Measuring the Virial Masses of Disk Galaxies

I present detailed models for the formation of disk galaxies, and investigate which observables are best suited as virial mass estimators. Contrary to naive expectations, the luminosities and circular velocities of disk galaxies are extremely poor indicators of total virial mass. Instead, I show that the product of disk scale length and rotation velocity squared yields a much more robust estimate. Finally, I show how this estimator may be used to put limits on the efficiencies of cooling and feedback during the process of galaxy formation.Comment: 6 pages, 2 fig. To appear in proceedings of "The Mass of Galaxies at Low and High Redshift", eds. R. Bender and A. Renzini (ESO Astrophysics Symposia, Springer-Verlag

Resolving Architectural Mismatches of COTS Through Architectural Reconciliation

The integration of COTS components into a system under development entails architectural mismatches. These have been tackled, so far, at the component level, through component adaptation techniques, but they also must be tackled at an architectural level of abstraction. In this paper we propose an approach for resolving architectural mismatches, with the aid of architectural reconciliation. The approach consists of designing and subsequently reconciling two architectural models, one that is forward-engineered from the requirements and another that is reverse-engineered from the COTS-based implementation. The final reconciled model is optimally adapted both to the requirements and to the actual COTS-based implementation. The contribution of this paper lies in the application of architectural reconciliation in the context of COTS-based software development. Architectural modeling is based upon the UML 2.0 standard, while the reconciliation is performed by transforming the two models, with the help of architectural design decisions.

The Mass Function and Average Mass Loss Rate of Dark Matter Subhaloes

We present a simple, semi-analytical model to compute the mass functions of dark matter subhaloes. The masses of subhaloes at their time of accretion are obtained from a standard merger tree. During the subsequent evolution, the subhaloes experience mass loss due to the combined effect of dynamical friction, tidal stripping, and tidal heating. Rather than integrating these effects along individual subhalo orbits, we consider the average mass loss rate, where the average is taken over all possible orbital configurations. This allows us to write the average mass loss rate as a simple function that depends only on redshift and on the instantaneous mass ratio of subhalo and parent halo. After calibrating the model by matching the subhalo mass function (SHMF) of cluster-sized dark matter haloes obtained from numerical simulations, we investigate the predicted mass and redshift dependence of the SHMF.We find that, contrary to previous claims, the subhalo mass function is not universal. Instead, both the slope and the normalization depend on the ratio of the parent halo mass, M, and the characteristic non-linear mass M*. This simply reflects a halo formation time dependence; more massive parent haloes form later, thus allowing less time for mass loss to operate. We analyze the halo-to-halo scatter, and show that the subhalo mass fraction of individual haloes depends most strongly on their accretion history in the last Gyr. Finally we provide a simple fitting function for the average SHMF of a parent halo of any mass at any redshift and for any cosmology, and briefly discuss several implications of our findings.Comment: Replaced to match version accepted for publication in MNRAS. Small section added that discusses higher-order moments of subhalo occupation distribution (including a new figure). Otherwise, few small change

Nuclear stellar discs in early-type galaxies --- II. Photometric properties

Hubble Space Telescope images of two early-type galaxies harboring both nuclear and outer stellar discs are studied in detail. By means of a photometric decomposition, the images of NGC 4342 and NGC 4570 are analyzed and the photometric properties of the nuclear discs investigated. We find a continuity of properties in the parameter space defined by the central surface brightness and the scalelength of discs in spirals, S0s and embedded discs in ellipticals, in the sense that the nuclear discs extend the observed disc properties even further towards smaller scalelengths and brighter central surface brightnesses. When including the nuclear discs, disc properties span more than four orders of magnitude in both scalelength and central surface brightness. The nuclear discs studied here are the smallest and brightest stellar discs known, and as such, they are as extreme in their photometric properties as Malin I, when compared to typical galactic discs that obey Freeman's law. We discuss a possible formation scenario in which the double-disc structure observed in these galaxies has been shaped by now dissolved bars. Based on the fact that the black holes known to exist in some of these galaxies have masses comparable to those of the nuclear discs, we explore a possible link between the black holes and the nuclear discs.Comment: 9 pages (TeX) with 7 figures (postscript). Accepted for publication in MNRA

A limit on behavioral plasticity in speech perception.

It is well attested that we perceive speech through the filter of our native language: a classic example is that of Japanese listeners who cannot discriminate between the American /l/ and /r/ and identify both as their own /r/ phoneme (Goto, 1971). Studies in the laboratory have shown, however, that perception of non-native speech sounds can be learned through training (Lively, Pisoni, Yamada, & Tohkura, 1994). This is consistent with neurophysiological evidence showing considerable experience-dependent plasticity in the brain at the first levels of sensory processing (Edeline & Weinberger, 1993; Kraus, et al., 1995; Merzenich & Sameshima, 1993; Weinberger, 1993). Outside of the laboratory, however, the situation seems to differ: we here report a study involving Spanish-Catalan bilingual subjects who have had the best opportunities to learn a new contrast but did not do it. Our study demonstrates a striking lack of behavioral plasticity: early and extensive exposure to a second language is not sufficient to attain the ultimate phonological competence of native speakers

Cloned interstitial stem cells grow as contiguous patches in hydra

The migration of interstitial cells was analyzed during the growth of stem cell clones in vivo. The spatial distribution of cloned cells was analyzed at a time by which extensive migration of interstitial cells could have occurred. All interstitial cell clones were found to form large contiguous patches of cells. The results indicate that there is little migration of large interstitial cells in undisturbed tissue during normal growth. This finding is surprising since numerous grafting experiments have shown extensive migration of these cells. The implications of finding nonrandomly distributed stem cells are discussed