arXiv subjects

L. Spinoglio

Publications and source records attributed to L. Spinoglio.

At least 73 records · Page 4Linked to original sources

arXiv2015

The bolometric and UV attenuation in normal spiral galaxies of the Herschel Reference Survey

The dust in nearby galaxies absorbs a fraction of the UV-optical-near-infrared radiation produced by stars. This energy is consequently re-emitted in the infrared. We investigate the portion of the stellar radiation absorbed by spiral galaxies from the HRS by modelling their UV-to-submillimetre spectral energy distributions. Our models provide an attenuated and intrinsic SED from which we find that on average 32 % of all starlight is absorbed by dust. We define the UV heating fraction as the percentage of dust luminosity that comes from absorbed UV photons and find that this is 56 %, on average. This percentage varies with morphological type, with later types having significantly higher UV heating fractions. We find a strong correlation between the UV heating fraction and specific star formation rate and provide a power-law fit. Our models allow us to revisit the IRX-AFUV relations, and derive these quantities directly within a self-consistent framework. We calibrate this relation for different bins of NUV-r colour and provide simple relations to relate these parameters. We investigated the robustness of our method and we conclude that the derived parameters are reliable within the uncertainties which are inherent to the adopted SED model. This calls for a deeper investigation on how well extinction and attenuation can be determined through panchromatic SED modelling.

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arXiv2015

The imprint of rapid star formation quenching on the spectral energy distributions of galaxies

[Abridged] In high density environment, the gas content of galaxies is stripped, leading to a rapid quenching of their star formation activity. This dramatic environmental effect is generally not taken into account in the SFHs usually assumed to perform spectral energy distribution (SED) fitting of these galaxies, yielding to a poor fit of their stellar emission and, consequently, a biased estimate of the SFR. We aim at reproducing the SFH of galaxies that underwent a rapid star formation quenching using a truncated delayed SFH that we implemented in the SED fitting code CIGALE. We show that the ratio between the instantaneous SFR and the SFR just before the quenching ($r_{SFR}$) is well constrained as long as rest frame UV data are available. This SED modelling is applied to the Herschel Reference Survey (HRS) containing isolated galaxies and sources falling in the dense environment of the Virgo cluster. The latter are HI-deficient due to ram pressure stripping. We show that the truncated delayed SFH successfully reproduces their SED while typical SFH assumptions fail. A good correlation is found between $r_{SFR}$ and HI-def, the parameter quantifying the gas deficiency of cluster galaxies, meaning that SED fitting results can be used to provide a tentative estimate of the gas deficiency of galaxies for which HI observations are not available. The HRS galaxies are placed on the SFR-$M_*$ diagram showing that the HI-deficient sources lie in the quiescent region confirming previous studies. Using the $r_{SFR}$ parameter, we derive the SFR of these sources before quenching and show that they were previously on the main sequence relation. We show that the $r_{SFR}$ parameter is also well recovered for deeply obscured high redshift sources, as well as in absence of IR data. SED fitting is thus a powerful tool to identify galaxies that underwent a rapid star formation quenching.

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arXiv2015

A possible link between the power spectrum of interstellar filaments and the origin of the prestellar core mass function

Two major features of the prestellar CMF are: 1) a broad peak below 1 Msun, presumably corresponding to a mean gravitational fragmentation scale, and 2) a characteristic power-law slope, very similar to the Salpeter slope of the stellar initial mass function (IMF) at the high-mass end. While recent Herschel observations have shown that the peak of the prestellar CMF is close to the thermal Jeans mass in marginally supercritical filaments, the origin of the power-law tail of the CMF/IMF at the high-mass end is less clear. Inutsuka (2001) proposed a theoretical scenario in which the origin of the power-law tail can be understood as resulting from the growth of an initial spectrum of density perturbations seeded along the long axis of filaments by interstellar turbulence. Here, we report the statistical properties of the line-mass fluctuations of filaments in nearby molecular clouds observed with Herschel using a 1-D power spectrum analysis. The observed filament power spectra were fitted by a power-law function $(P_{true}(s) \propto s^α)$ after removing the effect of beam convolution at small scales. A Gaussian-like distribution of power-spectrum slopes was found centered at -1.6, close to that of the one-dimensional velocity power spectrum generated by subsonic Kolomogorov turbulence (-1.67). An empirical correlation, $P^{0.5}(s_0) \propto ^{1.4 \pm 0.1} $, was also found between the amplitude of each filament power spectrum $P(s_0)$ and the mean column density along the filament $ $. Finally, the dispersion of line-mass fluctuations along each filament $σ_{\rm M_{line}}$ was found to scale with the physical length $L$ of the filament, roughly as $σ_{M_{line}} \propto L^{0.7}$. Overall, our results are consistent with the suggestion that the bulk of the CMF/IMF results from the gravitational fragmentation of filaments.

