arXiv subjects

M. A. Hendry

Publications and source records attributed to M. A. Hendry.

30 records · Page 2Linked to original sources

arXiv1997

One Arc Degree Core Substructure of the Virgo Cluster

The results of an analysis of the substructure of 1 degree field galaxies of the Virgo cluster by means of the S-tree technique developed by Gurzadyan et al, are represented. The existence of 3 main subgroups is shown and their dynamical parameters are obtained. The mass centers of the subgroups appear to be aligned in the direction parallel to the elongation of the Virgo cluster. The morphological analysis shows some domination of dwarf galaxies in the subgroup A containing M 87, and existence of two spiral galaxies N4425 and N4461; the latter fact can be crucial for the estimation of the distance of the Virgo cluster by means of the search of Cepheids in that spirals.

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arXiv1997

The Hubble Diagram of Type Ia Supernovae in Non-Uniform Pressure Universes

We use the redshift-magnitude relation, as derived by Dcabrowski (1995), for the two exact non-uniform pressure spherically symmetric Stephani universes with the observer positioned at the center of symmetry, to test the agreement of these models with recent observations of high redshift type Ia supernovae (SNIa), as reported in Perlmutter et al. (1997). By a particular choice of model parameters, we show that these models give an excellent fit to the observed redshifts and (corrected) B band apparent magnitudes of the SNIa data, but for an age of the Universe which is typically about two Gyr greater than in the corresponding Friedmann model. Based on a value of $H_0 \sim 65$ and assuming $Λ\geq 0$, the P97 data implies a Friedmann age of at most 13 Gyr and in fact a best-fit (for $q_0 = 0.5$) age of only 10 Gyr. Our Stephani models, on the other hand, can give a good fit to the P97 data with an age of up to 15 Gyr and could, therefore, significantly alleviate the conflict between recent cosmological and astrophysical age predictions. The choice of model parameters is quite robust: one requires only that the non-uniform pressure parameter, $a$, in one of the models is negative and satisfies $|a| \lte$ 3 km$^2$ s$^{-2}$ Mpc$^{-1}$. By allowing slightly larger, negative, values of $a$ one may `fine tune' the model to give an even better fit to the P97 data.

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arXiv1997

New Determination of the Hubble Parameter Using the Principle of Terrestrial Mediocrity

Measurements of the linear diameters of 12 nearby spiral galaxies with distances determined from primary indicators suggest that both the Milky Way Galaxy and M31 are in the middle of the range of sizes for such galaxies. By comparing the measured linear diameters of these nearby systems with the inferred diameters of a sample of more than 3000 spirals with known redshifts, we conclude that the most likely value of the Hubble Parameter lies in the range 50 - 55 km/s Mpc^{-1}.

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arXiv1997

The Milky Way is just an average spiral

It has been thought for many years that the Milky Way is an overly large spiral galaxy. Using Cephied distances to 17 spiral galaxies we calculate the true linear diameters of those galaxies. These diameters are then compared to that of the Milky Way which is found to be, at most, an averagely sized spiral galaxy. When compared to galaxies of approximately the same Hubble type ($2<T<6$) the Milky Way is found to be slightly undersized. This suggests that the Hubble parameter is at the lower end of the currently accepted range of possibilities.

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arXiv1997

Period-luminosity and period-luminosity-colour relations for Mira variables at maximum light

In this paper we confirm the existence of period-luminosity (PL) and period-luminosity-colour (PLC) relations at maximum light for O and C Mira variables in the LMC. We demonstrate that in the J and H bands the maximum light PL relations have a significantly smaller dispersion than their counterparts at mean light, while the K band and bolometric PL relations have a dispersion comparable to that at mean light. In the J, H and K bands the fitted PL relations for the O Miras are found to have smaller dispersion than those for the C Miras, at both mean and maximum light, while the converse is true for the relations based on bolometric magnitudes. The inclusion of a non-zero log period term is found to be highly significant in all cases except that of the C Miras in the J band, for which the data are found to be consistent with having constant absolute magnitude. This suggests the possibility of employing C Miras as standard candles. We suggest both a theoretical justification for the existence of Mira PL relations at maximum light and a possible explanation of why these relations should have a smaller dispersion than at mean light. The existence of such maximum light relations offers the possibility of extending the range and improving the accuracy of the Mira distance scale to Galactic globular clusters and to other galaxies.

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arXiv1996

A deep search for radio emission from three X-ray pulsars : are radio emission and X-ray pulsations anticorrelated ?

We present results from a deep search for radio emission from the X-ray pulsar systems GX 1+4, GS 0834-430 and 4U 0115+63 which have variously been suggested to possess radio jets and to be good candidates for propellor ejection mechanisms. None of these sources is detected at their optical positions, to 3 sigma limits of a few hundred micro-Jy. This places upper limits on their radio luminosities and thus on the internal energy and numbers of any relativistic electrons which are three to four orders of magnitude below those of radio-bright X-ray binaries such as SS 433 and Cyg X-3. Spectral and structural information on the proposed `radio lobes' of GX 1+4 make their association with the source unlikely. The lack of detected radio emission from any X-ray pulsar system is discussed statistically, and it is found that X-ray pulsations and radio emission from X-ray binaries are strongly anti-correlated.

