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The dominant source of electromagnetic energy in the Universe today (over ultraviolet, optical and near-infrared wavelengths) is starlight. However, quantifying the amount of starlight produced has proven difficult due to interstellar dust grains which attenuate some unknown fraction of the light. Combining a recently calibrated galactic dust model with observations of 10,000 nearby galaxies we find that (integrated over all galaxy types and orientations) only (11 +/- 2)% of the 0.1 micron photons escape their host galaxies; this value rises linearly (with log(lambda)) to (87 +/- 3)% at 2.1 micron. We deduce that the energy output from stars in the nearby Universe is (1.6+/-0.2) x 10^{35} W Mpc^{-3} of which (0.9+/-0.1) x 10^{35} W Mpc^{-3} escapes directly into the inter-galactic medium. Some further ramifications of dust attenuation are discussed, and equations that correct individual galaxy flux measurements for its effect are provided., Comment: Accepted by ApJ Letters

Modelling the UV/optical - infrared/submm SEDs of spiral galaxies observed with Herschel will be an essential tool to quantitatively interpret these observations in terms of the present and past star-formation activity of these systems. In this lecture we describe the SED modelling technique we have developed, its applications and tests of its predictions. We show that both the panchromatic SED modelling of individual galaxies and the B-band attenuation-inclination relation of large statistical samples suggest that spiral galaxies in the nearby Universe behave as optically thick systems in their global properties and large-scale distribution of light (central face-on B-band opacity of approx. 4). However disk galaxies are very inhomogeneous systems, having both optically thick components (e.g. spiral arms), and optically thin components (e.g. the interarm regions), the latter making galaxies transparent to background galaxies., Comment: invited lecture; to appear in Proceedings of the lectures given at the Les Houche Winter School "Astronomy in the submillimeter and far infrared domains with the Herschel Space Observatory"

Based on our sample of 10095 galaxies with bulge-disc decompositions we derive the empirical B-band internal attenuation--inclination relation for galaxy discs and their associated central bulges. Our results agree well with the independently derived dust models of Tuffs et al., leading to a direct constraint on the mean opacity of spiral discs of Tau_B^f = 3.8 +/- 0.7 (central face-on B-band opacity). Depending on inclination, the B-band attenuation correction varies from 0.2 -- 1.1 mag for discs and from 0.8 -- 2.6 mag for bulges. We find that, overall, 37 per cent of all B-band photons produced in discs in the nearby universe are absorbed by dust, a figure that rises to 71 per cent for bulge photons. The severity of internal dust extinction is such that one must incorporate internal dust corrections in all optical studies of large galaxy samples. This is particularly pertinent for optical HST comparative evolutionary studies as the dust properties will also be evolving. We use the new results to revise our recent estimates of the spheroid and disc luminosity functions. From our best fitting dust models we derive a redshift zero cosmic dust density of rho_{dust} ~ (5.3 +/- 1.7) x 10^5, h M_{odot} Mpc^-3. This implies that (0.0083 +/- 0.0027), h per cent of the baryons in the Universe are in the form of dust and (11.9 +/- 1.7), h per cent (Salpeter-`lite' IMF) are in the form of stars (~58 per cent reside in galaxy discs, ~10 per cent in red elliptical galaxies, ~29 per cent in classical galaxy bulges and the remainder in low luminosity blue spheroid systems/components). [Abridged], Comment: Accepted for publication in MNRAS (18 pages, 16 figures). This paper, the MGC data catalogues, and associated papers are available from the MGC website at http://www.eso.org/~jliske/mgc/