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arXiv2015

Filaments in the Lupus molecular clouds

We have studied the filaments extracted from the column density maps of the nearby Lupus 1, 3, and 4 molecular clouds, derived from photometric maps observed with the Herschel satellite. Filaments in the Lupus clouds have quite low column densities, with a median value of $\sim$1.5$\times$10$^{21}$ cm$^{-2}$ and most have masses per unit length lower than the maximum critical value for radial gravitational collapse. Indeed, no evidence of filament contraction has been seen in the gas kinematics. We find that some filaments, that on average are thermally subcritical, contain dense cores that may eventually form stars. This is an indication that in the low column density regime, the critical condition for the formation of stars may be reached only locally and this condition is not a global property of the filament. Finally, in Lupus we find multiple observational evidences of the key role that the magnetic field plays in forming filaments, and determining their confinement and dynamical evolution.

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arXiv2015

Detection of two power-law tails in the probability distribution functions of massive GMCs

We report the novel detection of complex high-column density tails in the probability distribution functions (PDFs) for three high-mass star-forming regions (CepOB3, MonR2, NGC6334), obtained from dust emission observed with Herschel. The low column density range can be fit with a lognormal distribution. A first power-law tail starts above an extinction (Av) of ~6-14. It has a slope of alpha=1.3-2 for the rho~r^-alpha profile for an equivalent density distribution (spherical or cylindrical geometry), and is thus consistent with free-fall gravitational collapse. Above Av~40, 60, and 140, we detect an excess that can be fitted by a flatter power law tail with alpha>2. It correlates with the central regions of the cloud (ridges/hubs) of size ~1 pc and densities above 10^4 cm^-3. This excess may be caused by physical processes that slow down collapse and reduce the flow of mass towards higher densities. Possible are: 1. rotation, which introduces an angular momentum barrier, 2. increasing optical depth and weaker cooling, 3. magnetic fields, 4. geometrical effects, and 5. protostellar feedback. The excess/second power-law tail is closely linked to high-mass star-formation though it does not imply a universal column density threshold for the formation of (high-mass) stars.

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arXiv2015

A census of dense cores in the Aquila cloud complex: SPIRE/PACS observations from the Herschel Gould Belt survey

We present and discuss the results of the Herschel Gould Belt survey observations in a ~11 deg^2 area of the Aquila molecular cloud complex at d~260 pc, imaged with the SPIRE/PACS cameras from 70 to 500 micron. We identify a complete sample of starless dense cores and embedded protostars in this region, and analyze their global properties and spatial distributions. We find a total of 651 starless cores, ~60% of which are gravitationally bound prestellar cores, and they will likely form stars in the future. We also detect 58 protostellar cores. The core mass function (CMF) derived for the prestellar cores is very similar in shape to the stellar initial mass function (IMF), supporting the earlier view that there is a close physical link between the IMF and the CMF. The global shift in mass scale observed between the CMF and the IMF is consistent with a typical star formation efficiency of ~40%. By comparing the numbers of starless cores to the number of young stellar objects, we estimate that the lifetime of prestellar cores is ~1 Myr. We find a strong correlation between the spatial distribution of prestellar cores and the densest filaments. About 90% of the Herschel-identified prestellar cores are located above a background column density corresponding to A_V~7, and ~75% of them lie within filamentary structures with supercritical masses per unit length >~16 M_sun/pc. These findings support a picture wherein the cores making up the peak of the CMF (and probably responsible for the base of the IMF) result primarily from the gravitational fragmentation of marginally supercritical filaments. Given that filaments appear to dominate the mass budget of dense gas at A_V>7, our findings also suggest that the physics of prestellar core formation within filaments is responsible for a characteristic "efficiency" SFR/M_dense ~5+-2 x 10^-8 yr^-1 for the star formation process in dense gas.

