Astrophys. J., 723, 993-1005 (2010/November-2)
The spatial extent of (U)LIRGs in the mid-infrared. I. The continuum emission.
DIAZ-SANTOS T., CHARMANDARIS V., ARMUS L., PETRIC A.O., HOWELL J.H., MURPHY E.J., MAZZARELLA J.M., VEILLEUX S., BOTHUN G., INAMI H., APPLETON P.N., EVANS A.S., HAAN S., MARSHALL J.A., SANDERS D.B., STIERWALT S. and SURACE J.A.
Abstract (from CDS):
We present an analysis of the extended mid-infrared (MIR) emission of the Great Observatories All-Sky LIRG Survey sample based on 5-15 µm low-resolution spectra obtained with the Infrared Spectrograph on Spitzer. We calculate the fraction of extended emission (FEE) as a function of wavelength for the galaxies in the sample, FEEλ, defined as the fraction of the emission which originates outside of the unresolved component of a source at a given distance. We find that the FEEλ varies from one galaxy to another, but we can identify three general types of FEEλ: one where FEEλ is constant, one where features due to emission lines and polycyclic aromatic hydrocarbons appear more extended than the continuum, and a third which is characteristic of sources with deep silicate absorption at 9.7 µm. More than 30% of the galaxies have a median FEEλ 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_13.2 µm_≲ 0.85). The large values of FEE_13.2 µm_ that we find in many LIRGs suggest that the extended component of their MIR continuum emission originates in scales up to 10 kpc and may contribute as much as the nuclear region to their total MIR luminosity. The mean size of the LIRG cores at 13.2 µm is 2.6 kpc. However, once the IR luminosity of the systems reaches the threshold of LIR∼ 1011.8 L☉, slightly below the regime of Ultra-luminous Infrared Galaxies (ULIRGs), all sources become clearly more compact, with FEE_13.2 µm_≲ 0.2, and their cores are unresolved. Our estimated upper limit for the core size of ULIRGs is less than 1.5 kpc. Furthermore, our analysis indicates that the compactness of systems with LIR≳ 1011.25 L☉ strongly increases in those classified as mergers in their final stage of interaction. The FEE_13.2 µm_ is also related to the contribution of an active galactic nucleus (AGN) to the MIR emission. 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_60 µm/f100 µm_) 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.
galaxies: active - galaxies: evolution - galaxies: interactions - galaxies: starburst - infrared: galaxies
VizieR on-line data:
<Available at CDS (J/ApJ/723/993): table1.dat>
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