We present infrared observations of 66 starburst galaxies over the full range of oxygen abundances observed in local star-forming galaxies, from image to 8.9. The data include imaging and ...spectroscopy from the Spitzer Space Telescope, supplemented by ground-based near-infrared imaging. We confirm a strong correlation of aromatic emission with metallicity, with a threshold at image. We show that the far-infrared color temperature of the large dust grains increases toward lower metallicity, peaking at a metallicity of 8 before turning over. We compute dust masses and compare them to H i masses from the literature to derive the ratio of atomic gas to dust, which increases by nearly 3 orders of magnitude between solar metallicity and a metallicity of 8, below which it flattens out. The abrupt change in aromatic emission at mid-infrared wavelengths thus appears to be reflected in the far-infrared properties, indicating that metallicity changes affect the composition of the full range of dust grain sizes that dominate the infrared emission. Although the great majority of galaxies show similar patterns of behavior as described above, there are three exceptions, SBS 0335-052E, Haro 11, and SHOC 391. Their infrared SEDs are dominated energetically by the mid- IR near 24 mum rather than by the 60-200 mum region. In addition, they have very weak near-infrared outputs and their SEDs are dominated by emission by dust at wavelengths as short as 1.8 mum. The latter behavior indicates that the dominant star-forming episodes in them are extremely young. The component of the ISM responsible for the usual far-infrared emission appears to be either missing or inefficiently heated in these three galaxies.
With the goal of investigating the degree to which the MIR emission traces the SFR, we analyze Spitzer 8 and 24 mu m data of star-forming regions in a sample of 33 nearby galaxies with available HST ...NICMOS images in the Pa alpha (1.8756 mu m) emission line. The galaxies are drawn from the SINGS sample and cover a range of morphologies and a factor similar to 10 in oxygen abundance. Published data on local low-metallicity starburst galaxies and LIRGs are also included in the analysis. Both the stellar continuum-subtracted 8 mu m emission and the 24 mu m emission correlate with the extinction-corrected Pa alpha line emission, although neither relationship is linear. Simple models of stellar populations and dust extinction and emission are able to reproduce the observed nonlinear trend of the 24 mu m emission versus number of ionizing photons, including the modest deficiency of 24 mu m emission in the low-metallicity regions, which results from a combination of decreasing dust opacity and dust temperature at low luminosities. Conversely, the trend of the 8 mu m emission as a function of the number of ionizing photons is not well reproduced by the same models. The 8 mu m emission is contributed, in larger measure than the 24 mu m emission, by dust heated by nonionizing stellar populations, in addition to the ionizing ones, in agreement with previous findings. Two SFR calibrations, one using the 24 mu m emission and the other using a combination of the 24 mu m and H alpha luminosities (Kennicutt and coworkers), are presented. No calibration is presented for the 8 mu m emission because of its significant dependence on both metallicity and environment. The calibrations presented here should be directly applicable to systems dominated by ongoing star formation.
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.
Interstellar dust and starlight are modeled for the galaxies of the project "Key Insights on Nearby Galaxies: A Far-Infrared Survey with Herschel." The galaxies were observed by the Infrared Array ...Camera and the Multiband Imaging Photometer for Spitzer on Spitzer Space Telescope, and the Photodetector Array Camera and Spectrometer and the Spectral and Photometric Imaging Receiver on Herschel Space Observatory. With data from 3.6 to 500 m, dust models are strongly constrained. Using a physical dust model, for each pixel in each galaxy we estimate (1) dust surface density, (2) dust mass fraction in polycyclic aromatic hydrocarbons (PAHs), (3) distribution of starlight intensities heating the dust, (4) total infrared (IR) luminosity emitted by the dust, and (5) IR luminosity originating in subregions with high starlight intensity. The dust models successfully reproduce the observed global and resolved spectral energy distributions. With the angular resolution of Herschel, we obtain well-resolved maps (available online) for the dust properties. As in previous studies, we find the PAH fraction to be an increasing function of metallicity, with a threshold oxygen abundance Z/Z 0.1, but we find the data to be fitted best with increasing linearly with above a threshold value of 0.15(O/H) . We obtain total dust masses for each galaxy by summing the dust mass over the individual map pixels; these "resolved" dust masses are consistent with the masses inferred from a model fit to the global photometry. The global dust-to-gas ratios obtained from this study are found to correlate with galaxy metallicities. Systems with Z/Z 0.5 have most of their refractory elements locked up in dust, whereas in systems with Z/Z 0.3 most of these elements tend to remain in the gas phase. Within galaxies, we find that is suppressed in regions with unusually warm dust with . With knowledge of one long-wavelength flux density ratio (e.g., f160/f500), the minimum starlight intensity heating the dust ( ) can be estimated to within ∼50%, despite a variation in of more than two orders of magnitude. For the adopted dust model, dust masses can be estimated to within ∼0.2 dex accuracy using the f160/f500 flux ratio and the integrated dust luminosity, and to ∼0.07 dex accuracy using the 500 m luminosity alone. There are additional systematic errors arising from the choice of dust model, but these are hard to estimate. These calibrated prescriptions for estimating starlight heating intensity and dust mass may be useful for studies of high-redshift galaxies.