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arXiv2015

Linking dust emission to fundamental properties in galaxies: The low-metallicity picture

In this work, we aim at providing a consistent analysis of the dust properties from metal-poor to metal-rich environments by linking them to fundamental galactic parameters. We consider two samples of galaxies: the Dwarf Galaxy Survey (DGS) and KINGFISH, totalling 109 galaxies, spanning almost 2 dex in metallicity. We collect infrared (IR) to submillimetre (submm) data for both samples and present the complete data set for the DGS sample. We model the observed spectral energy distributions (SED) with a physically-motivated dust model to access the dust properties. Using a different SED model (modified blackbody), dust composition (amorphous carbon), or wavelength coverage at submm wavelengths results in differences in the dust mass estimate of a factor two to three, showing that this parameter is subject to non-negligible systematic modelling uncertainties. For eight galaxies in our sample, we find a rather small excess at 500 microns (< 1.5 sigma). We find that the dust SED of low-metallicity galaxies is broader and peaks at shorter wavelengths compared to more metal-rich systems, a sign of a clumpier medium in dwarf galaxies. The PAH mass fraction and the dust temperature distribution are found to be driven mostly by the specific star-formation rate, SSFR, with secondary effects from metallicity. The correlations between metallicity and dust mass or total-IR luminosity are direct consequences of the stellar mass-metallicity relation. The dust-to-stellar mass ratios of metal-rich sources follow the well-studied trend of decreasing ratio for decreasing SSFR. The relation is more complex for highly star-forming low-metallicity galaxies and depends on the chemical evolutionary stage of the source (i.e., gas-to-dust mass ratio). Dust growth processes in the ISM play a key role in the dust mass build-up with respect to the stellar content at high SSFR and low metallicity. (abridged)

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arXiv2015

Herschel Observations of the W3 GMC (II): Clues to the Formation of Clusters of High-Mass Stars

The W3 GMC is a prime target for investigating the formation of high-mass stars and clusters. This second study of W3 within the HOBYS Key Program provides a comparative analysis of subfields within W3 to further constrain the processes leading to the observed structures and stellar population. Probability density functions (PDFs) and cumulative mass distributions (CMDs) were created from dust column density maps, quantified as extinction Av. The shape of the PDF, typically represented with a lognormal function at low Av "breaking" to a power-law tail at high Av, is influenced by various processes including turbulence and self-gravity. The breaks can also be identified, often more readily, in the CMDs. The PDF break from lognormal (Av(SF)= 6-10 mag) appears to shift to higher Av by stellar feedback, so that high-mass star-forming regions tend to have higher PDF breaks. A second break at Av > 50 mag traces structures formed or influenced by a dynamic process. Because such a process has been suggested to drive high-mass star formation in W3, this second break might then identify regions with potential for hosting high-mass stars/clusters. Stellar feedback appears to be a major mechanism driving the local evolution and state of regions within W3. A high initial star formation efficiency in a dense medium could result in a self-enhancing process, leading to more compression and favourable star-formation conditions (e.g., colliding flows), a richer stellar content, and massive stars. This scenario would be compatible with the "convergent constructive feedback" model introduced in our previous Herschel study.

