Spatial distribution of far infrared emission in spiral galaxies I. Relation with radio continuum emission

We use high resolution IRAS and 20 cm radio continuum (RC) images of a sample of 22 spiral galaxies to study the correlation between the far infra-red (FIR) and RC emissions within the galactic disks. A combination of exponential and gaussian profiles rather than a single exponential profile is found to be a better representation of the observed intensity profiles in the two bands. The gaussian component, which we show is not due to the effects of limited beam-resolution, contains more than 60% of the total flux in majority of the galaxies. The dominance of the gaussian component suggests that the nuclear star forming regions and the bulge stars are more important contributors to the emission in the two bands, rather than the outer exponential stellar disks. The RC profile is flatter compared to the FIR profile, resulting in a decrease of their ratio, Q60, away from the center. However, the Q60 increases in the extreme outer parts, where the dispersion in the FIR and RC correlation is also higher than in the central regions. The global Q60 and its dispersion match those in the inner parts of the galaxies. These results imply that the observed tight correlation in the global quantities reflects processes in the inner regions only where OB stars and the associated Type II supernovae control the FIR and RC emission. In the outer parts heating of very small dust grains by the old disk stars provides a secondary component in the FIR emission, without associated RC emission. The edge-on galaxy NGC3079 shows extended FIR and RC emissions along its minor axis, probably associated with the nuclear starburst activity.Comment: Uses aas2pp4.sty and epsfig.sty, 21 pages. Figure 1 is spread over 22 pages and is available at http://www.inaoep.mx/~ydm/preprint.html To appear in Astronomical Journal, September 199

Spatial distribution of Far infrared emission in spiral galaxies II. Heating sources and gas-to-dust ratio

We study the radial distribution of the temperature of the warm dust and gas-to-dust mass ratios in a sample of 22 spiral galaxies. The heating capabilities of the diffuse interstellar radiation field (ISRF), based on Desert et al. model, are investigated in 13 of the sample galaxies. In general, the temperature of the warm dust decreases away from the center, reaches a minimum value at the mid-disk and increases again in the outer parts of galaxies. Heating a mixture of small and big grains by the ISRF is able to explain the observed behavior qualitatively. However, ultraviolet photons from recent star formation events are necessary for a detailed matching of the warm dust temperature profiles. Very small grains contribute typically more than 50% to the observed flux at 60 micron beyond half the disk radius in galaxies. Optical depth profiles, derived from the observed 60 micron and warm dust temperature profiles, peak at or close to the galactic center. In 13 of the galaxies, where dust temperature profiles are modeled, we obtain gas-to-dust mass ratio profiles, after correction for the contaminating effects of very small grains. The gas-to-dust mass ratio decreases by a factor of 8 from the center to the optical isophotal radius, where the value approaches the local galactic value. We demonstrate that the observed steep gradient is a result of the over-estimation of the molecular mass, and can be flattened out to within a factor of 2, if the molecular hydrogen mass (H2) is recomputed assuming a metallicity dependent conversion factor from CO intensity to H2 column density. The flattened radial profiles indicate a global gas-to-dust ratio of around 300, which is within a factor of two of the local galactic value.Comment: Uses aas2pp4.sty and epsfig.sty, 27 pages. To appear in Astronomical Journal, September 199

Radio continuum and far-infrared emission of spiral galaxies: Implications of correlations

Researchers present a study extending the correlation seen between radio continuum and far-infrared emissions from spiral galaxies to a lower frequency of 408 MHz and also as a function of radio spectral index. The tight correlation seen between the two luminosities is then used to constrain several parameters governing the emissions such as the changes in star formation rate and mass function, frequency of supernovae that are parents of the interstellar electrons and factors governing synchrotron radio emission

Guided Modes of Elliptical Metamaterial Waveguides

The propagation of guided electromagnetic waves in open elliptical metamaterial waveguide structures is investigated. The waveguide contains a negative-index media core, where the permittivity, $\epsilon$ and permeability $\mu$ are negative over a given bandwidth. The allowed mode spectrum for these structures is numerically calculated by solving a dispersion relation that is expressed in terms of Mathieu functions. By probing certain regions of parameter space, we find the possibility exists to have extremely localized waves that transmit along the surface of the waveguide

A search for a consistent model for the electromagnetic spectrum of the crab nebula

An attempt is made to search for a consistent model to explain the electromagnetic spectrum of the Crab nebula (Tau A). It is assumed that there is a continuous injection of electrons at the centre of the nebula with an energy spectrum E-1.54 as evidenced by radio data. This spectrum must steepen to a slope larger than 2 at some energy Ei in order to ensure that the energy input into electrons remains finite. The spectrum must also steepen beyond an energy Ec depending on the magnetic field because of synchrotron energy losses. Two types of models are considered: Class I, in which the whole nebula is characterised by a uniform magnetic field, and Class II, in which besides the general field H0, small filamentary regions of strong field Hs are postulated. In models of Class I, the best fit to the observed data is obtained when E t >E c and H0⋍5×10-4 gauss. However, this predicts a decrease in X-ray source size beyond ~40 KeV. There are two possibilities of Class II model depending on the residence time of electrons in strong field regions being small or large. The former case explains the flattening in the optical spectrum

Far-infrared observations of Circinus and NGC 4945 galaxies

Circinus and NGC 4945 are two galaxies luminous in the infrared and are characterized by compact non thermal radio nuclei, deep silicate absorption features and unusually strong water vapor maser luminosities. Moorwood and Glass (1984) have observed these galaxies extensively in the 1 to 20 micron range. In the far-infrared, observations up to 100 microns are available from the Infrared Astronomy Satellite (IRAS). In order to study the cool dust component of these galaxies, researchers observed them at 150 microns using the Tata Institute of Fundamental Research (TIFR) 100 cm balloon-borne telescope. Here, they report observations along with deconvolved maps at 50 and 100 microns obtained from the Chopped Photometric Channel (CPC) on board IRAS

Dust in a few southern H II regions

The property of dust in four southern H II region/molecular cloud complexes (RCW 108, RCW 57, RCW 122, and G351.6-1.3) was discussed. These regions were observed at an effective wavelength of 150 micron using TIFR balloon borne 1 m telescope and deconvolved maps with a resolution of 1 min were obtained. The data were combined with other available data to derive the properties of the infrared emitting dust in these regions