We build, as far as theory will permit, self consistent model HII regions around central clusters of aging stars. These produce strong emission line diagnostics applicable to either individual HII regions in galaxies, or to the integrated emission line spectra of disk or starburst galaxies. The models assume that the expansion and internal pressure of individual HII regions is driven by the net input of mechanical energy from the central cluster, be it through winds or supernova events. This eliminates the ionization parameter as a free variable, replacing it with a parameter which depends on the ratio of the cluster mass to the pressure in the surrounding interstellar medium. These models explain why HII regions with low abundances have high excitation, and demonstrate that at least part of the warm ionized medium is the result of overlapping faint, old, large, and low pressure HII regions. We present line ratios (at both optical and IR wavelengths) which provide reliable abundance diagnostics for both single HII regions or for integrated galaxy spectra, and we find a number that can be used to estimate the mean age of the cluster stars exciting individual HII regions., Comment: 22 pages. 18 figures. Accepted for publication in Astrophysical journal Supplements. Electronic tabular material is available on request to Michael.Dopita@anu.edu.au

We examine, from a theoretical viewpoint, how the physical parameters of HII regions are controlled in both normal galaxies and in starburst environments. These parameters are the HII region luminosity function, the time-dependent size, the covering fraction of molecular clouds, the pressure in the ionized gas and the ionization parameter. The factors which control them are the initial mass function of the exciting stars, the cluster mass function, the metallicity and the mean pressure in the surrounding interstellar medium. We investigate the sensitivity of the H$\alpha$ luminosity to the IMF, and find that this can translate to more than a factor two variation in derived star formation rates. The molecular cloud dissipation timescale is estimated from a case study of M17 to be $\sim1$ Myr for this object. Based upon HII luminosity function fitting for nearby galaxies, we suggest that the \HII region cluster mass function is fitted by a log-normal form peaking at $\sim 100 M_{\odot}$. The cluster mass function continues the stellar IMF to higher mass. The pressure in the HII regions is controlled by the mechanical luminosity flux from the central cluster. Since this is closely related to the ionizing photon flux, we show that the ionization parameter is not a free variable, and that the diffuse ionized medium may be composed of many large, faint and old HII regions. Finally, we derive theoretical probability distributions for the ionization parameter as a function of metallicity and compare these to those derived for SDSS galaxies., Comment: 36 pages, 8 figures. To be published in Astrophysical Journal, August 2006

Following on from IRAS, ISO has provided a huge advancement in our knowledge of the phenomenology of the infrared (IR) emission of normal galaxies and the underlying physical processes. Highlights include: the discovery of an extended cold dust emission component, present in all types of gas-rich galaxies and carrying the bulk of the dust luminosity; the definitive characterisation of the spectral energy distribution in the IR, revealing the channels through which stars power the IR light; the derivation of realistic geometries for stars and dust from ISO imaging; the discovery of cold dust associated with HI extending beyond the optical body of galaxies; the remarkable similarity of the near-IR (NIR)/ mid-IR (MIR) SEDs for spiral galaxies, revealing the importance of the photo-dissociation regions in the energy budget for that wavelength range; the importance of the emission from the central regions in shaping up the intensity and the colour of the global MIR luminosity; the discovery of the ``hot'' NIR continuum emission component of interstellar dust; the predominance of the diffuse cold neutral medium as the origin for the main interstellar cooling line, [CII] 158 micron, in normal galaxies., Comment: 47 pages, 15 figures, to be published in the ISO Special Issue of Space Science Reviews: "ISO science legacy - a compact review of ISO major achievements", Springer 2005. See http://www.iso.vilspa.esa.es/science/SSR/ for a higher resolution version and for all papers in the volume

We give an overview of the factors shaping the ultraviolet (UV)/optical - far-infrared (FIR)/submillimeter (submm) spectral energy distributions (SEDs) of normal (non-starburst) galaxies. Particular emphasis is placed on the influen ce of the geometry of dust and stars on the propagation of light through the int erstellar medium. Although strong constraints can be placed on the amount and la rge scale distribution of dust in disks from the appearance of the galaxies in t he optical/UV range, this dust does not account for the observed amplitude and c olour of the FIR/submm radiation. Additional, optically thick components of dust associated with the young stellar population on large and small scales are requ ired to account for the complete UV/optical - FIR/submm SEDs. Self-consistent mo dels for the calculation of SEDs of spiral galaxies are reviewed, and their pred ictions for the dust emission and the attenuation of starlight are compared and contrasted., Comment: Invited review presented at "The Spectral Energy Distribution of Gas-Rich Galaxies: Confronting Models with Data", Heidelberg, 4-8 Oct. 2004 (to be published in AIP Conf. Ser.); for a version with full resolution figures go to http://edoc.mpg.de/207562.0