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arXiv1996

Thresholds on star formation and the chemical evolution of galactic discs: cosmochronology and the age of the galaxy

In this paper we analyse different chronometers based on the models of chemical evolution developed in Chamcham, Pitts \& Tayler (1993; hereafter CPT) and Chamcham \& Tayler (1994; hereafter CT). In those papers we discussed the ability of our models to reproduce the observed G-dwarf distribution in the solar neighbourhood, age-metallicity relation and radial chemical abundance gradients. We now examine their response to the predictions of cosmochronology. We use the recent production ratios of the actinide pairs $^{235}$U/$^{238}$U and $^{232}$Th/$^{238}$U provided by Cowan, Thielemann \& Truran (1991) and the observed abundance ratios from Anders \& Grevesse (1989) to determine the duration of nucleosynthesis in the solar neighbourhood, and thus to determine maximum likelihood estimates and confidence intervals for the infall parameter, $β$, which controls the growth rate of the disc in our models. We compare our predictions for the age of the disc with the age of the galaxy estimated from models of white dwarf cooling and from the age of globular clusters. From our statistical analysis we find that these three methods of age prediction appear to be consistent for a range of maximum likelihood values of $β$ which is in good agreement with the values considered in CPT and CT, which were found to give a good fit to the observational data examined in those papers. We also briefly consider the consistency of our results with the age of the universe predicted in different cosmological models -- a topic which we will investigate more fully in future work.

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arXiv1996

Cepheids, supernovae and the value of $H_0$

The calibration of the Type Ia supernova distances using HST observations of Cepheid variables is discussed. A new maximum likelihood method of calibration is applied to derive the PL relation for a composite sample of Cepheids in the LMC and in the SNIa host galaxies NGC5253 and IC4182. Our results show that the calibration of the Cepheid PL relation is robust both to sampling error and to luminosity and period selection effects. Hence, the outstanding uncertainty in deriving estimates of $H_0$ from SNIa remains the dispersion of the SNIa luminosity function, and not unresolved systematic errors in its Cepheid calibration.

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arXiv1995

A physical distance indicator for spiral galaxies

In this paper we derive a Tully Fisher relation from measured I band photometry and H$α$ rotation curves of a large survey of southern sky spiral galaxies, obtained in Persic \& Salucci (1995) by deprojecting and folding the raw H$α$ data of Mathewson, Ford \& Buchhorn (1992). We calibrate the relation by combining several of the largest clusters in the survey, using an iterative maximum likelihood procedure to account for observational selection effects and Malmquist bias. We also incorporate a simple model for the line of sight depth of each cluster. Our results indicate a Tully Fisher relation of intrinsic dispersion $\sim0.3$ mag, corresponding to a distance error dispersion of $13\%$. Application of this relation to mapping the large scale velocity field is underway.

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arXiv1995

Distance Estimation in Cosmology

In this paper we outline the framework of mathematical statistics with which one may study the properties of galaxy distance estimators. We describe, within this framework, how one may formulate the problem of distance estimation as a Bayesian inference problem, and highlight the crucial question of how one incorporates prior information in this approach. We contrast the Bayesian approach with the classical `frequentist' treatment of parameter estimation, and illustrate -- with the simple example of estimating the distance to a single galaxy in a redshift survey -- how one can obtain a significantly different result in the two cases. We also examine some examples of a Bayesian treatment of distance estimation -- involving the definition of Malmquist corrections -- which have been applied in recent literature, and discuss the validity of the assumptions on which such treatments have been based.

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arXiv1994

Bias Minimisation in POTENT

Much current interest in cosmology has revolved around the peculiar velocity field of galaxies. Many methods have been proposed to derive this, probably the most prominent being POTENT (Dekel et al. 1990). However, this is prone to many forms of systematic error or bias. Here, we examine some of the methods currently used to minimise these biases, particularly Malmquist corrections, and highlight some of their pitfalls. As an alternative we propose an iterative scheme based on Monte Carlo simulations of POTENT that shows noticeable improvement over the other methods considered.

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arXiv1994

Optimal Reconstruction of the Velocity and Density Field: Potent and Max-Flow Algorithms

Although Potent purports to use only radial velocities in reconstructing the potential velocity field of galaxies, the derivation of transverse components is implicit in the smoothing procedures adopted. Thus the possibility arises of using nonradial line integrals to derive a smoothed velocity field. For an inhomogeneous galaxy distribution the optimal path for integration need not be radial, and can be obtained using max-flow algorithms. In this paper we describe how one may use Dijkstra's algorithm to obtain this optimal path and velocity field, and present the results of applying the algorithm to a realistic spatial distribution of galaxies. These results show that the method has limited effect due to the large smoothing scales employed in Potent. However, the viability of the technique is demonstrated and, finally, we discuss other possible methods involving averaging over an ensemble of non-radial paths for improving a potential velocity field derived from redshifts.

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