Observations of evolution in the dust-to-metal ratio allow us to constrain the dominant dust processing mechanisms. In this work, we present a study of the dust-to-metal and dust-to-gas ratios in a ...sub-sample of ~500 DustPedia galaxies. Using literature and MUSE emission line fluxes, we derived gas-phase metallicities (oxygen abundances) for over 10 000 individual regions and determine characteristic metallicities for each galaxy. We study how the relative dust, gas, and metal contents of galaxies evolve by using metallicity and gas fraction as proxies for evolutionary state. The global oxygen abundance and nitrogen-to-oxygen ratio are found to increase monotonically as galaxies evolve. Additionally, unevolved galaxies (gas fraction >60%, metallicity 12 + log(O∕H) < 8.2) have dust-to-metal ratios that are about a factor of 2.1 lower (a factor of six lower for galaxies with gas fraction >80%) than the typical dust-to-metal ratio (Md∕MZ ~ 0.214) for more evolved sources. However, for high gas fractions, the scatter is larger due to larger observational uncertainties as well as a potential dependence of the dust grain growth timescale and supernova dust yield on local conditions and star formation histories. We find chemical evolution models with a strong contribution from dust grain growth describe these observations reasonably well. The dust-to-metal ratio is also found to be lower for low stellar masses and high specific star formation rates (with the exception of some sources undergoing a starburst). Finally, the metallicity gradient correlates weakly with the HI-to-stellar mass ratio, the effective radius and the dust-to-stellar mass ratio, but not with stellar mass.
Dynamic and thermal processes regulate the structure of the multiphase interstellar medium (ISM), and ultimately establish how galaxies evolve through star formation. Thus, to constrain ISM models ...and better understand the interplay of these processes, it is of great interest to measure the thermal pressure ( ) of the diffuse, neutral gas. By combining C ii 158 m, H i, and CO data from 31 galaxies selected from the Herschel KINGFISH sample, we have measured thermal pressures in 534 predominantly atomic regions with typical sizes of ∼1 kiloparsec. We find a distribution of thermal pressures in the K cm−3 range. For a sub-sample of regions with conditions similar to those of the diffuse, neutral gas in the Galactic plane, we find thermal pressures that follow a log-normal distribution with a median value of Pth/k 3600 K cm−3. These results are consistent with thermal pressure measurements using other observational methods. We find that increases with radiation field strength and star formation activity, as expected from the close link between the heating of the gas and the star formation rate. Our thermal pressure measurements fall in the regime where a two-phase ISM with cold and warm neutral media could exist in pressure equilibrium. Finally, we find that the midplane thermal pressure of the diffuse gas is about ∼30% of the vertical weight of the overlying ISM, consistent with results from hydrodynamical simulations of self-regulated star formation in galactic disks.
Context. To compute the star formation rate (SFR) of galaxies from the rest-frame ultraviolet (UV), it is essential to take the obscuration by dust into account. To do so, one of the most popular ...methods consists in combining the UV with the emission from the dust itself in the infrared (IR). Yet, different studies have derived different estimators, showing that no such hybrid estimator is truly universal. Aims. In this paper we aim at understanding and quantifying what physical processes fundamentally drive the variations between different hybrid estimators. In so doing, we aim at deriving new universal UV+IR hybrid estimators to correct the UV for dust attenuation at local and global scales, taking the intrinsic physical properties of galaxies into account. Methods. We use the CIGALE code to model the spatially resolved far-UV to far-IR spectral energy distributions of eight nearby star-forming galaxies drawn from the KINGFISH sample. This allows us to determine their local physical properties, and in particular their UV attenuation, average SFR, average specific SFR (sSFR), and their stellar mass. We then examine how hybrid estimators depend on said properties. Results. We find that hybrid UV+IR estimators strongly depend on the stellar mass surface density (in particular at 70 μm and 100 μm) and on the sSFR (in particular at 24 μm and the total infrared). Consequently, the IR scaling coefficients for UV obscuration can vary by almost an order of magnitude: from 1.55 to 13.45 at 24 μm for instance. This result contrasts with other groups who found relatively constant coefficients with small deviations. We exploit these variations to construct a new class of adaptative hybrid estimators based on observed UV to near-IR colours and near-IR luminosity densities per unit area. We find that they can reliably be extended to entire galaxies. Conclusions. The new estimators provide better estimates of attenuation-corrected UV emission than classical hybrid estimators published in the literature. Taking naturally variable impact of dust heated by old stellar populations into account, they constitute an important step towards universal estimators.