Following on from IRAS, the ISOPHOT instrument on board the Infrared Space Observatory (ISO) has provided a huge advancement in our knowledge of the phenomenology of the far-infrared (FIR) emission of normal galaxies and the underlying physical processes. Highlights include: the discovery of an extended cold dust emission component, present in all types of gas-rich galaxies and carrying the bulk of the dust luminosity; the definitive characterisation of the spectral energy distribution in the FIR, revealing the channels through which stars power the FIR light; the derivation of realistic geometries for stars and dust from ISOPHOT imaging and the discovery of cold dust associated with HI extending beyond the optical body of galaxies., Comment: Invited review presented at "The Spectral Energy Distribution of Gas-Rich Galaxies: Confronting Models with Data", Heidelberg, 4-8 Oct. 2004 (to be published in AIP Conf. Ser.); for a version with full resolution figures go to http://edoc.mpg.de/207563.0

In this paper, we combine the stellar spectral synthesis code STARBURST 99, the nebular modelling code MAPPINGS IIIq, a 1-D dynamical evolution model of \HII regions around massive clusters of young stars and a simplified model of synchrotron emissivity to produce purely theoretical self-consistent synthetic spectral energy distributions (SEDs) for (solar metallicity) starbursts lasting some $10^8$ years. These SEDs extend from the Lyman Limit to beyond 21 cm. We find that two ISM parameters control the form of the SED; the pressure in the diffuse phase of the ISM (or, equivalently, its density), and the molecular cloud dissipation timescale. We present detailed SED fits to Arp 220 and NGC 6240, and we give the predicted colors for starburst galaxies derived from our models for the IRAS and the Spitzer Space Observatory MIPS and IRAC instruments. Our models reproduce the spread in observed colors of starburst galaxies. Finally, we present absolute calibrations to convert observed fluxes into star formation rates in the UV (GALEX), at optical wavelengths (H$\alpha$), and in the IR (IRAS or the Spitzer Space Observatory). (Abstract Truncated), Comment: 56 pages, 16 figures, accepted by The Apstrophysical Journal For version with full, colour figures go to http://www.mso.anu.edu.au/~bgroves/starburst/

We present new calculations of the attenuation of stellar light from spiral galaxies using geometries for stars and dust which can reproduce the entire spectral energy distribution from the UV to the FIR/submm and can also account for the surface brightness distribution in both the optical/NIR and FIR/submm. The calculations are based on the model of Popescu et al. (2000), which incorporates a dustless stellar bulge, a disk of old stars with associated diffuse dust, a thin disk of young stars with associated diffuse dust, and a clumpy dust component associated with star-forming regions in the thin disk. The attenuations, which incorporate the effects of multiple anisotropic scattering, are derived separately for each stellar component, and presented in the form of easily accessible polynomial fits as a function of inclination, for a grid in optical depth and wavelength. The wavelength range considered is between 912 AA and 2.2 micron, sampled such that attenuation can be conveniently calculated both for the standard optical bands and for the bands covered by GALEX. The attenuation characteristics of the individual stellar components show marked differences between each other. A general formula is given for the calculation of composite attenuation, valid for any combination of the bulge-to-disk ratio and amount of clumpiness. As an example, we show how the optical depth derived from the variation of attenuation with inclination depends on the bulge-to-disk ratio. Finally, a recipe is given for a self-consistent determination of the optical depth from the Halpha/Hbeta line ratio., Comment: accepted for publication in Astronomy and Astrophysics; 20 pages, 9 figures; for the paper with a high resolution version of Fig. 1 (12.28 Mb), see http://edoc.mpg.de/59626

