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A Bressan - One of the best experts on this subject based on the ideXlab platform.

  • starburst evolution free free absorption in the Radio Spectra of luminous iras galaxies
    Monthly Notices of the Royal Astronomical Society, 2010
    Co-Authors: M Clemens, O Vega, A Bressan, Anna M. M. Scaife
    Abstract:

    We describe Radio observations at 244 and 610 MHz of a sample of 20 luminous and ultra-luminous 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.

  • modeling the Spectral energy distribution of ulirgs i the Radio Spectra
    arXiv: Astrophysics, 2007
    Co-Authors: M Clemens, O Vega, A Bressan, G L Granato, L Silva, P Panuzzo
    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.

O Vega - One of the best experts on this subject based on the ideXlab platform.

  • starburst evolution free free absorption in the Radio Spectra of luminous iras galaxies
    Monthly Notices of the Royal Astronomical Society, 2010
    Co-Authors: M Clemens, O Vega, A Bressan, Anna M. M. Scaife
    Abstract:

    We describe Radio observations at 244 and 610 MHz of a sample of 20 luminous and ultra-luminous 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.

  • Starburst evolution: free-free absorption in the Radio Spectra of luminous IRAS galaxies
    Monthly Notices of the Royal Astronomical Society, 2010
    Co-Authors: Marcel Clemens, O Vega, Anna M. M. Scaife, Alessandro Bressan
    Abstract:

    We describe Radio observations at 244 and 610 MHz of a sample of 20 luminous and ultra-luminous IRAS galaxies. These are a sub-set 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 HII region lifetimes. We find that the ionized gas lifetimes are an order of magnitude larger than plausible lifetimes for individual HII 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 modify the intrinsic synchrotron spectrum so that it steepens toward higher frequencies, the often observed deficit in fluxes higher than ~10 GHz would be much reduced.

  • Modeling the Spectral energy distribution of ULIRGs - I. The Radio Spectra
    Astronomy & Astrophysics, 2007
    Co-Authors: Marcel Clemens, O Vega, G L Granato, L Silva, Alessandro Bressan, P Panuzzo
    Abstract:

    Aims. We aim to constrain new starburst/AGN models of IRAS bright galaxies via their Spectral energy distribution from the nearinfrared to the Radio. To this end, we determine the Radio Spectra for a sample of 31 luminous and ultraluminous IRAS galaxies (LIRGs/ULIRGs). Methods. We present here new high frequency VLA observations at 22.5 GHz and 8.4 GHz and also derive fluxes at other Radio frequencies from archival data. Together with Radio data from the literature, we construct the Radio spectrum for each source. In the selection of data we have made every effort to ensure that these fluxes neither include contribution s from nearby objects, nor underestimate the flux due to high interferometer resolutio n. Results. From our sample of well-determined 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 f requencies, 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, fre e-free emission to make a significant contribution to the Radio flux a t higher Radio frequencies. Despite this trend, the Radio sp ectral indices between 8.4 and 22.5 GHz are flatter for sources with higher va lues of the far-infrared (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. Conclusions. The youngest LIRGs and ULIRGs are characterized by flatter av erage Radio Spectral indices from 1.4 to 22.5 GHz, and by a larger contribution to their high frequency, Radio s pectra from free-free emission. However, the youngest sources are not those that have the greatest free-free absorption at low rad io 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.

  • modeling the Spectral energy distribution of ulirgs i the Radio Spectra
    arXiv: Astrophysics, 2007
    Co-Authors: M Clemens, O Vega, A Bressan, G L Granato, L Silva, P Panuzzo
    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.

Alessandro Bressan - One of the best experts on this subject based on the ideXlab platform.

  • Starburst evolution: free-free absorption in the Radio Spectra of luminous IRAS galaxies
    Monthly Notices of the Royal Astronomical Society, 2010
    Co-Authors: Marcel Clemens, O Vega, Anna M. M. Scaife, Alessandro Bressan
    Abstract:

    We describe Radio observations at 244 and 610 MHz of a sample of 20 luminous and ultra-luminous IRAS galaxies. These are a sub-set 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 HII region lifetimes. We find that the ionized gas lifetimes are an order of magnitude larger than plausible lifetimes for individual HII 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 modify the intrinsic synchrotron spectrum so that it steepens toward higher frequencies, the often observed deficit in fluxes higher than ~10 GHz would be much reduced.

