arXiv subjects

B. T. Draine

Publications and source records attributed to B. T. Draine.

At least 73 records · Page 4Linked to original sources

arXiv2012

A Study of Heating and Cooling of the ISM in NGC 1097 with Herschel-PACS and Spitzer-IRS

NGC 1097 is a nearby Seyfert 1 galaxy with a bright circumnuclear starburst ring, a strong large-scale bar and an active nucleus. We present a detailed study of the spatial variation of the far infrared (FIR) [CII]158um and [OI]63um lines and mid-infrared H2 emission lines as tracers of gas cooling, and of the polycyclic aromatic hydrocarbon (PAH) bands as tracers of the photoelectric heating, using Herschel-PACS, and Spitzer-IRS infrared spectral maps. We focus on the nucleus and the ring, and two star forming regions (Enuc N and Enuc S). We estimated a photoelectric gas heating efficiency ([CII]158um+[OI]63um)/PAH in the ring about 50% lower than in Enuc N and S. The average 11.3/7.7um PAH ratio is also lower in the ring, which may suggest a larger fraction of ionized PAHs, but no clear correlation with [CII]158μm/PAH(5.5 - 14um) is found. PAHs in the ring are responsible for a factor of two more [CII]158um and [OI]63um emission per unit mass than PAHs in the Enuc S. SED modeling indicates that at most 25% of the FIR power in the ring and Enuc S can come from high intensity photodissociation regions (PDRs), in which case G0 ~ 10^2.3 and nH ~ 10^3.5 cm^-3 in the ring. For these values of G0 and nH PDR models cannot reproduce the observed H2 emission. Much of the the H2 emission in the starburst ring could come from warm regions in the diffuse ISM that are heated by turbulent dissipation or shocks.

astro-ph.GA↗

arXiv2012

Resolving the Far-IR Line Deficit: Photoelectric Heating and Far-IR Line Cooling in NGC 1097 and NGC 4559

The physical state of interstellar gas and dust is dependent on the processes which heat and cool this medium. To probe heating and cooling of the ISM over a large range of infrared surface brightness, on sub-kiloparsec scales, we employ line maps of [C \ii] 158 $μ$m, [O \one] 63 $μ$m, and [N \ii] 122 $μ$m in NGC 1097 and NGC 4559, obtained with the PACS spectrometer onboard {\it Herschel}. We matched new observations to existing Spitzer-IRS data that trace the total emission of polycyclic aromatic hydrocarbons (PAHs). We confirm at small scales in these galaxies that the canonical measure of photoelectric heating efficiency, ([C \ii] + [O \one])/TIR, decreases as the far-infrared color, $νf_ν$(70 $μ$m)/$νf_ν$(100 $μ$m), increases. In contrast, the ratio of far-infrared (far-IR) cooling to total PAH emission, ([C \ii] + [O \one])/PAH, is a near constant $\sim$6% over a wide range of far-infrared color, 0.5 \textless\ $νf_ν$(70 $μ$m)/$νf_ν$(100 $μ$m) $\lesssim$ 0.95. In the warmest regions, where $νf_ν$(70 $μ$m)/$νf_ν$(100 $μ$m) $\gtrsim$ 0.95, the ratio ([C \ii] + [O \one])/PAH drops rapidly to 4%. We derived representative values of the local UV radiation density, $G_0$, and the gas density, $n_H$, by comparing our observations to models of photodissociation regions. The ratio $G_0/n_H$, derived from fine-structure lines, is found to correlate with the mean dust-weighted starlight intensity, $ $ derived from models of the IR SED. Emission from regions that exhibit a line deficit is characterized by an intense radiation field, indicating that small grains are susceptible to ionization effects. We note that there is a shift in the 7.7 / 11.3 $μ$m PAH ratio in regions that exhibit a deficit in ([C \ii] + [O \one])/PAH, suggesting that small grains are ionized in these environments.

astro-ph.CO↗

arXiv2011

Herschel Far-Infrared and Sub-millimeter Photometry for the KINGFISH Sample of Nearby Galaxies

