A physical model of the infrared-to-radio correlation in galaxies

Hélou, G.; Bicay, M. D.

doi:10.1086/173146

Cited by 126 publications

(142 citation statements)

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“…However, the 21 cm continuum contours clearly show a better correlation to the molecular gas. This correlation underscores the importance of gas density and associated magnetic fields in the extraction of synchrotron emission from cosmic rays that diffuse from their source with a characteristic $1 kpc scale length (Bicay, Helou, & Condon 1989;Bicay & Helou 1990;Helou & Bicay 1993). However, the separation between the major axis of the CO bar and the ridge of H ii regions is less than the CO beamwidth, and the lack of a CO/21 cm offset may be due to the 1 0 resolution data used in the comparison.…”

Section: Co Gas and Star Formation Tracers: Morphology Comparisonsmentioning

confidence: 90%

CO in the Disk of the Barred Spiral Galaxy M83: CO (1–0), CO (2–1), and Neutral Gas

Crosthwaite¹,

Turner²,

Buchholz³

et al. 2002

The Astronomical Journal

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We present '' on-the-fly '' maps of CO (1-0) and CO (2-1) in the barred spiral galaxy M83. The entire optical disk is filled with CO emission in both lines, including interarm regions. For a '' standard '' conversion factor the total molecular mass is 2:5 Â 10 9 M , 4% of the total dynamical mass. Molecular gas constitutes 80% of total gas mass within the inner 10 0 and 30% overall. CO (1-0) emission and 21 cm continuum emission are extremely well correlated spatially in regions where H 2 dominates the gas surface density. The spatial correlation between CO (1-0) and 100 lm emission is not as good. While the kinematics of the outer H i disk is severely nonaxisymmetrically perturbed, the kinematics of the CO disk and inner H i disk is well behaved and circular. The molecular disk appears to have a distinct hard edge at 5 0 . The hard edge is accompanied by a steep decline in the total gas surface density at that radius, although the H i does continue outward at a lower level. This edge is coincident with the onset of strong warping of the outer disk. The mean ratio of CO (2-1)/ CO (1-0) emission is $1, high by Galactic standards. Peaks in this ratio in interarm regions suggest that the CO emission is optically thin there, among other possibilities. Thus, M83 is unusual in that it has a relatively axisymmetric, almost entirely molecular, inner 10 0 gas disk surrounded by a warped, entirely atomic, outer gas disk. A sharp discontinuity separates the inner and outer gas disks. This kind of gas distribution is not seen in other late-type, gas-rich spirals.

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Section: Co Gas and Star Formation Tracers: Morphology Comparisonsmentioning

confidence: 90%

CO in the Disk of the Barred Spiral Galaxy M83: CO (1–0), CO (2–1), and Neutral Gas

Crosthwaite¹,

Turner²,

Buchholz³

et al. 2002

The Astronomical Journal

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“…Their results were then related to the physical model of Helou & Bicay (1993). Although this method was found to improve the spatial correlation between the distributions of infrared and radio emission by reducing the residuals in the ratio maps, a single kernel failed to work perfectly for an entire galaxy disk as evidenced by the recognizable structures (i.e., spiral arms) left in the residual maps.…”

Section: Two-component Image-smearing Analysismentioning

confidence: 99%

“…A value of À1 describes how diffusion would occur for hard scattering off particles (i.e., the hard sphere approximation). On the other hand, ¼ À 1 3 best describes soft scattering off of structures, specifically, the case in which scattering is dominated by localized B perturbations whose spatial density is proportional to the density of the gas ( Helou & Bicay 1993). In reality, both scattering processes are likely active, which will correspond to an intermediate value of ; this value will likely vary within and among galaxies.…”

Section: Comparison With Scaling Relationsmentioning

confidence: 99%

Connecting Far‐Infrared and Radio Morphologies of Disk Galaxies: Cosmic‐Ray Electron Diffusion After Star Formation Episodes

