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Register a new userThe Spatial Extent of (U)LIRGs in the mid-Infrared I: The Continuum Emission
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T. Diaz-Santos
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We present an analysis of the extended mid-infrared (MIR) emission of the Great Observatories All-Sky LIRG Survey (GOALS) sample based on 5-15um low resolution spectra obtained with the IRS on Spitzer. We calculate the fraction of extended emission as a function of wavelength for the galaxies in the sample, FEE_lambda. We can identify 3 general types of FEE_lambda: one where it is constant, one where features due to emission lines and PAHs appear more extended than the continuum, and a third which is characteristic of sources with deep silicate absorption at 9.7um. More than 30% of the galaxies have a median FEE_lambda larger than 0.5 implying that at least half of their MIR emission is extended. Luminous Infrared Galaxies (LIRGs) display a wide range of FEE in their warm dust continuum (0<=FEE_13.2um<=0.85). The large values of FEE_13.2um that we find in many LIRGs suggest that their extended MIR continuum emission originates in scales up to 10kpc. The mean size of the LIRG cores at 13.2um is 2.6kpc. However, once the LIR of the systems reaches the threshold of ~10^11.8Lsun, all sources become clearly more compact, with FEE_13.2um<=0.2, and their cores are unresolved. Our estimated upper limit for the core size of ULIRGs is less than 1.5kpc. The analysis indicates that the compactness of systems with LIR>~10^11.25Lsun strongly increases in those classified as mergers in their final stage of interaction. The FEE_13.2um is also related to the contribution of an active galactic nucleus (AGN) to the MIR. Galaxies which are more AGN-dominated are less extended, independently of their LIR. We finally find that the extent of the MIR continuum emission is correlated with the far-IR IRAS log(f_60um/f_100um) color. This enables us to place a lower limit to the area in a galaxy from where the cold dust emission may originate, a prediction which can be tested soon with the Herschel Space Telescope.
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We use high resolution IRAS and 20 cm radio continuum (RC) images of a sample of 22 spiral galaxies to study the correlation between the far infra-red (FIR) and RC emissions within the galactic disks. A combination of exponential and gaussian profiles rather than a single exponential profile is found to be a better representation of the observed intensity profiles in the two bands. The gaussian component, which we show is not due to the effects of limited beam-resolution, contains more than 60% of the total flux in majority of the galaxies. The dominance of the gaussian component suggests that the nuclear star forming regions and the bulge stars are more important contributors to the emission in the two bands, rather than the outer exponential stellar disks. The RC profile is flatter compared to the FIR profile, resulting in a decrease of their ratio, Q60, away from the center. However, the Q60 increases in the extreme outer parts, where the dispersion in the FIR and RC correlation is also higher than in the central regions. The global Q60 and its dispersion match those in the inner parts of the galaxies. These results imply that the observed tight correlation in the global quantities reflects processes in the inner regions only where OB stars and the associated Type II supernovae control the FIR and RC emission. In the outer parts heating of very small dust grains by the old disk stars provides a secondary component in the FIR emission, without associated RC emission. The edge-on galaxy NGC3079 shows extended FIR and RC emissions along its minor axis, probably associated with the nuclear starburst activity.
We report the detection of 6.2um polycyclic aromatic hydrocarbon (PAH) and rest-frame 4-7um continuum emission in the z=4.055 submillimeter galaxy GN20, using the Infrared Spectrograph (IRS) on board the Spitzer Space Telescope. This represents the first detection of PAH emission at z>4. The strength of the PAH emission feature is consistent with a very high star formation rate of ~1600Msun/yr. We find that this intense starburst powers at least ~1/3 of the faint underlying 6um continuum emission, with an additional, significant (and perhaps dominant) contribution due to a power-law-like hot dust source, which we interpret to likely be a faint, dust-obscured active galactic nucleus (AGN). The inferred 6um AGN continuum luminosity is consistent with a sensitive upper limit on the hard X-ray emission as measured by the Chandra X-Ray Observatory if the previously undetected AGN is Compton-thick. This is in agreement with the finding at optical/infrared wavelengths that the galaxy and its nucleus are heavily dust-obscured. Despite the strong power-law component enhancing the mid-infrared continuum emission, the intense starburst associated with the photon-dominated regions that give rise to the PAH emission appears to dominate the total energy output in the infrared. GN20 is one of the most luminous starburst galaxies known at any redshift, embedded in a rich protocluster of star-forming galaxies. This investigation provides an improved understanding of the energy sources that power such exceptional systems, which represent the extreme end of massive galaxy formation at early cosmic times.
We present T-ReCS high spatial resolution N-band (8-13 micron) spectroscopy of the central regions (a few kpc) of 3 local LIRGs. The nuclear spectra show deep 9.7 micron silicate absorption feature and the high ionization [SIV]10.5 micron emission line, consistent with their optical classification as AGN. The two LIRGs with unresolved mid-IR emission do not show PAH emission at 11.3 micron in their nuclear spectra. The spatially resolved mid-IR spectroscopy of NGC 5135 allows us to separate out the spectra of the Seyfert nucleus, an HII region, and the diffuse region between them on scales of less than 2.5 arcsec ~ 600 pc. The diffuse region spectrum is characterized by strong PAH emission with almost no continuum, whereas the HII region shows PAH emission with a smaller equivalent width as well as [NeII]12.8 micron line.
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