We present deep maps of dust emission from the edge-on spiral galaxy NGC 891, obtained with the ISOPHOT instrument on board the Infrared Space Observatory in broad band filters with reference wavelengths centered on 170 and 200 micron. Using new processing methods to remove the effects of detector transients from the data, we detect cold dust at high dynamic range. The observed surface brightness distribution and colour profile of the far-infrared (FIR) emission are found to be in good agreement with predictions for their counterparts derived from the model of Popescu et al. (2000a). Thus, NGC 891 is the first galaxy for which an intrinsic distribution of dust and stars could be found which simultaneously accounts for both the optical/near-IR and FIR morphologies., Comment: accepted for publication in Astronomy & Astrophysics

Spiral galaxies in the Local Universe are commonly observed to be embedded in extended disks of neutral hydrogen - the so called ``extended HI disks''. Based on observations made using the ISOPHOT instrument on board the Infrared Space Observatory, we report the first detection of cold dust in the extended HI disk of a spiral galaxy. The detection was achieved through a dedicated deep Far-Infrared observation of a large field encompassing the entire HI disk of the edge-on spiral galaxy NGC 891. Our discovery indicates that the extended HI disk of NGC 891 is not primordial in origin., Comment: accepted for publication in Astronomy and Astrophysics Letters

We review the morphological and spectral energy distribution characteristics of the dust continuum emission (emitted in the 40-200 micron spectral range) from normal galaxies, as revealed by detailed ISOPHOT mapping observations of nearby spirals and by ISOPHOT observations of the integrated emissions from representative statistical samples in the local universe., Comment: 7 pages with 5 figures included (eps format); invited review talk held at the ESA Workshop ``Exploiting the ISO Data Archive. Infrared Astronomy in the Internet Age'', Siguenza, Spain 24-27 June, 2002

We present results from a new modelling technique which can account for the observed optical/NIR - FIR/submm spectral energy distributions (SEDs) of normal star-forming galaxies in terms of a minimum number of essential parameters specifying the star-formation history and geometrical distribution of stars and dust. The model utilises resolved optical/NIR images to constrain the old stellar population and associated dust, and geometry-sensitive colour information in the FIR/submm to constrain the spatial distributions of young stars and associated dust. It is successfully applied to the edge-on spirals NGC891 and NGC5907. In both cases the young stellar population powers the bulk of the FIR/sub-mm emission. The model also accounts for the observed surface brightness distribution and large-scale radial brightness profiles in NGC891 as determined using the Infrared Space Observatory (ISO) at 170 and 200 mcrions and at 850 micron using SCUBA., Comment: 20 pages (Latex), Highlight talk at the Joint European and National Astronomical Meeting JENAM 2001, to be published in Reviews in Modern Astronomy 15: Five Days of Creation: Astronomy with Large Telescopes from Ground and Base. Germany : Astronomische Gesellschaft, 2002

We present deep diffraction-limited far-infrared (FIR) strip maps of a sample of 63 galaxies later than S0 and brighter than B_T 16.8, selected from the Virgo Cluster Catalogue of Binggeli, Sandage & Tammann. The ISOPHOT instrument on board the Infrared Space Observatory was used to achieve sensitivities typically an order of magnitude deeper than IRAS in the 60 and 100 micron bands and to reach the confusion limit at 170 microns. The averaged 3 sigma upper limits for integrated flux densities of point sources at 60, 100 and 170 microns are 43, 33 and 58 mJy, respectively. 54 galaxies (85.7 percent) are detected at least at one wavelength, and 40 galaxies (63.5 percent) are detected at all three wavelengths. The highest detection rate (85.7 percent) is in the 170 micron band. In many cases the galaxies are resolved, allowing the scale length of the infrared disks to be derived from the oversampled brightness profiles in addition to the spatially integrated emission. The data presented should provide the basis for a variety of statistical investigations of the FIR spectral energy distributions of gas rich galaxies in the local universe spanning a broad range in star-formation activity and morphological types, including dwarf systems and galaxies with rather quiescent star formation activity., Comment: 53 pages (Latex), Fig. 10 is only available for the journal paper, to be published in ApJS, v139, March 2002