New far-infrared and sub-millimeter photometry from the Herschel Space Observatory is presented for 61 nearby galaxies from the Key Insights on Nearby Galaxies: A Far-Infrared Survey with Herschel (KINGFISH) sample. The spatially-integrated fluxes are largely consistent with expectations based on Spitzer far-infrared photometry and extrapolations to longer wavelengths using popular dust emission models. Dwarf irregular galaxies are notable exceptions, as already noted by other authors, as their 500um emission shows evidence for a sub-millimeter excess. In addition, the fraction of dust heating attributed to intense radiation fields associated with photo-dissociation regions is found to be (21+/-4)% larger when Herschel data are included in the analysis. Dust masses obtained from the dust emission models of Draine & Li are found to be on average nearly a factor of two higher than those based on single-temperature modified blackbodies, as single blackbody curves do not capture the full range of dust temperatures inherent to any galaxy. The discrepancy is largest for galaxies exhibiting the coolest far-infrared colors.

astro-ph.CO↗

arXiv2011

KINGFISH -- Key Insights on Nearby Galaxies: A Far-Infrared Survey with Herschel: Survey Description and Image Atlas

The KINGFISH project (Key Insights on Nearby Galaxies: a Far-Infrared Survey with Herschel) is an imaging and spectroscopic survey of 61 nearby (d < 30 Mpc) galaxies, chosen to cover a wide range of galaxy properties and local interstellar medium (ISM) environments found in the nearby Universe. Its broad goals are to characterize the ISM of present-day galaxies, the heating and cooling of their gaseous and dust components, and to better understand the physical processes linking star formation and the ISM. KINGFISH is a direct descendant of the Spitzer Infrared Nearby Galaxies Survey (SINGS), which produced complete Spitzer imaging and spectroscopic mapping and a comprehensive set of multi-wavelength ancillary observations for the sample. The Herschel imaging consists of complete maps for the galaxies at 70, 100, 160, 250, 350, and 500 microns. The spectal line imaging of the principal atomic ISM cooling lines ([OI]63um, [OIII]88um, [NII]122,205um, and [CII]158um) covers the subregions in the centers and disks that already have been mapped in the mid-infrared with Spitzer. The KINGFISH and SINGS multi-wavelength datasets combined provide panchromatic mapping of the galaxies sufficient to resolve individual star-forming regions, and tracing the important heating and cooling channels of the ISM, across a wide range of local extragalactic ISM environments. This paper summarizes the scientific strategy for KINGFISH, the properties of the galaxy sample, the observing strategy, and data processing and products. It also presents a combined Spitzer and Herschel image atlas for the KINGFISH galaxies, covering the wavelength range 3.6 -- 500 microns. All imaging and spectroscopy data products will be released to the Herschel user generated product archives.

astro-ph.CO↗

arXiv2011

Spitzer IRS Detection of Molecular Hydrogen Rotational Emission Towards Translucent Clouds

Using the Infrared Spectrograph on board the Spitzer Space Telescope, we have detected emission in the S(0), S(1), and S(2) pure-rotational (v=0-0) transitions of molecular hydrogen (H2) towards 6 positions in two translucent high Galactic latitude clouds, DCld 300.2-16.9 and LDN 1780. The detection of these lines raises important questions regarding the physical conditions inside low-extinction clouds that are far from ultraviolet radiation sources. The ratio between the S(2) flux and the flux from PAHs at 7.9 microns averages 0.007 for these 6 positions. This is a factor of about 4 higher than the same ratio measured towards the central regions of non-active Galaxies in the Spitzer Infrared Nearby Galaxies Survey (SINGS). Thus the environment of these translucent clouds is more efficient at producing rotationally excited H2 per PAH-exciting photon than the disks of entire galaxies. Excitation analysis finds that the S(1) and S(2) emitting regions are warm (T >300K), but comprise no more than 2% of the gas mass. We find that UV photons cannot be the sole source of excitation in these regions and suggest mechanical heating via shocks or turbulent dissipation as the dominant cause of the emission. The clouds are located on the outskirts of the Scorpius-Centaurus OB association and may be dissipating recent bursts of mechanical energy input from supernova explosions. We suggest that pockets of warm gas in diffuse or translucent clouds, integrated over the disks of galaxies, may represent a major source of all non-active galaxy H2 emission.

astro-ph.GA↗

arXiv2011

Common-Resolution Convolution Kernels for Space- and Ground-Based Telescopes

Multi-wavelength study of extended astronomical objects requires combining images from instruments with differing point spread functions (PSFs). We describe the construction of convolution kernels that allow one to generate (multi-wavelength) images with a common PSF, thus preserving the colors of the astronomical sources. We generate convolution kernels for the cameras of the Spitzer Space Telescope, Herschel Space Observatory, Galaxy Evolution Explorer (GALEX), Wide-field Infrared Survey Explorer (WISE), ground-based optical telescopes (Moffat functions and sum of Gaussians), and Gaussian PSFs. These kernels allow the study of the Spectral Energy Distribution (SED) of extended objects, preserving the characteristic SED in each pixel. The convolution kernels and the IDL packages used to construct and use them are made publicly available.

