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The SPICA mid- and far-infrared telescope will address fundamental issues in our understanding of star formation and ISM physics in galaxies. A particular hallmark of SPICA is the outstanding sensitivity enabled by the cold telescope, optimised detectors, and wide instantaneous bandwidth throughout the mid- and far-infrared. The spectroscopic, imaging, and polarimetric observations that SPICA will be able to collect will help in clarifying the complex physical mechanisms which underlie the baryon cycle of galaxies. In particular, (i) the access to a large suite of atomic and ionic fine-structure lines for large samples of galaxies will shed light on the origin of the observed spread in star-formation rates within and between galaxies, (ii) observations of HD rotational lines (out to similar to 10 Mpc) and fine structure lines such as [C II] 158 mu m (out to similar to 100 Mpc) will clarify the main reservoirs of interstellar matter in galaxies, including phases where CO does not emit, (iii) far-infrared spectroscopy of dust and ice features will address uncertainties in the mass and composition of dust in galaxies, and the contributions of supernovae to the interstellar dust budget will be quantified by photometry and monitoring of supernova remnants in nearby galaxies, (iv) observations of far-infrared cooling lines such as [O I] 63 mu m from star-forming molecular clouds in our Galaxy will evaluate the importance of shocks to dissipate turbulent energy. The paper concludes with requirements for the telescope and instruments, and recommendations for the observing strategy.

We derive total (atomic + molecular) hydrogen densities in giant molecular clouds (GMCs) in the nearby spiral galaxy M33 using a method that views the atomic hydrogen near regions of recent star formation as the product of photodissociation. Far-ultraviolet (FUV) photons emanating from a nearby OB association produce a layer of atomic hydrogen on the surfaces of nearby GMCs. Our approach provides an estimate of the total hydrogen density in these GMCs from observations of the excess FUV emission that reaches the GMC from the OB association and of the excess 21-cm radio H I emission produced after these FUV photons convert H-2 into H I on the GMC surface. The method provides an alternative approach to the use of CO emission as a tracer of H-2 in GMCs and is especially sensitive to a range of densities well below the critical density for CO(1-0) emission. We describe our 'PDR method' in more detail and apply it using GALEX FUV and Very Large Array 21-cm radio data to obtain volume densities in a selection of GMCs in the nearby spiral galaxy M33. We have also examined the sensitivity of the method to the linear resolution of the observations used; the results obtained at 20 pc are similar to those for the larger set of data at 80-pc resolution. The cloud densities we derive range from 1 to 500 cm(-3), with no clear dependence on the galactocentric radius; these results are generally similar to those obtained earlier in the cases of M81, M83 and M101 using the same method.

We derive total (atomic + molecular) hydrogen densities in giant molecular clouds (GMCs) in the nearby spiral galaxy M33 using a method that views the atomic hydrogen near regions of recent star formation as the product of photodissociation. Far-ultraviolet (FUV) photons emanating from a nearby OB association produce a layer of atomic hydrogen on the surfaces of nearby GMCs. Our approach provides an estimate of the total hydrogen density in these GMCs from observations of the excess FUV emission that reaches the GMC from the OB association and of the excess 21-cm radio H I emission produced after these FUV photons convert H-2 into H I on the GMC surface. The method provides an alternative approach to the use of CO emission as a tracer of H-2 in GMCs and is especially sensitive to a range of densities well below the critical density for CO(1-0) emission.We describe our 'PDR method' in more detail and apply it using GALEX FUV and Very Large Array 21-cm radio data to obtain volume densities in a selection of GMCs in the nearby spiral galaxy M33. We have also examined the sensitivity of the method to the linear resolution of the observations used; the results obtained at 20 pc are similar to those for the larger set of data at 80-pc resolution. The cloud densities we derive range from 1 to 500 cm(-3), with no clear dependence on the galactocentric radius; these results are generally similar to those obtained earlier in the cases of M81, M83 and M101 using the same method.