We have statistically analyzed the spatially integrated far-infrared (FIR) emissions of the complete volume- and luminosity-limited sample of late-type (later than S0) Virgo cluster galaxies measured using the Infrared Space Observatory (ISO) by Tuffs et al. (2002) in bands centered on 60, 100 and 170 microns. 30 out of 38 galaxies detected at all three wavelengths contain a cold dust emission component, present within all morphological types of late-type systems ranging from early giant spirals to Blue Compact Dwarfs (BCDs), and which could not have been recognized by IRAS. The temperatures of the cold dust were found to be broadly distributed, with a median of 18 K, some 8-10 K lower than would have been predicted from IRAS. The derived total dust masses are correspondingly increased by factors of typically 6 - 13. A particularly significant result concerns the low dust temperatures (ranging down to less than 10 K) and large dust masses associated with the Im and BCD galaxies in our sample. We propose two scenarios to account for the FIR characteristics of these systems., Comment: 38 pages (Latex); 9 figures; complete version of Fig. 2 can be found at http://nedwww.ipac.caltech.edu/level5/Sept01/Popescu/Popescu_contents.html; to be published in ApJ, v567, 2002

We describe a new tool for the analysis of the UV to the sub-millimeter (sub-mm) spectral energy distribution (SED) of spiral galaxies. We use a consistent treatment of grain heating and emission, solve the radiation transfer problem for a finite disk and bulge, and self-consistently calculate the stochastic heating of grains placed in the resulting radiation field. We use this tool to analyse the well-studied nearby edge-on spiral galaxy NGC891. First we investigate whether the old stellar population in NGC891, along with a reasonable assumption about the young stellar population, can account for the heating of the dust and the observed far-infrared (FIR) and sub-mm emission. The dust distribution is taken from the model of Xilouris et al. (1999), who used only optical and near-IR observations to determine it. We have found that such a simple model cannot reproduce the SED of NGC891, especially in the sub-mm range. It underestimates by a factor of 2-4 the observed sub-mm flux. A number of possible explanations exist for the missing sub-mm flux. We investigate a few of them and demonstrate that one can reproduce the observed SED in the FIR and the sub-mm quite well, as well as the observed radial profile at 850 microns. For the models calculated we give the relative proportion of the dust radiation powered by the old and young stellar populations as a function of FIR/sub-mm wavelength. In all models we find that the dust is predominantly heated by the young stellar population., Comment: 15 pages, accepted for publication in Astronomy and Astrophysics

We make predictions for the diffuse far-infrared (FIR) emission from dust in the intracluster medium (ICM) of the Virgo cluster using detailed information on potential dust sources, grain heating and sputtering rates available for this cluster from recent optical and X-ray studies. We show that the infrared emission from dust injected by sources inside the core region of the Virgo cluster is a factor of 10 below the detection limit of currently available telescopes. The outer regions of dynamically young clusters like the Virgo cluster have a further potential source of intracluster grains since they are still accreting freshly infalling spiral galaxies which are presumably contained in an accreting intergalactic medium (IGM). We show that cosmic ray driven winds from the infalling spirals can inject grains into a subvirial IGM that is external to the observed X-ray-emitting ICM. Sputtering during the injection process and in the IGM is weak, so that the injected grains should accumulate in the IGM until the infall brings them into contact with the hot ICM. This effect dominates the dust injection rate from known sources embedded in the hot Virgo ICM. Thus, any detection of diffuse IR emission would probe the current dust accretion rate for the cluster, acting as an indicator of the youth and the dynamical state of the cluster. The predictions for the Virgo cluster are generalised to other clusters and the possibility of detection of dynamically young clusters at cosmological distances is discussed., Comment: 19 pages, accepted for publication in Astronomy and Astrophysics