Murphy

Hélou

Kenney

et al. 2008

ApJ

Self Cite

111

182

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We present results on the ISM properties of 29 galaxies based on a comparison of Spitzer far-infrared and Westerbork Synthesis Radio Telescope radio continuum imagery. Of these 29 galaxies, 18 are close enough to resolve at P1 kpc scales at 70 m and 22 cm. We extend the approach of our earlier work of smoothing infrared images to approximate cosmic-ray (CR) electron spreading and thus largely reproduce the appearance of radio images. Using a wavelet analysis, we decompose each 70 m image into one component containing the star-forming structures and a second one for the diffuse disk. The components are smoothed separately, and their combination compared to a free-free corrected 22 cm radio image; the scale lengths are then varied to best match the radio and smoothed infrared images. We find that late-type spirals having high amounts of ongoing star formation benefit most from the two-component method. We also find that the disk component dominates for galaxies having low star formation activity, whereas the structure component dominates at high star formation activity. We propose that this result arises from an age effect rather than from differences in CR electron diffusion due to varying ISM parameters. The bulk of the CR electron population in actively star-forming galaxies is significantly younger than that in less active galaxies due to recent episodes of enhanced star formation; these galaxies are observed within $10 8 yr since the onset of the most recent star formation episode. The sample irregulars have anomalously low best-fit scale lengths for their surface brightnesses compared to the rest of the sample spirals, which we attribute to enhanced CR electron escape.

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“…The origin of this relation is likely related to the prevalent energy-loss mechanisms for cosmic-ray electrons, such as synchrotron radiation and inverse Compton scattering (Condon 1992; Thompson et al 2006;Lacki et al 2010), escape of cosmic rays from the galaxy (Helou & Bicay 1993) and potentially additional processes like ionization or bremsstrahlung (Longair 1994;Murphy 2009). Due to the underlying complexity of the physical situation, it is thus surprising that a well-defined correlation emerges over nearly five orders of magnitude in luminosity (e.g.…”

Section: Introductionmentioning

confidence: 99%

A new interpretation of the far-infrared – radio correlation and the expected breakdown at high redshift

Schleicher

Beck

2013

A&A

115

143

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Context. Observations of galaxies up to z ∼ 2 show a tight correlation between far-infrared and radio continuum emission, suggesting a relation between star formation activity and magnetic fields in the presence of cosmic rays. Aims. We explain the far-infrared -radio continuum correlation by relating star formation and magnetic field strength in terms of turbulent magnetic field amplification, where turbulence is injected by supernova explosions from massive stars. We assess the potential evolution of this relation at high redshift, and explore the impact on the far-infrared -radio correlation. Methods. We calculate the expected amount of turbulence in galaxies based on their star formation rates, and infer the expected magnetic field strength due to turbulent dynamo amplification. We calculate the timescales for cosmic ray energy losses via synchrotron emission, inverse Compton scattering, ionization and bremsstrahlung emission, probing up to which redshift strong synchrotron emission can be maintained. Results. We find that the correlation between star formation rate and magnetic field strength in the local Universe can be understood as a result of turbulent magnetic field amplification. The ratio of radio to far-infrared surface brightness is expected to increase with total field strength. A continuation of the correlation is expected towards high redshifts. If the typical gas density in the interstellar medium increases at high z, we expect an increase of the magnetic field strength and the radio emission, as indicated by current observations. Such an increase would imply a modification, but not a breakdown of the far-infrared -radio correlation. We expect a breakdown at the redshift when inverse Compton losses start dominating over synchrotron emission. For a given star formation surface density, we calculate the redshift where the far-infrared -radio correlation will break down, yielding z ∼ (Σ SFR /0.0045 M kpc −2 yr −1 ) 1/(6−α/2) . In this relation, the parameter α describes the evolution of the characteristic ISM density in galaxies as (1 + z) α . We note that observed frequencies of 1−10 GHz are particularly well-suited to explore this relation, as bremsstrahlung losses could potentially dominate at low frequencies. Conclusions. Both the possible raise of the radio emission at high redshift and the final breakdown of the far-infrared -radio correlation at a critical redshift will be probed by the Square Kilometre Array (SKA) and its pathfinders, while the typical ISM density in galaxies will be probed with ALMA. The combined measurements will thus allow a verification of the model proposed here.

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A physical model of the infrared-to-radio correlation in galaxies

Cited by 126 publications

References 0 publications

CO in the Disk of the Barred Spiral Galaxy M83: CO (1–0), CO (2–1), and Neutral Gas

CO in the Disk of the Barred Spiral Galaxy M83: CO (1–0), CO (2–1), and Neutral Gas

Connecting Far‐Infrared and Radio Morphologies of Disk Galaxies: Cosmic‐Ray Electron Diffusion After Star Formation Episodes

A new interpretation of the far-infrared – radio correlation and the expected breakdown at high redshift

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