We present the first results of a pilot study aimed at understanding the influence of bars on the evolution of galaxy discs through the study of their stellar content. We examine here the kinematics, star formation history, mass-weighted, luminosity-weighted and single stellar population (SSP) equivalent ages and metallicities for four galaxies ranging from lenticulars to late-type spirals. The data employed extend to 2-3 disc scalelengths, with S/N(angstrom) > 50, where S/N stands for the signal-to-noise ratio. Several techniques are explored to derive star formation histories and SSP-equivalent parameters, each of which is shown to be compatible. We demonstrate that the age-metallicity degeneracy is highly reduced by using spectral fitting techniques - instead of indices - to derive these parameters. Our results are robust to the choices of stellar population models. We found that the majority of the stellar mass in our sample is composed of old (similar to 10 Gyr) stars. This is true in the bulge and disc regions, even beyond 2 disc scalelengths. In the bulge region, we find that the young, dynamically cold, structures produced by the presence of the bar (e. g. nuclear discs or rings) are responsible for shaping the bulges' age and metallicity gradients, as suggested by Peletier et al. In the disc region, a larger fraction of young stars are present in the external parts of the disc compared with the inner parts. The disc growth is therefore compatible with a moderate inside-out formation scenario, where the luminosity-weighted age changes from similar to 10 Gyr in the centre to similar to 4 Gyr at 2 disc scalelengths, depending upon the galaxy. However, the presence of substructures, like star-forming rings, can produce stellar population trends that are not directly related to the growing of the disc but to the bar potential. For two galaxies, we compare the metallicity and age gradients of the disc major-axis with that of the bar, finding very important differences. In particular, the stellar population of the bar is more similar to that of the bulge than to that of the disc, indicating that, at least in those two galaxies, bars formed long ago and have survived to the present day.

We present the first results of a pilot study aimed at understanding the influence of bars on the evolution of galaxy discs through the study of their stellar content. We examine here the kinematics, star formation history, mass-weighted, luminosity-weighted and single stellar population (SSP) equivalent ages and metallicities for four galaxies ranging from lenticulars to late-type spirals. The data employed extend to 2-3 disc scalelengths, with S/N(angstrom) > 50, where S/N stands for the signal-to-noise ratio. Several techniques are explored to derive star formation histories and SSP-equivalent parameters, each of which is shown to be compatible. We demonstrate that the age-metallicity degeneracy is highly reduced by using spectral fitting techniques - instead of indices - to derive these parameters. Our results are robust to the choices of stellar population models. We found that the majority of the stellar mass in our sample is composed of old (similar to 10 Gyr) stars. This is true in the bulge and disc regions, even beyond 2 disc scalelengths. In the bulge region, we find that the young, dynamically cold, structures produced by the presence of the bar (e. g. nuclear discs or rings) are responsible for shaping the bulges' age and metallicity gradients, as suggested by Peletier et al. In the disc region, a larger fraction of young stars are present in the external parts of the disc compared with the inner parts. The disc growth is therefore compatible with a moderate inside-out formation scenario, where the luminosity-weighted age changes from similar to 10 Gyr in the centre to similar to 4 Gyr at 2 disc scalelengths, depending upon the galaxy. However, the presence of substructures, like star-forming rings, can produce stellar population trends that are not directly related to the growing of the disc but to the bar potential. For two galaxies, we compare the metallicity and age gradients of the disc major-axis with that of the bar, finding very important differences. In particular, the stellar population of the bar is more similar to that of the bulge than to that of the disc, indicating that, at least in those two galaxies, bars formed long ago and have survived to the present day.

An overview is given of ISO results on regions of high excitation ISM and gas, i.e. H II regions, the Galactic Centre and Supernova Remnants. IR emission due to fine-structure lines, molecular hydrogen, silicates, polycyclic aromatic hydrocarbons and dust are summarised, their diagnostic capabilities illustrated and their implications highlighted.

We present a radio continuum survey at 1400 and 350 MHz of a region of 2degrees x 2degrees centered on the Cygnus OB2 association (d = 1.7 kpc), using the Westerbork Synthesis Radio Telescope (WSRT) with angular resolutions of, respectively, 13" and 55". The resulting 5 sigma flux-density limits of, respectively, similar to2 mJy and similar to10-15 mJy are a significant improvement over previous surveys. We detected 210 discrete sources with sizes less than 1.9theta(beam) (beam size), 98 of which at both frequencies. We also detected 28 resolved sources (sizes > 1.9theta(beam)) still having well-defined peak intensities. The observed spectral index alpha(350)(1400) distribution and source count strongly suggest an excess of sources of Galactic origin in the direction of Cyg OB2. We have searched for positional coincidences of the detected sources in our list with other radio, infrared, and optical objects from various surveys by using the likelihood ratio (LR) method. Furthermore, we looked for objects that show characteristics of either optically thick stellar winds (alpha greater than or similar to +0.6), or nonthermal emission (-1 less than or similar to alpha less than or similar to +0.6) and/or variable spectral flux density. The LR method resulted in 108 identifications. Eighty unidentified sources, i.e., similar to2/3, show characteristics of sources of Galactic origin, 10 of which may be stars. The remaining unidentified sources are probably of extragalactic origin. We identified one source with the O7 star Cyg OB2-335 and consider it to be a candidate colliding-wind binary. We also identify 19 point sources with known infrared and optical objects: these have nearly. at or inverted spectral indices, and some of them show flux-density variability. Follow-up multifrequency monitoring of these sources will be important in establishing the reality of the flux variabilities and to assess the nature of these sources.