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arXiv2015

The JCMT Gould Belt Survey: First results from the SCUBA-2 observations of the Ophiuchus molecular cloud and a virial analysis of its prestellar core population

In this paper we present the first observations of the Ophiuchus molecular cloud performed as part of the James Clerk Maxwell Telescope (JCMT) Gould Belt Survey (GBS) with the SCUBA-2 instrument. We demonstrate methods for combining these data with previous HARP CO, Herschel, and IRAM N$_{2}$H$^{+}$ observations in order to accurately quantify the properties of the SCUBA-2 sources in Ophiuchus. We produce a catalogue of all of the sources found by SCUBA-2. We separate these into protostars and starless cores. We list all of the starless cores and perform a full virial analysis, including external pressure. This is the first time that external pressure has been included in this level of detail. We find that the majority of our cores are either bound or virialised. Gravitational energy and external pressure are on average of a similar order of magnitude, but with some variation from region to region. We find that cores in the Oph A region are gravitationally bound prestellar cores, while cores in the Oph C and E regions are pressure-confined. We determine that N$_{2}$H$^{+}$ is a good tracer of the bound material of prestellar cores, although we find some evidence for N$_{2}$H$^{+}$ freeze-out at the very highest core densities. We find that non-thermal linewidths decrease substantially between the gas traced by C$^{18}$O and that traced by N$_{2}$H$^{+}$, indicating the dissipation of turbulence at higher densities. We find that the critical Bonnor-Ebert stability criterion is not a good indicator of the boundedness of our cores. We detect the pre-brown dwarf candidate Oph B-11 and find a flux density and mass consistent with previous work. We discuss regional variations in the nature of the cores and find further support for our previous hypothesis of a global evolutionary gradient across the cloud from southwest to northeast, indicating sequential star formation across the region.

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arXiv2015

The Herschel Dwarf Galaxy Survey: I. Properties of the low-metallicity ISM from PACS spectroscopy

The far-infrared (FIR) lines are key tracers of the physical conditions of the interstellar medium (ISM) and are becoming workhorse diagnostics for galaxies throughout the universe. Our goal is to explain the differences and trends observed in the FIR line emission of dwarf galaxies compared to more metal-rich galaxies. We present Herschel PACS spectroscopic observations of the CII157um, OI63 and 145um, OIII88um, NII122 and 205um, and NIII57um fine-structure cooling lines in a sample of 48 low-metallicity star-forming galaxies of the guaranteed time key program Dwarf Galaxy Survey. We correlate PACS line ratios and line-to-LTIR ratios with LTIR, LTIR/LB, metallicity, and FIR color, and interpret the observed trends in terms of ISM conditions and phase filling factors with Cloudy radiative transfer models. We find that the FIR lines together account for up to 3 percent of LTIR and that star-forming regions dominate the overall emission in dwarf galaxies. Compared to metal-rich galaxies, the ratios of OIII/NII122 and NIII/NII122 are high, indicative of hard radiation fields. In the photodissociation region (PDR), the CII/OI63 ratio is slightly higher than in metal-rich galaxies, with a small increase with metallicity, and the OI145/OI63 ratio is generally lower than 0.1, demonstrating that optical depth effects should be small on the scales probed. The OIII/OI63 ratio can be used as an indicator of the ionized gas/PDR filling factor, and is found ~4 times higher in the dwarfs than in metal-rich galaxies. The high CII/LTIR, OI/LTIR, and OIII/LTIR ratios, which decrease with increasing LTIR and LTIR/LB, are interpreted as a combination of moderate FUV fields and low PDR covering factor. Harboring compact phases of low filling factor and a large volume filling factor of diffuse gas, the ISM of low-metallicity dwarf galaxies has a more porous structure than that in metal-rich galaxies.

