Total Far-infrared Research Articles

Extragalactic Magnetism with SOFIA (SALSA Legacy Program). VII. A Tomographic View of Far-infrared and Radio Polarimetric Observations through MHD Simulations of Galaxies

The structure of magnetic fields in galaxies remains poorly constrained, despite the importance of magnetism in the evolution of galaxies. Radio synchrotron and far-infrared (FIR) polarization and polarimetric observations are the best methods to measure galactic scale properties of magnetic fields in galaxies beyond the Milky Way. We use synthetic polarimetric observations of a simulated galaxy to identify and quantify the regions, scales, and interstellar medium (ISM) phases probed at FIR and radio wavelengths. Our studied suite of magnetohydrodynamical cosmological zoom-in simulations features high-resolutions (10 pc full-cell size) and multiple magnetization models. Our synthetic observations have a striking resemblance to those of observed galaxies. We find that the total and polarized radio emission extends to approximately double the altitude above the galactic disk (half-intensity disk thickness of h I radio ∼ h PI radio = 0.23 ± 0.03 kpc) relative to the total FIR and polarized emission that are concentrated in the disk midplane (h I FIR ∼ h PI FIR = 0.11 ± 0.01 kpc). Radio emission traces magnetic fields at scales of ≳300 pc, whereas FIR emission probes magnetic fields at the smallest scales of our simulations. These scales are comparable to our spatial resolution and well below the spatial resolution (<300 pc) of existing FIR polarimetric measurements. Finally, we confirm that synchrotron emission traces a combination of the warm neutral and cold neutral gas phases, whereas FIR emission follows the densest gas in the cold neutral phase in the simulation. These results are independent of the ISM magnetic field strength. The complementarity we measure between radio and FIR wavelengths motivates future multiwavelength polarimetric observations to advance our knowledge of extragalactic magnetism.

Detections of [C ii] 158 μm and [O iii] 88 μm in a Local Lyman Continuum Emitter, Mrk 54, and Its Implications to High-redshift ALMA Studies* * Based on observations done with FIFI-LS on SOFIA.

We present integral field, far-infrared (FIR) spectroscopy of Mrk 54, a local Lyman continuum emitter, obtained with FIFI-LS on the Stratospheric Observatory for Infrared Astronomy. This is only the second time, after Haro 11, that [C ii] 158 μm and [O iii] 88 μm spectroscopy of the known LCEs have been obtained. We find that Mrk 54 has a strong [C ii] emission that accounts for ∼1% of the total FIR luminosity, whereas it has only moderate [O iii] emission, resulting in the low [O iii]/[C ii] luminosity ratio of 0.22 ± 0.06. In order to investigate whether [O iii]/[C ii] is a useful tracer of f esc (LyC escape fraction), we examine the correlations of [O iii]/[C ii] and (i) the optical line ratio of O32 ≡ [O iii] 5007 Å/[O ii] 3727 Å, (ii) specific star formation rate, (iii) [O iii] 88 μm/[O i] 63 μm ratio, (iv) gas-phase metallicity, and (v) dust temperature based on a combined sample of Mrk 54 and the literature data from the Herschel Dwarf Galaxy Survey and the LITTLE THINGS Survey. We find that galaxies with high [O iii]/[C ii] luminosity ratios could be the result of high ionization (traced by O32), bursty star formation, high ionized-to-neutral gas volume filling factors (traced by [O iii] 88 μm/[O i] 63 μm), and low gas-phase metallicities, which is in agreement with theoretical predictions. We present an empirical relation between the [O iii]/[C ii] ratio and f esc based on the combination of the [O iii]/[C ii] and O32 correlation, and the known relation between O32 and f esc. The relation implies that high-redshift galaxies with high [O iii]/[C ii] ratios revealed by the Atacama Large Millimeter/submillimeter Array may have f esc ≳ 0.1, significantly contributing to the cosmic reionization.