astro-ph.IM↗

arXiv2011

The Spitzer Spectroscopic Survey of the Small Magellanic Cloud (S4MC): Probing the Physical State of Polycyclic Aromatic Hydrocarbons in a Low-Metallicity Environment

We present results of mid-infrared spectroscopic mapping observations of six star-forming regions in the Small Magellanic Cloud from the Spitzer Spectroscopic Survey of the SMC (S4MC). We detect the mid-IR emission from polycyclic aromatic hydrocarbons (PAHs) in all of the mapped regions, greatly increasing the range of environments where PAHs have been spectroscopically detected in the SMC. We investigate the variations of the mid-IR bands in each region and compare our results to studies of the PAH bands in the SINGS sample and in a sample of low-metallicity starburst galaxies. PAH emission in the SMC is characterized by low ratios of the 6-9 micron features relative to the 11.3 micron feature and weak 8.6 and 17.0 micron features. Interpreting these band ratios in the light of laboratory and theoretical studies, we find that PAHs in the SMC tend to be smaller and less ionized than those in higher metallicity galaxies. Based on studies of PAH destruction, we argue that a size distribution shifted towards smaller PAHs cannot be the result of processing in the interstellar medium, but instead reflects differences in the formation of PAHs at low metallicity. Finally, we discuss the implications of our observations for our understanding of the PAH life-cycle in low-metallicity galaxies---namely that the observed deficit of PAHs may be a consequence of PAHs forming with smaller average sizes and therefore being more susceptible to destruction under typical interstellar medium conditions.

astro-ph.CO↗

arXiv2011

Spinning Dust Emission: Effects of irregular grain shape, transient heating and comparison with WMAP results

Planck is expected to answer crucial questions on the early Universe, but it also provides further understanding on anomalous microwave emission. Electric dipole emission from spinning dust grains continues to be the favored interpretation of anomalous microwave emission. In this paper, we present a method to calculate the rotational emission from small grains of irregular shape with moments of inertia $I_{1}> I_{2}> I_{3}$. We show that a torque-free rotating irregular grain with a given angular momentum radiates at multiple frequency modes. The resulting spinning dust spectrum has peak frequency and emissivity increasing with the degree of grain shape irregularity, which is defined by $I_{1}:I_{2}:I_{3}$. We discuss how the orientation of dipole moment $\bmu$ in body coordinates affects the spinning dust spectrum for different regimes of internal thermal fluctuations. We show that the spinning dust emissivity for the case of strong thermal fluctuations is less sensitive to the orientation of $\bmu$ than in the case of weak thermal fluctuations. We calculate spinning dust spectra for a range of gas density and dipole moment. The effect of compressible turbulence on spinning dust emission intensity is investigated. We show that the emission intensity in a turbulent medium increases by a factor from 1.2-1.4 relative to that in a uniform medium, as sonic Mach number $M_{s}$ increases from 2-7. Finally, spinning dust parameters are constrained by fitting our improved model to five-year {\it Wilkinson Microwave Anisotropy Probe} cross-correlation foreground spectra, for both the H$α$-correlated and 100 $μ$m-correlated emission spectra.

astro-ph.GA↗

arXiv2011

Calibrating Extinction-Free Star Formation Rate Diagnostics with 33GHz Free-Free Emission in NGC6946

Abridged: Using free-free emission measured in the Ka-band (26-40GHz) for 10 star-forming regions in the nearby galaxy NGC6946, including its starbursting nucleus, we compare a number of SFR diagnostics that are typically considered to be unaffected by interstellar extinction: i.e., non-thermal radio (i.e., 1.4GHz), total infrared (IR; 8-1000um), and warm dust (i.e., 24um) emission, along with the hybrid (obscured + unobscured) indicators of Hα+24um and UV+IR. The 33GHz free-free emission is assumed to provide the most accurate measure of the current SFR. Among the extranuclear star-forming regions, the 24um, Hα+24um and UV+IR SFR calibrations are in good agreement with the 33GHz free-free SFRs. However, each of the SFR calibrations relying on some form of dust emission overestimate the nuclear SFR by a factor of ~2. This is more likely the result of excess dust heating through an accumulation of non-ionizing stars associated with an extended episode of star formation in the nucleus rather than increased competition for ionizing photons by dust. SFR calibrations using the non-thermal radio continuum yield values which only agree with the free-free SFRs for the nucleus, and underestimate the SFRs from the extranuclear star-forming regions by a factor of ~2. This result likely arises from the CR electrons decaying within the starburst region with negligible escape compared to the young extranuclear star-forming regions. Finally, we find that the SFRs estimated using the total 33GHz emission agree well with the free-free SFRs due to the large thermal fractions present at these frequencies even when local diffuse backgrounds are not removed. Thus, rest-frame 33GHz observations may act as a reliable method to measure the SFRs of galaxies at increasingly high redshift without the need of ancillary radio data to account for the non-thermal emission.