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arXiv2015

The Herschel Comprehensive (U)LIRG Emission Survey (HerCULES): CO Ladders, fine structure lines, and neutral gas cooling

(Ultra) Luminous Infrared Galaxies ((U)LIRGs) are objects characterized by their extreme infrared (8-1000 $μ$m) luminosities ($L_{LIRG}>10^{11} $L$_\odot$ and $L_{ULIRG}>10^{12}$ L$_\odot$). The Herschel Comprehensive ULIRG Emission Survey (HerCULES; PI van der Werf) presents a representative flux-limited sample of 29 (U)LIRGs that spans the full luminosity range of these objects (10$^{11}\leq L_\odot \geq10^{13}$). With the \emph{Herschel Space Observatory}, we observe [CII] 157 $μ$m, [OI] 63 $μ$m, and [OI] 145 $μ$m line emission with PACS, CO J=4-3 through J=13-12, [CI] 370 $μ$m, and [CI] 609 $μ$m with SPIRE, and low-J CO transitions with ground-based telescopes. The CO ladders of the sample are separated into three classes based on their excitation level. In 13 of the galaxies, the [OI] 63 $μ$m emission line is self absorbed. Comparing the CO excitation to the IRAS 60/100 $μ$m ratio and to far infrared luminosity, we find that the CO excitation is more correlated to the far infrared colors. We present cooling budgets for the galaxies and find fine-structure line flux deficits in the [CII], [SiII], [OI], and [CI] lines in the objects with the highest far IR fluxes, but do not observe this for CO $4\leq J_{upp}\leq13$. In order to study the heating of the molecular gas, we present a combination of three diagnostic quantities to help determine the dominant heating source. Using the CO excitation, the CO J=1-0 linewidth, and the AGN contribution, we conclude that galaxies with large CO linewidths always have high-excitation CO ladders, and often low AGN contributions, suggesting that mechanical heating is important.

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arXiv2014

The relationship between polycyclic aromatic hydrocarbon emission and far-infrared dust emission from NGC 2403 and M83

We examine the relation between polycyclic aromatic hydrocarbon (PAH) emission at 8 microns and far-infrared emission from hot dust grains at 24 microns and from large dust grains at 160 and 250 microns in the nearby spiral galaxies NGC 2403 and M83 using data from the Spitzer Space Telescope and Herschel Space Observatory. We find that the PAH emission in NGC 2403 is better correlated with emission at 250 microns from dust heated by the diffuse interstellar radiation field (ISRF) and that the 8/250 micron surface brightness ratio is well-correlated with the stellar surface brightness as measured at 3.6 microns. This implies that the PAHs in NGC 2403 are intermixed with cold large dust grains in the diffuse interstellar medium (ISM) and that the PAHs are excited by the diffuse ISRF. In M83, the PAH emission appears more strongly correlated with 160 micron emission originating from large dust grains heated by star forming regions. However, the PAH emission in M83 is low where the 24 micron emission peaks within star forming regions, and enhancements in the 8/160 micron surface brightness ratios appear offset relative to the dust and the star forming regions within the spiral arms. This suggests that the PAHs observed in the 8 micron band are not excited locally within star forming regions but either by light escaping non-axisymmetrically from star forming regions or locally by young, non-photoionising stars that have migrated downstream from the spiral density waves. The results from just these two galaxies show that PAHs may be excited by different stellar populations in different spiral galaxies.

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arXiv2014

Insights into gas heating and cooling in the disc of NGC 891 from Herschel far-infrared spectroscopy

We present Herschel PACS and SPIRE spectroscopy of the most important far-infrared cooling lines in the nearby edge-on spiral galaxy, NGC 891: [CII] 158 $μ$m, [NII] 122, 205 $μ$m, [OI] 63, 145 $μ$m, and [OIII] 88 $μ$m. We find that the photoelectric heating efficiency of the gas, traced via the ([CII]+[OII]63)/$F_{\mathrm{TIR}}$ ratio, varies from a mean of 3.5$\times$10$^{-3}$ in the centre up to 8$\times$10$^{-3}$ at increasing radial and vertical distances in the disc. A decrease in ([CII]+[OII]63)/$F_{\mathrm{TIR}}$ but constant ([CII]+[OI]63)/$F_{\mathrm{PAH}}$ with increasing FIR colour suggests that polycyclic aromatic hydrocarbons (PAHs) may become important for gas heating in the central regions. We compare the observed flux of the FIR cooling lines and total IR emission with the predicted flux from a PDR model to determine the gas density, surface temperature and the strength of the incident far-ultraviolet (FUV) radiation field, $G_{0}$. Resolving details on physical scales of ~0.6 kpc, a pixel-by-pixel analysis reveals that the majority of the PDRs in NGC 891's disc have hydrogen densities of 1 < log ($n$/cm$^{-3}$) < 3.5 experiencing an incident FUV radiation field with strengths of 1.7 < log $G_0$ < 3. Although these values we derive for most of the disc are consistent with the gas properties found in PDRs in the spiral arms and inter-arm regions of M51, observed radial trends in $n$ and $G_0$ are shown to be sensitive to varying optical thickness in the lines, demonstrating the importance of accurately accounting for optical depth effects when interpreting observations of high inclination systems. With an empirical relationship between the MIPS 24 $μ$m and [NII] 205 $μ$m emission, we estimate an enhancement of the FUV radiation field strength in the far north-eastern side of the disc.

