Starburst evolution: free–free absorption in the radio spectra of luminous IRAS galaxies
Mon. Not. R. Astron. Soc.
Starburst evolution: free-free absorption in the radio spectra of luminous IRAS galaxies
M. S. Clemens 2
A. Scaife 0 1
O. Vega 4
A. Bressan 2 3 4
0 Dublin Institute for Advanced Studies , 31 Fitzwilliam Place, Dublin 2 , Ireland
1 Astrophysics Group, Cavendish Laboratory , J. J. Thomson Avenue, Cambridge CB3 0HE
2 INAF-Osservatorio Astronomico di Padova , Vicolo dell'Osservatorio, 5, 35122 Padova , Italy
3 SISSA-ISAS, International School for Advanced Studies , ia Beirut 4, 34014 Trieste , Italy
4 INAOE , Luis Enrique Erro 1, 72840 Tonantzintla, Puebla , Mexico
A B S T R A C T We describe radio observations at 244 and 610 MHz of a sample of 20 luminous and ultraluminous IRAS galaxies. These are a subset of a sample of 31 objects that have well-measured radio spectra up to at least 23 GHz. The radio spectra of these objects below 1.4 GHz show a great variety of forms and are rarely a simple power-law extrapolation of the synchrotron spectra at higher frequencies. Most objects of this class have spectral turn-overs or bends in their radio spectra. We interpret these spectra in terms of free-free absorption in the starburst environment. Several objects show radio spectra with two components having free-free turn-overs at different frequencies (including Arp 220 and Arp 299), indicating that synchrotron emission originates from regions with very different emission measures. In these sources, using a simple model for the supernova rate, we estimate the time for which synchrotron emission is subject to strong free-free absorption by ionized gas and compare this to expected H II region lifetimes. We find that the ionized gas lifetimes are an order of magnitude larger than the plausible lifetimes for individual H II regions. We discuss the implications of this result and argue that those sources which have a significant radio component with strong free-free absorption are those in which the star formation rate is still increasing with time. We note that if ionization losses are important, the resulting curvature of the radio spectrum would much reduce the often observed deficit in fluxes above ∼10 GHz.
galaxies; active - infrared; galaxies - radio continuum; galaxies
1 I N T R O D U C T I O N
The radio emission from star-forming galaxies is the result of
two emission processes, non-thermal synchrotron, and thermal
bremsstrahlung, or free–free emission. Synchrotron is by far the
dominant component in the 1–10 GHz range and is characterized
by a power-law emission spectrum, f ν ∝ ν−α, where α 0.8. Free–
free emission has an almost flat spectrum and the emission falls
with frequency only as ν−0.1. Therefore, towards higher
frequencies free–free emission becomes increasingly important and radio
spectra are expected to show a flattening beyond ∼10 GHz.
In luminous infrared galaxies (LIRGs) and ultra-luminous
infrared galaxies (ULIRGs), although such a flattening is sometimes
seen, the majority of radio spectra, even when measured to 23 GHz,
show no such flattening. In fact, there is a tendency for the spectra
to steepen (Clemens et al. 2008). Despite this, Clemens et al. (2008)
showed that the spectral index between 8.4 and 23 GHz is flatter for
sources with high values of the far-infrared–radio flux ratio, q. As
these sources, with an excess of infrared relative to radio emission,
are expected to be younger, they would be expected to have
proportionately more free–free gas and thus shallower spectral indices at
high frequencies.
As we will see later, the radio spectra of LIRGs and ULIRGs
also hold surprises towards low frequencies, with spectra showing
a variety of bends and turn-overs. From a theoretical standpoint,
there is more than one way in which the radio spectrum may flatten
towards low frequencies. There are those processes that absorb
radio photons, free–free absorption and synchrotron self-absorption
and those that cause energy losses of the relativistic electron
population directly. Of these, ionization losses have been considered
by Thompson et al. (2006) as a cause for the flattening of starburst
radio spectra towards low frequencies. However, as illustrated by
these authors, the flattening is rather gradual. Our data show
evidence of rapid changes in spectral index and also inversions of the
spectra, much more consistent with an absorption process with a
strong energy dependence.
Both free–free absorption and synchrotron self-absorption could
rapidly absorb the power-law synchrotron spectrum towards low
frequencies. However, synchrotron self-absorption requires extremely
high brightness temperatures in order to be significant, and these
are just not attained (with the possible exception of UGC 8058) in
our sources (Condon et al. 1991). We therefore consider free–free
absorption to be the defining mechanism for the spectral shapes of
this sample, especially at low radio frequencies.
Here we describe new observations of a sample of LIRGs and
ULIRGs at 244 and 610 MHz made (...truncated)