A SOFIA Survey of [C ii] in the Galaxy M51. I. [C ii] as a Tracer of Star Formation

Abstract We present a [C ii] 158 μm map of the entire M51 (including M51b) grand design spiral galaxy observed with the Far Infrared Field-Imaging Line Spectrometer (FIFI-LS) instrument on board the Stratospheric Observatory For Infrared Astronomy (SOFIA). We compare the [C ii] emission with the total far-infrared (TIR) intensity and star formation rate (SFR) surface density maps (derived using Hα and 24 μm emission) to study the relationship between [C ii] and the star formation activity in a variety of environments within M51 on scales of 16″ corresponding to ∼660 pc. We find that [C ii] and the SFR surface density are well correlated in the central, spiral arm, and inter-arm regions. The correlation is in good agreement with that found for a larger sample of nearby galaxies at kpc scales. We find that the SFR, and [C ii] and TIR luminosities in M51, are dominated by the extended emission in M51's disk. The companion galaxy M51b, however, shows a deficit of [C ii] emission compared with the TIR emission and SFR surface density, with [C ii] emission detected only in the SW part of this galaxy. The [C ii] deficit is associated with an enhanced dust temperature in this galaxy. We interpret the faint [C ii] emission in M51b to be a result of suppressed star formation in this galaxy, while the bright mid- and far-infrared emission, which drive the TIR and SFR values, are powered by other mechanisms. A similar but less-pronounced effect is seen at the location of the black hole in M51's center. The observed [C ii] deficit in M51b suggests that this galaxy is a valuable laboratory to study the origin of the apparent [C ii] deficit observed in ultra-luminous galaxies.

Are High-redshift Galaxies Hot? Temperature of z &gt; 5 Galaxies and Implications for Their Dust Properties

Abstract Recent studies have found a significant evolution and scatter in the relationship between the UV spectral slope (β UV) and the infrared excess (IRX; L IR/L UV) at z &gt; 4, suggesting different dust properties of these galaxies. The total far-infrared (FIR) luminosity is key for this analysis, but it is poorly constrained in normal (main-sequence) star-forming z &gt; 5 galaxies, where often only one single FIR point is available. To better inform estimates of the FIR luminosity, we construct a sample of local galaxies and three low-redshift analogues of z &gt; 5 systems. The trends in this sample suggest that normal high-redshift galaxies have a warmer infrared (IR) spectral energy distribution (SED) compared to average z &lt; 4 galaxies that are used as priors in these studies. The blueshifted peak and mid-IR excess emission could be explained by a combination of a larger fraction of metal-poor interstellar medium being optically thin to ultraviolet (UV) light and a stronger UV radiation field due to high star formation densities. Assuming a maximally warm IR SED suggests a 0.6 dex increase in total FIR luminosities, which removes some tension between the dust attenuation models and observations of the IRX−β relation at z &gt; 5. Despite this, some galaxies still fall below the minimum IRX−β relation derived with standard dust cloud models. We propose that radiation pressure in these highly star-forming galaxies causes a spatial offset between dust clouds and young star-forming regions within the lifetime of O/B stars. These offsets change the radiation balance and create viewing-angle effects that can change UV colors at fixed IRX. We provide a modified model that can explain the location of these galaxies on the IRX−β diagram.

HERUS: A CO ATLAS FROM SPIRE SPECTROSCOPY OF LOCAL ULIRGs

ABSTRACT We present the Herschel SPIRE Fourier Transform Spectroscopy (FTS) atlas for a complete flux-limited sample of local ultraluminous infrared galaxies (ULIRGs) as part of the HERschel Ultra Luminous InfraRed Galaxy Survey (HERUS). The data reduction is described in detail and was optimized for faint FTS sources ,with particular care being taken for the subtraction of the background, which dominates the continuum shape of the spectra. To improve the final spectra, special treatment in the data reduction has been given to any observation suffering from artifacts in the data caused by anomalous instrumental effects. Complete spectra are shown covering 200–671 μm, with photometry in the SPIRE bands at 250, 350, and 500 μm. The spectra include near complete CO ladders for over half of our sample, as well as fine structure lines from [C i] 370 μm, [C i] 609 μm, and [N ii] 205 μm. We also detect H2O lines in several objects. We construct CO spectral line energy distributions (SLEDs) for the sample, and compare their slopes with the far-infrared (FIR) colors and luminosities. We show that the CO SLEDs of ULIRGs can be broadly grouped into three classes based on their excitation. We find that the mid-J (5 &lt; J &lt; 8) lines are better correlated with the total FIR luminosity, suggesting that the warm gas component is closely linked to recent star formation. The higher J transitions do not linearly correlate with the FIR luminosity, consistent with them originating in hotter, denser gas that is unconnected to the current star formation. We conclude that in most cases more than one temperature component is required to model the CO SLEDs.

