arXiv subjects

P. Alexander

Publications and source records attributed to P. Alexander.

25 records · Page 2Linked to original sources

arXiv2000

The SCUBA Local Universe Galaxy Survey I: First Measurements of the Submillimetre Luminosity and Dust Mass Functions

We have used SCUBA to observe a complete sample of 104 galaxies selected at 60 microns from the IRAS BGS and we present here the 850 micron measurements. Fitting the 60,100 and 850 micron fluxes with a single temperature dust model gives the sample mean temperature T=36 K and beta = 1.3. We do not rule out the possibility of dust which is colder than this, if a 20 K component was present then our dust masses would increase by factor 1.5-3. We present the first measurements of the luminosity and dust mass functions, which were well fitted by Schechter functions (unlike those 60 microns). We have correlated many global galaxy properties with the submillimetre and find that there is a tendancy for less optically luminous galaxies to contain warmer dust and have greater star formation efficiencies (cf. Young 1999). The average gas-to-dust ratio for the sample is 581 +/- 43 (using both atomic and molecular hydrogen), significantly higher than the Galactic value of 160. We believe this discrepancy is due to a cold dust component at T < 20 K. There is a suprisingly tight correlation between dust mass and the mass of molecular hydrogen as estimated from CO measurements, with an intrinsic scatter of ~50%.

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arXiv1999

The heating of the ICM by powerful radio sources

We present a model for the compression and heating of the ICM by powerful radio galaxies and quasars. Based on a self-similar model of the dynamical evolution of FRII-type objects we numerically integrate the hydrodynamic equations governing the flow of the shocked ICM in between the bow shock and the radio lobes of these sources. The resulting gas properties are presented and discussed. The X-ray emission of the shocked gas is calculated and is found to be in agreement with observations. The enhancement of the X-ray emission of cluster gas due to the presence of powerful radio galaxies may play an important role in the direct detection of cluster gas at high redshifts.

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arXiv1998

FRII radio galaxies with z<0.3 -- II. Beaming and unification

In a previous paper we presented measurements of the properties of jets and cores in a large sample of FRII radio galaxies with z<0.3. Here we test, by means of Monte Carlo simulations, the consistency of those data with models in which the prominences of cores and jets are determined by relativistic beaming. We conclude that relativistic beaming is needed to explain the relationships between core and jet prominences, and that speeds between 0.5 and 0.7c on kpc scales provide the best fits to the data.

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arXiv1998

Cosmic ray propagation and star formation history of NGC 1961

We present new radio continuum data at 4 frequencies on the supermassive, peculiar galaxy NGC 1961. These observations allow us to separate the thermal and the nonthermal radio emission and to determine the nonthermal spectral index distribution. This spectral index distribution in the galactic disk is unusual: at the maxima of the radio emission the synchrotron spectrum is very steep, indicating aged cosmic ray electrons. Away from the maxima the spectrum is much flatter. The steep spectrum of the synchrotron emission at the maxima indicates that a strong decline of the star formation rate has taken place at these sites. The extended radio emission is a sign of recent cosmic ray acceleration, probably by recent star formation. We suggest that a violent event in the past, most likely a merger or a collision with an intergalactic gas cloud, has caused the various unusual features of the galaxy.

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arXiv1998

FR II radio galaxies with z<0.3 -- I. Properties of jets, cores and hot spots

In previous papers we have discussed high-resolution observations of a large sample of powerful radio galaxies with z<0.3. Jets are detected in up to 80 per cent of the sample, and radio cores in nearly all the objects; in addition, we are able to resolve the hot spots in most sources. In this paper we present measurements of the radio properties of these components. The prominences of the jets detected do not appear to be a function of radio luminosity, providing the clearest evidence yet that the reported low detection rate of jets in radio galaxies has been an artefact of low-sensitivity observations. We find a positive correlation between the total source length and core prominence in the narrow-line radio galaxies. We have found evidence for a relationship between hot spot size and total source size, but few other significant relationships between hot spot properties and those of the jets or lobes. We compare our measurements to those of Bridle et al (1994), based on observations of a sample of quasars, and argue that the results are consistent with a modification of the unified model in which the broad-line radio galaxies are the low-luminosity counterparts of quasars, although the situation is complicated by contamination with low-excitation radio galaxies which appear to have radio properties different from the high-excitation objects. We discuss the classes of empirical model that can be fitted to the dataset.

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arXiv1997

The jets in 3C 296

We present observations made with the VLA at 1.5 and 8.4 GHz of the nearby FRI radio galaxy 3C296. The most recent models of FRI radio galaxies suggest that substantial deceleration must take place in their jets, with strongly relativistic velocities on parsec scales giving place to at most mildly relativistic velocities on scales of tens of kiloparsecs. The region over which this deceleration takes place is therefore of considerable interest. By considering the side-to-side asymmetries of the jets of 3C296, we constrain the region of strong deceleration in the source. Our observations show evidence that the jets have slow edges surrounding faster central spines. We discuss the implications of our observations for models of the magnetic field structure in these objects.

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arXiv1996

Constraints on Cosmic Ray propagation from Radio Continuum data of NGC 2146

We present high-sensitivity multi-frequency radio continuum observations of the starburst galaxy NGC~2146. We have fitted these data with a three-dimensional diffusion model. The model can describe the radio emission from the inner disk of NGC~2146 well, indicating that diffusion is the dominant mode of propagation in this region. Our results are indicating that NGC~2146 has recently undergone a starburst, the star forming activity being located in a central bar. The spatial variation of the radio emission and of the spectral index yield tight constraints on the diffusion coefficient $D_0$ and the energy dependence of the diffusion. Away from the central bar of the galaxy the radio emission becomes filamentary and the diffusion model was found to be a poor fit to the data in these regions; we attribute this to different transport processes being important in the halo of the galaxy.

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