Aims. We compare the far-infrared to sub-millimetre dust emission properties measured in high Galactic latitude cirrus with those determined in a sample of 204 late-type DustPedia galaxies. The aim ...is to verify if it is appropriate to use Milky Way dust properties to derive dust masses in external galaxies. Methods. We used Herschel observations and atomic and molecular gas masses to estimate ϵ(250 μm), the disc-averaged dust emissivity at 250 μm, and from this, the absorption cross section per H atom σ(250 μm) and per dust mass κ(250 μm). The emissivity ϵ(250 μm) requires one assumption, which is the CO-to-H2 conversion factor, and the dust temperature is additionally required for σ(250 μm); yet another constraint on the dust-to-hydrogen ratio D/H, depending on metallicity, is required for κ(250 μm). Results. We find ϵ(250 μm) = 0.82 ± 0.07 MJy sr−1 (1020 H cm−2)−1 for galaxies with 4 < F(250 μm)/F(500 μm) < 5. This depends only weakly on the adopted CO-to-H2 conversion factor. The value is almost the same as that for the Milky Way at the same colour ratio. Instead, for F(250 μm)/F(500 μm) > 6, ϵ(250 μm) is lower than predicted by its dependence on the heating conditions. The reduction suggests a variation in dust emission properties for spirals of earlier type, higher metallicity, and with a higher fraction of molecular gas. When the standard emission properties of Galactic cirrus are used for these galaxies, their dust masses might be underestimated by up to a factor of two. Values for σ(250 μm) and κ(250 μm) at the Milky Way metallicity are also close to those of the cirrus. Mild trends of the absorption cross sections with metallicity are found, although the results depend on the assumptions made.
ABSTRACT
The dust mass absorption coefficient, κd is the conversion function used to infer physical dust masses from observations of dust emission. However, it is notoriously poorly constrained, and ...it is highly uncertain how it varies, either between or within galaxies. Here we present the results of a proof-of-concept study, using the DustPedia data for two nearby face-on spiral galaxies M 74 (NGC 628) and M 83 (NGC 5236), to create the first ever maps of κd in galaxies. We determine κd using an empirical method that exploits the fact that the dust-to-metals ratio of the interstellar medium is constrained by direct measurements of the depletion of gas-phase metals. We apply this method pixel-by-pixel within M 74 and M 83, to create maps of κd. We also demonstrate a novel method of producing metallicity maps for galaxies with irregularly sampled measurements, using the machine learning technique of Gaussian process regression. We find strong evidence for significant variation in κd. We find values of κd at 500 $\mu$m spanning the range 0.11–0.25 ${\rm m^{2}\, kg^{-1}}$ in M 74, and 0.15–0.80 ${\rm m^{2}\, kg^{-1}}$ in M 83. Surprisingly, we find that κd shows a distinct inverse correlation with the local density of the interstellar medium. This inverse correlation is the opposite of what is predicted by standard dust models. However, we find this relationship to be robust against a large range of changes to our method – only the adoption of unphysical or highly unusual assumptions would be able to suppress it.
Context. Star formation activity is an important driver of galaxy evolution and is influenced by the physical properties of the interstellar medium. Dwarf galaxies allow us to understand how the ...propagation of radiation and the physical conditions of the different ISM phases are affected by the low-metallicity environment. Aims. Our objective is to investigate the physical properties of the ionized gas of the low-metallicity dwarf galaxy, IC 10, at various spatial scales: from individual H II regions to the entire galaxy scale and examine whether diagnostics for integrated measurements introduce bias in the results. Methods. We modeled the ionized gas combining the mid- and far-infrared fine-structure cooling lines observed with Spitzer/IRS and Herschel/PACS, with the photoionization code CLOUDY. The free parameters of the models are the age of the stellar cluster, the density, and the ionization parameter of the ionized gas as well as the depth of the cloud. The latter is used to investigate the leakage of the ionizing photons from the analyzed regions of IC 10. We investigated H II regions in the main star-forming body, on scales of ~25 pc, three in the main star-forming region in the center of the galaxy and two on the first arc. We then considered larger sizes on the scale of ~200 pc. Results. Most clumps have almost-identical properties, density ~102–102.6 cm−3, ionization parameter between 10−2.2 and 10−1.6, and age of the stellar cluster ~5.5 Myr. All of them are matter-bounded regions, allowing ionizing photons to leak. The relatively uniform physical properties of the clumps suggest a common origin for their star formation activity, which could be related to the feedback from stellar winds or supernovae of a previous generation of stars. The properties derived for ~200 pc size “zones” have similar properties as the H II regions they encompass, but with the larger regions tending to be more radiation bounded. Finally, we investigated the fraction of CII 157.7 μm, SiII 34.8 μm and FeII 25.9 μm emission arising from the ionized gas phase and we find that most of the emission originates from the neutral gas, not from the ionized gas.