We present a radio continuum survey at 1400 and 350 MHz of a region of 2degrees x 2degrees centered on the Cygnus OB2 association (d = 1.7 kpc), using the Westerbork Synthesis Radio Telescope (WSRT) with angular resolutions of, respectively, 13" and 55". The resulting 5 sigma flux-density limits of, respectively, similar to2 mJy and similar to10-15 mJy are a significant improvement over previous surveys. We detected 210 discrete sources with sizes less than 1.9theta(beam) (beam size), 98 of which at both frequencies. We also detected 28 resolved sources (sizes > 1.9theta(beam)) still having well-defined peak intensities. The observed spectral index alpha(350)(1400) distribution and source count strongly suggest an excess of sources of Galactic origin in the direction of Cyg OB2.We have searched for positional coincidences of the detected sources in our list with other radio, infrared, and optical objects from various surveys by using the likelihood ratio (LR) method. Furthermore, we looked for objects that show characteristics of either optically thick stellar winds (alpha greater than or similar to +0.6), or nonthermal emission (-1 less than or similar to alpha less than or similar to +0.6) and/or variable spectral flux density. The LR method resulted in 108 identifications. Eighty unidentified sources, i.e., similar to2/3, show characteristics of sources of Galactic origin, 10 of which may be stars. The remaining unidentified sources are probably of extragalactic origin.We identified one source with the O7 star Cyg OB2-335 and consider it to be a candidate colliding-wind binary. We also identify 19 point sources with known infrared and optical objects: these have nearly. at or inverted spectral indices, and some of them show flux-density variability. Follow-up multifrequency monitoring of these sources will be important in establishing the reality of the flux variabilities and to assess the nature of these sources.

We present high-angular-resolution, far-infrared images of the Small Magellanic Cloud (SMC). The images were reconstructed from data obtained with the Infrared Astronomical Satellite (IRAS), using the Pyramid Maximum Entropy algorithm. The angular resolutions of the images are 1 arcmin at 12, 25 and 60 mu m, and 1.7 arcmin at 100 mu m, which provides an excellent complement to the recent neutral hydrogen (H I) survey from the Australia Telescope Compact Array, which has a resolution of 1.6 arcmin. We use the 60- and 100-mu m data, together with a standard dust model, to derive a temperature map and a dust column density distribution. Dust temperatures range from 23 to 45 K, which is, on average, warmer than for the Galaxy, although there is a deficit of warm, very small dust grains in the SMC. The total amount of cool dust in the SMC is 1.8(-0.2)(+1.3) x 10(4) M.. There is a significant variation of the observed dust-to-gas ratio across the SMC. This may be caused by corresponding variations in metallicity, but this explanation is not supported by the limited available data in the literature. Alternatively, there may be a large amount of molecular hydrogen (H(2)) in the SMC, similar to 8 x 10(8) M., and/or a large amount (similar to 10(6) M.) of cold dust (less than or equal to 15 K) not seen by IRAS or COBE/DIRBE. The average dust-to-atomic gas ratio is, by mass, 8.2 x 10(-5), which is a factor of similar to 30 below the Galactic value. The spatial power spectrum of the dust column density image can be approximated by P(d)(k) proportional to k(-3.1), which shows that, as with the H I power spectrum, there is no preferred scale size for dust clouds. A spatial coherence analysis shows that, over a wide range of spatial scales (greater than or similar to 50 pc), H I and dust are well correlated (> 60 per cent).

We present high-resolution interferometric data of the inner 30' of the H I gas of the irregular galaxy NGC 4449. In addition to the previously known two counter-rotating gas systems, the H I exhibits an unusual ring morphology and high velocity dispersions in the outer part of the optical galaxy. In addition the position angle and inclination of the outer gas are moderately different from those of the optical galaxy. The position angle and inclination of the inner gas system are approximately similar to those of the inner optical galaxy only if strong deviations from ordered motion in the center are ignored. The H I interstellar medium appears to be fragmented into clouds, which are similar to clouds observed in other galaxies. There is very little correlation between the H I surface density and H alpha surface brightness. The optical isophotes of the galaxy define three different and distinct position angles and inclinations. The inner structure is similar in properties to a bar structure that includes a large fraction of the optical galaxy. The nature of the middle structure is unclear. Exterior to these bars are unusual faint filamentary structures. The V- and J-band surface photometry are well fit by an exponential disk to low surface brightness levels, including the bar structures. However, the central surface brightness is unusually bright. The star formation activity compared over three different time scales as a function of radius is fairly consistent. In the optical there are also unusual extensions at low light levels in the outskirts of the galaxy. Many characteristics of the gas and stars are consistent with a picture in which NGC 4449 has been disturbed by an external perturber, and the gas has not yet returned to equilibrium.