The dust properties of bubble H II regions as seen byHerschel

Because of their relatively simple morphology, "bubble" HII regions have been instrumental to our understanding of star formation triggered by HII regions. With the far-infrared (FIR) spectral coverage of the Herschel satellite, we can access the wavelengths where these regions emit the majority of their energy through their dust emission. At Herschel wavelengths 70 micron to 500 micron, the emission associated with HII regions is dominated by the cool dust in their photodissociation regions (PDRs). We find average dust temperatures of 26K along the PDRs, with little variation between the HII regions in the sample, while local filaments and infrared dark clouds average 19K and 15K respectively. Higher temperatures lead to higher values of the Jeans mass, which may affect future star formation. The mass of the material in the PDR, collected through the expansion of the HII region, is between ~300 and ~10,000 Solar masses for the HII regions studied here. These masses are in rough agreement with the expected masses swept up during the expansion of the \hii\ regions. Approximately 20% of the total FIR emission is from the direction of the bubble central regions. This suggests that we are detecting emission from the "near-side" and "far-side" PDRs along the line of sight and that bubbles are three-dimensional structures. We find only weak support for a relationship between dust temperature and beta, of a form similar to that caused by noise and calibration uncertainties alone.

Herschel★ observations of the Centaurus cluster - the dynamics of cold gas in a cool core

Brightest cluster galaxies (BCGs) in the cores of galaxy clusters have distinctly different properties from other low redshift massive ellipticals. The majority of the BCGs in cool-core clusters show signs of active star formation. We present observations of NGC 4696, the BCG of the Centaurus galaxy cluster, at far-infrared (FIR) wavelengths with the Herschel space telescope. Using the PACS spectrometer, we detect the two strongest coolants of the interstellar medium, CII at 157.74 micron and OI at 63.18 micron, and in addition NII at 121.90 micron. The CII emission is extended over a region of 7 kpc with a similar spatial morphology and kinematics to the optical H-alpha emission. This has the profound implication that the optical hydrogen recombination line, H-alpha, the optical forbidden lines, NII 6583 Angstrom, the soft X-ray filaments and the far-infrared CII line all have the same energy source. We also detect dust emission using the PACS and SPIRE photometers at all six wavebands. We perform a detailed spectral energy distribution fitting using a two-component modified black-body function and find a cold 19 K dust component with mass 1.6x10^6 solar mass and a warm 46 K dust component with mass 4.0x10^3 solar mass. The total FIR luminosity between 8 micron and 1000 micron is 7.5x10^8 solar luminosity, which using Kennicutt relation yields a low star formation rate of 0.13 solar mass per yr. This value is consistent with values derived from other tracers, such as ultraviolet emission. Combining the spectroscopic and photometric results together with optical H-alpha, we model emitting clouds consisting of photodissociation regions (PDRs) adjacent to ionized regions. We show that in addition to old and young stellar populations, there is another source of energy, such as cosmic rays, shocks or reconnection diffusion, required to excite the H-alpha and CII filaments.

Importance of far-infrared mapping in a spiral galaxy: AKARI observation of M81

The importance of the far-infrared (FIR) mapping is demonstrated for a face-on spiral galaxy, M81, by analyzing its imaging data at 65, 90, and 140 {\mu}m taken by AKARI. Basic products are the dust temperature map, the dust optical depth map, and the colour-colour diagram. The main features are as follows. (i) The dust temperature derived from the total fluxes at 90 {\mu}m and 140 {\mu}m reflects the relatively low temperatures seen in the interarm and spiral arms excluding the warm spots, rather than the high temperatures in warm spots and the centre. This indicates that the total FIR luminosity is dominated by the dust heated by the general interstellar radiation field. (ii) The galaxy is more extended at 140 {\mu}m than at the other shorter wavelengths, which reflects the radial dust temperature gradient. (iii) The dust optical depth derived from the FIR mapping is broadly consistent with that estimated from the FIR-to-ultraviolet luminosity ratio. (iv) The FIR colour-colour diagramis useful to identify a 'contamination' of warm dust. The existence of small-scale warm star-forming regions is supported in the bright spots along the spiral arms. This contamination also leads to an underestimate of dust optical depth (or dust column density).