When combined with infrared observations with the Spitzer telescope (3 to 160 μm), the Herschel
Space Observatory now fully samples the thermal dust emission up to 500 μm and enables us to better ...estimate the total infrared-submm energy budget (L
TIR) of nearby galaxies. We present new empirical calibrations to estimate resolved and integrated total infrared luminosities from Spitzer and Herschel bands used as monochromatic or combined tracers. We base our calibrations on resolved elements of nearby galaxies (3 to 30 Mpc) observed with Herschel. We perform a resolved spectral energy distribution (SED) modelling of these objects using the Draine & Li dust models and investigate the influence of the addition of Spectral and Photometric Imaging Receiver (SPIRE) measurements in the estimation of L
TIR. We find that using data up to 250 μm leads to local L
TIR values consistent with those obtained with a complete coverage (up to 500 μm) within ±10 per cent for most of our resolved elements. We then study the distribution of energy in the resolved SEDs of our galaxies. The bulk of energy (30-50 per cent) is contained in the 70-160 μm band. The 24-70 μm fraction decreases with increasing metallicity. The 160-1100 μmsubmillimetre band can account for up to 25 per cent of the L
TIR in metal-rich galaxies. We investigate the correlation between the total infrared (TIR) surface brightnesses/luminosities and monochromatic Spitzer and Herschel surface brightnesses/luminosities. The three Photodetector Array Camera and Spectrometer (PACS) bands can be used as reliable monochromatic estimators of the L
TIR, the 100 μm band being the most reliable monochromatic tracer. There is also a strong correlation between the SPIRE 250 μm and L
TIR, although with more scatter than for the PACS relations. We also study the ability of our monochromatic relations to reproduce integrated L
TIR of nearby galaxies as well as L
TIR of z ∼ 1-3 sources. Finally, we provide calibration coefficients that can be used to derive TIR surface brightnesses/luminosities from a combination of Spitzer and Herschel surface brightnesses/fluxes and analyse the associated uncertainties.
Although it accounts only for a small fraction of the baryonic mass, dust has a profound impact on the physical processes at play in galaxies. Thus, to understand the evolution of galaxies, it is ...essential not only to characterize dust properties per se, but also in relation to global galaxy properties. To do so, we derive the dust properties of galaxies in a volume limited, K-band selected sample, the Herschel Reference Survey (HRS). We gather infrared photometric data from 8 μm to 500 μm from Spitzer, WISE, IRAS, and Herschel for all of the HRS galaxies. Draine & Li (2007, ApJ, 663, 866) models are fit to the data from which the stellar contribution has been carefully removed. We find that our photometric coverage is sufficient to constrain all of the parameters of the Draine & Li models and that a strong constraint on the 20−60 μm range is mandatory to estimate the relative contribution of the photo-dissociation regions to the infrared spectral energy distribution (SED). The SED models tend to systematically underestimate the observed 500 μm flux densities, especially for low-mass systems. We provide the output parameters for all of the galaxies, i.e., the minimum intensity of the interstellar radiation field, the fraction of polycyclic aromatic hydrocarbon (PAH), the relative contribution of PDR and evolved stellar population to the dust heating, the dust mass, and the infrared luminosity. For a subsample of gas-rich galaxies, we analyze the relations between these parameters and the main integrated properties of galaxies, such as stellar mass, star formation rate, infraredluminosity, metallicity, Hα and H-band surface brightness, and the far-ultraviolet attenuation. A good correlation between the fraction of PAH and the metallicity is found, implying a weakening of the PAH emission in galaxies with low metallicities and, thus, low stellar masses. The intensity of the diffuse interstellar radiation field and the H-band and Hα surface brightnesses are correlated, suggesting that the diffuse dust component is heated by both the young stars in star-forming regions and the diffuse evolved population. We use these results to provide a new set of infrared templates calibrated with Herschel observations on nearby galaxies and a mean SED template to provide the z = 0 reference for cosmological studies. For the same purpose, we place our sample on the SFR − M∗ diagram. The templates are compared to the most popular infrared SED libraries, enlightening a large discrepancy between all of them in the 20−100 μm range.
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