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Register a new userA simple connection between the near- and mid-infrared emission of galaxies and their star-formation rates
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We have measured the near-infrared colors and the fluxes of individual pixels in 68 galaxies common to the Spitzer Infrared Nearby Galaxies Survey and the Large Galaxy Atlas Survey. Each galaxy was separated into regions of increasingly red near-infrared colors. In the absence of dust extinction and other non-stellar emission, stellar populations are shown to have relatively constant NIR colors, independent of age. In regions of high star formation, the average intensity of pixels in red-excess regions (at 1.25, 3.6, 4.5, 5.6, 8.0 and 24 micron) scales linearly with the intrinsic intensity of Halpha emission, and thus with the star-formation rate within the pixel. This suggests that most NIR-excess regions are not red because their light is being depleted by absorption. Instead, they are red because additional infrared light is being contributed by a process linked to star-formation. This is surprising because the shorter wavelength bands in our study (1.25 micron-5.6 micron) do not probe emission from cold (10-20 K) and warm (50-100 K) dust associated with star-formation in molecular clouds. However, emission from hot dust (700-1000 K) and/or Polycyclic Aromatic Hydrocarbon molecules can explain the additional emission seen at the shorter wavelengths in our study. The contribution from hot dust and/or PAH emission at 2-5micron and PAH emission at 5.6 and 8.0 micron scales linearly with warm dust emission at 24 micron and the intrinsic Halpha emission. Since both are tied to the star-formation rate, our analysis shows that the NIR excess continuum emission and PAH emission at ~1-8 micron can be added to spectral energy distribution models in a very straight-forward way, by simply adding an additional component to the models that scales linearly with star-formation rate.
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The Halpha and mid-infrared mean disk surface brightnesses are compared in a sample of nearby spirals observed by ISOCAM. This shows that, in spiral disks, dust emission at 7 and 15 microns provides a reasonable star formation tracer. The fact that the 15 to 7 micron flux ratio is nearly constant in various global exciting conditions indicates a common origin, namely the aromatic infrared band carriers, and implies that at these wavelengths, dust emission from the disks of normal galaxies is dominated by photodissociation regions and not by HII regions themselves. We use this newly-found correlation between the mid-infrared and the Halpha line to investigate the nature of the link between the far-infrared (60 and 100 microns) and Halpha. Although the separation of the central regions from the disk is impossible to achieve in the far-infrared, we show that a circumnuclear contribution to the dust emission, having no equivalent counterpart in Halpha, is most likely responsible for the well-known non-linearity between far-infrared and Halpha fluxes in spiral galaxies. We derive a calibration of 7 and 15 micron fluxes in terms of star formation rates from a primary calibration of Halpha in the literature, and also outline the applicability limits of the proposed conversion, which should not be blindly extrapolated to objects whose nature is unknown.
We use the near--infrared Brgamma hydrogen recombination line as a reference star formation rate (SFR) indicator to test the validity and establish the calibration of the {it Herschel} PACS 70 mu m emission as a SFR tracer for sub--galactic regions in external galaxies. Brgamma offers the double advantage of directly tracing ionizing photons and of being relatively insensitive to the effects of dust attenuation. For our first experiment, we use archival CFHT Brgamma and Ks images of two nearby galaxies: NGC,5055 and NGC,6946, which are also part of the {it Herschel} program KINGFISH (Key Insights on Nearby Galaxies: a Far-Infrared Survey with Herschel). We use the extinction corrected Brgamma emission to derive the SFR(70) calibration for H{sc ii} regions in these two galaxies. A comparison of the SFR(70) calibrations at different spatial scales, from 200 pc to the size of the whole galaxy, reveals that about 50% of the total 70mu m emission is due to dust heated by stellar populations that are unrelated to the current star formation. We use a simple model to qualitatively relate the increase of the SFR(70) calibration coefficient with decreasing region size to the star formation timescale. We provide a calibration for an unbiased SFR indicator that combines the observed Halpha with the 70 mu m emission, also for use in H{sc ii} regions. We briefly analyze the PACS 100 and 160 mu m maps and find that longer wavelengths are not as good SFR indicators as 70mu m, in agreement with previous results. We find that the calibrations show about 50% difference between the two galaxies, possibly due to effects of inclination.