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arXiv2014

Quantifying the Heating Sources for Mid-infrared Dust Emissions in Galaxies: The Case of M 81

With the newly available SPIRE images at 250 and 500 micron from Herschel Space Observatory, we study quantitative correlations over a sub-kpc scale among three distinct emission components in the interstellar medium of the nearby spiral galaxy M 81 (NGC 3031): (a) $I_{8}$ or $I_{24}$, the surface brightness of the mid-infrared emission observed in the Spitzer IRAC 8 or MIPS 24 micron band, with $I_8$ and $I_{24}$ being dominated by the emissions from Polycyclic Aromatic Hydrocarbons (PAHs) and very small grains (VSGs) of dust, respectively; (b) $I_{500}$, that of the cold dust continuum emission in the Herschel SPIRE 500 micron band, dominated by the emission from large dust grains heated by evolved stars, and (c) $I_{{\rm H}α}$, a nominal surface brightness of the H$α$ line emission, from gas ionized by newly formed massive stars. The results from our correlation study, free from any assumption on or modeling of dust emissivity law or dust temperatures, present solid evidence for significant heating of PAHs and VSGs by evolved stars. In the case of M 81, about 67% (48%) of the 8 micron (24 micron) emission derives its heating from evolved stars, with the remainder attributed to radiation heating associated with ionizing stars.

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arXiv2014

The Herschel exploitation of local galaxy Andromeda (HELGA) V: Strengthening the case for substantial interstellar grain growth

In this paper we consider the implications of the distributions of dust and metals in the disc of M31. We derive mean radial dust distributions using a dust map created from Herschel images of M31 sampling the entire far-infrared (FIR) peak. Modified blackbodies are fit to approximately 4000 pixels with a varying, as well as a fixed, dust emissivity index (beta). An overall metal distribution is also derived using data collected from the literature. We use a simple analytical model of the evolution of the dust in a galaxy with dust contributed by stellar sources and interstellar grain growth, and fit this model to the radial dust-to-metals distribution across the galaxy. Our analysis shows that the dust-to-gas gradient in M31 is steeper than the metallicity gradient, suggesting interstellar dust growth is (or has been) important in M31. We argue that M31 helps build a case for cosmic dust in galaxies being the result of substantial interstellar grain growth, while the net dust production from stars may be limited. We note, however, that the efficiency of dust production in stars, e.g., in supernovae (SNe) ejecta and/or stellar atmospheres, and grain destruction in the interstellar medium (ISM) may be degenerate in our simple model. We can conclude that interstellar grain growth by accretion is likely at least as important as stellar dust production channels in building the cosmic dust component in M31.