Far-Infrared Properties of Blue Compact Dwarf Galaxies Observed with AKARI/Far-Infrared Surveyor (FIS)

Abstract We report on the basic far-infrared (FIR) properties of eight blue compact dwarf galaxies (BCDs) observed by AKARI. We have measured the fluxes at the four FIS bands (wavelengths of 65$\mu$m, 90$\mu$m, 140$\mu$m, and 160$\mu$m). Based on these fluxes, we have estimated basic quantities of dust: dust temperature, dust mass, and total FIR luminosity. We find that the typical dust temperature of the BCD sample is systematically higher than that of normal spiral galaxies, although there is a large variety. The interstellar radiation field estimated from the dust temperature ranges up to 100-times the galactic value. This confirms the concentrated star-forming activity in BCDs. The star-formation rate can be evaluated from the FIR luminosity as 0.01-0.5$M_{\odot}$yr$^{-1}$. Combining this quantity with the gas mass taken from the literature, we have estimated the gas consumption timescales (gas mass divided by the star-formation rate), which have proved to span a wide range from 1Gyr to 100Gyr. A natural interpretation of this large variety can be provided by intermittent star-formation activity. We finally show the relation between dust-to-gas ratio and metallicity (we utilized our estimate of dust mass, and took other necessary quantities from the literature). There is a positive correlation between dust-to-gas ratio and metallicity, as expected from chemical evolution models.

The Properties of the Mid‐ to Far‐Infrared Emission in the Large Magellanic Cloud

We investigate the properties of mid- to far-infrared emission in the Large Magellanic Cloud (LMC) based on the COBE DIRBE and IRAS Sky Survey Atlas data sets. We focus on the properties of far-infrared (FIR) thermal emission carried by submicron dust and mid-infrared (MIR) excess emission by very small grains in the LMC in comparison to those in the Milky Way. We carefully estimate the dust temperature by decomposing the various structures including young star clusters, supergiant shells, and CO molecular clouds and examine the MIR to FIR spectral energy distribution (SED) associated with each structure in the LMC. The amount of the MIR excess emission in 12 and 25 ?m relative to the total FIR emission (FFIR) in the LMC is generally smaller than that in the Milky Way, which confirms the scarcity of very small grains suggested by past studies on the extinction curve. The characteristics of 25 and 60 ?m excess emission proportional to the square of the incident radiation field strength appear around young star clusters in the LMC. This sequence can be reproduced by the superposition of dust emission under different radiation field strengths ranging from 1 to 105 times of that in the solar vicinity. Finally, the MIR to FIR SEDs in CO molecular clouds in the LMC are discussed. Through these attempts, we succeed in completely explaining the characteristics of mid- to far-infrared SEDs given by the infrared broadband photometric data sets in terms of the superposition of the standard stochastic heating dust model.