We present a sample of low-resolution 5-38 km Spitzer IRS spectra of the inner few square kiloparsecs of 59 nearby galaxies spanning a large range of star formation properties. A robust method for ...decomposing mid-infrared galaxy spectra is described and used to explore the behavior of PAH emission and the prevalence of silicate dust extinction. Evidence for silicate extinction is found in6 of the sample, at strengths that indicate that most normal galaxies undergo A sub(V) 3 mag averaged over then-centers. The contribution of PAH emission to the total infrared power is found to peak near 10% and extend up to 620% and is suppressed at metallicities Z Z /4, as well as in low-luminosity AGN environments. Strong interband PAH feature strength variations (2-5 times) are observed, with the presence of a weak AGN and, to a lesser degree, increasing metallicity shifting power to the longer wavelength bands. A peculiar PAH emission spectrum with markedly diminished 5-8 km features arises among the sample solely in systems with relatively hard radiation fields harboring low-luminosity AGNs. The AGNs may modify the emitting grain distribution and provide the direct excitation source of the unusual PAH emission, which cautions against using absolute PAH strength to estimate star formation rates in systems harboring active nuclei. Alternatively, the low star formation intensity often associated with weak AGNs may affect the spectrum. The effect of variations in the mid-infrared spectrum on broadband infrared surveys is modeled and points to more than a factor of 2 uncertainty in results that assume a fixed PAH emission spectrum, for redshifts z = 0-2.5.
We examine colors from 3.6 to 24 mu m as a function of metallicity (O/H) for a sample of 34 galaxies. The galaxies range over 2 orders of magnitude in metallicity. They display an abrupt shift in the ...8 mu m-to-24 mu m color for metallicities between one-third and one-fifth of the solar value. The mean 8-to-24 mu m flux density ratio below and above 12 + log (O/H) = 8.2 is 0.08 plus or minus 0.04 and 0.70 plus or minus 0.53, respectively. We use mid-IR colors and spectroscopy to demonstrate that the shift is primarily due to a decrease in the 8 mu m flux density, as opposed to an increase in the 24 mu m flux density. This result is most simply interpreted as being due to a weakening at low metallicity of the mid-IR emission bands usually attributed to PAHs (polycyclic aromatic hydrocarbons) relative to the small-grain dust emission. However, existing empirical spectral energy distribution models cannot account for the observed short-wavelength (below 8 mu m) colors of the low-metallicity galaxies merely by reducing the strength of the PAH features; some other emission source (e.g., hot dust) is required.
The Spitzer Infrared Nearby Galaxies Survey (SINGS) is carrying out a comprehensive multiwavelength survey on a sample of 75 nearby galaxies. The 1-850 km spectral energy distributions (SEDs) are ...presented using broadband imaging data from Spitzer, 2MASS, ISO, IRAS, and SCUBA. The infrared colors derived from the globally integrated Spitzer data are generally consistent with the previous generation of models that were developed using global data for normal star-forming galaxies, although significant deviations are observed. Spitzer's excellent sensitivity and resolution also allow a detailed investigation of the infrared SEDs for various locations within the three large, nearby galaxies NGC 3031 (M81), NGC 5194 (M51), and NGC 7331. A wide variety of spectral shapes is found within each galaxy, especially for NGC 3031, the closest of the three targets and thus the galaxy for which the smallest spatial scales can be explored. Strong correlations exist between the local star formation rate and the infrared colors f sub(u)(70 km)/f sub(u)(160 km) and f sub(u)(24 km)/f sub(u)(160 km), suggesting that the 24 and 70 km emission are useful tracers of the local star formation activity level. Preliminary evidence indicates that variations in the 24 km emission, and not variations in the emission from polycyclic aromatic hydrocarbons at 8 km, drive the variations in the f sub(u)(8.0 km)/f sub(u)(24 km) colors within NGC 3031, NGC 5194, and NGC 7331. If the galaxy-to-galaxy variations in SEDs seen in our sample are representative of the range present at high redshift, then extrapolations of total infrared luminosities and star formation rates from the observed 24 km flux will be uncertain at the factor of 5 level (total range). The corresponding uncertainties using the redshifted 8.0 km flux (e.g., observed 24 km flux for a z = 2 source) are factors of 10-20. Considerable caution should be used when interpreting such extrapolated infrared luminosities.
