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Modeling the spectral energy distribution of ULIRGs I: the radio spectra | | M.S. Clemens
; O. Vega
; A. Bressan
; G.L. Granato
; L. Silva
; P. Panuzzo
; | | Date: |
1 Aug 2007 | | Abstract: | As a constraint for new starburst/AGN models of IRAS bright galaxies we
determine the radio spectra of 31 luminous and ultraluminous IRAS galaxies
(LIRGs/ULIRGs). We construct the radio spectra using both new and archival
data. From our sample of radio spectra we find that very few have a straight
power-law slope. Although some sources show a flattening of the radio spectral
slope at high frequencies the average spectrum shows a steepening of the radio
spectrum from 1.4 to 22.5 GHz. This is unexpected because in sources with high
rates of star formation we expect flat spectrum, free-free emission to make a
significant contribution to the radio flux at higher radio frequencies. Despite
this trend the radio spectral indices between 8.4 and 22.5 GHz are flatter for
sources with higher values of the FIR-radio flux density ratio q, when this is
calculated at 8.4 GHz. Therefore, sources that are deficient in radio emission
relative to FIR emission (presumably younger sources) have a larger thermal
component to their radio emission. However, we find no correlation between the
radio spectral index between 1.4 and 4.8 GHz and q at 8.4 GHz. Because the low
frequency spectral index is affected by free-free absorption, and this is a
function of source size for a given mass of ionized gas, this is evidence that
the ionized gas in ULIRGs shows a range of densities. The youngest LIRGs and
ULIRGs are characterized by a larger contribution to their high-frequency radio
spectra from free-free emission. However, the youngest sources are not those
that have the greatest free-free absorption at low radio frequencies. The
sources in which the effects of free-free absorption are strongest are instead
the most compact sources. Although these have the warmest FIR colours, they are
not necessarily the youngest sources. | | Source: | arXiv, 0708.0149 | | Services: | Forum | Review | PDF | Favorites | |
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