The star formation rate (SFR) is one of the most fundamental parameters of galaxies, but nearly all of the standard SFR diagnostics are difficult to measure in active galaxies because of contamination from the active galactic nucleus (AGN). Being less sensitive to dust extinction, the mid-infrared fine-structure lines of [NeII] 12.81 micron and [NeIII] 15.56 micron effectively trace the SFR in star-forming galaxies. These lines also have the potential to serve as a reliable SFR indicator in active galaxies, provided that their contribution from the AGN narrow-line region can be removed. We use a new set of photoionization calculations with realistic AGN spectral energy distributions and input assumptions to constrain the magnitude of [NeII] and [NeIII] produced by the narrow-line region for a given strength of [NeV] 14.32 micron. We demonstrate that AGNs emit a relatively restricted range of [NeII]/[NeV] and [NeIII]/[NeV] ratios. Hence, once [NeV] is measured, the AGN contribution to the low-ionization Ne lines can be estimated, and the SFR can be determined from the strength of [NeII] and [NeIII]. We find that AGN host galaxies have similar properties as compact extragalactic HII regions, which indicates that the star formation in AGN hosts is spatially concentrated. This suggests a close relationship between black hole accretion and nuclear star formation. We update the calibration of [NeII] and [NeIII] strength as a SFR indicator, explicitly considering the effects of metallicity, finding very good relations between Ne fractional abundances and the [NeIII]/[NeII] ratio for different metallicities, ionization parameters, and starburst ages. Comparison of neon-based SFRs with independent SFRs for active and star-forming galaxies shows excellent consistency with small scatter ($sim0.18$ dex).
A summary is presented for 130 galaxies observed with the Herschel PACS instrument to measure fluxes for the [CII] 158 um emission line. Sources cover a wide range of active galactic nucleus to starburst classifications, as derived from polycyclic aromatic hydrocarbon (PAH) strength measured with the Spitzer Infrared Spectrograph. Redshifts from [CII] and line to continuum strengths (equivalent width of [CII]) are given for the full sample, which includes 18 new [CII] flux measures. Calibration of L([CII)]) as a star formation rate (SFR) indicator is determined by comparing [CII] luminosities with mid-infrared [NeII] and [NeIII] emission line luminosities; this gives the same result as determining SFR using bolometric luminosities of reradiating dust from starbursts: log SFR = log L([CII)]) - 7.0, for SFR in solar masses per year and L([CII]) in solar luminosities. We conclude that L([CII]) can be used to measure SFR in any source to a precision of ~ 50%, even if total source luminosities are dominated by an AGN component. The line to continuum ratio at 158 um, EW([CII]), is not significantly greater for starbursts (median EW([CII]) = 1.0 um) compared to composites and AGN (median EW([CII]) = 0.7 um), showing that the far infrared continuum at 158 um scales with [CII] regardless of classification. This indicates that the continuum at 158 um also arises primarily from the starburst component within any source, giving log SFR = log vLv(158 um) - 42.8 for SFR in solar masses per year and vLv(158 um) in erg per sec.
We investigate the use of mid-infrared PAH bands, continuum and emission lines as probes of star-formation and AGN activity in a sample of 100 `normal and local (z~0.1) galaxies. The MIR spectra were obtained with the Spitzer IRS as part of the Spitzer-SDSS-GALEX Spectroscopic Survey (SSGSS) which includes multi-wavelength photometry from the UV to the FIR and optical spectroscopy. The spectra were decomposed using PAHFIT (Smith et al. 2007), which we find to yield PAH equivalent widths (EW) up to ~30 times larger than the commonly used spline methods. Based on correlations between PAH, continuum and emission line properties and optically derived physical properties (gas phase metallicity, radiation field hardness), we revisit the diagnostic diagram relating PAH EWs and [NeII]/[OIV] and find it more efficient as distinguishing weak AGNs from star-forming galaxies than when spline decompositions are used. The luminosity of individual MIR component (PAH, continuum, Ne and molecular hydrogen lines) are found to be tightly correlated to the total IR luminosity and can be used to estimate dust attenuation in the UV and in Ha lines based on energy balance arguments.
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