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Register a new userCool dust heating and temperature mixing in nearby star-forming galaxies
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Physical conditions of the interstellar medium in galaxies are closely linked to the ambient radiation field and the heating of dust grains. In order to characterize dust properties in galaxies over a wide range of physical conditions, we present here the radial surface brightness profiles of the entire sample of 61 galaxies from Key Insights into Nearby Galaxies: Far-Infrared Survey with Herschel (KINGFISH). The main goal of our work is the characterization of the grain emissivities, dust temperatures, and interstellar radiation fields responsible for heating the dust. After fitting the dust and stellar radial profiles with exponential functions, we fit the far-infrared spectral energy distribution (SED) in each annular region with single-temperature modified black bodies using both variable (MBBV) and fixed (MBBF) emissivity indices beta, as well as with physically motivated dust models. Results show that while most SED parameters decrease with radius, the emissivity index beta also decreases with radius in some galaxies, but in others is increasing, or rising in the inner regions and falling in the outer ones. Despite the fixed grain emissivity (average beta~ 2.1) of the physically-motivated models, they are well able to accommodate flat spectral slopes with beta<= 1. We find that flatter slopes (beta<= 1.5) are associated with cooler temperatures, contrary to what would be expected from the usual Tdust-beta degeneracy. This trend is related to variations in Umin since beta and Umin are very closely linked over the entire range in Umin sampled by the KINGFISH galaxies: low Umin is associated with flat beta<=1. Both these results strongly suggest that the low apparent beta values (flat slopes) in MBBV fits are caused by temperature mixing along the line-of-sight, rather than by intrinsic variations in grain properties. Abstract truncated for arXiv.
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The infrared spectral energy distributions (SEDs) of main-sequence galaxies in the early universe (z > 4) is currently unconstrained as infrared continuum observations are time consuming and not feasible for large samples. We present Atacama Large Millimetre Array (ALMA) Band 8 observations of four main-sequence galaxies at z ~ 5.5 to study their infrared SED shape in detail. Our continuum data (rest-frame 110$rm mu m$, close to the peak of infrared emission) allows us to constrain luminosity weighted dust temperatures and total infrared luminosities. With data at longer wavelengths, we measure for the first time the emissivity index at these redshifts to provide more robust estimates of molecular gas masses based on dust continuum. The Band 8 observations of three out of four galaxies can only be reconciled with optically thin emission redward of rest-frame 100$rm mu m$. The derived dust peak temperatures at z ~ 5.5 (38$pm$8K) are elevated compared to average local galaxies, however, 5-10K below what would be predicted from an extrapolation of the trend at $z<4$. This behaviour can be explained by decreasing dust abundance (or density) towards high redshifts, which would cause the infrared SED at the peak to be more optically thin, making hot dust more visible to the external observer. From the 850$rm mu m$ dust continuum, we derive molecular gas masses between $10^{10}$ and $10^{11},{rm M_{odot}}$ and gas fractions (gas over total mass) of 30-80% (gas depletion times of 100-220Myrs). All in all, our results provide a first measured benchmark SED to interpret future millimetre observations of normal, main-sequence galaxies in the early Universe.
We present infrared views of the environmental effects on the dust properties in star-forming (SF) galaxies at z ~ 0, using the AKARI Far-Infrared Surveyor (FIS) all-sky map and the large spectroscopic galaxy sample from Sloan Digital Sky Survey (SDSS) Data Release 7 (DR7). We restrict the sample to those within the redshift range of 0.05 < z < 0.07 and the stellar mass range of 9.2 < log_10 (M_star/M_solar). We select SF galaxies based on their H_alpha equivalent width (EW_Ha> 4 A) and emission line flux ratios. We perform far-infrared (FIR) stacking analyses by splitting the SDSS SF galaxy sample according to their stellar mass, specific SFR (SSFR_SDSS), and environment. We derive total infrared luminosity (LIR) for each subsample using the average flux densities at WIDE-S (90 micron) and WIDE-L (140 micron) bands, and then compute IR-based SFR (SFR_IR) from L_IR. We find a mild decrease of IR- based SSFR (SSFR_IR) amongst SF galaxies with increasing local density (~0.1-dex level at maximum), which suggests that environmental effects do not instantly shut down the SF activity in galaxies. We also derive average dust temperature (T_dust) using the flux densities at 90 micron and 140 micron bands. We confirm a strong positive correlation between T_dust and SSFR_IR, consistent with recent studies. The most important finding of this study is that we find a marginal trend that T_dust increases with increasing environmental galaxy density. Although the environmental trend is much milder than the SSFR-T_dust correlation, our results suggest that the environmental density may affect the dust temperature in SF galaxies, and that the physical mechanism which is responsible for this phenomenon is not necessarily specific to cluster environments because the environmental dependence of T_dust holds down to relatively low-density environments.