The Interaction of the Galactic Center Arches Cluster and the Thermal Filaments

We present Infrared Space Observatory (ISO) measurements of the far-infrared (FIR) fine-structure lines [S IV] (10.5 μm), [S III] (33.5 μm), [O III] (51.8 and 88.4 μm), [N III] (57.3 μm), [N II] (121.9 μm), [O I] (63.2 and 145.5 μm), and [C II] (157.7 μm) at five positions on the thermal ``Arched'' Filaments in the Galactic center region in order to investigate the influence of the massive, young Arches cluster on the interstellar medium. Serendipitous measurements of OH (119.3 μm) are also presented. Radio recombination line and ISO spectroscopic measurements provide strong evidence that the Arches cluster is responsible for the excitation of the Arched Filaments. The observations reported here were selected to provide measurements close to the Arches cluster and at uniformly increasing projected distances over the range 0-9 pc. Within the measurement uncertainties, the diagnostic line ratios most sensitive to excitation and least sensitive to density fluctuations, [O III]/[S III] and [S IV]/[S III], appear to decrease with projected distance from the Arches cluster. In agreement with other studies, this provides additional evidence that the cluster is the primary ionization source for the Arched Filaments. The total FIR luminosity also generally falls with projected distance from the Arches cluster. The best predictions from photoionization models for the rapid decrease in the measured line flux ratios with projected distance from the Arches cluster, however, are only accurate to a factor of 1.5, including allowance for additional offset along the line of sight. The measurements of lines formed predominantly in photodissociation regions are not well matched by current models. High-resolution spectra indicate gas moving at -70 km s-1 only at the location of the Arches cluster, significantly different from the predominant -30 km s-1 gas seen in the underlying molecular cloud.

Dust masses and star formation in bright IRAS galaxies

We address the problem of modeling the far-infrared (FIR) spectrum and deriving the star-formation rate (SFR) and the dust mass of spiral galaxies. We use the realistic physical model of Popescu et al. ([CITE]) to describe the overall ultra-violet (UV), optical and FIR spectral energy distribution (SED) of a spiral galaxy. The model takes into account the 3-dimensional old and young stellar distributions in the bulge and the disk of a galaxy, together with the dust geometry. The geometrical characteristics of the galaxy and the intrinsic optical and near-infrared spectra are determined by the galaxy's observed K-band photometry. The UV part of the spectrum is assumed to be proportional to the SFR through the use of population synthesis models. By solving the radiative transfer equation, we are able to determine the absorbed energy, the dust temperature and the resulting FIR spectrum. The model has only three free parameters: SFR, dust mass, and the fraction of the UV radiation which is absorbed locally by dense dust in the HII regions. Using this model, we are able to fit well the FIR spectra of 62 bright IRAS galaxies from the “SCUBA Local Universe Galaxy Survey" of Dunne et al. ([CITE]). As a result, we are able to determine, among others, their SFR and dust mass. We find that, on average, the SFR (in absolute units), the star-formation efficiency, the SFR surface density and the ratio of FIR luminosity over the total intrinsic luminosity, are larger than the respective values of typical spiral galaxies of the same morphological type. We also find that the mean gas-to-dust mass ratio is close to the Galactic value, while the average central face-on optical depth of these galaxies in the V band is 2.3. Finally, we find a strong correlation between SFR or dust mass and observed FIR quantities like total FIR luminosity or FIR luminosity at 100 and 850 . These correlations yield well-defined relations, which can be used to determine a spiral galaxy's SFR and dust-mass content from FIR observations.

The far-infrared/radio correlation in the ISO era

We present the correlation between the far-infrared (FIR) and radio emissions from a composite sample of 72 nearby normal galaxies observed with the ISOPHOT instrument on board the Infrared Space Observatory. The galaxies in the sample have measurements at three FIR wavelengths (60, 100 and 170 micron), which allowed a direct determination of the warm and cold FIR emission components. This is the first time that the correlation has been established for the total FIR luminosity, of which most is carried by the cold dust component predominantly emitting longwards of the spectral coverage of IRAS. The slope of this correlation is slightly non-linear (1.10+/-0.03). Separate correlations between the warm and cold FIR emission components and the radio emission have also been derived. The slope of the warm FIR/radio correlation was found to be linear (1.03+/-0.03). For the cold FIR/radio correlation we found a slightly non-linear (1.13+/-0.04) slope. We qualitatively interpret the correlations in terms of star formation rate and find that both the FIR and radio emissions may be consistent with a non-linear dependence on star formation rate for galaxies not undergoing starburst activity.