  • Modeling the Spectral energy distribution of ULIRGs - I. The Radio Spectra
    Astronomy & Astrophysics, 2007
    Co-Authors: Marcel Clemens, O Vega, G L Granato, L Silva, Alessandro Bressan, P Panuzzo
    Abstract:

    Aims. We aim to constrain new starburst/AGN models of IRAS bright galaxies via their Spectral energy distribution from the nearinfrared to the Radio. To this end, we determine the Radio Spectra for a sample of 31 luminous and ultraluminous IRAS galaxies (LIRGs/ULIRGs). Methods. We present here new high frequency VLA observations at 22.5 GHz and 8.4 GHz and also derive fluxes at other Radio frequencies from archival data. Together with Radio data from the literature, we construct the Radio spectrum for each source. In the selection of data we have made every effort to ensure that these fluxes neither include contribution s from nearby objects, nor underestimate the flux due to high interferometer resolutio n. Results. From our sample of well-determined 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 f requencies, 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, fre e-free emission to make a significant contribution to the Radio flux a t higher Radio frequencies. Despite this trend, the Radio sp ectral indices between 8.4 and 22.5 GHz are flatter for sources with higher va lues of the far-infrared (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. Conclusions. The youngest LIRGs and ULIRGs are characterized by flatter av erage Radio Spectral indices from 1.4 to 22.5 GHz, and by a larger contribution to their high frequency, Radio s pectra from free-free emission. However, the youngest sources are not those that have the greatest free-free absorption at low rad io 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.

Marcel Clemens - One of the best experts on this subject based on the ideXlab platform.

  • Starburst evolution: free-free absorption in the Radio Spectra of luminous IRAS galaxies
    Monthly Notices of the Royal Astronomical Society, 2010
    Co-Authors: Marcel Clemens, O Vega, Anna M. M. Scaife, Alessandro Bressan
    Abstract:

    We describe Radio observations at 244 and 610 MHz of a sample of 20 luminous and ultra-luminous IRAS galaxies. These are a sub-set 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 HII region lifetimes. We find that the ionized gas lifetimes are an order of magnitude larger than plausible lifetimes for individual HII 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 modify the intrinsic synchrotron spectrum so that it steepens toward higher frequencies, the often observed deficit in fluxes higher than ~10 GHz would be much reduced.

  • Modeling the Spectral energy distribution of ULIRGs - I. The Radio Spectra
    Astronomy & Astrophysics, 2007
    Co-Authors: Marcel Clemens, O Vega, G L Granato, L Silva, Alessandro Bressan, P Panuzzo
    Abstract:

    Aims. We aim to constrain new starburst/AGN models of IRAS bright galaxies via their Spectral energy distribution from the nearinfrared to the Radio. To this end, we determine the Radio Spectra for a sample of 31 luminous and ultraluminous IRAS galaxies (LIRGs/ULIRGs). Methods. We present here new high frequency VLA observations at 22.5 GHz and 8.4 GHz and also derive fluxes at other Radio frequencies from archival data. Together with Radio data from the literature, we construct the Radio spectrum for each source. In the selection of data we have made every effort to ensure that these fluxes neither include contribution s from nearby objects, nor underestimate the flux due to high interferometer resolutio n. Results. From our sample of well-determined 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 f requencies, 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, fre e-free emission to make a significant contribution to the Radio flux a t higher Radio frequencies. Despite this trend, the Radio sp ectral indices between 8.4 and 22.5 GHz are flatter for sources with higher va lues of the far-infrared (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. Conclusions. The youngest LIRGs and ULIRGs are characterized by flatter av erage Radio Spectral indices from 1.4 to 22.5 GHz, and by a larger contribution to their high frequency, Radio s pectra from free-free emission. However, the youngest sources are not those that have the greatest free-free absorption at low rad io 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.

Anna M. M. Scaife - One of the best experts on this subject based on the ideXlab platform.

  • starburst evolution free free absorption in the Radio Spectra of luminous iras galaxies
    Monthly Notices of the Royal Astronomical Society, 2010
    Co-Authors: M Clemens, O Vega, A Bressan, Anna M. M. Scaife
    Abstract:

    We describe Radio observations at 244 and 610 MHz of a sample of 20 luminous and ultra-luminous 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.

  • Starburst evolution: free-free absorption in the Radio Spectra of luminous IRAS galaxies
    Monthly Notices of the Royal Astronomical Society, 2010
    Co-Authors: Marcel Clemens, O Vega, Anna M. M. Scaife, Alessandro Bressan
    Abstract:

    We describe Radio observations at 244 and 610 MHz of a sample of 20 luminous and ultra-luminous IRAS galaxies. These are a sub-set 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 HII region lifetimes. We find that the ionized gas lifetimes are an order of magnitude larger than plausible lifetimes for individual HII 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 modify the intrinsic synchrotron spectrum so that it steepens toward higher frequencies, the often observed deficit in fluxes higher than ~10 GHz would be much reduced.