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arXiv2014

The identification of dust heating mechanisms in nearby galaxies using Herschel 160/250 and 250/350 micron surface brightness ratios

We examined variations in the 160/250 and 250/350 micron surface brightness ratios within 24 nearby (<30 Mpc) face-on spiral galaxies observed with the Herschel Space Observatory to identify the heating mechanisms for dust emitting at these wavelengths. The analysis consisted of both qualitative and quantitative comparisons of the 160/250 and 250/350 micron ratios to H alpha and 24 micron surface brightnesses, which trace the light from star forming regions, and 3.6 micron emission, which traces the light from the older stellar populations of the galaxies. We find broad variations in the heating mechanisms for the dust. In one subset of galaxies, we found evidence that emission at <=160 microns (and in rare cases potentially at <=350 microns) originates from dust heated by star forming regions. In another subset, we found that the emission at >=250 microns (and sometimes at >=160 microns) originates from dust heated by the older stellar population. In the rest of the sample, either the results are indeterminate or both of these stellar populations may contribute equally to the global dust heating. The observed variations in dust heating mechanisms does not necessarily match what has been predicted by dust emission and radiative transfer models, which could lead to overestimated dust temperatures, underestimated dust masses, false detections of variability in dust emissivity, and inaccurate star formation rate measurements.

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arXiv2014

The Herschel Exploitation of Local Galaxy Andromeda (HELGA): IV. Dust scaling relations at sub-kpc resolution

The imprints of dust-starlight interactions are visible in scaling relations between stellar characteristics, star formation parameters and dust properties. We aim to examine dust scaling relations on a sub-kpc resolution in the Andromeda galaxy (M31) by comparing the properties on a local and global scale to other galaxies of the local universe. New Herschel observations are combined with available data from GALEX, SDSS, WISE and Spitzer to construct a dataset covering UV to submm wavelengths. We work at the resolution of the SPIRE $500\; μ$m beam, with pixels corresponding to physical regions of 137 x 608 pc in the galaxy's disk. A panchromatic spectral energy distribution was modelled for each pixel and several dust scaling relations are investigated. We find, on a sub-kpc scale, strong correlations between $M_d/M_\star$ and NUV-r, and between $M_d/M_\star$ and $μ_\star$ (the stellar mass surface density). Striking similarities with corresponding relations based on integrated galaxies are found. We decompose M31 in four macro-regions based on their FIR morphology; the bulge, inner disk, star forming ring and the outer disk. All regions closely follow the galaxy-scale average trends. The specific star formation characteristics we derive for these macro-regions give strong hints of an inside-out formation of the bulge-disk geometry, as well as an internal downsizing process. However, within each macro-region, a great diversity in individual micro-regions is found. Furthermore, we confirm that dust in the bulge of M31 is heated only by the old stellar populations. In general, the local dust scaling relations indicate that the dust content in M31 is maintained by a subtle interplay of past and present star formation. The similarity with galaxy-based relations strongly suggests that they are in situ correlations, with underlying processes that must be local in nature. (Abriged)

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arXiv2014

Extreme dust disks in Arp 220 as revealed by ALMA

We present new images of Arp 220 from the Atacama Large Millimeter/submillimeter Array with the highest combination of frequency (691 GHz) and resolution ($0.36 \times 0.20^{\prime\prime}$) ever obtained for this prototypical ultraluminous infrared galaxy. The western nucleus is revealed to contain warm (200 K) dust that is optically thick ($τ_{434μm} = 5.3$), while the eastern nucleus is cooler (80 K) and somewhat less opaque ($τ_{434μm} = 1.7$). We derive full-width half-maximum diameters of $ 76 \times \le 70$ pc and $123 \times 79$ pc for the western and eastern nucleus, respectively. The two nuclei combined account for ($83 ^{+65}_{-38}$ (calibration) $^{+0}_{-34}$ (systematic))% of the total infrared luminosity of Arp 220. The luminosity surface density of the western nucleus ($ \log(σT^4) = 14.3\pm 0.2 ^{+0}_{-0.7}$ in units of L$_\odot$ kpc$^{-2}$) appears sufficiently high to require the presence of an AGN or a "hot starburst", although the exact value depends sensitively on the brightness distribution adopted for the source. Although the role of any central AGN remains open, the inferred mean gas column densities of $0.6-1.8 \times 10^{25}$ cm$^{-2}$ mean that any AGN in Arp 220 must be Compton-thick.

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