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The decomposition of the 21 cm rotation curve of galaxies into contribution from the disk and dark halo depends on the adopted mass to light ratio (M/L) of the disk. Given the vertical velocity dispersion (σz) of stars in the disk and its scale height (hz), the disk surface density and hence the M/L can be estimated. Earlier works have used this technique to conclude that galaxy disks are submaximal. Here we address an important conceptual problem: star-forming spirals have an old (kinematically hot) disk population and a young cold disk population. Both of these populations contribute to the integrated light spectra from which σz is measured. The measured scale height hz is for the old disk population. In the Jeans equation, σz and hz must pertain to the same population. We have developed techniques to extract the velocity dispersion of the old disk from integrated light spectra and from samples of planetary nebulae. We present the analysis of the disk kinematics of the galaxy NGC 628 using IFU data in the inner regions and planetary nebulae as tracers in the outer regions of the disk. We demonstrate that using the scale height of the old thin disk with the vertical velocity dispersion of the same population, traced by PNe, results in a maximal disk for NGC 628. Our analysis concludes that previous studies underestimate the disk surface mass density by ~ 2, sufficient to make a maximal disk for NGC 628 appear like a submaximal disk.
We have found a correlation between the M / L global gradients and the structural parameters of the luminous components of a sample of 19 early-type galaxies. Such a correlation supports the hypothesis that there is a connection between the dark matter content and the evolution of the baryonic component in such systems.
Planetary nebulae (PNe) may be the most promising tracers in the halos of early-type galaxies. We have used multi-object spectrographs on the WHT and the VLT, and the new Planetary Nebula Spectrograph on the WHT, to obtain hundreds of PN velocities in a small sample of nearby galaxies. These ellipticals show weak halo rotation, which may be consistent with ab initio models of galaxy formation, but not with more detailed major merger simulations. the galaxies near L* show evidence of a universal declining velocity dispersion profile, and dynamical models indicate the presence of little dark matter within 5 Reff—implying halos either not as massive or not as centrally concentrated as CDM predicts.
It has been suggested that the peanut-shaped bulges seen in some edge-on disk galaxies are produced when bars in these galaxies buckle. This paper reviews the modelling which seeks to show how bars buckle, and I present a very simple new model which captures the essential physics of this process. I then discuss the problems in establishing observationally the connection between peanut-shaped bulges and bars. Confirmation of the link has proved difficult because boxy bulges are only apparent in edge-on galaxies whereas bars are only easily detectable in more face-on systems. Finally, I present a new technique which avoids this difficulty by searching for the distinctive kinematic signature of an edgeon bar; application of this method to spectra of peanut-shaped bulges reveals that they are, indeed, associated with hidden bars.
The decomposition of the 21 cm rotation curve of galaxies into contribution from the disk and dark halo depends on the adopted mass to light ratio (M/L) of the disk. Given the vertical velocity dispersion (σz) of stars in the disk and its scale height (h), the disk surface density and hence the M/L can be estimated. Earlier works have used this technique to conclude that galaxy disks are submaximal. Here we address an important conceptual problem: σz and h must pertain to the same population. Our analysis concludes that previous studies underestimate the disk surface mass density by ~ 2, sufficient to make a maximal disk for M74 appear like a submaximal disk.
We examined whether preschoolers' ontological knowledge would influence lexical extension. In Experiment 1, four-year-olds were presented with a novel label for either an object with eyes described as an animal, or the same object without eyes described as a tool. In the animal condition, children extended the label to similar-shaped objects, whereas in the tool condition, children extended the label to similar-function objects. In Experiment 2, when four-year-olds were presented with objects with eyes described as tools, they extended the label on the basis of shared function. These experiments suggest that preschoolers' conceptual knowledge guides their lexical extension.
NMAGIC is a parallel implementation of our made-to-measure (χ2M2M) algorithm for constructing N-particle models of stellar systems from observational data, which extends earlier ideas by Syer & Tremaine (1996). The χ2M2M algorithm properly accounts for observational errors, is flexible, and can be applied to various systems and geometries. We show its ability to reproduce the internal dynamics of an oblate isotropic rotator model and report on the modeling of the dark matter (DM) halo of NGC 3379 combining SAURON and PN.S kinematic data. The χ2M2M algorithm is practical, reliable and can be applied to various dynamical systems without symmetry restrictions. We conclude that χ2M2M holds great promise for unraveling the internal dynamics of bulges.
We examine the dark matter properties of nearby early-type galaxies using planetary nebulae (PNe) as mass probes. We have designed a specialised instrument, the Planetary Nebula Spectrograph (PN.S) operating at the William Herschel telescope, with the purpose of measuring PN velocities with best efficiency. The primary scientific objective of this custom-built instrument is the study of the PN kinematics in 12 ordinary round galaxies. Preliminary results showing a dearth of dark matter in ordinary galaxies (Romanowsky et al. 2003) are now confirmed by the first complete PN.S datasets. On the other hand early-type galaxies with a “regular” dark matter content are starting to be observed among the brighter PN.S target sample, thus confirming a correlation between the global dark-to-luminous mass virial ratio (fDM = MDMM*) and the galaxy luminosity and mass.
Based on FORS2-VLT long-slit spectroscopy, the analysis of the central absorption line indices of 9 S0 galaxies in the Fornax Cluster is presented. Central indices correlate with central velocity dispersions (σ0) as observed in ellipticals (E). However, the stellar population properties of these S0s indicates that the observed trends are produced by relative differences in age and α-element abundances and not in metallicity ([Fe/H]) as previous studies have found in E galaxies. The observed scatter in the line indices versus σ0 relations can be partially explained by the rotationally-supported nature of many of these systems. The presence of tighter line indices vs. maximum (circular) rotational velocity (VMAX) relations confirms this statement. It was also confirmed that the dynamical mass is the driving physical property of all these correlations and in our Fornax S0s it has to be estimated assuming rotational support.
We present early results from the UK ROSAT Deep and Extended Deep Surveys. A total of 240 faint X-ray sources have been detected, most of which are expected to be QSOs and Seyfert galaxies at redshifts z < 3, although normal galaxies and starburst galaxies are also present. We will use these surveys, together with our parallel VLA 20cm & 6cm radio surveys and multicolour optical CCD surveys, to determine the evolution of the faint end of the X-ray and optical luminosity functions (LF) of QSOs, study the multiwaveband emission mechanisms of QSOs, map their distribution over a ‘wedge’ of high redshift sky, and investigate the X-ray evolution of distant clusters of galaxies.
The Multiwaveband Surveys.
The ROSAT survey was performed in a region of high-latitude sky of very low, and uniform, Galactic column density (71019 cm−2), as determined by our 21cm and IRAS 100µm measurements. The deep survey reaches a limiting X-ray flux of 410−15 erg cm−2s−1 (0.5–2keV) over a 40 arcmin diameter region of sky and contains 96 faint X-ray sources. The extended survey stretches over a 4° × 40 arcmin strip starting from the position of the deep survey, with a limiting flux of 10−14 erg cm−2s−1 (0.5–2 keV).
Deep VLA radio maps at 20cm (and at 6cm in the deep survey area only) have been constructed to flux limits of 0.5 mJy on the deep survey field and 2mJy on the extended survey.
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