astro-ph.CO↗

arXiv2011

On Radiation Pressure in Static, Dusty HII Regions

Radiation pressure acting on gas and dust causes HII regions to have central densities that are lower than the density near the ionized boundary. HII regions in static equilibrium comprise a family of similarity solutions, parametrized by 3 parameters: beta, gamma, and the product (Q_0 n_rms); beta characterizes the stellar spectrum, gamma characterizes the dust/gas ratio, Q_0 is the ionizing output from the star (photons/s), and n_rms is the rms density within the ionized region. Adopting standard values for beta and gamma, varying (Q_0 n_rms) generates a one-parameter family of density profiles, ranging from nearly uniform density (small Q_0 n_rms), to hollow-sphere HII regions (large Q_0 n_rms). When (Q_0 n_rms) exceeds 10^{52} cm^{-3} s^{-1}, dusty HII regions have conspicuous central cavities, even if no stellar wind is present. For given beta, gamma and (Q_0 n_rms), a fourth quantity, which can be Q_0, determines the overall size and density of the HII region. Examples of density and emissivity profiles are given. We show how quantities of interest -- such as the peak-to-center emissivity ratio, the rms-to-mean density ratio, the edge-to-rms density ratio, and the fraction of the ionizing photons absorbed by the gas -- depend on the 3 parameters beta, gamma, and (Q_0 n_rms). For dusty HII regions, compression of the gas and dust into an ionized shell results in a substantial increase in the fraction of the >13.6 eV photons that actually ionize H (relative to a uniform density HII region with the same dust/gas ratio and density n=n_rms). We discuss the extent to which radial drift of dust grains in HII regions can alter the dust-to-gas ratio. The applicability of these solutions to real HII regions is discussed.

astro-ph.GA↗

arXiv2011

The Displaced Dusty ISM of NGC 3077: Tidal Stripping in the M 81 Triplet

We present the detection of extended (~30 kpc^2) dust emission in the tidal \hi\ arm near NGC 3077 (member of the M\,81 triplet) using SPIRE on board Herschel. Dust emission in the tidal arm is typically detected where the \hi\ column densities are >10^21 cm^-2. The SPIRE band ratios show that the dust in the tidal arm is significantly colder (~13 K) than in NGC 3077 itself (~31 K), consistent with the lower radiation field in the tidal arm. The total dust mass in the tidal arm is ~1.8 x 10^6 M_sun (assuming beta=2), i.e. substantially larger than the dust mass associated with NGC 3077 (~2 x 10^5 M_sun). Where dust is detected, the dust-to-gas ratio is 6+/-3 x 10^-3, consistent within the uncertainties with what is found in NGC 3077 and nearby spiral galaxies with Galactic metallicities. The faint HII regions in the tidal arm can not be responsible for the detected enriched material and are not the main source of the dust heating in the tidal arm. We conclude that the interstellar medium (atomic HI, molecules and dust) in this tidal feature was pre-enriched and stripped off NGC 3077 during its recent interaction (~3 x 10^8 yr ago) with M 82 and M 81. This implies that interaction can efficiently remove heavy elements and enriched material (dust, molecular gas) from galaxies. As interactions were more frequent at large lookback times, it is conceivable that they could substantially contribute (along with galactic outflows) to the enrichment of the intergalactic medium.

astro-ph.CO↗

arXiv2010

H-alpha and Free-Free Emission from the WIM

Recent observations have found the ratio of H-alpha to free-free radio continuum to be surprisingly high in the diffuse ionized ISM (the so-called WIM), corresponding to an electron temperature of only ~3000K. Such low temperatures were unexpected in gas that was presumed to be photoionized. We consider a 3-component model for the observed diffuse emission, consisting of a mix of (1) photoionized gas, (2) gas that is recombining and cooling, and (3) cool H I gas. This model can successfully reproduce the observed intensities of free-free continuum, H-alpha, and collisionally-excited lines such as NII 6583. To reproduce the low observed value of free-free to H-alpha, the PAH abundance in the photoionized regions must be lowered by a factor ~3, and ~20% of the diffuse H-alpha must be reflected from dust grains, as suggested by Wood & Reynolds (1999).

