almost model-independent way using the V−H colour map and the SAURON Mgb absorption line map of Ganda et al. The maps show that AV ranges from 0 to 2 mag, in the centre from 0 to 1.5 mag, in agreement with the models of Tuffs et al. We describe the surface brightness profiles as the superposition of an exponential disc and a Sérsic bulge. The bulges are small (0.1–2.5 kpc), and show a shape parameter n ranging from ≈0.7 to 3, with a mean value smaller than two: well below the value for the classical de Vaucouleurs bulges. Most galaxies (16 out of 18) show a central light excess above the Sérsic fit to the bulge, which can be interpreted as a nuclear cluster, as shown by previous studies. We provide zero-order estimates for the magnitude of these components. We discuss the correlations among the structural galaxy parameters and with other relevant quantities, such as Hubble type and stellar velocity dispersion. We compare these results with a recent paper by Graham & Worley, who present a summary of most of the NIR surface photometry of spirals in the literature. For both early- and late-type spirals, bulge luminosity strongly correlates with central velocity dispersion; at constant velocity dispersion, later type bulges are larger and less dense, and have lower Sérsic n-values.

A Pathfinder Instrument for Precision Radial Velocities in the Near-Infrared

ABSTRACTWe have designed and tested an in-plane echelle spectrograph configured to investigate precision radial velocities from ground-based near-infrared observations. The spectrograph operates across the spectral range of 0.9–1.7 μm at a spectral resolution of R = 50,000, and uses a liquid nitrogen-cooled HAWAII 1 K detector. Repeated measurements of the Earth’s rotation via integrated sunlight with two different instrument arrangements in the near-infrared Y band have produced radial velocities with ∼10 m s-1 rms over a period of several hours. The most recent instrument configuration has achieved an unbinned rms of 7 m s-1 and suggests that infrared radial velocity precisions may be able to approach those achieved at optical wavelengths.

Entry Flashes of Cometary Fragments in Jovian Upper Atmosphere

Short lasting flashes, called as First Precursor (PC1), were observed by some ground-based near-infrared observations for the impacts of large-sized fragments of comet Shoemaker-Levy 9 (SL9) in July 1994. The impact detections by the spacecraft Galileo [2, 7] about 10 seconds after the PC1 detections by ground based telescopes, combined with the far-side impacts of SL9 fragments as viewed from Earth, suggest that the source of the PC1 should be located in the Jovian upper atmosphere above the limb, at which the atmospheric pressure is extremely low. Thus, an important problem on the PC1 is how does the falling cometary fragment, which is a huge meteorite, emit near-infrared in the extremely thin atmosphere. The ablation model, which is usually used for an impact bolide, can only estimate flux from the bolide in a dense atmosphere at visible wavelength. In this paper, we assume that the PC1s are thermal radiation from the fragments and attempt quantitative estimations of the PCI fluxes using a simple entry flash model.

Ground-Based Near-Infrared Observations of Global Solar Oscillations

Near-infrared (0.7-1.0 μm) observations of solar brightness oscillations were performed in 1983-1991 at the Crimean Solar Tower with the use of two (16 × 16 and 32 × 32) photodiode arrays. In 1991 new observations of p-modes were made simultaneously in two spectral ranges, near 0.7 and 1.0 μm. The data is analysed to check for the presence of solar variation with the 160-min period. It is found that the mean relative amplitude for the 160-min solar irradiance variation at 0.73 - 1.65 μm wavelengths, ~ 2 × 10–6, is much lower than the upper limits set by the ACRIM and IPHIR space experiments.