A New Empirical Method for Estimating the Far-Infrared Flux of Galaxies

Abstract We propose a new empirical method to estimate the total far-infrared flux of galaxies from the spectral energy distribution (SED) at wavelengths of $ \lambda \leq 100 \, {\mu \mathrm {m}}$. It is difficult to derive the total far-infrared luminosity from only the IRAS data, though it is one of the most important properties of galaxies. Observations by Infrared Telescope in Space (IRTS) indicate that the SED of the diffuse emission from the Galactic plane in this wavelength region can be derived from the $ 60 \, {\mu \mathrm {m}}$ to $ 100 \, {\mu \mathrm {m}}$ color. This empirical SED relation was improved in order to obtain a better fit to the Galactic plane data for $ I_\nu(60 \, {\mu \mathrm {m}}) / I_\nu(100 \, {\mu \mathrm {m}}) \gt 0.6$, and applied to 96 IRAS galaxies for which ISOPHOT and KAO data are available at $ \lambda \gt 100 \, {\mu \mathrm {m}}$. As a result, the empirical relation well describes the far-infrared (FIR) SED for a majority of galaxies. Additionally, the total FIR flux for $ \lambda \geq 40 \, {\mu \mathrm {m}}$ was derived from the flux densities at 60 and $ 100 \, {\mu \mathrm {m}}$ by using this model. For the 96 IRAS galaxies, the uncertainty in the total far-infrared flux of the present method is 26%. The present method is more accurate than the previous one widely used to derive the total infrared flux from the IRAS 60 and $ 100 \, {\mu \mathrm {m}}$ data.

Distribution of Cold Dust in Orion A and B

Large scale far-infrared (FIR) observations of the Orion complex at 205 and 138 micron are presented with an aim of studying the distribution of cold (< 25 K) dust. The maps in these FIR bands extend over approximately 3600 sq. arcmin and cover regions around OMC-1, 2, 3 in Orion A and NGC 2023 and NGC 2024 in Orion B. Some limited regions have also been mapped at 57 micron. A total of 15 sources in Orion A and 14 in Orion B (south) have been identified from our FIR maps. Dust temperature distribution in both Orion A and Orion B (south) have been determined reliably using the maps at 205 & 138 micron obtained from simultaneous observations using almost identical beams (1.6 dia). These temperatures have been used to generate map of the optical depth at 150 micron, for the Orion B region. The coldest source detected is in OMC-3 and has a temperature of about 15 K. The diffuse FIR emission in the different sub-regions is found to vary between 25% to 50% of the total FIR emission from that sub-region.

The Dust Content and Opacity of Actively Star‐forming Galaxies

We present far-infrared (FIR) photometry at 150 and 205 micron(s) of eight low-redshift starburst galaxies obtained with the Infrared Space Observatory (ISO) ISOPHOT. Five of the eight galaxies are detected in both wave bands, and these data are used, in conjunction with IRAS archival photometry, to model the dust emission at lambda approximately greater than 40 microns. The FIR spectral energy distributions (SEDs) are best fitted by a combination of two modified Planck functions, with T approx. 40 - 55 K (warm dust) and T approx. 20-23 K (cool dust) and with a dust emissivity index epsilon = 2. The cool dust can be a major contributor to the FIR emission of starburst galaxies, representing up to 60% of the total flux. This component is heated not only by the general interstellar radiation field, but also by the starburst itself. The cool dust mass is up to approx. 150 times larger than the warm dust mass, bringing the gas-to-dust ratios of the starbursts in our sample close to Milky Way values, once resealed for the appropriate metallicity. The ratio between the total dust FIR emission in the range 1-1000 microns and the IRAS FIR emission in the range 40 - 120 microns is approx. 1.75, with small variations from galaxy to galaxy. This ratio is about 40% larger than previously inferred from data at millimeter wavelengths. Although the galaxies in our sample are generally classified as bright, for four of them the UV energy emerging shortward of 0.2 microns is less than 15% of the FIR energy. On average, about 30% of the bolometric flux is coming out in the UV-to-near-IR wavelength range; the rest is emitted in the FIR. Energy balance calculations show that the FIR emission predicted by the dust reddening of the UV-to-near-IR stellar emission is within a factor of approx. 2 of the observed value in individual galaxies and within 20% when averaged over a large sample. If our sample of local starbursts is representative of high-redshift (z approx. greater than 1), UV - bright star-forming galaxies, these galaxies' FIR emission will be generally undetected in submillimeter surveys, unless: (1) their bolometric luminosity is comparable to or larger than that of ultraluminous FIR galaxies and (2) their FIR SED contains a cool dust component.