astro-ph.GA↗

arXiv2010

Nearby Galaxies with Spitzer

We review the main advances brought by the Spitzer Space Telescope in the field of nearby galaxies studies, concentrating on a few subject areas, including: (1) the physics of the Polycyclic Aromatic Hydrocarbons that generate the mid-infrared features between ~3.5 micron and ~20 micron; (2) the use of the mid- and far-infrared emission from galaxies as star formation rate indicators; and (3) the improvement of mid-infrared diagnostics to discriminate between thermal (star-formation) and non-thermal (AGN) emission in galaxies and galaxy centers.

astro-ph.CO↗

arXiv2010

Improving the model of emission from spinning dust: effects of grain wobbling and transient spin-up

Observations continue to support the interpretation of the anomalous microwave foreground as electric dipole radiation from spinning dust grains as proposed by Draine and Lazarian (1998ab). In this paper we present a refinement of the original model by improving the treatment of a number of physical effects. First, we consider a disk-like grain rotating with angular velocity at an arbitrary angle with respect to the grain symmetry axis and derive the rotational damping and excitation coefficients arising from infrared emission, plasma-grain interactions and electric dipole emission. The angular velocity distribution and the electric dipole emission spectrum for grains is calculated using the Langevin equation, for cases both with and without fast internal relaxation. Our results show that, the peak emissivity of spinning dust, compared to earlier studies, increases by a factor of ~2 for the Warm Neutral Medium (WNM), the Warm Ionized Medium (WIM), the Cold Neutral Medium (CNM) and the Photodissociation Region (PDR), and by a factor ~4 for Reflection Nebulae (RN). The frequency at the emission peak also increases by factors ~1.4 to ~2 for these media. The increased emission and peak frequency result from the non-sphericity of grain shape and from the anisotropy in damping and excitation along directions parallel and perpendicular to the grain symmetry axis. Second, we provide a detailed numerical study including transient spin-up of grains by single-ion collisions. The impulses broaden the emission spectrum and increase the peak emissivity for the CNM, WNM and WIM. In addition, we present an improved treatment of rotational excitation and damping by infrared emission.

astro-ph.GA↗

arXiv2010

Mapping far-IR emission from the central kiloparsec of NGC 1097

Using photometry of NGC 1097 from the Herschel PACS (Photodetector Array Camera and Spectrometer) instrument, we study the resolved properties of thermal dust continuum emission from a circumnuclear starburst ring with a radius ~ 900 pc. These observations are the first to resolve the structure of a circumnuclear ring at wavelengths that probe the peak (i.e. lambda ~ 100 micron) of the dust spectral energy distribution. The ring dominates the far-infrared (far-IR) emission from the galaxy - the high angular resolution of PACS allows us to isolate the ring's contribution and we find it is responsible for 75, 60 and 55% of the total flux of NGC 1097 at 70, 100 and 160 micron, respectively. We compare the far-IR structure of the ring to what is seen at other wavelengths and identify a sequence of far-IR bright knots that correspond to those seen in radio and mid-IR images. The mid- and far-IR band ratios in the ring vary by less than +/- 20% azimuthally, indicating modest variation in the radiation field heating the dust on ~ 600 pc scales. We explore various explanations for the azimuthal uniformity in the far-IR colors of the ring including a lack of well-defined age gradients in the young stellar cluster population, a dominant contribution to the far-IR emission from dust heated by older (> 10 Myr) stars and/or a quick smoothing of local enhancements in dust temperature due to the short orbital period of the ring. Finally, we improve previous limits on the far-IR flux from the inner ~ 600 pc of NGC 1097 by an order of magnitude, providing a better estimate of the total bolometric emission arising from the active galactic nucleus and its associated central starburst.