We present high-resolution interferometric data of the inner 30' of the H I gas of the irregular galaxy NGC 4449. In addition to the previously known two counter-rotating gas systems, the H I exhibits an unusual ring morphology and high velocity dispersions in the outer part of the optical galaxy. In addition the position angle and inclination of the outer gas are moderately different from those of the optical galaxy. The position angle and inclination of the inner gas system are approximately similar to those of the inner optical galaxy only if strong deviations from ordered motion in the center are ignored. The H I interstellar medium appears to be fragmented into clouds, which are similar to clouds observed in other galaxies. There is very little correlation between the H I surface density and H alpha surface brightness. The optical isophotes of the galaxy define three different and distinct position angles and inclinations. The inner structure is similar in properties to a bar structure that includes a large fraction of the optical galaxy. The nature of the middle structure is unclear. Exterior to these bars are unusual faint filamentary structures. The V- and J-band surface photometry are well fit by an exponential disk to low surface brightness levels, including the bar structures. However, the central surface brightness is unusually bright. The star formation activity compared over three different time scales as a function of radius is fairly consistent. In the optical there are also unusual extensions at low light levels in the outskirts of the galaxy. Many characteristics of the gas and stars are consistent with a picture in which NGC 4449 has been disturbed by an external perturber, and the gas has not yet returned to equilibrium.

The Square Kilometre Array (SKA) will have a low frequency component (SKA-low) which has as one of its main science goals the study of the redshifted 21cm line from the earliest phases of star and galaxy formation in the Universe. This 21cm signal provides a new and unique window on both the formation of the first stars and accreting black holes and the later period of substantial ionization of the intergalactic medium. The signal will teach us fundamental new things about the earliest phases of structure formation, cosmology and even has the potential to lead to the discovery of new physical phenomena. Here we present a white paper with an overview of the science questions that SKA-low can address, how we plan to tackle these questions and what this implies for the basic design of the telescope., Comment: Accepted for publication in Experimental Astronomy, reformatted to 57 pages, some updated and improved figures and minor changes and updates to the text

Models of the chemical evolution of the Milky Way suggest that: the observed abundances of elements heavier than helium ('metals') require a continuous infall of gas with metallicity (metal abundance) about 0.1 times the solar value. An infall rate integrated over the entire disk of the Milky Way of similar to 1 solar mass per year can solve the 'G-dwarf problem'-the observational fact that the metallicities of most long-lived stars near the Sun lie in a relatively narrow range(1-3). This infall dilutes the enrichment arising from the production of heavy elements in stars, and thereby prevents the metallicity of the interstellar medium from increasing steadily with time. However, in other spiral galaxies, the low-metallicity gas needed to provide this infall has been observed only in associated dwarf galaxies(4) and in the extreme outer disk of the Milky Way(5,6). In the distant Universe, low-metallicity hydrogen clouds (known as 'damped Ly alpha absorbers') are sometimes seen near galaxies(7,8). Here we report a metallicity of 0.09 times solar for a massive cloud that is falling into the disk of the Milky Way. The mass now associated with this cloud represents an infall per unit area of about the theoretically expected rate, and similar to 0.1-0.2 times the amount required for the whole Galaxy.

Models of the chemical evolution of the Milky Way suggest that: the observed abundances of elements heavier than helium ('metals') require a continuous infall of gas with metallicity (metal abundance) about 0.1 times the solar value. An infall rate integrated over the entire disk of the Milky Way of similar to 1 solar mass per year can solve the 'G-dwarf problem'-the observational fact that the metallicities of most long-lived stars near the Sun lie in a relatively narrow range(1-3). This infall dilutes the enrichment arising from the production of heavy elements in stars, and thereby prevents the metallicity of the interstellar medium from increasing steadily with time. However, in other spiral galaxies, the low-metallicity gas needed to provide this infall has been observed only in associated dwarf galaxies(4) and in the extreme outer disk of the Milky Way(5,6). In the distant Universe, low-metallicity hydrogen clouds (known as 'damped Ly alpha absorbers') are sometimes seen near galaxies(7,8). Here we report a metallicity of 0.09 times solar for a massive cloud that is falling into the disk of the Milky Way. The mass now associated with this cloud represents an infall per unit area of about the theoretically expected rate, and similar to 0.1-0.2 times the amount required for the whole Galaxy.