The Nature of the Far-Infrared Emission from Seyfert Galaxies

Optical R-band imaging data and far-infrared (FIR) Infrared Space Observatory data from Seyfert galaxies are combined to show that the compact optical emission from the nuclear and circumnuclear region correlates reasonably well with the warm FIR emission that is observed in these objects, while the extended optical emission, which essentially originates in the host galaxy disk, correlates with the cold FIR emission that peaks at around 100 μm. We further show that while the compact optical emission in all the objects studied always accounts for less than half of the total R-band emission, the warm FIR emission, which is attributed to the nuclear and circumnuclear region, dominates the total FIR luminosity in some objects.

Correlation of the radio continuum intensity with the FIR luminosity and its implication for dust heating sources and physical processes in galaxies

The tight correlation between the far-infrared (FIR) luminosities and the radio continuum intensities of late-type galaxies can be shown to be not only a mass-scaling (or “richness”) effect. It rather depends on intrinsic properties like star-formation rate per unit mass, connecting différent physical processes in a galaxy. While the FIR emission is thermal radiation of dust grains heated by stellar UV and optical light, the radio continuum consists of thermal Bremsstrahlung, and nonthermal synchrotron radiation from relativistic electrons. The dominance of the so-called cool component of the total FIR radiation can be understood by the absorption of non-ionizing UV emission from intermediate massive stars (5–20 M.) which also contribute dominantly to the galaxian supernova rates. The relativistic electrons are therefore generated as a consequence of supernova explosions whose dynamical influence on galaxian gas motions (“turbulence”) in turn affects the generation of the magnetic fields in which the synchrotron emission occurs. Most galaxian disks are optically thick for their own UV emission, ionizing and non-ionizing. Similarly, energetic electrons lose most of their energy by Inverse Compton and synchrotron losses in galaxian disks and their halos. Therefore the independence of morphology, size, color, etc. of the FIR/radio correlation is basically explained by a “calorimeter theory”. However a residual “radio-quiet” FIR emission due to dust-absorbed optical emission from old, low-mass stars appears necessary to explain the non-linearity of the correlation. The abnormal FIR-to-radio ratios of clustered galaxies are interpreted by an active interaction between these galaxies and presumably existing dense intracluster fragments.

Far-infrared luminosity functions of normal galaxies

A volume-limited sample (&lt;1400 km s^-1^ corrected for local motion) is constructed from the Zwicky catalog and IRAS data base to examine the far-infrared (FIR) luminosity functions of normal galaxies, and to investigate possible relationships between FIR emission and galaxy morphology. Quantitative and unbiased treatment is provided by "survival analysis" statistical methods. We find that the FIR distributions of normal galaxies are better fit by lognormal than Schechter functions. The total FIR emissivity (8 to 115 microns) of normal galaxies is approximately equal to half their emission in the B plus V optical bands. Normal galaxy FIR emission is uncorrelated with the basic S0-Sm Hubble sequence of spiral galaxy morphology, but appears to be affected by de Vaucouleurs' revised morphological classifications based on inner rings and S-shaped arms. Spirals with bars and inner rings are systematically fainter than unbarred spirals. We suggest that bars and rings reduce the amount or spatially confine the dust in spiral disks, resulting in lower efficiency conversion of optical and UV photons into the infrared.

Far infrared emission from three new planetary nebulae

As dust emission in the far infrared (FIR) is a characteristic property of planetary nebulae we searched the Infrared Astronomical Satellite (IRAS) point-source catalogue for confirmatory evidence on the two new possible planetary nebulae S 68 and 248 - 5 identified by Fesen, Gull & Heckathorn (1983) and the high-excitation planetary nebula 76 + 36 detected by Sanduleak (1983). We identify the nebulae 248 - 5 and 76 + 36 with IRAS sources 07404 - 3240 and 17125 + 4919, respectively and have determined their dust temperature, total FIR emission and optical depth. We also set a lower limit ranging in value from 1.2 × 10-6 to 3.7 × 10-5 forMdust/Mbd of the nebula 248 - 5 depending on whether its grain material is silicate or graphite. S 68 could not be identified with an IRAS source.