We present a spectrophotometric catalogue of 90 emission-line galaxies (ELGs) discovered during an objective - prism survey that aimed to search for dwarf galaxies within the voids (Popescu et al. 1996, 1998). The paper presents line ratios, equivalent widths and absolute fluxes for the emission-lines seen in the spectra of the galaxies. A list of newly discovered Wolf-Rayet galaxies is presented. Many objects included in the catalogue have low metallicity and the extreme cases are proposed as candidates for very low metallicity galaxies., Comment: 14 pages, to appear in Astronomy and Astrophysics Suppl. Series

We present a study of the star formation properties of a sample of emission line galaxies (ELGs) with respect to their environment. This study is part of a bigger project that aimed to find galaxies in voids and to investigate the large scale structure of the ELGs. A survey for ELGs was therefore conducted with the result that 16 galaxies have been found in very low density environments, of which 8 ELGs were found in two very well defined nearby voids. The sample presented here contains some galaxies identified in voids, as well as in the field environment that delimited the voids. These ELGs are all Blue Compact Galaxies (BCGs), and all void galaxies are also dwarfs. Both void and field sample contain the same mixture of morphological subtypes of BCDs, from the extreme Searle-Sargent galaxies to the Dwarf-Amorphous Nuclear-Starburst galaxies. The main result of this study is that field and void galaxies seem to have similar star formation rates (SFR), similar ratios between the current SFR and their average past SFR and similar mean SFR surface densities. There is no trend in metallicity, in the sense that void galaxies would have lower metallicities than their field counterparts. The field-cluster dichotomy is also discussed using available results from the literature, since our sample does not cover the cluster environment., Comment: 10 pages, accepted for publication in Astronomy and Astrophysics

We present the properties of the supernova SN 1995ah discovered in a Blue Compact Dwarf galaxy (BCD) around 10 days after the maximum. This is the first supernova event observed in a BCD. The photometric and spectroscopic data suggest that SN 1995ah is a Type II supernova and could belong to the rare Bright SNII Linear subclass, for which ~=-18.9+-0.6 at maximum light., Comment: 6 pages, latex

We present the results of a search for emission-line galaxies (ELGs) towards nearby voids, based on a sample selected on the HQS (Hamburg Quasar Survey) - III-aJ objective-prism plates. The survey was based on the presence of emission-lines and therefore has the advantage to detect very faint objects (with all the flux in the emission-lines and almost no continuum), that would be missed by an apparent magnitude limited survey. We found objects as faint as B=20.5 and M(B)=(-15.0,-12.0). The observational data are given in Popescu et al. (1996) and in the present paper we consider a complete subsample from these data. We analyse the spatial distribution of our sample of ELGs in comparison with the distribution of normal galaxies. Overall both distributions trace the same structures. Nevertheless we also found a few ELGs in voids, from which at least 8 lie in the very well defined nearby voids. The isolated galaxies seem to form fainter structures that devide the larger voids into smaller ones. {}From our estimates of the expected number of void galaxies we conclude that we did not find an underlying homogenous void population., Comment: 15 pages, LATEX file

We present the observational results of our search for emission-line galaxies (ELG) towards nearby voids. In order to find ELG, we started a survey using the IIIa-J objective prism plates from the Hamburg QSO Survey. The plates are digitized and an automatic procedure was applied to select the candidates. Digitized direct plates were used to determine coordinates and to reject overlaps between spectra. A total area of 1248 square degrees was scanned, distributed in four different regions. All the selected objects were observed with follow-up spectroscopy. We have obtained a final sample of 203 objects, of which 196 are emission-line galaxies, four are galaxies with absorption lines and three are QSOs. Almost half of our objects are newly discovered ones and three quarters of the given redshifts are new. Our sample contains mainly high ionization galaxies and is less sensitive in the detection of low-ionization objects. The apparent magnitudes, as derived from the objective prism plates, range between 15.0