astro-ph.CO↗

arXiv2010

Enhanced dust heating in the bulges of early-type spiral galaxies

Stellar density and bar strength should affect the temperatures of the cool (T ~ 20-30 K) dust component in the inner regions of galaxies, which implies that the ratio of temperatures in the circumnuclear regions to the disk should depend on Hubble type. We investigate the differences between cool dust temperatures in the central 3 kpc and disk of 13 nearby galaxies by fitting models to measurements between 70 and 500 microns. We attempt to quantify temperature trends in nearby disk galaxies, with archival data from Spitzer/MIPS and new observations with Herschel/SPIRE, which were acquired during the first phases of the Herschel observations for the KINGFISH (key insights in nearby galaxies: a far-infrared survey with Herschel) sample. We fit single-temperature modified blackbodies to far-infrared and submillimeter measurements of the central and disk regions of galaxies to determine the temperature of the component(s) emitting at those wavelengths. We present the ratio of central-region-to-disk-temperatures of the cool dust component of 13 nearby galaxies as a function of morphological type. We find a significant temperature gradient in the cool dust component in all galaxies, with a mean center-to-disk temperature ratio of 1.15 +/- 0.03. The cool dust temperatures in the central ~3 kpc of nearby galaxies are 23(+/-3)% hotter for morphological types earlier than Sc, and only 9(+/-3)% hotter for later types. The temperature ratio is also correlated with bar strength, with only strongly barred galaxies having a ratio over 1.2. The strong radiation field in the high stellar density of a galactic bulge tends to heat the cool dust component to higher temperatures, at least in early-type spirals with relatively large bulges, especially when paired with a strong bar.

astro-ph.CO↗

arXiv2010

Far-Infrared Line Imaging of the Starburst Ring in NGC 1097 with the Herschel/PACS Spectrometer

NGC 1097 is a nearby SBb galaxy with a Seyfert nucleus and a bright starburst ring. We study the physical properties of the interstellar medium (ISM) in the ring using spatially resolved far-infrared spectral maps of the circumnuclear starburst ring of NGC 1097, obtained with the PACS spectrometer on board the Herschel Space Telescope. In particular, we map the important ISM cooling and diagnostic emission lines of [OI] 63 $μ$m, [OIII] 88 $μ$m, [NII] 122 $μ$m, [CII] 158 $μ$m and [NII] 205 $μ$m. We observe that in the [OI] 63 $μ$m, [OIII] 88 $μ$m, and [NII] 122 $μ$m line maps, the emission is enhanced in clumps along the NE part of the ring. We observe evidence of rapid rotation in the circumnuclear ring, with a rotation velocity of ~220$ km s$^{-1}$ (inclination uncorrected) measured in all lines. The [OI] 63 $μ$m/[CII] 158 $μ$m ratio varies smoothly throughout the central region, and is enhanced on the northeastern part of the ring, which may indicate a stronger radiation field. This enhancement coincides with peaks in the [OI] 63 $μ$m and [OIII] 88 $μ$m maps. Variations of the [NII] 122 $μ$m/[NII] 205 $μ$m ratio correspond to a range in the ionized gas density between 150 and 400 cm$^{-3}$.

astro-ph.CO↗

arXiv2010

The Calibration of Monochromatic Far-Infrared Star Formation Rate Indicators

(Abridged) Spitzer data at 24, 70, and 160 micron and ground-based H-alpha images are analyzed for a sample of 189 nearby star-forming and starburst galaxies to investigate whether reliable star formation rate (SFR) indicators can be defined using the monochromatic infrared dust emission centered at 70 and 160 micron. We compare recently published recipes for SFR measures using combinations of the 24 micron and observed H-alpha luminosities with those using 24 micron luminosity alone. From these comparisons, we derive a reference SFR indicator for use in our analysis. Linear correlations between SFR and the 70 and 160 micron luminosity are found for L(70)>=1.4x10^{42} erg/s and L(160)>=2x10^{42} erg/s, corresponding to SFR>=0.1-0.3 M_sun/yr. Below those two luminosity limits, the relation between SFR and 70 micron (160 micron) luminosity is non-linear and SFR calibrations become problematic. The dispersion of the data around the mean trend increases for increasing wavelength, becoming about 25% (factor ~2) larger at 70 (160) micron than at 24 micron. The increasing dispersion is likely an effect of the increasing contribution to the infrared emission of dust heated by stellar populations not associated with the current star formation. The non-linear relation between SFR and the 70 and 160 micron emission at faint galaxy luminosities suggests that the increasing transparency of the interstellar medium, decreasing effective dust temperature, and decreasing filling factor of star forming regions across the galaxy become important factors for decreasing luminosity. The SFR calibrations are provided for galaxies with oxygen abundance 12+Log(O/H)>8.1. At lower metallicity the infrared luminosity no longer reliably traces the SFR because galaxies are less dusty and more transparent.

astro-ph.CO↗