New far-infrared and submillimeter 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 500 mu m emission shows evidence for a submillimeter excess. In addition, the fraction of dust heating attributed to intense radiation fields associated with photodissociation regions is found to be (21 + or - 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.
The survey description and the near-, mid-, and far-infrared flux properties are presented for the 258 galaxies in the Local Volume Legacy (LVL). LVL is a Spitzer Space Telescope legacy program that ...surveys the local universe out to 11 Mpc, built upon a foundation of ultraviolet, H Delta *a, and Hubble Space Telescope imaging from 11HUGS (11 Mpc H Delta *a and Ultraviolet Galaxy Survey) and ANGST (ACS Nearby Galaxy Survey Treasury). LVL covers an unbiased, representative, and statistically robust sample of nearby star-forming galaxies, exploiting the highest extragalactic spatial resolution achievable with Spitzer. As a result of its approximately volume-limited nature, LVL augments previous Spitzer observations of present-day galaxies with improved sampling of the low-luminosity galaxy population. The collection of LVL galaxies shows a large spread in mid-infrared colors, likely due to the conspicuous deficiency of 8 Delta *mm polycyclic aromatic hydrocarbon emission from low-metallicity, low-luminosity galaxies. Conversely, the far-infrared emission tightly tracks the total infrared emission, with a dispersion in their flux ratio of only 0.1 dex. In terms of the relation between the infrared-to-ultraviolet ratio and the ultraviolet spectral slope, the LVL sample shows redder colors and/or lower infrared-to-ultraviolet ratios than starburst galaxies, suggesting that reprocessing by dust is less important in the lower mass systems that dominate the LVL sample. Comparisons with theoretical models suggest that the amplitude of deviations from the relation found for starburst galaxies correlates with the age of the stellar populations that dominate the ultraviolet/optical luminosities.
Physical dust models are presented for 65 galaxies in SINGS that are strongly detected in the four IRAC bands and three MIPS bands. For each galaxy we estimate (1) the total dust mass, (2) the ...fraction of the dust mass contributed by PAHs, and (3) the intensity of the starlight heating the dust grains. We find that spiral galaxies have dust properties resembling the dust in the local region of the Milky Way, with similar dust-to-gas ratio and similar PAH abundance. The observed SEDs, including galaxies with SCUBA photometry, can be reproduced by dust models that do not require "cold" (T 10 K) dust. The dust-to-gas ratio is observed to be dependent on metallicity. In the interstellar media of galaxies with Ao identical with 12 + log sub(10)(O/H) > 8.1, grains contain a substantial fraction of interstellar Mg, Si, and Fe. Galaxies with Ao < 8.1 and extended H I envelopes in some cases appear to have global dust-to-gas ratios that are low for their measured oxygen abundance, but the dust-to-gas ratio in the regions where Infrared emission is detected generally appears to be consistent with a substantial fraction of interstellar Mg, Si, and Fe being contained in dust. The PAH index q sub(PAH), the fraction of the dust mass in the form of PAHs, correlates with metallicity. The nine galaxies in our sample with Ao < 8.1 have a median q sub(PAH) = 1.0%, whereas galaxies with Ao > 8.1 have a median q sub(pah) = 3.55%. The derived dust masses favor a value X approximately 4 x 10 super(20) cm super(-2) (K km s super(-1)) super(-1) for the CO-to-H sub(2) conversion factor. Except for some starbursting systems (Mrk 33, Tol 89, NGC 3049), dust in the diffuse ISM dominates the IR power.
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