We use new ALMA observations to investigate the connection between dense gas fraction, star formation rate, and local environment across the inner region of four local galaxies showing a wide range of molecular gas depletion times. We map HCN (1-0), HCO$^+$ (1-0), CS (2-1), $^{13}$CO (1-0), and C$^{18}$O (1-0) across the inner few kpc of each target. We combine these data with short spacing information from the IRAM large program EMPIRE, archival CO maps, tracers of stellar structure and recent star formation, and recent HCN surveys by Bigiel et al. and Usero et al. We test the degree to which changes in the dense gas fraction drive changes in the SFR. $I_{HCN}/I_{CO}$ (tracing the dense gas fraction) correlates strongly with $I_{CO}$ (tracing molecular gas surface density), stellar surface density, and dynamical equilibrium pressure, $P_{DE}$. Therefore, $I_{HCN}/I_{CO}$ becomes very low and HCN becomes very faint at large galactocentric radii, where ratios as low as $I_{HCN}/I_{CO} sim 0.01$ become common. The apparent ability of dense gas to form stars, $Sigma_{SFR}/Sigma_{dense}$ (where $Sigma_{dense}$ is traced by the HCN intensity and the star formation rate is traced by a combination of H$alpha$ and 24$mu$m emission), also depends on environment. $Sigma_{SFR}/Sigma_{dense}$ decreases in regions of high gas surface density, high stellar surface density, and high $P_{DE}$. Statistically, these correlations between environment and both $Sigma_{SFR}/Sigma_{dense}$ and $I_{HCN}/I_{CO}$ are stronger than that between apparent dense gas fraction ($I_{HCN}/I_{CO}$) and the apparent molecular gas star formation efficiency $Sigma_{SFR}/Sigma_{mol}$. We show that these results are not specific to HCN.
We aim to characterize the relationship between dust properties. We also aim to provide equations to estimate accurate dust properties from limited observational datasets. We assemble a sample of 1,630 nearby (z<0.1) galaxies-over a large range of Mstar, SFR - with multi-wavelength observations available from wise, iras, planck and/or SCUBA. The characterization of dust emission comes from SED fitting using Draine & Li dust models, which we parametrize using two components (warm and cold ). The subsample of these galaxies with global measurements of CO and/or HI are used to explore the molecular and/or atomic gas content of the galaxies. The total Lir, Mdust and dust temperature of the cold component (Tc) form a plane that we refer to as the dust plane. A galaxys sSFR drives its position on the dust plane: starburst galaxies show higher Lir, Mdust and Tc compared to Main Sequence and passive galaxies. Starburst galaxies also show higher specific Mdust (Mdust/Mstar) and specific Mgas (Mgas/Mstar). The Mdust is more closely correlated with the total Mgas (atomic plus molecular) than with the individual components. Our multi wavelength data allows us to define several equations to estimate Lir, Mdust and Tc from one or two monochromatic luminosities in the infrared and/or sub-millimeter. We estimate the dust mass and infrared luminosity from a single monochromatic luminosity within the R-J tail of the dust emission, with errors of 0.12 and 0.20dex, respectively. These errors are reduced to 0.05 and 0.10 dex, respectively, if the Tc is used. The Mdust is correlated with the total Mism (Mism propto Mdust^0.7). For galaxies with Mstar 8.5<log(Mstar/Msun) < 11.9, the conversion factor alpha_850mum shows a large scatter (rms=0.29dex). The SF mode of a galaxy shows a correlation with both the Mgass and Mdust: high Mdust/Mstar galaxies are gas-rich and show the highest SFRs.
It remains a major challenge to derive a theory of cloud-scale ($lesssim100$ pc) star formation and feedback, describing how galaxies convert gas into stars as a function of the galactic environment. Progress has been hampered by a lack of robust empirical constraints on the giant molecular cloud (GMC) lifecycle. We address this problem by systematically applying a new statistical method for measuring the evolutionary timeline of the GMC lifecycle, star formation, and feedback to a sample of nine nearby disc galaxies, observed as part of the PHANGS-ALMA survey. We measure the spatially-resolved ($sim100$ pc) CO-to-H$alpha$ flux ratio and find a universal de-correlation between molecular gas and young stars on GMC scales, allowing us to quantify the underlying evolutionary timeline. GMC lifetimes are short, typically 10-30 Myr, and exhibit environmental variation, between and within galaxies. At kpc-scale molecular gas surface densities $Sigma_{rm H_2}geqslant8$M$_{odot}$pc$^{-2}$, the GMC lifetime correlates with time-scales for galactic dynamical processes, whereas at $Sigma_{rm H_2}leqslant8$M$_{odot}$pc$^{-2}$ GMCs decouple from galactic dynamics and live for an internal dynamical time-scale. After a long inert phase without massive star formation traced by H$alpha$ (75-90% of the cloud lifetime), GMCs disperse within just 1-5 Myr once massive stars emerge. The dispersal is most likely due to early stellar feedback, causing GMCs to achieve integrated star formation efficiencies of 4-10% These results show that galactic star formation is governed by cloud-scale, environmentally-dependent, dynamical processes driving rapid evolutionary cycling. GMCs and HII regions are the fundamental units undergoing these lifecycles, with mean separations of 100-300 pc in star-forming discs. Future work should characterise the multi-scale physics and mass flows driving these lifecycles.
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